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Review

The Use of Copper-Based Delafossite to Improve Hydrogen Production Performance: A Review

by
Hasnae Chfii
*,
Amal Bouich
* and
Bernabé Mari Soucase
Departament de Física Aplicada, Institut de Disseny i Fabricació (IDF), Universitat Politècnica de València, 46022 Valencia, Spain
*
Authors to whom correspondence should be addressed.
Hydrogen 2024, 5(1), 39-58; https://doi.org/10.3390/hydrogen5010004
Submission received: 25 December 2023 / Revised: 24 January 2024 / Accepted: 24 January 2024 / Published: 30 January 2024
(This article belongs to the Special Issue Feature Papers in Hydrogen (Volume 2))
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Abstract

:
This review paper reports on the use of Delafossite as a layer between perovskite-based solar cells to improve hydrogen production efficiency and make the process easier. The investigation delves into the possible breakthroughs in sustainable energy generation by investigating the synergistic interplay between Delafossite and solar technology. This investigation covers copper-based Delafossite material’s properties, influence on cell performance, and function in the electrolysis process for hydrogen production. Some reports investigate the synthesis and characterizations of delafossite materials and try to improve their performance using photo electrochemistry. This work sheds light on the exciting prospects of Delafossite integration using experimental and analytical methodologies.

1. Introduction

Hydrogen (H2) stands out as a crucial, eco-friendly, abundant, and safe alternative in the realm of renewable energy, poised to address the escalating need for sustainable and clean power. Widely regarded as one of the most environmentally friendly energy sources, hydrogen holds significant promise in mitigating both the energy crisis and climate change [1,2,3,4,5,6,7,8,9]. Furthermore, its application in electricity generation in various industrial settings brings the added benefit of minimal nitrous and sulfur oxide emissions. Notwithstanding these advantages, challenges persist in the efficient processing and storage of hydrogen, hindering the full realization of a hydrogen-based energy economy. Notably, security issues about hydrogen storage, transportation, and transportation remain common [10,11,12,13,14,15,16,17]. Current conventional hydrogen storage methods, employing various metal hydrides, fall short in terms of storage capacity. Hence, there is a pressing need to develop a hydrogen storage system based on fuel cells to address this limitation [18,19,20,21,22,23,24]. H2 fuel cell systems, characterized by their low carbon emissions and high efficiency, are emerging as compelling alternatives to internal combustion engines in the automotive sector [25,26,27,28,29,30,31]. The environmental benefits, increased efficiency, and burgeoning global interest in hydrogen fuel cell systems underscore their growing prominence in recent years. Photocatalysts, relying on either photo-electrocatalysis or direct photocatalysis, engage in processes that hinge on incoming light-induced electron production and transport between electrodes (Figure 1).
When electrodes are made of a photoactive material, including a semiconductor (SC), this method can be implemented. The photoanode in this configuration is made of an n-type SC, whereas the photocathode is made of a p-type SC. Figure 1 depicts the operating principle of the resultant photoelectrochemical cell (PEC). The core idea of a photocatalysis system is the promotion of water breakdown utilizing solar energy, propelling the photoactive material to generate photoexcited charge carriers, ultimately yielding hydrogen. The process unfolds in the following steps: [5] (i) The photoanode absorbs solar light, producing holes and electrons. (ii) Holes as well as electrons travel from one electrode to the next. (iii) Chemical processes make it easier to remove H2 and O2 from water molecules [31,32,33,34,35].
In accordance with the analysis conducted by the International Energy Agency (IEA) on the global total final energy consumption, the share of hydrogen energy in overall energy consumption is anticipated to rise to approximately 2% by 2030 and around 10% by 2050. This escalation is imperative for achieving the net-zero plan concurrently with a substantial reduction in the consumption of nonrenewable fossil fuels. Furthermore, the levels of total final energy consumption derived from renewables are expected to witness an increase of about 0.51% by 2030 and approximately 7.41% by 2050 compared to the levels in 2020. The growing demand for renewables will propel electricity consumption, surging from roughly 19.13% in 2020 to around 26.28% in 2030 and a notable 49.23% in 2050. It is noteworthy that intermittent renewables can be converted into electricity through power stations integrated into grids, with surplus electricity being employed in the production of hydrogen through electrolysis. The effective interconversion of hydrogen and electricity is facilitated by fuel cells and electrolysers. In light of these dynamics, it is evident that hydrogen is poised to assume a pivotal and irreplaceable role in shaping the future landscape of long-term energy systems [36].
In the quest for an ideal semiconductor (SC) material, certain criteria must be met to ensure optimal performance. Specifically, the material’s band gap (Eg) must surpass {E°(O2/H2O)–E°(H2O/H2)}, with the conduction band (ECB) situated below E°(H2O/H2) and the valence band (EVB) positioned above E°(O2/H2O). Accounting for overpotentials that frequently reach 1 V, a material’s Eg must exceed 2.2 eV for effective water cleavage. Notably, many stable oxides are exclusively absorbed in the UV region, making them impracticable for efficient solar spectrum use since their Eg values are too high. To overcome this problem, research attention has recently shifted to narrow-band-gap SCs, particularly in the context of H2 generation. The exploration of novel materials using electronic bands formed from cationic orbitals is an interesting method. The oxides, which have the general formula Cu+M3+O2 and crystallize in the delafossite structure, have VB and CB bands that originate from Cu 3d orbitals with d-d photo transitions. M represents rare earth or a three-dimensional metal in this context. These materials are pH-insensitive and may be precisely matched with electrolyte levels at a specific pH [37,38,39,40,41].
Delafossite compounds, notably CuMO2 (Figure 2), in which the M sites are filled by boron group elements, transition metals, or rare-earth elements, have attracted a lot of attention over the past few decades. This renewed attention stems from their prospective uses, which range from serving as electrodes for hydrogen generation via photoelectrochemical (PEC) water splitting to functioning as transparent conductive oxides in optoelectronic devices. Additionally, these compounds are being explored for their potential roles in spintronic and ferroelectric devices [42,43,44,45,46,47,48].

