Advances in Phosphorus-Based Catalysts for Urea Electrooxidation: A Pathway to Sustainable Waste to Energy Conversion Through Electrocatalysis

Abstract

The electrocatalytic oxidation of urea has gained significant attention as a promising pathway for sustainable energy conversion and wastewater treatment that could address the dual goals of waste remediation and renewable energy generation. Phosphorous function groups-based catalysts have been introduced as potential electrode materials for enhancing the urea electrocatalytic oxidation reaction (UEOR) due to their unique structural properties, high stability, and tunable electronic characteristics. This review presents recent advancements in phosphorous-based catalysts (phosphates/phosphides) for UEOR. It highlights the development of novel phosphorous materials, synthesis approaches, and electrocatalytic insights into urea electrooxidation on phosphorous-based materials surfaces. Key topics include the role of different metal phosphates, surface modifications, and compositional optimizations to improve electrocatalytic efficiency and durability. Through a critical evaluation of current research trends and technological progress, this review underscores the potential of phosphate-based catalysts as environmentally friendly and efficient alternatives for sustainable waste-to-energy conversion via UEOR. The review concludes with a perspective on future directions for optimizing phosphate catalysts, scaling up practical applications, and integrating UEOR systems into renewable energy infrastructures.

1. Introduction

The growing global emphasis on sustainable energy and environmental remediation has placed catalysis points and their applications at the forefront of research, which could drive the development of efficient and environmentally friendly catalytic systems [1,2]. Among various catalytic processes, urea electrooxidation has garnered significant attention as a dual-purpose approach that addresses waste remediation and energy conversion [3,4]. This process offers the potential to transform urea-rich wastewater, which is a persistent pollutant into valuable energy resources, aligning with green chemistry and circular economy [5].

Phosphate-based catalysts have been presented as a promising type of material in this context, owing to their unique structural attributes, high chemical stability, and tunable electronic properties [6,7]. These features make them suitable candidates for enhancing the electrocatalytic systems such as electrooxidation of small molecules [8], as shown in Figure 1, such as hydrazine electrooxidation [9,10], alcohol oxidation [11], and urea electrooxidation reaction (UEOR) [12]. Unlike conventional catalytic systems, phosphate-based materials exhibit advantages such as structural versatility [13], compatibility with surface modifications [14], and the potential for high catalytic efficiency [15,16].

This review delves into the advancements in phosphate-based catalysts for UEOR, providing a comprehensive exploration of their synthesis strategies, structural optimizations, and mechanistic insights. Special emphasis is placed on the role of different metal phosphates, surface functionalization techniques, and compositional innovations in improving the catalytic activity, selectivity, and durability of these materials. By critically analyzing current trends and progress, the review underscores the promising role of phosphate-based catalysts in sustainable waste-to-energy conversion technologies. The discussion concludes by addressing challenges in scaling up applications and integrating these catalysts into renewable energy systems, which could offer a perspective on future directions in this exciting catalytic field.

2. Chemistry of Phosphates

Phosphates are a class of chemical materials derived from phosphoric acid (H₃PO₄), which forms various salts and esters when combined with metals or organic molecules [17]. They play an essential role in biological [18], geological [19], and industrial systems [20], contributing to energy transfer [21], mineralization processes [22], 3D printing [23], and fertilizer production [24]. Understanding the types and structures of phosphates offers insight into their diverse applications and natural significance [25]. At the heart of all phosphate compounds is the phosphate ion (PO₄³⁻), a tetrahedral structure where a phosphorus atom is covalently bonded to four O atoms [26]. In this arrangement, one oxygen atom forms a double bond with phosphorus, and the other three are single-bonded and carry negative charges, which provide more reactivity and the capability of forming various chemical bonds. This flexibility allows phosphates to exist in multiple forms, each with distinct structural and functional characteristics. Phosphates can be categorized into three main types: orthophosphates, polyphosphates, and organophosphates [27]. Orthophosphates (PO₄³⁻) are the simplest form and serve as the foundation for other phosphate materials. They are prevalent in biological systems, notably in adenosine triphosphate (ATP), where they store and transfer energy within cells [28]. Orthophosphates also exist in minerals such as apatite [29], which is a primary source of phosphorus in nature and an essential component of bones and teeth [30]. The second type of phosphate is polyphosphates, which are chains or rings of phosphate units linked by oxygen bridges [31]. These compounds include pyrophosphate (P₂O₇⁴⁻) and tripolyphosphate (P₃O₁₀⁵⁻) and can be synthesized by condensation reactions involving orthophosphates. Polyphosphates are widely used in industrial applications, including detergents, water softeners, and food additives [32]. In living organisms, they act as energy reservoirs and are involved in processes like cell signaling and bone mineralization [33]. The third type of phosphate is organophosphates, which consist of phosphate groups covalently bound to organic molecules [34]. These compounds play a pivotal role in biochemistry, as seen in nucleotides, DNA, RNA, and phospholipids that form cellular membranes [35]. