Applications of Ferric Oxide in Water Splitting by Electrolysis: A Comprehensive Review

Abstract

In water electrolysis, the use of an efficient catalyst derived from earth-abundant materials which is cost-effective and stable is essential for the economic sustainability of hydrogen production. A wide range of catalytic materials have been reported upon so far, among which Fe₂O₃ stands out as one of the most credible candidates in terms of cost and abundance. However, Fe₂O₃ faces several limitations due to its poor charge transfer properties and catalytic ability; thus, significant modifications are essential for its effective utilization. Considering the future of water electrolysis, this review provides a detailed summary of Fe₂O₃ materials employed in electrolytic applications with a focus on critically assessing the key electrode modifications that are essential for the materials’ utilization as efficient electrocatalysts. With this in mind, Fe₂O₃ was implemented in a heterojunction/composite, doped, carbon supported, crystal facet tuned system, as well as in metal organic framework (MOF) systems. Furthermore, Fe₂O₃ was utilized in alkaline, seawater, anion exchange membrane, and solid oxide electrolysis systems. Recently, magnetic field-assisted water electrolysis has also been explored. This comprehensive review highlights the fact that the applicability of Fe₂O₃ in electrolysis is limited, and hence, intense and strategically focused research is vital for converting Fe₂O₃ into a commercially viable, cost-effective, and efficient catalyst material.

1. Introduction

The energy derived from fossil fuels was the main driving force for the industrial and economic growth of the 19th century [1]. At present, roughly 60 to 80 percent of the total energy produced and consumed in developed countries is derived from fossil fuels such as coal, oil, and natural gas, supporting infrastructure systems such as electricity generation and transportation, as well as industry [2]. The past century shows the importance of fossil fuels and energy sources regarding the navigation of human advancement through industrialization and economic growth. However, the extensive usage of fossil fuels poses serious concerns in terms of greenhouse gases (GHG) and pollution, which cause an imbalance in nature, leading to environmental calamities [3]. Moreover, fossil fuels are non-renewable and geopolitically controlled natural resources which pose challenges in terms of constant extraction and controlled supply chains; thus, their replacement is inevitable [4]. Importantly, a profound shift toward carbon-free, renewable, and sustainable energy resources is essential in terms of safeguarding the environment and supporting economic stability along with industrial growth. Hence, the current focus of research presents an unprecedented opportunity to regulate global warming via the development of carbon-neutral technologies.

Among the currently available green (or renewable) energy contenders, hydrogen has been found to be the ideal option to replace fossil fuels [5]. The abundance of atomic and molecular hydrogen on Earth in a wide range of naturally available molecules such as water, fossil fuels, etc. makes it a potential candidate as a future renewable energy source [6]. The superiority of hydrogen as a source of energy compared with other sources is due to its extensive utilization in wider applications that provide energy (heat/electricity) with a high specific energy density [7] and to the fact that it releases carbon-free water as a byproduct, e.g., when used in fuel cells [8,9,10]. Hence, the demand for hydrogen is significantly increasing and is expected to continue to rise in the coming decades owing to industrial applications in addition to applications in energy infrastructure. The availability and green production of hydrogen in all parts of the globe (water), combined with sustainable routes of extraction, make it an ideal energy source for energy independence, regardless of geopolitical situations. Given its advantages and demand, the extensive research advancements in hydrogen-driven energy and industrial utilization could experience exponential growth due to the commitment of major economic players to shift toward carbon-free technologies.

Advancement toward a hydrogen infrastructure has gained significant attention and funding recently, as it plays an important role in achieving the Paris Agreement target set for the reduction in emissions by 2050 [11]. To achieve this target, reducing the cost of hydrogen is crucial; this depends on hydrogen production technologies, storage, transportation, and utilization. Recent developments reveal that the rapid and onsite/remote availability of hydrogen from a wide range of production technologies is capable of significantly reducing storage and transportation expenses. However, the choice of suitable hydrogen production technologies is a vital factor that influences the overall cost of hydrogen (LCOH—levelized cost of hydrogen). Figure 1 represents the various types of hydrogen produced around the globe, along with their sources of production and the amount of carbon emissions associated with these production technologies. Note that, except for green hydrogen, other methods of hydrogen production (black, blue, brown, and grey) use fossil fuels as a source of hydrogen; only green hydrogen uses water as a source (if renewables and water electrolysis are used). Moreover, except for blue and green hydrogen technologies, all other methodologies emit enormous amounts of GHG and are thus not recommended as sources of energy in the future. Notably, the reduction in carbon emissions (such as carbon dioxide, CO₂) for blue hydrogen is due to the carbon capture and sequestration (CCS) processes linked to those hydrogen production systems.

Among large-scale hydrogen production technologies, the steam methane reforming (SMR) method dominates as the most cost-effective production technology (>99%) [12]. However, the utilization of fossil fuels in SMR leads to enormous carbon emissions. Even though SMR technology could be converted to blue hydrogen by combining it with CCS, its dependence on fossil fuels makes it a geopolitical challenge and expensive (due to carbon capture processing) compared to the other technologies. Therefore, the use of carbon-free sources and methods, along with the use of renewable energy, is essential for achieving cost-effective, pilot-scale hydrogen production in order to fulfil the emission target and achieve cost reductions. In particular, rigorous research and development are essential to explore, develop, and exploit greener hydrogen production technologies that could meet future demands and are cost effective.

2. Overview of Electrochemical Water Splitting

Electrochemical water splitting is a process of water oxidation and reduction to produce O₂ and H₂ under an applied potential bias/electricity, as shown in Equation (1). The phenomenon of the decomposition of molecules using an applied current was first observed in 1789 and was later applied to in water electrolysis in 1800 by Nicholson and Carlisle [13]. Later, water electrolysis methodology was optimized and further developed into a mature technology for increased efficiency and practicality. Recently, the importance of water electrolysis has gained significant attention, given that it is carbon-free approach to hydrogen production utilizing renewable and sustainable resources. For electrolysis, the water required for H₂ and O₂ production could be sourced from the sea or any water body, and electricity generated using green and inexpensive renewable sources, such as solar and wind, could provide the required electrical energy to split the water molecules. Thermodynamically, the energy required to split water into H₂ and O₂ is shown in the below equation.

H₂O + Electricity (237.2 kJ mol⁻¹) + Heat (48.6 kJ mol⁻¹) → H₂ + ½ O₂

(1)

The above equation represents an ideal condition required in terms of the energy, electrodes, and charge dynamics required to split water into H₂ and O₂. Electrochemically (thermodynamic cell voltage), a minimum bias voltage of 1.229 V vs. a standard hydrogen electrode (SHE) is required to oxidize water at the anode to produce O₂ and to produce H₂ gas at the cathode surface. However, under normal experimental conditions, the required voltage has been found to be higher than the theoretical values (>1.229 V vs. SHE) to overcome the ohmic resistance of the electrolyte and the reaction kinetics in the electrochemical cell. The process of water electrolysis in an electrochemical cell is shown in Figure 2. To split water, the electrode materials are connected to the anode and cathode supply of the power output and are immersed in a suitable electrolyte. Under the applied electricity, the water molecules are split into O₂ via oxygen evolution reaction (OER) and H₂ through hydrogen evolution reaction (HER) at the anode and cathode surfaces, respectively. Generally, the anode and cathode compartments are separated using a separator/diaphragm/membrane to avoid the mixing of H₂ and O₂ gases and to maintain the flow of ions between the compartments.

Water electrolysis can be achieved in a wide range of electrolytes of different pH values with the utilization of efficient and stable electrodes. In acidic electrolytes, the water is oxidized at the anode surface, which gives two electrons, whereas the H⁺ ions are diffused through the separator/membrane toward the cathode to combine with electrons to yield H₂ gas. In neutral and alkaline electrolytes, OH⁻ ions are consumed to produce O₂ gas along with two electrons and a water molecule, which are reduced to H₂ at the cathode surface with the release of OH⁻ ions. The constant production of H₂ and O₂ is balanced by the production and diffusion of the relevant ions toward the respective electrode surface (Figure 2). In water electrolysis, the use of electrolytes is essential to provide significant conductivity between the electrodes, i.e., to decrease the ohmic resistance and create a feasible ionic environment at the electrode surface for effective OER and HER activity. Hence, the utilization of electrolytes with high ionic mobility is essential to decrease the thermodynamic barrier and energy requirement to oxidize and reduce water molecules. Generally, potassium hydroxide (KOH), sulfuric acid (H₂SO₄), sodium sulfate (Na₂SO₄), phosphate buffers, etc. are used as electrolytes in the water electrolysis systems.

3. Electrolysis Techniques

Electrolysis techniques are generally classified based on the setup and components utilized, as shown in Figure 3, among which alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE), anion exchange membrane water electrolysis (AEMWE), and solid oxide electrolysis cells (SOECs) are the most widely used methods (Figure 3). In AWE, the anode and cathode half-cell reactions take place (under the applied cell potentials) in different compartments separated by diaphragms/separators that allow the effective diffusion of OH⁻ ions from the cathode (after HER) to the anode side to carry out the OER process. AWE is generally operated with very high pH electrolytes (up to 5–7 M) using Ni-based electrodes. Due to its simplistic design and minimal component requirements, AWE is widely used in hydrogen production at pilot/industrial scales. In PEMWE, the anode and cathode sides are separated by a PEM (usually Nafion^®), which facilitates the effective diffusion of H⁺ ions. During the electrolysis, the water molecules are oxidized at the anode side, producing H⁺ ions that are transferred to the cathode side via the PEM for the water reduction reaction. For the electrolysis, acidic electrolytes are passed through the anode side containing an OER (iridium dioxide, IrO₂) catalyst. At the cathode side, platinum (Pt) nanoparticle-supported on carbon black (Pt/C) catalysts are widely employed as electrodes for HER processes. The PEM setup involves the use of gas diffusion layers (GDL), which yield high-purity H₂ compared to that produced using AWE.

The AEMWE was developed by combining the advantages of AWE and PEMWE. The AEMWE is a membrane-based alkaline electrolyzer that produces high-purity H₂ in mild basic conditions without the use of platinum group metal (PGM) catalysts. During electrolysis, the basic electrolyte is fed into both the anode and cathode side to produce H₂, whereas the OH⁻ ions diffuse toward the anode through the AEM to generate O₂. Recently, AEMWE has demonstrated promising breakthroughs, indicating its commercial viability and pilot-scale production capability. SOEC systems are operated at very high temperatures (>700 °C) to split water into O₂ and H₂. At elevated temperatures, the water is fed in the form of steam at the cathode side, where H₂ is produced. A solid ceramic membrane was used to separate the anode and cathode sides, allowing the effective diffusion of O^2– ions from the cathode to the anode cell. Similar to AEMWE, SOEC has exhibited promising commercial viability, indicating its potential contribution to hydrogen production in the future.