2. Delafossite Used in Water-Splitting Systems for Hydrogen Production

In the presence of water, separated electrons participate in the reduction reaction, converting water molecules (H2O) into hydrogen gas (H2). Simultaneously, the holes contribute to the oxidation reaction, producing oxygen gas (O2) [49,50,51,52,53,54].
-
Reduction Reaction:
2H2O + 2e− → H2 + 2OH−
-
Oxidation Reaction:
4H2O → 4e− + O2 + 4H+
-
Photoelectrochemical (PEC) System:
Photo electrochemistry is dependent on the excited state of a semiconductor achieved using suitable light quantities (hv > Eg). This excitation automatically generates an electric junction force between the two phases, allowing electron–hole pairs produced by photons to be separated. Photoelectrons in the conduction band generate cathodic hydrogen on the surface catalyst, but photo holes in the valence band move in opposite directions to aid in the anodic decomposition process [55,56,57,58,59]. This simultaneous oxidation and reduction process takes place on opposite poles within the crystallite, mimicking a photoelectrochemical diode and eliminating the need for elaborate and costly electrochemical equipment. The electrolytic solution closes the circuit with little electrical resistance [60,61,62,63,64]. Because of its fascinating features, including its ability to modulate optical gaps and lattice constants, the CuMO2 family has sparked interest in light–energy conversion. A notable example is CuYO2, representing a host lattice with the ability to intercalate anionic species, a focal point in hydrogen energy research [65,66,67,68,69].
A cost-effective approach to harnessing solar energy for fuel generation via water splitting involves utilizing a D4-type photoelectrochemical (PEC) tandem cell. This configuration consists of a connected n-type semiconductor photoanode and a p-type semiconductor photocathode. Despite the success in developing robust and cost-effective photoanode materials (e.g., WO3, Fe2O3, and BiVO4) with respectable photocurrents and high stability, the search for reliable and affordable p-type photocathodes continues [70,71,72,73,74,75]. Several p-type semiconductors, including p-Si, p-GaInP2, and p-WSe2, have exhibited exceptional performance. However, their widespread production faces challenges due to expensive and energy-intensive deposition or processing procedures. Notably, copper-based materials like p-Cu2O, p-CuGaSe2 (CGS), and p-Cu2ZnSnS4 (CZTS) have recently emerged as promising alternatives for p-type photocathodes, garnering attention for their potential in this role. These materials, which are based on plentiful amounts of copper, work well, even with solution-based deposition processes, allowing for large-scale deposition. However, difficulties continue since chalcogenides sometimes require a hazardous cadmium sulfide overlayer, and stability concerns, notably regarding copper(I) oxide, have developed [76,77,78].
An effective technique for addressing these difficulties includes covering the absorbent material with a conformal protective layer. Ensuring the hermeticity of this layer is crucial to prevent the degradation of the absorbing layer and maintain a thin profile to mitigate resistance losses. However, this approach introduces complexity to the manufacturing process and elevates the cost of the photoelectrodes [79,80,81,82,83,84]. To facilitate the development of cost-effective and easily scalable photoelectrochemical (PEC) devices, it is imperative to identify an intrinsically stable p-type material for water photoreduction. In this context, copper-based delafossite materials like CuCrO2 [85,86,87,88], CuAlO2 [86,87,89], CuGaO2 [87,88], and CuFeO2 [87,88] have demonstrated notable water stability. Furthermore, CuFeO2 [90,91] and CuRhO2 have recently been postulated to be possible water reduction photocathodes.
The reductive HER mechanism in copper-based delafossite materials involves intricate processes occurring at the semiconductor–electrolyte interface, with potential structural reconstructions influencing the overall efficiency of the reaction. As photons with energy greater than the bandgap (hv > Eg) are absorbed by the semiconductor, electron–hole pairs are generated, initiating the HER process. In the specific case of CuMO2 (where M represents various metal elements), the conduction band electrons are harnessed for the reduction of protons to form hydrogen gas, while the photo-generated holes may be involved in concurrent anodic processes.
Before HER initiation, the delafossite structure, characterized by layered arrangements of metal cations, may undergo transformations in response to interaction with the electrolyte. This interaction can lead to changes in the oxidation states of copper ions and alterations in the coordination environment, potentially facilitating the subsequent HER steps.
During and after an HER, the semiconductor–electrolyte interface plays a crucial role in governing the reaction kinetics. The intricate interplay involves the adsorption of protons, charge transfer processes, and the release of hydrogen gas. Any structural reconstruction during the HER may impact the catalytic sites, influencing the overall efficiency and stability of the photocatalyst.
This mechanistic insight provides a foundation for understanding the complexities of reductive HERs in copper-based delafossite materials. It is essential to consider these processes to optimize the performance of these materials in sustainable energy applications.

3. CuFeO2 Delafossite Materials

As emphasized by Mathieu S et al., the literature underscores CuFeO2’s nature as a promising candidate for solar water reduction. However, it is crucial to continue to attempt to optimize doping and enhance charge separation efficiency in sacrificial electrolytes. Additionally, the semiconductor interface requires optimization using suitable overlayers or catalysts to reduce the reported overpotential necessary for water reduction. The study by Mathieu S et al. showcased a sol–gel-based method for producing thin films of p-type delafossite CuFeO2 on FTO glass. Our citrate–nitrate technique outperforms previously described methods for preparing CuFeO2 for photoelectrochemical (PEC) research [92]. These advantages encompass straightforward solution processing of the films, a comparatively low heating temperature required for delafossite phase generation, and the ability to tailor layer thickness. Rigorous physical characterization confirmed the films’ purity and crystal phase. Notably, CuFeO2 films exhibited an excellent band-edge location, featuring an optical bandgap energy conducive to high-efficiency water splitting in an integrated tandem system. As shown in Figure 3, in the presence of sacrificial electron acceptors, the J–V curves of unaltered CuFeO2 electrodes demonstrated record photocurrent densities for bare films, with a noteworthy photocurrent initiation at +0.9 V versus a reversible hydrogen electrode (RHE). Furthermore, the incident photon-to-current efficiency onset was determined to be 830 nm. The bare electrodes displayed impressive durability under operating settings for several days, distinguishing CuFeO2 from other photocathodes. Typically, protective coatings are essential to ensure the survival of materials under corrosive conditions, particularly in reductive environments [93,94].
Another pertinent study conducted by Carrier et al. showcased a straightforward electrochemical synthesis method for thin CuFeO2 electrodes. Their research demonstrated the thermodynamic possibility of generating H2 photoelectrochemically while absorbing the whole visible spectrum [95]. Future efforts will concentrate on adjusting deposition conditions; we are exploring alternative techniques for creating CuFeO2 electrodes with optimal thicknesses and morphologies. Additionally, our research involves identifying suitable H2 evolution catalysts for integration onto the surface of CuFeO2 (Figure 4). This strategy attempts to improve the effectiveness of CuFeO2 photocathodes for splitting water using photoelectrochemical techniques [96].
Furthermore, we are conducting investigations into the utilization of p-CuFeO2 as a photocathode for the reduction of CO2, exploring the potential advantage of its less-favorable catalytic characteristics for H2 evolution. The current work investigated the photoelectrochemical generation of H2 without stirring utilizing electrode-type CuFeO2, ruling out the potential of mechano-catalytically creating H2. Nevertheless, due to the limited steady-state photocurrent attained for water reduction, generating a measurable quantity of H2 from a thin CuFeO2 film using the current setup proved challenging [97]. Attempts to boost H2 production by increasing illumination intensity failed because increased intensity resulted in large concentrations of photogenerated electrons on the outside of the CuFeO2, leading to photo corrosion. Surprisingly, the inclusion of O2 in the electrolyte reduced severe photo corrosion when more powerful light was used. An idea was developed wherein creating a photocurrent in an environment of O2, which improves CuFeO2 photostability under high light, would increase the possibility of detecting H2. Even with a poor photocurrent-to-H2 conversion efficiency, any amount of H2 discovered in this experiment would indicate the thermodynamic feasibility of solar H2 synthesis by CuFeO2 [98].