In water electrolysis systems, the catalysts and electrodes employed for OER and HER activities play a vital role in determining the overall efficiency, stability, and long-term operation potential. Noble metals (PGMs) are widely known for their efficient catalytic activity toward OER and HER and thus are routinely used in commercial and pilot-scale electrolyzers. In particular, the noble metals show significantly low overpotential (η) to split water into H₂ and O₂ in comparison to the non-noble metal catalysts. Hence, a non-noble metal catalyst needs comparatively higher overpotentials to perform OER and HER. Despite the efficiency factor, the cost, abundance, and geographical conditions need to be considered for commercial acceptance, along with cost reductions. Hence, catalysts derived from non-noble metals are recommended, as these are cost-effective, abundant, and stable. With this in mind, the catalysts/electrodes play a crucial role in determining the hydrogen production cost, and thus, the viability of commercial hydrogen. However, non-noble catalysts show limited catalytic activity, and thus, various modifications and different strategies are essential [14,15,16] for their effective implementation in electrolyzers. Therefore, the choice of a suitable catalyst and the required modifications are critical in developing efficient catalysts for water splitting in electrolyzers.

4. Fe₂O₃

The use of metal oxides in OER processes is known to be an effective approach, as the surface of metal oxides (MOx) provides essential sites for reaction intermediates to be adsorbed to initiate OER activity. Specifically, the adsorption and desorption of essential reaction intermediates on the metal oxide surface is an important step toward achieving water splitting reactions. Reaction intermediates with optimum adsorption energies at the oxide surface hold the key to improving water splitting efficiency [17]. Moreover, the conductive nature of MOx means that it serves as an effective charge diffusion/transfer medium. MOx can be obtained with different nanostructures, crystal phase/planes, and doped, and it can be anchored to a wide range of catalysts/substrates. Hence, the use of MOx in electrocatalytic processes offers a promising strategy for hydrogen production. Among metal oxides, Fe₂O₃ is an abundant mineral form of Fe metal (in addition to other minerals) in nature. The hematite mineral form of Fe₂O₃ is the naturally occurring form of iron, a known primary element extracted from the earth’s crust. In general, Fe₂O₃ has a dark red or brown color and has various polymorphs. Fe₂O₃ is known for its relatively weak ferromagnetic properties at room temperature; it also exhibits antiferromagnetic properties below 260 K. Even though the magnetic properties of Fe₂O₃ are not known to directly influence its other applications, they have been found to affect its carrier transport properties and optoelectronic characterizations, owing to the various spin configurations. Furthermore, Fe₂O₃ exhibits strong photon absorption in the visible yellow region to UV, with a photon transmission in the orange to the infrared region that results in the typical red appearance. Fe₂O₃ has a band gap of 2.1 eV, which accounts for the absorption of ~40% of sunlight and thus presents semiconductor properties. Fe₂O₃ is highly stable and can be easily dissolved/reacted with most acids. The other forms of iron oxides generally found in nature are FeO (iron (II) oxide) and Fe₃O₄ (magnetite). Due to its abundance, cost, absorbance, stability, and non-toxic nature, Fe₂O₃ is effectively utilized in a wide range of applications [18], including as an anode material in batteries [19], in supercapacitors [20], in gas-sensing [21], in electrochemical sensors [22], for photoelectrochemical water splitting [23], dye degradation [24], in biomedical applications [25], and in electrolysis [26]. In particular, considering its OER capabilities under the applied bias potential (with or without illumination), Fe₂O₃-based electrodes have been used for water-splitting applications using both photoelectrochemical [27] and electrocatalytic modes [28]. Furthermore, Fe₂O₃ has been effectively employed in biomedical applications due to its superior permeability and stability. In particular, Fe₂O₃ has been utilized to transfer therapeutic agents through tissues and in controlled drug-releasing agents. Due to its non-toxic nature, Fe₂O₃ is well suited as nano-carriers for drugs that could limit and regulate the drug dosage and supply, resulting in more effective treatments. The utilization of Fe₂O₃ in medicine offers effective treatments for drug release and limits overdosages to certain body sites. Recent work on Fe₂O₃ applications has led to Fe₂O₃ being recognized as a superior catalyst molecule when employed directly and as a support material. Owing to its unique physiochemical properties, two different valence oxidation states, and surface properties, Fe₂O₃ is being effectively employed in energy applications. Therefore, the use of Fe₂O₃-based materials as electrodes, photoelectrodes, and catalysts could provide a promising opportunity for cheap hydrogen production technologies.

Fe₂O₃ is found in α-, β-, γ-, and ε- polymorphs, having different properties and stabilities. Among the polymorphs, the α phase has been widely studied owing to its stability and feasible application in charge storage devices and as pigments in paints [29,30]. β-Fe₂O₃ contains a bixbyite structure and is a rarely studied polymorph due to its intermediary metastable condition, which transforms to α-Fe₂O₃ at higher temperatures [31]. The γ polymorph of Fe₂O₃, also known as maghemite, contains a cubic structure and shows ferromagnetic properties [32]. The ε-Fe₂O₃ polymorph is a well-known magnetic material exhibiting a non-centrosymmetric orthorhombic structure [33]. Among the polymorphs, both α and γ show interchangeable crystalline forms, in which α-Fe₂O₃ becomes dominant and stable at higher temperatures when treated in an inert atmosphere and with a reducing agent [34].

Hematite (Fe₂O₃) is commonly described as having a hexagonal crystal structure. However, the fundamental crystal system is found to have a rhombohedral structure, that is, a hexagonal unit cell with specific parameters is used to describe the rhombohedral lattice of hematite Fe₂O₃. A typical crystal structure of hematite Fe₂O₃ is presented in Figure 4 (generated using Vesta [35]). The rhombohedral crystal system of hematite shows $\stackrel{-}{\mathrm{R}}$3c space groups which could be transformed and described using hexagonal coordinates. Note that even though the underlying crystal structure of hematite Fe₂O₃ is rhombohedral, it is often described in terms of a hexagonal lattice system for convenience, as shown in Figure 4. In hematite α-Fe₂O₃ having $\stackrel{-}{\mathrm{R}}$3c space group, six equivalent O²⁻ atoms are bonded to one Fe³⁺ atom to form FeO₆ octahedra, whereas the lattice O²⁻ ions appear to be bonded to four equivalent Fe³⁺ atoms. The O²⁻ atoms bonded to Fe³⁺ form a mixture of distorted edge and corner-sharing OFe₄ trigonal pyramids, as shown in Figure 4, whereas the FeO₆ octahedra is found to form an edge and corner-sharing, along with a distorted face configuration with the neighboring octahedra.

4.1. Water Splitting Applications of Fe₂O₃

Recently, Fe₂O₃ has been extensively used in water-splitting applications in both photoelectrochemical and electro-catalytical modes. Among the widely explored transition metals, Fe demonstrates considerably superior electrocatalytic activity, including HER, OER, and the oxygen reduction reaction (ORR), even compared to widely used Co and Ni-based materials [36,37,38]. Owing to its valency between the (III) and (II) states that offers exceptional catalytic properties and stability in comparison with the Ni and Co-based metal oxides, Fe possesses a feasible reversible redox capability [39]. Figure 5a shows the number of research articles published (photoelectrochemical and electrocatalytic modes) on the use of Fe₂O₃ electrodes for water splitting in recent decades. A gradual increase in the number of articles indicates that Fe₂O₃ is gaining significant attention owing to its semiconductor and catalytic properties. The extensive utilization of Fe₂O₃ in photoelectrochemical and photocatalytic applications is evident, owing to its absorbance and band edge properties that facilitate OER activity using photogenerated charges. Figure 5b shows the number of articles published during the last decade concerning the electrocatalytic water-splitting activity of Fe₂O₃, indicating the importance of Fe₂O₃ in water-splitting applications. In electrocatalytic water splitting systems, Fe₂O₃ materials with relevant modifications, including heterojunction with oxides/carbon/hydroxide/MOF systems, doping, oxygen vacancy tuning, etc., have been employed. In addition, Fe₂O₃ has exhibited significant activity as a catalyst in AEM and SOEC systems.

Table 1. Fe₂O₃ electrodes employed in electrolysis and their experimental and electrochemical properties.