4. CuCrO2 Delafossite Materials

In a notable study, S. Saad et al. investigated the technical feasibility of photochemical hydrogen (H2) evolution by employing a mix of CuCrO2 powder in electrolytes composed of water with various reducing agents (SO32−, (insert another reducing agent), and S2O32−). Electrochemical assessments were conducted at 23 °C using a Pyrex cell fitted with a Pt counter electrode and an electrode made of saturated calomel (SCE). A deoxygenated promoting electrolyte (1 M KOH) was used, and potentials were recorded using a Voltalab 201 potentiostat. The photoactivity experiments were conducted in a 600 mL double-jacketed cell at 50 ± 0.1 °C, which was illuminated by three 200 W tungsten lamps. Gas chromatography was used to identify hydrogen, and a coaxial switched burette was used to estimate its volume. A Jenway 6051 spectrometer was used to calorimetrically quantify the concentration of yellow polysulfide Sn2− at max = 520 nm. Once-distilled water and A.R.-grade reagents were used to make solutions that could be neutralized with Na2HPO4 and NaH2PO4 (Figure 5) [99].
The chosen oxide exhibits significant corrosion resistance and possesses an optimal band gap (Eg) of 1.32 eV. The deliberate introduction of a small quantity of oxygen was anticipated to induce the partial oxidation of Cu+ to Cu2+, resulting in the material acquiring p-type semiconductivity. The oxidation of S2 is critical for photocorrosion inhibition, and H2 evolution rises concurrently with the synthesis of Sn2− polysulfides. Notably, when p-CuCrO2 is coupled with in situ created n-Cu2O, a large amount of H2 is produced. H2 is primarily liberated on CuCrO2, whereas S2 is oxidized on the Cu2O surface. The Cu2O/CuCrO2 hetero system has been improved in terms of several physical properties [100].
The photoactivity was found to be conditional and reducing the synthesis temperature using the nitrate approach enhanced specific surface area (Ssp). Due to electrons’ poor mobility, the arrival of electrons at the contact was designated as the rate-determining phase in the framework of photoelectrochemical H2 generation [101].
CuCrO2, a p-type semiconductor with a delafossite structure, has received a lot of interest in relation to photocatalysis and solar cells because of its effective absorption of visible light. While cuprous oxide (Cu2O) shows potential as a photocathode, its chemical stability is limited due to its sensitivity to corrosion when exposed to light. CuCrO2, on the other hand, stands out as an attractive option for photocatalytic applications due to its outstanding chemical stability, p-type semiconductor characteristics, and ability to absorb visible light. Previous research has demonstrated its capacity to create hydrogen from water and remove metal ions when exposed to visible light [102]. Notably, Mg-doped CuCrO2 has demonstrated improved photocatalytic hydrogen generation activity. However, issues such as reduced hydrogen synthesis after short light exposure necessitate additional investigation.
Yi Ma et al. have investigated the photocatalytic H2 generation capability of CuCrO2 in conjunction with co-catalysts, using water and ethanol as sacrificial reagents. Their study revealed that CuCrO2 has a consistent H2 generation rate over a 3 h timeframe (Figure 6). Their study proceeded to investigate the photoelectrochemical (PEC) properties of CuCrO2, confirming its p-type semiconductor features. A cathodic photocurrent, a common property of light excitation, was detected and grew stronger as the calcination temperature rose [103,104,105,106]. According to the findings, greater calcination temperatures minimize flaws by functioning as recombination centers. CuCrO2’s steady hydrogen generation activity makes it a suitable material for solar-to-energy applications. However, co-catalysts, notably Pt, are thought to be required for effective hydrogen creation during water splitting. Combinations with other semiconductor materials may increase the stability and efficiency of CuCrO2, potentially leading to the creation of efficient p–n junction materials [107].