Catalyst/Electrode	Synthesis Method	Electrolyte	Stability	Overpotential mV vs. SHE	Tafel Slope (mV dec−1)	Ref
Ni and Zn-doped Fe2O3	Combustion method	0.1 M KOH	-	OER: 350 (for 10 mA cm−2)	OER: 26	[40]
Se-Fe2O3@Ni/NiO	Thermal method	1 M KOH	21 h	OER: 205 (for 10 mA cm−2)	OER: 36	[41]
P-doped Fe2O3/ZnO	Chemical bath deposition/hydrothermal/CVD	1 M KOH	24 h	OER: 250 HER: 139 (for 10 mA cm−2)	OER:40 HER: 66	[42]
Zn and S co-doped Fe2O3	Hydrothermal/CVD	1 M KOH	100 h	OER: 350 (for 500 mA cm−2)	OER: 47.5	[43]
Co3O4/Fe3O4	Co-precipitation method	0.1 M KOH	8 h	OER: - HER: -	OER: 62 HER: 102	[44]
C-doped CoFe2O4/Fe2O3	Calcination approach	1 M KOH	5.5 h	OER: 260 HER: 236 (for 20 mA cm−2)	OER: 183 HER: 146	[45]
CoMo/Fe2O3	Hydrothermal/Electrodeposition methods	1 M KOH	100 h	OER: 266 HER: 71 (for 10 mA cm−2)	OER: 54 HER: 85	[46]
Fe2O3@CuO	Hydrothermal	1 M KOH	25 h	OER: 230 HER: 130 (for 10 mA cm−2)	OER: 54 HER: 77	[47]
IrO2–Fe2O3	Thermal decomposition	0.5 M H2SO4	600 cycles	HER: 78 (for 10 mA cm−2)	HER: 36.2	[48]
RuO2–Fe2O3	Synthesis thermal treatment	1 M KOH	-	OER: 292 (for 500 mA cm−2)	OER: 56.08 HER: −43	[49]
RuO2/Fe2O3	Synthesis	1 M KOH	18 h	OER: 386 HER: −239 (for 10 mA cm−2)	OER: 67 HER: 97	[50]
Fe2O3@NiO	Hydrothermal	1 M KOH	20 h	OER: 224 HER: 187 (for 10 mA cm−2)	OER: 20 HER: 53.8	[51]
WO3/Fe2O3-NiO	Chemical etching reaction and decomposition	1 M KOH	100 h	OER: 211 (for 100 mA cm−2)	OER: 39.5	[52]
Fe2O3/Ni(OH)2	Electrodeposition	1 M KOH	18 h	OER: 291 (for 10 mA cm−2)	OER: 53.7	[53]
Ni1Fe2@Fe2O3@C	High-temperature calcination	1 M KOH	30 h	OER: 271 (for 10 mA cm−2)	OER: 78	[54]
Fe2O3-MnO	Sol-gel method	1 M KOH	1000 cycles	OER: 370 (for 10 mA cm−2)	OER: 66	[55]
Fe2O3ǁǁFeP	Electrochemical oxidation/solution self-assembly/pyrolysis	1 M KOH	20 h	Hydrazine overpotential: 0.61 V HER: 138 (for 10 mA cm−2)	OER: 179.2 HER: 63.9	[56]
Fe2O3/FeP	Hydrothermal	1 M KOH	12 h	OER: 264 (for 10 mA cm−2)	OER: 47	[57]
Fe2O3/FeS	Hydrothermal	1 M KOH	10 h	OER: 264 (for 40 mA cm−2)	OER: 90	[58]
FeS/Fe2O3	Chemical etching/solvothermal	1 M KOH	10 h	OER: 266.5 (for 10 mA cm−2)	OER: 51.17	[59]
Fe2Se3/Fe2O3	Synthesis	1 M KOH	24 h	OER: 160 (for 20 mA cm−2)	OER: 30.02	[60]
Fe2O3/CNT	Co-precipitation method	1 M KOH	12h	OER: 383 (for 10 mA cm−2)	OER: 62	[61]
Fe/Fe2O3-Fe-N-doped C	Pyrolysis	0.1 M KOH	10h	OER: 0.69 V vs Ag/AgCl (for 10 mA cm−2)	OER:77.5	[62]
Fe2O3/g-C3N4	Thermal method	0.5 M KOH	10 min	OER: 425 (for 10 mA cm−2)	OER: 280	[63]
Fe2O3-C	Pyrolysis	1M KOH	48 h	HER: 245 (for 10 mA cm−2)	HER: 76.6	[64]
Ni3S2/Fe2O3/N-doped carbon	Thermal process/CVD	1 M KOH	200 h	OER: 188 (52 mA cm−2) HER: 78 (10 mA cm−2)	OER: 64.3 HER: 115.8	[65]
Fe2O3/N-graphene	Co-deposition (hydrothermal and electrodeposition)	1 M KOH	22 h	OER: 313 (for 100 mA cm−2)	OER: 81	[66]
Fe2O3/MWCNT	Pulsed laser ablation	1M KOH	10 h	OER: 310 (for 10 mA cm−2)	OER: 20.35	[67]
MOF-Fe2O3	Solvothermal	1 M KOH	20 h	OER: 439 HER: 230 (for 10 mA cm−2)	OER: 99 HER: 100	[68]
Fe2O3/NiO	Chemical bath deposition	1 M KOH	50 h	OER: 182 (for 10 mA cm−2)	OER: 45
		Sea water		OER: 291 (for 1 A cm−2)		[69]
		Domestic sewage		OER: 329 (for 1 A cm−2)
P-Fe2O3-CoP	Hydrothermal-gas-phase phosphorization process	Freshwater	12 h	OER: 250 HER: 219 (for 10 mA cm−2)	OER: 42 HER: 79	[70]
		Seawater		OER: 270 HER: 152 (for 10 mA cm−2)	OER: 59 HER: 95
RuNi-Fe2O3/IF	Hydrothermal	1 M KOH	100 h	OER: 329 HER: 75 (for 100 mA cm−2)	OER: 60.85 HER: 85.08	[71]
		1 M KOH + seawater		OER: 424 HER: 298 (for 100 mA cm−2)	OER: 69.58 HER: 114.31
Co-Fe2O3 (AEM)	Microwave-assisted hydrothermal	1.0 M KOH	500 h	OER: 310 (for 10 mA cm−2)	OER: 29	[72]

shows the literature reports on Fe₂O₃ in electrocatalytic water splitting applications. Additionally, the sections below provide a detailed discussion of various catalytic activity and electrochemical characterizations.

4.2. DFT Studies on the OER Activity of Fe₂O₃

Density functional theory (DFT) calculations offer a powerful tool to provide significant information regarding the surface and intrinsic properties of different elements/materials. In the case of Fe₂O₃, different surface orientations/crystal planes exhibit different atomic arrangements, distance defects, and atom densities which, in turn, affect their electronic structure and reactivity. Specifically, the adsorption of OER intermediates, their catalytic nature, and the stability of different facets/planes demonstrate important parameters in determining the efficiency of the water splitting. With this in mind, a wide range of DFT studies have been reported, highlighting the effects of different facets and their OER activities. Mainly, the (0001) facet of Fe₂O₃ has been reported to be more favorable for OER compared to other facets. The OER activity on the (0001) facet of completely hydroxylated Fe₂O₃ favors [73] the effective adsorption of reaction intermediates, resulting in enhanced OER activity. Furthermore, the doping of Fe₂O₃ (containing a fully hydroxylated surface) at the Fe and O sites using metal ions and anions, respectively, shows improved activity. For example, Ni and Co doping in Fe₂O₃ lead to the formation of a thermodynamically favored surface for effective adsorption reaction intermediates and increased efficiency of OER activity, resulting in low overpotentials. In-depth studies have indicated that active anion dopants at O lattice sites could improve the stability of the holes [73]. Furthermore, the surface states present at the (0001) facet of Fe₂O₃ have been found to affect the OER activity. The formation of a stable peroxo Fe-O-O-Fe adsorbate at the Fe₂O₃ surface has been found to be possible, whereas the presence of a Fe-O∙ type bond could be effective and is one of the possible intermediate pathways for the OER activity at the (0001) surface [74].

Similarly, DFT studies performed on the different exposed facets of Fe₂O₃ [75] have indicated a specific favorable OER orientation. Several facets of Fe₂O₃ have been studied, including (211), (021), (101), (210), and (100), which have, in general, been found to be less dominant in terms of OER activity. The calculations were performed by considering the free energy and overpotential values that dictate the four electron-coupled electron transfer reactions with OER intermediates. Studies revealed that the different orientations showed different free energy and overpotential values owing to the activity of intermediate species adsorbing on the facets, resulting in varied stability concerning the free energy values, which affects the resulting OER performance. Specifically, the orientation of Fe₂O₃ along (100) has been found to show relatively decreased overpotential values compared to the (211), (021), (101), and (210) orientations. Notably, the surface site in Fe₂O₃ that is responsible for the bridging of two Fe atoms on the (100) surface is favorable for adsorbate interactions, leading the enhanced OER activity. Interestingly, the (110) facet of Fe₂O₃ is known to be one of the favorable orientations for OER activity, as demonstrated by Zhang et al. [76] because, at the (100) facet, a noticeable competing OER reaction intermediate was observed at a single site between the O-O coupling and the formation of OOH. Here, O–O coupling refers to the involvement of two adjacent terminal Os at a dual site, leading to O–O bond formation. Furthermore, the presence of oxygen vacancies plays a critical role in decreasing the OER overpotentials [76]. Conclusively, DFT studies have indicated that the possible reaction pathways and favorable facets of Fe₂O₃ could be modified via the surface state, vacancies, and doping.

4.3. Crystal Facet and Morphology Tuning in Fe₂O₃

The morphology and the exposed crystal plane of Fe₂O₃ have a significant influence on the OER activity [77]. Generally, the interpretation of the structure−activity of electrodes/catalysts is cumbersome when analyzed using conventional electrochemical techniques. In most cases, interpretations of the results have been based on the average activity rather than a specific site or plane of activity, yielding an unambiguous conclusion. Therefore, for better clarity and more accurate interpretations, the direct mapping of electrochemical activities on the isolated crystals is essential. Using scanning electrochemical cell microscopy, Li et al. [77] proposed a single-entity electrochemical strategy to investigate the site-specific OER activity of Fe₂O₃. In their study, a single nanorod crystal of Fe₂O₃ was considered to monitor the OER activity; heterogeneity between and within the single nanorod crystal was observed. The single crystal Fe₂O₃ nanorods were obtained using the hydrothermal method. The produced nanorods has 0.5–1.5 μm rod lengths with a width of ~150 nm. The high-resolution morphology study confirmed significant electrochemical activity owing to the {001} plane present in the nanorod body. Accordingly, the enhanced OER activity was recorded at the {100} facet at the tip, which exhibited significantly increased activity. Hence the facet-dependent OER activity indicated that producing longer Fe₂O₃ nanorods with desired exposed facets would be beneficial compared to the other morphologies. Importantly, the proposed single-entity and sub-entity mapping strategy were shown to be an effective route to explore the intrinsic, surface, and facet-dependent catalytic activity; this approach could be broadly extended to other metal oxide nanostructures. The most important aspect of that study was the interpretation of the structural characteristics of most reactive OER sites on the surface of Fe₂O₃, which is crucial to optimize and engineer catalytic properties which could be applied to electrolyzers. Within the framework of a nanostructure, a crystal plane with active reactive sites is the key that needs to be recognized by engineering a specific nanoarchitecture which is generally hidden, shadowed, or removed/decreased in scale due to the presence of other nonreactive sites/planes.

4.4. Doped Fe₂O₃ Catalysts

The properties of metal oxides can be tuned/engineered to enhance electrochemical properties by introducing cations/anions into the crystal via doping [78]. In the case of Fe₂O₃, the intrinsic and surface properties could be altered/modified by doping. Generally, doping is achieved using a cation or an anion (or both) that displaces Fe or O atoms in the lattice. Doping could be performed during the synthesis process or in subsequent steps at high temperatures that ensure the effective displacement of Fe or O atoms from their lattice locations. The strategy of doping has advantages for materials like Fe₂O₃, which are abundant and cheap materials that show poor catalytic properties in their base form. Based on the nature of the dopant material, the properties of Fe₂O₃ could be tuned/engineered for optimum efficiency. For example, the utilization of Ni or Zn as a redox and non-redox active species, respectively, has been found to have a significant influence on the catalytic activity of Fe₂O₃, as noted by Shah et al [40]. Pristine and doped (Ni and Zn) Fe₂O₃ were obtained using a flame combustion method, followed by ball milling at 25 Hz for 15 min. Even though Ni is known to be effective as a dopant, Zn doping in Fe₂O₃ yielded significantly higher electrocatalytic properties. A low overpotential value of 350 mV was observed to reach 10 mA cm⁻². The experimental and theoretical calculations indicated that the Ni dopant sites had a redox characteristic, whereas the Zn dopant sites on Fe₂O₃ showed a non-redox characteristic. With this in mind, Ni tends to lower the energetic barrier of the OER process, resulting in improved catalytic activity compared to pristine samples. However, in the case of Zn doping, a unique reaction pathway was observed, which is termed as proton-coupled electron transfer; this approach is thermodynamically favored and thus showed enhanced OER activity. Pristine F₂O₃ offers routine four-electron oxidation reaction sites, while Zn-Fe₂O₃ provides a rapid and effective reaction site that utilizes efficient two-site reaction pathways. Here, the Zn site was shown to possess a non-redox characteristic, as it maintained a constant charge during the water oxidation process.

Based on the nature of the dopant, doping amount, and site, a significant alteration of the surface and bulk properties of Fe₂O₃ could be achieved. For example, the doping of Se in Fe₂O₃ linked to Ni/NiO particles was reported to show efficient OER activity [41]. The synthesis of Se-doped Fe₂O₃ on a Ni/NiO nanostructure was achieved using a thermal method that supported in situ deposition directly on carbon fibers. The formation of a solid solution was not observed in the proposed method due to the limited etching of Fe to the carbon support, leading to the doping of Se in Fe₂O₃ that inhibited the diffusion to Ni system to form Se-Fe₂O₃-Ni/NiO. This characterization indicated that the employed technique successfully incorporated nanoparticles of Se-Fe₂O₃-Ni/NiO into the porous structure of the carbon fiber. LSV measurements performed in 1M KOH indicated the production of 10 mA cm⁻² current density at an impressively lower overpotential value of 205 mV for the OER. Furthermore, a low Tafel slope of 36 mV dec⁻¹ was recorded, with a low charge transfer resistance at the electrolyte interface. The superior conducting property of the carbon substrate and the systematic arrangement of Fe₂O₃@Ni/NiO nano junction systems in the porous carbon substrate structure, along with favorable interfaces (Se-Ni/NiO and Se-Fe₂O₃), led to an impressive electrochemical result, confirming the efficient OER activity of the Se-Fe₂O₃-Ni/NiO electrode.

The introduction of nonmetal/anion dopants into a Fe₂O₃ lattice is known to improve its electrocatalytic properties [79]. Unlike doping with Se, doping with P transforms Fe₂O₃ into an efficient bifunctional catalyst which has been utilized only as an OER catalyst [42]. Previous studies have shown that the introduction of PO₄ into Fe₂O₃ improves the catalytic activity; this could be synthesized by performing electrochemical cycles in phosphate buffers [80]. The doping of P in iron oxides is known to regulate the electrical properties [79,81] which, in turn, effects the electrocatalytic efficiency. Generally, a support material is required for Fe₂O₃ owing to its limitation regarding the mass transfer process. In such a case, different metal oxides and morphologies have been adopted as support materials for the Fe₂O₃ catalyst. For example, the utilization of ZnO nanotubes as a support material for P–Fe₂O₃ is known to boost the catalytic activity, as the 1-dimensional tube structure of the support oxides (ZnO) provides effective pathways for the charges. On the other hand, the doping of Fe₂O₃ by anions such as P has been found to increase the donor density and improve the charge transfer process. A ZnO-coated P-Fe₂O₃ nanostructure was fabricated using chemical bath deposition (ZnO nanotube), followed by solution processing (Fe) and chemical vapor deposition (for P doping). During the water splitting experiments, ZnO-coated P-Fe₂O₃ generated 10 mA cm⁻² of current density at low OER overpotential at 250 mV, whereas the HER overpotential was observed at 139 mV. The applicability of ZnO/P-Fe₂O₃ was evaluated by testing the overall water splitting in two electrode systems, in which the HER and OER processes were effectively performed at a lower bias potential of 1.62 V. Furthermore, the prolonged electrolysis ability of the electrode over six days was established, indicating the stability and versatility of the ZnO-coated P-Fe₂O₃ bifunctional catalyst. Similarly, P-doped Fe₂O₃ linked to CoP was also explored for overall water splitting, including seawater electrolysis (discussed in later sections). Liu et al. [43] demonstrated the doping of both anion and metal ions into Fe₂O₃ for enhanced water-splitting applications. Zn was used as a cation, whereas S was employed as an anion dopant, which was simultaneously introduced into Fe₂O₃ linked to Fe₃O₄ on the iron foam. The Zn and S doped-Fe₂O₃/Fe₃O₄ (on iron foam) was found to undergo directional reconstruction into FeOOH to transform into Zn and S doped-Fe₃O₄-FeOOH (on iron foam). Even though Zn/S/Fe₃O₄-FeOOH showed the highest OER efficiency, its Fe₂O₃ counterpart showed significant water-splitting efficiency. Conclusively, doping Fe₂O₃ by anions and/or metal ions seems to be the key to enhancing the water-splitting activity and stability.

4.5. Fe₂O₃ in Heterojunction Systems

Constructing a heterojunction system with Fe₂O₃ offers significant advantages, e.g., providing support to the catalyst, acting as the main catalyst to support active sites, providing oxygen-deficient defects, etc., thereby improving the electrocatalytic activity. Specifically, the utilization of Fe₂O₃ as a support material for the catalyst is expected to decrease the quantity of loading of the main catalyst, which becomes vital in the case of expensive and rare metal-based catalysts. For example, the loading of a Co₃O₄ catalyst could be significantly reduced by using Fe₂O₃ as a support [44]. Even though experimental results have shown that Fe₃O₄ acts as a superior support to Co₃O₄ compared to Fe₂O₃, it still behaves more efficiently than the base Fe₂O₃ and Co₃O₄ electrodes, indicating the synergistic effect of using Fe₂O₃ as a support [44]. In the case of C-doped CoFe₂O₄ linked to Fe₂O₃, a significantly lower overpotential requirement was noted [45], that is, the C-doped CoFe₂O₄/Fe₂O₃ required 260 mV of overpotential to produce an OER current density of 100 mA cm⁻² and an HER overpotential of 236 mV, indicating excellent catalytic activity for overall water splitting. Similarly, the conjugation of Mo in addition to Co (as an alloy) with Fe₂O₃ has shown promising water-splitting results [46] owing to its bifunctional catalytic properties. The fabrication of a CoMo-Fe₂O₃ heterojunction on a nickel foam involved two sections, in which Fe₂O₃ was first deposited on the nickel foam using the hydrothermal method, followed by CoMo alloy deposition using the electrodeposition technique. The Fe₂O₃ nanostructure on the nickel foam appeared to have a nanosheet morphology, whereas the CoMo alloy showed spherical microparticles distributed evenly on the Fe₂O₃ nanosheets. The unique two-dimensional structure with spherical particles in the CoMo-Fe₂O₃ system could result in increased penetration of electrolyte ions, leading to an increase in the number of reaction sites, and thus, enhanced electrocatalytic activity. The electrochemical characterization results of CoMo-Fe₂O₃ revealed an impressive current density value of 50 mA cm⁻² for the OER process at the lowest overpotential of 266 mV. On the other hand, the 10 mA cm⁻² of HER current was recorded at an potential value of 71 mV. The Tafel slope for OER and HER activity was found to be 54 and 85 mV dec⁻¹, respectively. The practicality of the CoMo/Fe₂O₃ catalyst was demonstrated by using two electrode setups for alkaline water splitting that exhibited a current density of 10 mA cm⁻² at a lower bias voltage of 1.5 V. A theoretical study on CoMo/Fe₂O₃ revealed the formation of an effective interface that facilitated efficient charge redistribution and transfer, leading to enhanced catalytic activity. Importantly, a remarkable stability of over 100 h was recorded under prolonged electrolysis conditions as a bifunctional catalyst. Alothman et al. [47] have proposed the Fe₂O₃@CuO heterojunction as a bifunction catalyst for water electrolysis. Using a simple hydrothermal method followed by calcination, Fe₂O₃@CuO catalysts were synthesized; these were then coated onto a nickel foam by forming an ink with nafion. The optimized Fe₂O₃@CuO electrode exhibited OER and HER overpotentials of 230 and 130 mV, respectively, reaching 10 mA cm⁻² of current density. Furthermore, a Tafel slope of 54 and 77 mV dec⁻¹ was observed in the OER and HER voltammograms, respectively.

Noble metal catalysts could be anchored to Fe₂O₃ to enhance its activity and bifunctionality and reduce costs. For example, Fe₂O₃ and IrO₂ are well known OER catalysts; in contrast, their HER activity is rarely exploited. However, a composite of Fe₂O₃ and IrO₂ was found to perform effectively as a HER catalyst, as demonstrated by Yang et al. [48]. A thermal decomposition method was used that effectively optimized the IrO₂ and Fe₂O₃ content. The electrochemical measurements conducted in H₂SO₄ (0.5 M) indicated that the adsorption of hydrogen was more efficient on the IrO₂/Fe₂O₃ composite compared to its individual counterparts, resulting in a shift of overpotential onset toward the 0 V vs. RHE, due to the Volmer–Heyrovsky mechanism. Similarly, Mosallaei et al. [49] developed a RuO₂–Fe₂O₃ catalyst using the Ru-ionic complex. The proposed strategy differentiates itself from the reported methods by not using common metal salts for the oxide catalyst synthesis. In this approach, the synthesized Ru and Fe complex (Ru(then)₃]₂[Fe(CN)₆] and [Ru(phen)₃] [Fe(CN)₅(NO)]) were calcined to yield the RuO₂–Fe₂O₃ nanocrystal catalyst. A morphological analysis indicated good crystallinity of RuO₂–Fe₂O₃ with an average particle size of 8–12 nm. In the OER and HER experiments, the RuO₂–Fe₂O₃ catalyst exhibited an overpotential value of 292 and −148 mV, respectively, to produce a current density of 10 mA cm⁻². The low Tafel slope values of 56.08 and −43 mV dec⁻¹ were calculated from the OER and HER process LSV curves, respectively. The low charge transfer resistance at the electrolyte interface indicated the utilization of holes and electrons for OER and HER activity, confirming the superior bi-functional catalytic activity of the RuO₂–Fe₂O₃ catalyst. A similar strategy was employed by Mosallaei et al. [50] to produce a RuO₂–Fe₂O₃ catalyst using metal complexes. In their protocol, an impregnation method was used by utilizing [Ru(dmbpy)₃]₃[Fe(CN)₆]₂, an ionic pentanuclear complex, followed by thermal decomposition to yield RuO₂–Fe₂O₃ on a reduced graphene oxide nanosheet support. The nanorod and cubic-shaped RuO₂–Fe₂O₃ were found to be evenly distributed on the reduced graphene oxide nanosheets, indicating the presence of effective diffusion pathways via the superior conductivity and charge transfer properties of the reduced graphene oxide nanosheets during the water splitting reactions. Owing to their superior catalytic and nanostructure properties, the RuO₂–Fe₂O₃ and highly reduced graphene oxide nanosheets deposited on glassy carbon electrode exhibited an OER and HER overpotential value of 386 and −239 mV and a Tafel slope value of 67 and −97 mV dec⁻¹, respectively, in a 1 M KOH electrolyte. Furthermore, significant stability of the catalysts over 16 h was observed during the chronoamperometric measurements. Conclusively, the systematically arranged RuO₂–Fe₂O₃ catalysts on highly reduced graphene oxide nanosheets were shown to offer significantly increased catalytic sites with increased reaction surface area and porosity, leading to a synergistic effect for enhanced overall water splitting.