5. CuAlO2 Delafossite Materials

A study focusing on hydrogen production utilizing CuAlO2 delafossite was conducted by N. Koriche and colleagues. They reported H2 generation via the simultaneous oxidation of inorganic compounds in a CuAlO2 solution, constituting a process that is particularly effective when targeting certain processes since S2− oxidizes faster than H2O. Nevertheless, the photovoltage created was inadequate for electrolysis, necessitating an extra voltage of about 0.5–0.75 V since the VB is significantly lower than the O2/H2O potential. Semiconductors typically exhibit a higher O2 overpotential, often close to 1 V [108].
CuAlO2 is a semiconductor of the p type with limited-mobility polarons due to its tiny band gap. Its catalytic aptitude for visible-light-induced H2 generation was studied in connection to the synthesis technique. The conduction band’s potential (−1.63 V versus SCE) is lower than the H2O/H2 level, allowing for spontaneous H2 evolution. Coprecipitation resulted in increases in oxide activity. Maximum H2 generation was reported in a 0.1 M S2− mixture at pH 13.72, with an output rate equal to 0.19 mL h−1 mg−1. As seen in Figure 7, this production increased in tandem with the formation of polysulfide Sn2− (nS2− + 2(n − 1) H2O Sn2− + (n − 1) H2 + 2(n − 1)OH−). Both processes proceeded concurrently, with no observable photoactivity under pH 7, showing that S2− played a critical role. Carriers were transferred iso-energetically between CuAlO2 and the electrolyte, with the help of produced band gap states acting as strong relays [109]. The overall reaction is governed by the momentary arrival rate of electrons at the contact, and the greater thermal photoactivity observed is connected to enhanced electronic mobility [110].
In another study, Smith and colleagues explored a thermal-photocatalytic process using dispersed CuAlO2 catalyst nanoparticles in water under conditions of sunlight exposure. In the experimental setup, depicted in Figure 8, sunlight was used as the primary source of solar radiation, aligning the hydrogen generation container axis with the sun [111]. Solar heating was complemented with electrical resistance heating to elevate the water temperature, and no electrodes were placed in the water to prevent ion currents that may cause photocorrosion. The technique was stable and did not produce any ion currents. The cylinder was linked to a gas chromatograph to test the H2 concentration throughout typical runs lasting one to multiple hours.
Due to the known influence of CuAlO2 band gaps on material preparation parameters, optical absorption tests conducted on thin layers of CuAlO2 catalyst powders utilized in H2 production experiments were undertaken [112]. Through absorption coefficient measurement, the films formed over a quartz plate formed using a catalyst that was an aqueous mixture produced both indirect and direct band gap values (Figure 9). The direct gap for the CuAlO2 particles used was determined to be 3.01 eV, while the indirect gap was 1.87 eV, making both gaps accessible to solar light. Even though indirect-gap transitions require phonon aid in bulk materials, they may be more frequent in tiny catalyst nanoparticles or at higher temperatures.
A previous study demonstrated photocatalytic H2 creation for CuAlO2 in sulfide-doped water at 321 K, with dismal findings of H2 generation decreasing to zero after a 20–30 min runtime. Notably, corrosiveness was not seen in the experimental environment of CuAlO2 in H2O. The measured thermal activation barrier was 0.94 eV. First-principles calculations showed that H2 desorption is the most actively demanding stage in the H2 production process. The proposed technique for producing H2 uses thermal desorption, which is facilitated by a drop in the adsorption barriers produced by solar radiation, allowing contributions to H2 production from the visible, ultraviolet, and infrared radiation portions of the solar spectrum [113].

6. CuRhO2 Delafossite Materials

The study of polycrystalline CuRhO2, acting like a photocathode for visible-light water splitting, reveals its distinct band edge locations that cover the redox potentials associated with water oxidation and reduction. Photogenerated band conduction electrons can decrease water energetically, whereas related valence band holes may oxygenate water and thus make O2. In an air-saturated solution, visible light triggers H2 generation with a 0.2 V underpotential [114]. H2 generation in an Ar-saturated solution, on the other hand, is unstable due to the decrease in the semiconductor producing Cu(s). Notably, in spite of the presence of oxygen, no bulk Cu(s) is discovered, implying that CuRhO2 may self-heal in the presence of air, resulting in steady H2 production with roughly 80% Faradaic efficiency.
Jin Gu researched the photoelectrochemistry and manufacture of a p-type CuRhO2 electrode [115]. The researchers investigated the photostability and water reduction behavior of oxygen- and argon-saturated mixtures in a pH = 14 electrolyte. Because of its capacity to enable electrode regeneration in the presence of O2 without affecting H2 production, an alkaline electrolyte was chosen (Figure 10). This ground-breaking self-healing semiconductor–electrolyte interface provides the electrode with great stability, which is critical for the sustained photoelectrolysis of water [116].
The photoelectrochemical reduction of water utilizing a CuRhO2 electrode during visible light irradiation lasted more than 8 h at a potential of −0.9 V versus SCE in 1 M of NaOH exposed to air. The system was stable, as demonstrated by H2 gas generation detected by the formation of bubbles at the surface of the electrode, which was validated through gas chromatography tests [118]. The electrolyte must be saturated with O2 (from air or a pure oxygen stream), and the pH must be basic, according to two important conditions for a stable H2-producing system. The increased air stability suggests the existence of an interface self-regeneration process.
A CuRhO2 delafossite photocathode is an ideal choice for water reduction due to its powerful visible-light responsiveness and intrinsic semiconductor of a p-type nature. Polycrystalline electrodes, which are easily produced using solid-state techniques, exhibit water reduction in 1 M of NaOH under exposure to visible light at an underpotential of 0.2 V. Electrode durability is demonstrated by a photocurrent of up to −1.0 mA/cm2 at −0.9 V in air for at least 8 h. This is the first time an oxygen-driven self-healing process at an electrode–electrolyte interface has been described, highlighting the material’s potential for producing solar fuels from water [119].

7. CuMnO2 Delafossite Materials

CuMnO2 has long been explored as a catalyst, particularly in oxygen and hydrogen evolution (OER and HER) processes. CuMnO2 particles were studied using cyclic voltammetry, electrochemical impedance spectroscopy, and linear sweep voltammetry in a 1 M KOH solution and compared to an Ag/AgCl electrode to demonstrate the possibility of current generation in an alkaline electrolyte with water as an electrolysis process. As proven in this study, the CuMnO2 electrode for OER and HER exhibits more efficient electrocatalytic activity in a three-electrode arrangement on a potentiostat/galvanostat workstation with electrode rotation than without rotation [120]. The calculation of several electrochemical characteristics for the material, such as total charges, charge accessibility, specific capacitance, electroactive surface area, and electrode surface charge change, offered an in-depth understanding of the material’s catalytic behavior. This study’s findings indicate the potential for HER applications in the cathodic area owing to p-type CuMnO2 as well as future research into OER stability in the anodic region, as Figure 11 depicts. The findings of this study might contribute to an upsurge in research interest in these earth-abundant complex transition metal oxides for further electrochemical applications [121,122,123].

8. CuYO2 Delafossite Materials

The CuYO2 catalyst has superior electrical conductivity, great stability, and better thermal stability due to its pseudo-delafossite structure. Cu+ ions with d-d transitions are found in its layered structure, resulting in a Cu+ inter-configuration ranging from 3d94s1 to 3d10. The catalyst is made up of tightly coiled multilayered octahedral sites with Y3+ ions that share common edges and interact with Cu+ ion films in vertical coordination [124]. CuYO2 additionally features incorporated anionic organisms, which are useful in hydrogen energy research. In our work, we used the self-combustion GNP method to make a CuYO2 nanopowder catalyst, which we then used in an MSR. CuYO2 nanopowder precipitated as synthesized in a delafossite structure with a nanosized, spherical shape. During the MSR, the CuYO2 nanopowder catalyst produced a significant amount of hydrogen while generating very little coke. Table 1 and Figure 12 show temperature profiles for CuCrO2 and CuFeO2 nanopowders, CuCrO2 bulk powder, and an industrial Cu/Al/Zn catalyst.
The MSR catalytic efficiency was investigated in a continuous flow reactor using nitrogen as an intermediate gas. GC-1000 gas separation with a TCD was used to examine the gas products. The H2 generation rate (mL STP min−1 g-cat−1) was used to assess the efficacy of the CuYO2 nanopowder catalyst [90,127,128,129,130,131].
A 25 cm quartz pipe with a 1.2 cm internal diameter was used in the testing, and nitrogen was used as a dilutant and gas carrier with a flow rate of 30 sccm. Gas chromatography was used to assess the rate of hydrogen creation, and the catalyst performed better with rotation during the OER and HER. These findings demonstrate the potential of CuYO2 nanopowder as a viable catalytic material for hydrogen generation applications, including fuel cells, battery devices, electrolysers, and solar water splitting [102,132,133,134].
-
Performance table:
Delafossite MaterialPhotocatalytic ApplicationKey Performance
Metrics		Notable Features
CuFeO2Solar water reductionBand-edge location, stability, CO2 reduction potentialSol–gel-based method, excellent band edge location, stability, and CO2 reduction capability
CuCrO2Photochemical H2 evolutionBand gap, stability visible light responsivenessEfficient absorption of visible light, stable H2 production, potential for co-catalyst integration
CuAlO2Hydrogen ProductionBand gap, catalytic aptitude for visible light induced H2 generationEffective H2 generation under visible light, dependence on S2− as a reducing agent
CuRhO2Visible light water splittingBand edge locations, photostability, self-healing interfaceUnique self-healing semiconductor/electrolyte interface, stable H2 production under visible light
CuMnO2Oxygen and hydrogen evolutionElectrocatalytic activity, stabilityBifunctional electrocatalysis for OER and HER, potential for further research
CuYO2Methane steam reformationH2 production rate, thermal stability, catalyst efficiencySuperior electrical conductivity, great stability, potential for hydrogen generation applications
-
Active Sites:
  • CuFeO2—Active sites are likely associated with the delafossite structure, with an emphasis on the optimized semiconductor interface using suitable overlayers or catalysts.
  • CuCrO2—Active sites include the CuCrO2 surface where H2 is primarily liberated, and S2 is oxidized on the Cu2O surface when coupled with in situ created n-Cu2O.
  • CuAlO2—Active sites involve the conduction band’s potential, allowing spontaneous H2 evolution, particularly in the presence of S2− as a reducing agent.
  • CuRhO2—Active sites are attributed to the polycrystalline CuRhO2 surface, with photogenerated band conduction electrons reducing water and valence band holes oxygenating water.
  • CuMnO2—Active sites for CuMnO2 are not explicitly mentioned in the provided text, but further research may focus on its electrocatalytic activity for oxygen and hydrogen evolution.
  • CuYO2—Active sites are likely associated with the pseudo-delafossite structure, where Cu+ ions with d-d transitions and Y3+ ions interact, facilitating hydrogen generation during methane steam reformation.
-
Summary and Comparison:
  • CuFeO2—stands out for its sol–gel-based production method, excellent band-edge location, stability, and potential for CO2 reduction;
  • CuCrO2—noteworthy for efficient absorption of visible light, stable H2 production, and potential for co-catalyst integration;
  • CuAlO2—effective in H2 generation under visible light, particularly with S2− as a reducing agent;
  • CuRhO2—unique for its self-healing semiconductor/electrolyte interface, providing stability for sustained photo electrolysis of water;
  • CuMnO2—limited details provided, emphasizing further research opportunities;
  • CuYO2—superior electrical conductivity, great stability, and potential for hydrogen generation applications during methane steam reforming.