Yan Sang, ref. [51] proposed a one-step hydrothermal fabrication method for a Fe₂O₃/NiO bifunctional catalyst for water splitting. The material morphology showed a systematic distribution of Fe₂O₃ nanoparticles (50–80 nm) on hexagonal-shaped NiO nanoflakes. The LSV measurements in 1.0 M KOH indicated that Fe₂O₃/NiO required an OER overpotential of 224 mV and a HER overpotential value of 187 mV to achieve 10 mA cm⁻² of current density. Furthermore, a Tafel slope of 20.0 and 53.8 mV dec⁻¹ was observed from the LSV data of the OER and HER activity, respectively. Importantly, Fe₂O₃/NiO demonstrated its bifunctional catalytic activity by exhibiting alkaline overall water splitting at a cell potential value of +1.63 V vs. SHE and over 20 h of stability. The enhanced OER and HER performance of the Fe₂O₃/NiO catalyst was ascribed to the optimized electronic structure of heterojunctions, owing to the optimum level of oxygen vacancies in Fe₂O₃ that favor efficient charge transfer to improve the catalytic activity. The introduction of WO₃ into the Fe₂O₃/NiO catalyst was found to decrease the required overpotential [52]. However, unlike Fe₂O₃/NiO, the WO₃/Fe₂O₃/NiO catalyst functioned as only an OER catalyst, rather than as a bifunctional catalyst. The adopted synthesis method involved simultaneous hydrothermal treatment, comprising an etching reaction and the decomposition of metal ions. Such a synthesis approach caused the regulation of the intrinsic electronic properties of the catalysts due to the presence of high-density heterometal-oxygen bridge sites at the interface. A remarkably low overpotential value of +211 mV was shown to achieve 100 mA cm⁻² of OER current density. Furthermore, a low Tafel slope of 39.5 mV dec⁻¹ indicated excellent catalytic properties, and a stability of over 100 h was observed. A detailed mechanistic study indicated that the presence of W in the catalysts sped up the oxidation of Ni to the 3+/4+ state from the 2+ state due to the partial electron transformation from the high valence W⁶⁺ state. Among the reaction intermediates, a weaker adsorption energy of OH* was expected on WO₃ and a stronger one on Fe₂O₃. Being a stronger Lewis acid, W⁶⁺ readily oxidizes the neighboring Ni²⁺ and Fe³⁺ via the withdrawal of electrons. This mechanism optimizes the adsorption energies of the reaction intermediates and thus enhances the catalytic activity [52]. Conclusively, the presence of W has been revealed to be the key factor in achieving the lower overpotential of OER activity in Fe₂O₃/NiO catalysts. However, in the case of Fe₂O₃/Ni(OH)₂ and Fe₂O₃/NiFe alloy nanoparticles, a slightly higher OER overpotential was observed [53,54] indicating that anchoring Fe₂O₃ with NiO and WO₃ offers efficient OER catalysis.

The conjugation of Fe₂O₃ with MnO enhances the OER activity, as reported by Kim et al. [55]. A simple sol-gel technique was proposed to synthesize individual and heterojunctions of MnO and Fe₂O₃. In their electrochemical investigation, the materials were coated onto a Ni foam substrate (as a current collector) in the electrolyte. The electrochemical tests conducted in alkaline media indicated that the MnO–Fe₂O₃ heterojunction reached 10 mA cm⁻² of current density at a bias potential of +1.60 V vs. SHE; this was ascribed to the overpotential value of +370 mV. Furthermore, a 66 mV dec⁻¹ Tafel slope was recorded with a cycle stability of over 1000 cycles. Even though the overall overpotential requirement at 10 mA of current density was not high enough compared to the literature, the heterojunction was found to perform significantly better than its individual counterparts. Notably, the underlying mechanism indicated the synergistic effect of Mn and Fe, which induce the oxidation of O₂ and the reduction of H₂, respectively.

Recently, Fe₂O₃ has been used to oxidize hydrazine in an asymmetric electrochemical cell with a FeP cathode to produce H₂ [56]. In that report, electrolyzed carbon paper was used as a substrate and current collector. The Fe₂O₃ nanostructures were deposited on the substrate (referred to as electro-oxidized carbon paper) via the self-assembly of iron phthalocyanine, followed by pyrolysis in air (Figure 6). For the cathode electrode, the Fe₂O₃ deposited on the substrate was further treated with NaH₂PO₂ in the CVD system for phosphidation, yielding FeP on electro-oxidized (EO) carbon paper (Figure 6). Both the Fe₂O₃ and FeP deposited on the EO carbon paper appeared to show a mosslike morphology, in which the latter showed a weaker surface due to the Kirkendall effect. The morphology appeared to consist of closely packed nanocrystals which provided effective catalytic sites, whereas the porous architecture led to the effective diffusion of electrolyte ions and provided effective gas evolution pathways. In the absence of hydrazine, the OER overpotential was observed at +1.56 V vs. SHE to produce 10 mA cm⁻². Both Tafel plots and impedance values showed decreased slope and charge transfer resistance values, respectively indicating the advantages of coupling Fe₂O₃ and FeP in a two-electrode system for hydrazine oxidation for H₂ production. In particular, to reach a hydrazine oxidation current density of 10 mA cm⁻² at the Fe₂O₃ anode, a potential of 664 mV was required with a Tafel slope value of 179.2 mV dec⁻¹ (Figure 7a,b). In contrast, a HER current density of 10 mA cm⁻² at FeP was achieved with an overpotential of +77 mV with a Tafel slope value of 63.9 mV dec⁻¹ (Figure 7c,d). The electrochemical tests conducted in two-electrode setups consisting of a Fe₂O₃ anode and a FeP cathode on electrooxidized (EO) carbon paper indicated an efficient hydrazine oxidation (at the anode) current density of 10 mA cm⁻² at an impressively lower overpotential of +930 mV, along with the evolution of H₂ gas at the FeP cathode. This study indicated that an alternate reaction for water oxidation with Fe₂O₃ could be beneficial in achieving higher current densities with lower overpotentials. Such systems are believed to represent a key strategy in the fabrication of membrane-less or decoupled electrolysis systems.

The Fe₂O₃ and FeP heterojunction systems could be synthesized using a hydrothermal method with iron and red phosphorus as precursors [57]. A study of the morphology of the hydrothermally synthesized Fe₂O₃-FeP revealed that FeP consists of a nanorod structure, whereas Fe₂O₃ forms as cubic nanoparticles. The morphology showed the presence of a porous system that could be utilized for the effective diffusion of electrolyte ions and the removal of the produced gases. The electrochemical experiments conducted in an alkaline electrolyte (1 M KOH) indicated an impressive OER overpotential of +264 mV to achieve 10 mA cm⁻² of current density. Furthermore, the low value of the Tafel slope and resistance value (at the electrode/electrolyte interface) indicated the excellent catalytic activity offered by the Fe₂O₃/FeP heterojunction. A prolonged water splitting test indicated a stability of over 12 h with two electrode setups, with a potential requirement of only +1.65 V vs. SHE.

Unlike FeP anchored to Fe₂O₃ systems, FeS linked to Fe₂O₃ showed a decreased efficiency, indicating that FeP serves as an effective junction material compared to FeS in terms of OER activity [58]. In contrast, FeS anchored to Fe₂O₃ demonstrated a significantly lower overpotential in the study reported by Guo et al. [59]. In that study [59], the FeS/Fe₂O₃ catalyst (on the iron foam) was fabricated using a semi-sacrificial template approach that involved a solvothermal approach to produce Fe₂O₃, followed by exposure to air and treatment with a sulfide precursor to form FeS. The FeS/Fe₂O₃ showed a nanosheet morphology with an average size of 0.8 µm. A high-resolution TEM analysis confirmed nanojunction formation between the Fe₂O₃ and FeS, with good crystallinity and uniform distribution of elements. The formation of heterojunctions between Fe₂O₃ and FeS offered an effective interface with a tuned electronic structure, leading to effective an electron-proton transfer process that increased the number of active sites with improved stability and OER activity. The optimized FeS/Fe₂O₃ system generated 10 mA cm⁻² of current density at a lower overpotential value of +266.5 mV. Moreover, enhanced stability of over 50 h was recorded with a Tafel value of 51.17 mV dec⁻¹. The efficient catalytic activity and stability indicated the potential applicability of this material in industrial scale hydrogen production via electrolysis. Similar to iron sulfide, iron selenide anchored to Fe₂O₃ is known to exhibit excellent electrocatalytic water-splitting activity, as demonstrated by Sohail et al. [60]. As such, Fe₂S₃ has been used in OER. However, unlike FeS-Fe₂O₃, Fe₂Se₃/Fe₂O₃ shows a remarkably low overpotential value of +160 mV for OER activity at 20 mA cm², implying the lowest applied potential of +1.31 V vs. SHE. Indeed, the overpotential value exhibited by Fe₂Se₃/Fe₂O₃ was found to be the lowest for the Fe₂O₃-based electrode. Furthermore, excellent stability over 24 h was observed during chronoamperometric tests, with a stable current density of 65 mA cm⁻² at a bias potential of +1.65 V vs. SHE.

4.6. Fe₂O₃ Supported on Carbon Materials

Fe₂O₃ is known for its efficient conductivity compared to other metal oxides. Hence, providing a support material becomes essential for the diffusion/transfer of charges during catalysis. In such cases, the tuning of the catalyst morphology and the conductive nature of the support materials are vital for improved electrocatalytic activity. In the case of Fe₂O₃, limiting its nanomorphology by strategically tuning the crystal size and nanoarchitecture is effective due to its limited carrier path length. Specifically, limiting the nanostructure of Fe₂O₃ to one or two dimensions can boost the catalytic activity by orders of magnitude compared to its microstructures [82]. In addition, providing a highly conductive platform such as graphene (including other carbon supports) to Fe₂O₃ will further support the catalytic activity and stability by effectively and rapidly diffusing the charges that are responsible for catalytic reactions and conductivity [83,84,85]. For example, anchoring Fe₂O₃ hollow nanorods to carbon nanotubes (CNT) has been found to enhance the OER activity [61]. A Fe₂O₃-CNT junction could be fabricated via the co-precipitation method using urea. Even though the achieved overpotential, Tafel slope, and stability values were +383 mV, 68 mV dec⁻¹, and 12 h, respectively, the values were less significant compared to those in other reports. The strategy of using a carbon support may be a solution to overcome two main challenges, i.e., the limited conductivity of Fe₂O₃ and the poor penetration of the electrolyte on the surface of the catalyst. Furthermore, the introduction of CNT supports the underlying faradaic process by decreasing the resistivity. As a result, the Fe₂O₃-CNT performed better as a catalyst than pristine Fe₂O₃.

Zang et al. [62] investigated the OER activity of Fe₂O₃ anchored to Fe-N-doped carbon nanosheets. A high-temperature facile pyrolysis (1000 °C) approach was proposed to yield mesoporous Fe/Fe₂O₃-Fe-N-doped carbon nanosheets using shrimp shells. The optimized catalyst on GCE exhibited an OER current density of 10 mA cm⁻² at +0.69 V (vs. Ag/AgCl) and a Tafel slope of 77.5 mV dec⁻¹ with a stability of 10 h. Notably, the OER performance of the Fe/Fe₂O₃-Fe-N-doped carbon nanosheets was significant compared to commercial RuO₂. Similarly, Fe₂O₃ deposited on graphitized-carbon nitride nanocomposites was used in OER with alkaline media [63]. The electrochemical results were less significant compared to those of Fe/Fe₂O₃-Fe-N-doped carbon nanosheets. However, Fe₂O₃ anchored to N-doped carbon nanomaterial showed HER activity in alkaline media [64]. That is, unlike other carbon supports on Fe₂O₃, the catalyst, as reported by Jiang et al. [64], showed HER activity. An electrochemical analysis performed in 1 M KOH revealed a HER current density of 10 mA cm⁻² at an overpotential of +245 mV with a Tafel slope value of 76.6 mV dec⁻¹. Furthermore, excellent stability of the catalyst was observed up to 48 h of water splitting.

The introduction of Ni₃S₂ along with Fe₂O₃ anchored to N-doped carbon composites has been shown to yield excellent OER and HER activities [65]. Even though Ni₃S₂ is well known for its catalytic activity, its lifespan tends to be short due to ineffective coupling to the back contact or substrates. To overcome this challenge, the utilization of Fe₂O₃ and N-doped carbon composite has been proposed. A facile thermal decomposition approach was used with essential precursors to fabricate a Ni₃S₂/Fe₂O₃/N-doped carbon composite. The characterization of the catalyst-deposited nickel foam revealed nanosheets of Ni₃S₂ containing Fe₂O₃ and N-doped carbon particles. An electrochemical analysis conducted in 1 M KOH indicated that the Ni₃S₂/Fe₂O₃/N-doped carbon composite exhibited an impressive OER overpotential of 188 mV for 52 mA cm⁻² of current density, along with a Tafel slope value of 64.3 mV dec-1. In addition, about 10 mA cm⁻² of HER current density was produced at an overpotential value of +74 mV with a Tafel slope of 115.8 mV dec⁻¹. Importantly, the electrode composite in the bifunctional mode in two electrode systems was found to achieve about 100 mA cm⁻² of current density at a bias potential of +1.605 V vs. SHE, which was supplied using an external battery. The superior catalytic activity achieved in this report indicates that the utilized synthesis approach and the composite anchored to Ni₃S₂ result in a strong coupling of the catalyst to the nickel foam substrate.

The simultaneous deposition of Fe₂O₃ and graphene oxide on a nickel foam substrate can be achieved using the co-deposition method [66]. The approach involves separate graphene oxide and Fe₂O₃ (sugar-cubic) synthesis using a modified Hummers method and hydrothermal method, respectively, followed by electrodeposition to deposit the materials on the Ni foam. An electrocatalytic characterization conducted in 1 M KOH showed an overpotential value of +313 mV to generate an OER current density of 100 mA cm⁻²; this was ascribed to the Tafel slope of 81 mV dec⁻¹. The improved OER overpotential value of Fe₂O₃/graphene oxide was ascribed to the increased surface area of the nitrogen-doped graphene with effective mechanical properties that provide stability and enhanced active sites during electrolysis. Furthermore, the presence of uniformly distributed Fe₂O₃ nanocrystals on nitrogen-doped graphene offered increased reaction sites, favoring an effective interface at the electrolyte and providing a solid foundation for charge collection and diffusion/transfer. Likewise, dispersing Fe₂O₃ on MWCNT could also provide similar advantages to graphene in terms of increasing the OER activity of Fe₂O₃ [67]. In other words, the presence of MWCNT as a support for Fe₂O₃ favors the adsorption of OER intermediates that drive efficient charge transfer kinetics. Surface-functionalized multiwall carbon nanotubes act as an effective support to the uniform deposition of Fe₂O₃, which was achieved by pulsed laser ablation deposition. The LSV measurements were noted to produce 10 mA cm⁻² of current density at a lower overpotential of +310 mV and a Tafel slope of 20.35 mV dec⁻¹, confirming the superior OER activity of Fe₂O₃/MWCNT compared to its counterparts. The presence of MWCNT decreased the energy barriers of the OER intermediates, which are rate-determining. However, a limited stability of only 10 h was observed, which may not be significant compared to other electrode combinations that support Fe₂O₃. Therefore, among the tested carbon supports for Fe₂O₃, Ni₃S₂ anchored to N-doped carbon composites was found to yield superior OER activity, in addition to HER activity.

4.7. Fe₂O₃ Linked to MOFs

Recently, MOFs have gained significant attention in energy applications owing to their unique architecture, i.e., containing organic ligands linked to metal clusters [86]. MOF offers tunable porosity with an enhanced specific surface area that could be engineered into specific morphologies depending on the application needs. Using the unique properties of Fe₂O₃, Niu et al. [68] proposed FeOx node-based MOFs with engineered intrinsic active sites and architectures with enhanced reaction areas that exhibited excellent OER, HER, and ORR activities. The procedure involves the simple solvothermal method that allows extensive tuning of bonding between the Fe and 1,4-dicarboxybenzene ligands to be undertaken via the utilization of different solvent systems and a wide range of metal sources. Such tuning provides effective control over the selective or desired morphologies, such as concave octahedral, rod-like, spindle-like, and octahedron-like, offering desirable properties (Figure 8).

Furthermore, a pyrolysis step was proposed to yield a graphite nanocomposite of Fe₂O₃ that displayed a wide range of exposed crystal planes having unique properties and reaction capabilities with retained original morphologies (Figure 8). Figure 9 shows the LSV, Tafel slope, and I-t stability plots of concave octahedral, rod-like, spindle-like, and octahedron-like MOF-Fe₂O₃ for OER and HER applications. Among the explored structures, the concave octahedral form of Fe₂O₃—MOF was found to possess an enhanced active site, owing to the favorable facets for improved electrocatalytic OER and HER activity, with enhanced stabilities, as shown in Figure 9 [68]. Importantly, the results of this report indicate the applicability of Fe₂O₃ in MOF systems in terms of electrocatalytic activity.

4.8. Sea/Domestic Wastewater Electrolysis

Sources of freshwater around the globe are valuable and limited, and thus, the utilization of seawater and domestic sewage in electrolysis is encouraged on the basis of cost efficiency and resource availability. However, the electrochemical water splitting of seawater is challenging due to the presence of unwanted and interfering ions, decreased conductivity, poisoning, and corrosion of the electrodes/catalysts [87,88]. The presence of chloride anions leads to the evolution of chlorine in the OER process that causes corrosion, poisons the anodic surface through the formation of insoluble precipitates, and compromises efficiency [89]. To inhibit the evolution of chlorine and selectively perform OER, a systematically designed catalyst needs to be implemented. Li et al. [69] proposed seawater and domestic sewage water electrolysis using a Fe₂O₃/NiO interface. A simple chemical bath deposition method was proposed for the synthesis of Fe₂O₃ on Ni foam-producing porous structures. The characterization of the catalyst indicated the formation of a FeNi oxide interface that exhibited superior OER activity. In a 0.1 M KOH electrolyte, the Fe₂O₃/NiO exhibited a 10 mA cm⁻² current density at an overpotential value of +182 mV but also a high current density of 1000 mA cm⁻² at an overpotential of +267 mV. Both the Tafel slopes and EIS measurements indicated low slope and charge transfer resistance values, respectively. An impressive stability of over 50 h was recorded in both seawater and domestic sewage water, indicating the versatility of the Fe₂O₃/NiO catalyst. The superiority of Fe₂O₃/NiO was further tested in two electrode setups using seawater and alkaline domestic sewage. For this, a NiMo₄/MoO₂ electrode was used as a cathode and compared with Pt/C in terms of practicality. With alkaline electrolytes including NaCl and sea water-containing mixtures, the Fe₂O₃/NiO catalyst performed significantly better compared to commercial RuO₂ OER and Pt/C HER electrode systems, indicating exceptional selectivity of Fe₂O₃/NiO in the OER process. Furthermore, a stability of over 50 h was observed, indicating no apparent corrosion of the electrodes in Cl⁻ containing electrolytes. Similarly, the potential application of Fe₂O₃/NiO was tested in alkaline sewage electrolysis, as demonstrated in Figure 10. Interestingly, to perform OER with 100 mA cm⁻² in alkaline sewage system, the Fe₂O₃/NiO utilized an overpotential of +222 mV, which was observed to be lower than that of the standard RuO₂ OER catalyst, both in three- and two-electrode setups (Figure 10a,b). With an elevated current density of 500 mA cm⁻², the bias potential was noted to be +1.752 V vs. SHE, which confirmed the pilot scale industrial feasibility of the Fe₂O₃/NiO catalyst. Mainly the catalyst performance was found to be consistent, despite the high chemical oxygen demand; this confirmed the anti-poisoning characteristic of Fe₂O₃/NiO as a catalyst. A photographic image of a Fe₂O₃/NiO anode catalyst connected to standard battery with a NiMo₄/MoO₂ cathode catalyst is shown in Figure 10, confirming the formation of gas bubbles. The I-t plot shown in Figure 10d further confirms the stability of Fe₂O₃/NiO for more than 30 h of operation, indicating the catalytic performance and selectivity of the catalyst.

Figure 9. (a,d) LSV curves (b,e) Tafel plots and, (c,f) I-t plots for morphologically tuned Fe₂O₃ in terms of OER and HER activity. Reused with permission from [68]. Copyright 2021, with permission from Elsevier.

Figure 9. (a,d) LSV curves (b,e) Tafel plots and, (c,f) I-t plots for morphologically tuned Fe₂O₃ in terms of OER and HER activity. Reused with permission from [68]. Copyright 2021, with permission from Elsevier.

Cui et al. [71] proposed an effective and cost-effective method for the synthesis of a Ru, Ni-doped Fe₂O₃ bifunctional catalyst for alkaline and seawater splitting. The synthesis protocol involves the use of precursors in autoclave and hydrothermal synthesis with systematic optimization regarding the ratio of Ni and Ru. The optimized Fe₂O₃ bifunctional catalyst exhibited 100 mA of OER and HER current density at an overpotential value of +329 and +75 mV, respectively, in a 1.0 M KOH electrolyte. In a two-electrode setup as the bifunctional electrode mode, the RuNi-Fe₂O₃ required a very low applied potential of +1.66 V vs. SHE in the standard alkaline electrolyte. Importantly, in the seawater electrolyte, a significantly low overpotential of +1.73 V vs. SHE was optimized, indicating the effectiveness of the catalyst in practical applications. The selectivity and stability were further proved by testing the RuNi-Fe₂O₃ for long-term seawater electrolysis, as shown in Figure 10e. The I-t plot (Figure 10e) suggests excellent stability over 100 h of seawater splitting, indicating the superior performance of RuNi-Fe₂O₃ as a bifunctional catalyst. The standalone device shown in Figure 10f shows a two-electrode setup with a RuNi-Fe₂O₃ bifunctional catalyst for seawater splitting. Furthermore, using commercial solar panels, the electrode system was utilized to perform seawater electrolysis using a RuNi-Fe₂O₃ bifunctional catalyst. The efficacy was compared with that of a standard RuO₂-Pt/C electrode system, as shown in Figure 10f. The rapid evolution of gas bubbles, along with the increase in balloon volume, confirmed that the proposed catalytic system could be effectively employed in seawater electrolysis and could replace the PGM electrodes in alkaline water electrolysis systems.