9. Conclusions

In conclusion, hydrogen (H2) emerges as a pivotal and eco-friendly alternative in the realm of renewable energy, addressing the escalating need for sustainable and clean power. Its significance lies in its potential to alleviate both the energy crisis and climate change, particularly in the industrial sector, where its application in electricity generation results in minimal nitrous and sulfur oxide emissions. However, despite its environmental benefits, challenges persist in the efficient processing and storage of hydrogen, posing obstacles to the full realization of a hydrogen-based energy economy.
One major impediment is the security issues surrounding hydrogen storage, transportation, and utilization, with current conventional storage methods falling short in terms of capacity. To address this limitation, the implementation of a hydrogen storage system based on fuel cells has been proposed, particularly the H2 fuel cell system, characterized by low carbon emissions and high efficiency. This technology represents a compelling alternative to internal combustion engines, especially in the automotive sector, contributing to the global interest and growth of hydrogen fuel cell systems in recent years.
The exploration of photocatalysts, specifically in the context of water splitting for hydrogen generation, is a key focus. The integration of photoelectrodes into PV-cell-powered configurations showcases the potential of photoelectrochemical (PEC) systems in harnessing solar energy for hydrogen production. However, the choice of semiconductor (SC) material is crucial, necessitating the satisfaction of specific criteria relating to aspects such as band gap and stability to ensure optimal performance.
Delafossite compounds, such as CuMO2, have garnered attention for their applications in hydrogen generation via PEC water splitting, as well as in optoelectronic devices. Notably, CuFeO2 has been highlighted as a promising candidate, demonstrating excellent band edge location and stability for efficient water splitting. Its potential extends beyond hydrogen production, as it also holds promise in reducing CO2, showcasing the versatility of delafossite materials in addressing multiple environmental challenges.
The conclusions of the studies also delve into the exploration of other delafossite materials, including CuCrO2, CuAlO2, CuRhO2, CuMnO2, and CuYO2. Each material exhibits unique properties and catalytic capabilities, contributing to the broader understanding of their potential applications in solar water splitting, oxygen and hydrogen evolution processes, and other areas. Notably, CuRhO2 stands out because of its self-healing semiconductor/electrolyte interface, representing a breakthrough in achieving stable and sustained photo-electrolysis of water.

Author Contributions

H.C.: Conceptualization, Methodology, Writing—original draft, Experimental works, Summarization, revision. A.B.: Interpretation, Methodology, Validation, Writing—original draft, Revision. B.M.S.: Visualization, Investigation, Supervision, Revision. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data are included in the manuscript.