P-doped Fe₂O₃ decorated on CoP could be utilized to split seawater into H₂ and O₂, as demonstrated by Cui et al. [70]. Specifically, P-Fe₂O₃ was utilized to overcome the poor H–OH moiety dissociation on CoP, as well asl the inefficient desorption of intermediate molecules. Nanosheets of P-Fe₂O₃-CoP were fabricated using two two-step processes involving hydrothermal and gas-phase phosphorization procedures. During the water splitting process, the presence of P as a dopant was found to fulfill two main purposes, i.e., the introduction of an essential interfacial structure and defects. The optimized catalyst exhibited superior catalytic activity compared to the standard RuO₂ catalyst. In seawater and freshwater electrolytes (containing 1 M KOH), a significantly low overpotential of +270 and +250 mV to generate 10 mA cm⁻² of current density was observed. Furthermore, when employed as a bifunctional catalyst in a two-electrode electrolysis assembly, the P-Fe₂O₃-CoP catalyst exhibited a required bias potential of +1.61 vs. SHE for fresh water and about +1.65 V vs. SHE for seawater splitting reactions (for 10 mA cm⁻²). The low Tafel slope values, decreased electrode/electrolyte charge transfer resistance, and prolonged stability further confirmed the efficacy of the P-Fe₂O₃-CoP catalyst in fresh water and seawater electrolysis.

4.9. Influence of Magnetic Fields on Catalysts

The utilization of magnetic fields during electrolysis is known to provide a significant catalysis effect by accelerating the OER and HER activities [90,91,92]. The application of magnetic fields during electrolysis could exert different effects on electrolysis systems, improving the efficiency by providing the electrocatalytic effect, magnetohydrodynamic influence, magnetothermal effect, bubble management, magnetic moment alignment, spin selective effect [93,94], etc. Specifically, the formation of gas bubbles at the electrode surface during electrolysis causes decreased efficiency. In the electrolysis industry, an estimated energy loss of ~40% is expected due to the accumulation and inefficient removal of bubbles from the electrode surface [95]. The ineffective removal of gas bubbles may block the electrode surface, causing increased ohmic resistance and inhibiting facile mass transfer and nucleation [96].

Recently, Li et al. [97] demonstrated an effective route of harnessing magnetic fields in catalytic materials to enhance the OER process. This was successfully achieved by employing γ-Fe₂O₃ electrodes, owing to their magnetic properties. Specifically, cubic spinel structured γ-Fe₂O₃ (maghemite) is widely known for its strong magnetic properties and is widely employed as a permanent magnetic material [98]. The authors proposed a super-hydrophilic γ-Fe₂O₃ hydrosol with tuned surface wettability and magnetism for application under magnetic fields. This was achieved by coating the γ-Fe₂O₃ hydrosol on different substrates having unique magnetic and surface properties, such as carbon cloth (CCloth), nickel foam (NF), heat-treated carbon cloth (CCloth-Treated), and carbon paper, using a simple dip-coating method. Figure 11 shows the different setups used to induce magnetic fields in Fe₂O₃ electrodes. For instance, the catalytic activity of super-hydrophilic γ-Fe₂O₃ on NF was confirmed by LSV, Tafel plot, and EIS measurements, as shown in Figure 12a–c, respectively. The γ-Fe₂O₃ exhibited a cathodic shift in the onset potential in the LSV and required only +1.6 V vs. SHE to achieve 50 mA cm⁻² of current density, whereas only Ni foam required a significant higher potential of +1.73 V vs. SHE to achieve the same result. The cathodic shift in potential confirmed that the presence of super-hydrophilic γ-Fe₂O₃ favored OER activity over bare Ni foam. Furthermore, the super-hydrophilic γ-Fe₂O₃ showed a decreased Tafel slope (Figure 12b) and polarization resistance (Figure 12c) compared to the untreated Ni foam. In addition, an 80° contact angle was observed for bare Ni foam (inset of Figure 12d), whereas the γ-Fe₂O₃ showed a super-hydrophilic characteristic (inset of Figure 12e). Conclusively, the surface super-hydrophilic property of γ-Fe₂O₃ ensures the effective release of O₂ bubbles at the surface, along with the efficient and rapid transport of the electrolyte toward the electrode surface, leading to improved OER activity.

The chronoamperometric measurements performed (γ-Fe₂O₃/Ni-foam) in the presence of a magnetic field showed impressive results, indicating the importance and advantages of the use of magnetic fields. Four different setups were tested based on the magnet field input direction and system configuration, as shown in Figure 12. The experimental results confirmed that irrespective of the setup, a significant increase in current density was observed under the influence of a magnetic field. Among the setups, the second system showed a comparatively more significant influence of magnetic fields on OER activity. In the case of diamagnetic substrates, negative results were obtained. Importantly, the use of magnetic fields in electrocatalytic systems was found to provide an added Lorentz and Kelvin force for the effective removal of O₂ bubbles with efficient transport of electrolytes based on the implemented setup and substrates [97].

4.10. Fe₂O₃ in AEMWE Systems

AEMWE has emerged as one of the promising technologies that balances the advantages of both AWE and PEMWE into a single system. AEMWEs are operated in moderately weak alkaline conditions compared to AWE and integrated into membrane-based electrolyzer configurations like PEMWE. Unlike PEMWE, AEMWE may be used in non-PGM-based catalysts without any requirement for acidic electrolytes. Hence, the catalysts integrated into AEMWE are crucial for efficiency and stability. In AEMWE, the cracking of catalysts was found to be one of the limitations that hinders long-term activity and stability. Recently, Chen et al. [72] indicated the importance of Fe₂O₃ (with Co) as an island-type hybrid catalyst in AEMWE electrolysis that significantly inhibits the cracking of the catalysts. Using a systematically developed microwave-assisted hydrothermal technique, an island-type hybrid catalyst containing a controlled ratio of Fe₂O₃ and Co was synthesized. In the AEMWE system, the optimized Fe₂O₃/Co was used as an anode catalyst, whereas Pt/C was used as a cathode catalyst, which was loaded onto a carbon cloth support using a hand painting technique. The commercially available Fumapem FAA-3 membrane (Fumatech, Bietigheim-Bissingen, Germany) was used in the membrane electrode assembly (MEA) stack. The characterization results showed that the Fe₂O₃ acted as a support on the Co catalyst, providing a nanoscale gap for the effective removal of product gases with the electrolyte fill-ups. The efficiency of the catalyst was found to be dependent on the ratio of Fe₂O₃ and Co, as shown in the LSV and Tafel plots (Figure 13a,b). Electrolysis testing carried out in 1 M KOH (Co-Fe₂O₃ of 60:30 ratio) produced an impressive current density of 200 mA cm⁻², which was found to be much higher than the catalyst without the Fe₂O₃ support layer. Stability studies conducted over a longer electrolysis period indicated an impressive stability over 200 h (Figure 13c), with a retention of over 90% of the initial current density. Moreover, the LSV plots measured before and after the stability tests indicated no significant difference, indicating the superior stability of the Co-Fe₂O₃ catalyst (Figure 13d).

4.11. Fe₂O₃ Supported Ceramics in SOEC

The solid-state structure of SOEC systems offers significant advantages over other hydrogen electrolyzer technologies, e.g., a unique ion diffusion mechanism, high electrode reactivity hydrogen purity, simplistic design, and possible commercial applicability [92]. The solid ceramic electrolyte in SOEC systems that separates the anode from the cathode compartments and provides effective diffusion pathways to O²⁻ ions is one of the important components that determines the overall efficiency and device stability. A wide range of ceramic electrolytes have been proposed in the last few years. Among the electrolytes, yttria-stabilized zirconia (YSZ) has been widely employed, owing to its low conductivity for electrons. In SOEC systems, the oxygen/cathode electrode materials play an important role and are generally doped with Sr for efficient oxygen ion transport and overall cell performance. The use of strontium-doped materials often aligns well with the other components of the SOEC, such as YSZ electrolytes, as evidenced in recent publications. However, during SOEC cell operation, the Sr ions are prone to diffuse and combine with the YSZ material (Zr) to yield SrZrO₃. The SrZrO₃ formed in the electrolyte is known to be an insulating phase, and its presence restricts the diffusion of oxygen ions through the solid electrolyte toward the cathode electrode. The significant hindrance to the diffusion of oxygen ions has been found to increase the polarization resistance, which influences the SOEC efficiency and device performance. Hence, to inhibit the formation of an insulating SrZrO₃ phase in SOEC electrolytes, an interlayer was introduced containing ceria between the Sr-oxygen/cathode electrode and YSZ electrolyte [93,94]. Conclusively, the interlayers play a crucial role in SOEC systems in terms of preventing Sr diffusion, and, at the same time, provide effective pathways for oxygen ion transportation [93,94].

Considering the importance of the interlayer, its properties are known to play a significant role in SOEC performance. For example, a material offering an increased density of oxygen ion conduction is an ideal option for increased performance. Recently, the utilization of oxides of transition metals has been shown to improve SOEC performance by acting as a grain boundary cleaning agent and a sintering aid for ceramic electrolytes [95,96,97]. Recently, Qu et al. [98] demonstrated the integration of Fe₂O₃ to a samarium-doped ceria (SDC) interlayer in a SOEC system to improve the ionic conductivity of the cathode electrode. Pristine and Fe₂O₃ doped SDC were synthesized using a glycine-nitrate method, whereas the YSZ electrolyte and (La_0.6Sr_0.4)_0.95Co_0.2Fe_0.8O₃-_δ (LSCF) were commercially acquired; the latter was used as the oxygen electrode. For the hydrogen electrode, a glycine-nitrate method was used to obtain NiO. The doped and undoped SDC were prepared by pressing the relevant powders into green sheets (1250 °C for 4 h at a pressure of 10 MPa). During symmetrical cell testing conducted at 800 °C, a significant advantage of Fe₂O₃ doping was observed in terms of oxygen electrode polarization resistance and electrolysis current. The electrolysis current measured at 1.5 V during the symmetrical cell testing revealed a current density of 0.3 A cm⁻² for the undoped system, which was found to be increased to 0.5 A following Fe₂O₃ doping. On the other hand, the oxygen electrode polarization resistance of undoped systems was noted to be 0.22 Ω cm⁻², showing a significant decrease to 0.09 Ω cm⁻² in the presence of the Fe₂O₃ dopant. Importantly, the introduction of a Fe₂O₃ interlayer dopant led to improved ionic conductivity, cathode electrode stability, and hydrogen production (calculated to be 132 mL cm⁻² h⁻¹ when undoped vs. 195 mL cm⁻² h⁻¹ when doped), which, in turn, boosted the efficiency of SOEC.