Acknowledgments

Amal Bouich acknowledges MCIN for providing funding support through Ministerio de Ciancia e Innovación (Spain) (MCIN/AEI/10.13039/501100011033) and NextGenerationEU.

Conflicts of Interest

The authors declare no conflicts of interest that could have appeared to influence the work reported in this paper. The authors declare that they have no known competing financial or personal interests.

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Figure 1. Photoelectrochemical (PEC) system scheme: integration of photoelectrode into PV-cell-powered configuration [32].
Figure 1. Photoelectrochemical (PEC) system scheme: integration of photoelectrode into PV-cell-powered configuration [32].
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Figure 2. Crystal model of CuMO2 delafossite, where (●) is Cu, (●) is M, and (●) is O.
Figure 2. Crystal model of CuMO2 delafossite, where (●) is Cu, (●) is M, and (●) is O.
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Figure 3. A view of many layers of CuFeO2 film on FTO, and chronoamperometry of a CuFeO2 electrode under O2-bubbling and chopped illumination conditions [70].
Figure 3. A view of many layers of CuFeO2 film on FTO, and chronoamperometry of a CuFeO2 electrode under O2-bubbling and chopped illumination conditions [70].
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Figure 4. A schematic and a cross section of p-CuFeO2 thin film on FTO [95].
Figure 4. A schematic and a cross section of p-CuFeO2 thin film on FTO [95].
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Figure 5. A device for measuring the volume of hydrogen in experimental settings [99].
Figure 5. A device for measuring the volume of hydrogen in experimental settings [99].
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Figure 6. Photocatalytic and photoelectrochemical (PEC) devices for water splitting [106].
Figure 6. Photocatalytic and photoelectrochemical (PEC) devices for water splitting [106].
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Figure 7. The volume of H2 evolved as a function of illumination time was investigated for different redox couples in buffered electrolytes [109].
Figure 7. The volume of H2 evolved as a function of illumination time was investigated for different redox couples in buffered electrolytes [109].
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Figure 8. The hydrogen generation apparatus (dimensions are in millimeters) [111].
Figure 8. The hydrogen generation apparatus (dimensions are in millimeters) [111].
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Figure 9. Investigation and determination of the band gaps for our CuAlO2 powder thin films [112].
Figure 9. Investigation and determination of the band gaps for our CuAlO2 powder thin films [112].
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Figure 10. Polycrystalline CuRhO2 as a photocathode for visible-light-induced water splitting [117].
Figure 10. Polycrystalline CuRhO2 as a photocathode for visible-light-induced water splitting [117].
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Figure 11. CuMnO2 bifunctional electrocatalysis: unraveling stability factors in water splitting for OER and HER [121].
Figure 11. CuMnO2 bifunctional electrocatalysis: unraveling stability factors in water splitting for OER and HER [121].
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Figure 12. The rates of hydrogen (H2) generation of CuYO2 nanopowders after being heated at various temperatures at a flow rate of 30 sccm [125].
Figure 12. The rates of hydrogen (H2) generation of CuYO2 nanopowders after being heated at various temperatures at a flow rate of 30 sccm [125].
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Table 1. The rates of hydrogen (H2) generation of CuYO2 nanopowders after being heated at various temperatures at a flow rate of 30 sccm.
Table 1. The rates of hydrogen (H2) generation of CuYO2 nanopowders after being heated at various temperatures at a flow rate of 30 sccm.
CatalystH2 Production Rate (mL STP min−1 g-cat−1)
250 °C300 °C350 °C400 °CRef.
CuYO21107.91735.651390.051375.2[95]
CuFeO2705.188464.8691452.9752010.600[125]
CuCrO2279.169753.2051480.3651720.300[126]
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Chfii, H.; Bouich, A.; Soucase, B.M. The Use of Copper-Based Delafossite to Improve Hydrogen Production Performance: A Review. Hydrogen 2024, 5, 39-58. https://doi.org/10.3390/hydrogen5010004

AMA Style

Chfii H, Bouich A, Soucase BM. The Use of Copper-Based Delafossite to Improve Hydrogen Production Performance: A Review. Hydrogen. 2024; 5(1):39-58. https://doi.org/10.3390/hydrogen5010004

Chicago/Turabian Style

Chfii, Hasnae, Amal Bouich, and Bernabé Mari Soucase. 2024. "The Use of Copper-Based Delafossite to Improve Hydrogen Production Performance: A Review" Hydrogen 5, no. 1: 39-58. https://doi.org/10.3390/hydrogen5010004

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