5. Potential Application in Commercial Electrolyzers

The commercial applicability of the catalyst in large-scale electrolysis is the most important aspect of the hydrogen electrolysis industry. Being a cheap, stable, and abundant material, Fe₂O₃ could be a potential candidate as a catalyst for the electrolysis industry in the future. In recent research, the applicability of Fe₂O₃ has been successfully tested in AWE, AEMWE, and SOEC systems. Specifically, in basic electrolytes and high-temperature ceramic systems, Fe₂O₃ showed significant stability. However, its poor stability in low pH conditions restricts its use in PEM-based electrolyzers. Owing to its intrinsic and extrinsic characteristics, Fe₂O₃ is mainly used in OER applications and bifunctional systems. Its utilization as solely a HER catalyst has not been widely studied. As discussed above, the direct utilization of Fe₂O₃ in pristine form has limited applicability. However, suitable modifications and/or coupling and alloying with other materials have extended its applicability, efficiency, and stability. Based on this strategy, a wide range of catalytic systems have been proposed and tested. However, studies are limited to three- and two-electrode systems in lab-scale applications. This indicates that the testing of Fe₂O₃ in electrolyzer test systems is lacking, and significant effort is essential in this direction to promote Fe₂O₃ as a catalyst for pilot scale systems in the future. Therefore, studies directed toward the testing of Fe₂O₃-based catalysts in electrolyzer stacks are encouraged. Recent research and advancements have indicated that Fe₂O₃ could be an ideal catalyst material for the AWE, AEM, and SOEC-based electrolyzers of the future. However, the intrinsic and extrinsic characteristics of Fe₂O₃ are mainly exploited in OER applications and bifunctional systems following strategic modifications. Hence, significant research and development of modification strategies of Fe₂O₃ are essential. As presented in the discussion section, the current stages of different modifications indicate improved efficiency, selectivity, and stability of Fe₂O₃-based electrodes in terms of OER activity. In comparison with other metal oxides/hydroxides/alloy-based electrodes, the research focus on Fe₂O₃ has been limited. Hence, the coming years could provide significant milestones in Fe₂O₃-based OER catalysts in electrolyzer systems if suitable modification strategies are employed.

Based on the aforementioned discussion, we provided a brief roadmap of the use of Fe₂O₃ in water electrolyzers, as shown Figure 14a. The primary step involves the synthesis of Fe₂O₃. The synthesis protocol has to be simple and scalable industrial scale production. The next step involves the essential modifications, because the adaptation of suitable modification dictates the efficiency and stability of the catalyst. In this step, the catalyst could be modified to suit bifunctional catalysis, in addition to offering OER activity. The modification protocols need to be designed carefully to meet the scalable parameters without compromising efficiency, stability, safety, or toxicity. After the essential modifications, testing needs to be performed to assess the efficiency, stability, and necessary conditions on a lab scale. The testing step mostly determines the important parameters and conditions with which the catalyst could be utilized more efficiently. The final step involves the implementation and testing of the catalyst system in AWE, AEMWE, SOEC, and seawater electrolysis stacks to determine its potential applicability.

6. Discussion

Figure 14b summarizes the applications and modes of implementation of Fe₂O₃ in electrocatalytic water splitting systems. The low cost of Fe₂O₃, as well as its abundance, geographical availability, and non-toxicity, are the main aspects that drive interest in it as a catalyst in energy applications. Fe₂O₃ is widely employed as an OER (in addition to ORR) catalyst and as a bifunctional catalyst when combined with suitable materials or following modifications. On the other hand, its stand-alone application to HER is limited due to its poor catalysis in water reduction. Overall, the intrinsic and surface properties of Fe₂O₃ allow its effective implementation in water oxidation rather than water reductions. Despite widely reported OER applications, Fe₂O₃ still requires essential modifications to boost its OER activity and stability. As shown in Figure 14b, several strategies have been proposed to boost the OER activity and to implement it as a bifunctional/HER catalyst, as discussed below.

Crystal facet tuning and morphology: Specific facets and certain morphologies are known to offer significantly superior catalytic activities in Fe₂O₃ when compared to other facets and morphologies. Therefore, tuning the morphology to maximize the favorable facets via a controlled synthesis process is vital for enhanced catalytic activity, which would provide efficient charge diffusion. Research on F₂O₃ in this direction is limited, and thus more efforts should be made on designing synthesis schemes to obtain facets with specific morphologies.

Doping: The intrinsic characteristics of Fe₂O₃ are not suitable for its widespread application in, for example, water-splitting reactions. In such cases, doping could emerge as a solution to increase the abundance of catalytic sites via defects or oxygen vacancies and the density of states, which modifies the intrinsic properties of Fe₂O₃ to a certain extent. Even though few attempts have been reported until now, exploring a wider range of dopants could provide new insights into the properties of Fe₂O₃, potentially expanding its water-splitting applications.

Multijunction systems (anchoring to other materials): Anchoring Fe₂O₃ to different oxides, sulfides, phosphides, etc. has been undertaken extensively owing to its multiple benefits. For example, anchoring PGM catalysts to Fe₂O₃ could limit the excessive utilization of PGM without compromising the catalytic extent. In such cases, Fe₂O₃ acts as an effective platform for catalytic reaction sites. Furthermore, the presence of different metal ions (in a heterojunction system) near the Fe atoms at the surface could influence the adsorption energy and reaction intermediates which ultimately favor electrocatalytic reactions with rapid reactivity and selectivity. Alternatively, exploring other reactions for water oxidation with Fe₂O₃ could be beneficial in terms of achieving higher current densities with lower overpotentials. Such systems are believed to represent a key strategy in fabricating membrane-less or decoupled electrolysis systems. Moreover, heterojunctions having an effective interface with a tuned electronic structure are beneficial for the electron/hole transfer process.

Carbon material supports: The limited charge conductivity of Fe₂O₃ could be overcome by providing a support material for improved electrocatalytic activity. Specifically, limiting the nanostructure of Fe₂O₃ to one or two dimensions and providing a highly conductive platform such as graphene (including other carbon supports) could boost the catalytic activity. Carbon-supported Fe₂O₃ not only overcomes the limited conductivity but also improves the penetration of the electrolyte, increases back contact, increases the number of reaction sites, expands the effective interface, and provides a solid foundation for charge collection and diffusion/transfer.

Fe₂O₃-MOF: By utilizing a controlled and systematic synthesis (precursors) approach, a wide range of Fe₂O₃-MOFs have been obtained, offering tunable porosity and morphology and increased active surface area, resulting in excellent OER and HER activities. However, very limited literature in this direction limits our understanding, and thus, more mechanistic insights are needed.

Seawater electrolysis: Seawater electrolysis is an ambitious target in electrolysis, as sources of freshwater are limited and are essential to human needs; thus, the utilization of seawater and domestic sewage water is encouraged. Such an implementation could also offer benefits in terms of cost, efficiency, and availability. Fe₂O₃ has shown a certain degree of water-splitting activity with significant stability and selectivity in some studies. Specifically, the Fe₂O₃ has been shown to withstand unwanted and interfering ions, which otherwise decrease conductivity and negatively affect the catalytic properties. In some experiments, Fe₂O₃ (anchored system) performed significantly better than commercial RuO₂ OER and Pt/C HER catalysts, indicating exceptional selectivity, stability, and efficiency in seawater electrolysis. A more detailed mechanistic study in this direction could potentially provide insights into Fe₂O₃ charge dynamics which, in turn, would benefit the development of highly efficient catalysts for seawater splitting in the future.

Fe₂O₃ under an applied magnetic field: Magnetic fields applied during electrolysis improve the catalysis by accelerating the OER and HER activities. Specifically, a magnetic field applied to γ-Fe₂O₃ alters the surface super-hydrophilic property that ensures the release of O₂ bubbles at the surface, along with efficient penetration of electrolyte. Additionally, the magnetic field provides an added Lorentz and Kelvin force for the removal of O₂ bubbles that ultimately results in improved water-splitting activity.

Fe₂O₃ in AEMWE: The cracking of catalysts in AEMWE affect their long-term activity and stability. This problem could be solved by using Fe₂O₃ (with Co) catalyst that inhibit cracking. Hence, Fe₂O₃ may play a crucial role in AEMWE in membrane-based electrolyzers in the future. Significant research in this direction is essential, as it could benefit fabrication of cost-effective electrolysis systems.

Fe₂O₃ in SOEC: The integration of Fe₂O₃ to a samarium-doped ceria (SDC) interlayer in SOEC systems improves the ionic conductivity of the cathode electrode. The introduction of Fe₂O₃ as an interlayer dopant improves the cathode electrode stability and hydrogen production. More dedicated research in this direction could provide new insights regarding the applicability of Fe₂O₃ in high-temperature electrolysis.

7. Summary and Outlook

The discussion presented in this review offers an important and underlying strategy regarding the use of Fe₂O₃ in electrocatalytic water splitting systems. Among the explored metal oxides, hematite provides the most cost-effective and efficient option for electrolysis applications. The abundance, non-toxicity, cost, and stability of basic electrolytes are the main advantages of Fe₂O₃ electrodes compared to other oxide counterparts. Thus, Fe₂O₃ offers enormous potential in future energy devices due to its versatile applicability. However, the intrinsic properties of Fe₂O₃ limit its application as an efficient catalyst. Despite its wider pertinence in photoelectrochemical water splitting applications, its implementation in electrolysis is limited; it is nonetheless a promising option, and thus, novel strategies in this direction are essential.

In electrolysis, Fe₂O₃ is widely used as an OER catalyst, in addition to bifunctional and HER catalysts, when combined with suitable materials and following the application of suitable modifications. The most common modification strategies applied to Fe₂O₃ include crystal facet and morphology tuning, doping, the creation of multijunction systems (anchoring with other materials), use with carbon supports, MOF linking, seawater electrolysis, the use of the magnetic field effect, and the application of AEMWE and SOEC systems. The various modifications implemented with Fe₂O₃ offer a wide range of advantages that benefit its OER, HER, and/or bifunctional characteristics. In light of this, the discussion presented here provides a comprehensive summary of advancements regarding the use of Fe₂O₃ catalysts with different modifications. Each modification provides unique changes and improvements in Fe₂O₃ to improve its capabilities. For example, Fe₂O₃ combined with Fe₂Se₃ or Ni₃S₂/N-doped carbon NiO shows the lowest overpotential (+160 and +188 mV) ever reported for the Fe₂O₃ in 1 M KOH, generating an OER current density of 20 and 52 mA cm⁻², whereas Co-Fe₂O₃ shows exceptional stability of over 500 h when utilized in AEMWE systems. Despite its excellent performance in water splitting, reports addressing its use in electrolysis are limited when compared with those for other oxides. Therefore, significant research is essential to realize the full potential of Fe₂O₃ as a cost-effective and efficient water-splitting catalyst. Until now, the main focus has been in the direction of multijunction systems, whereas the doping strategy, facet tuning, and magnetic field effects have been rarely discussed. Therefore, a more significant focus in these directions could open up further potential of Fe₂O₃ as an efficient water-splitting catalyst. The thorough discussion on Fe₂O₃ presented here offers a solid platform for future research regarding the extensive modifications required to develop a highly efficient, stable, and cheap catalyst for electrocatalytic OER and HER. Importantly, the valuable insights presented here will offer detailed guidelines for the efficient design of Fe₂O₃-based catalysis systems.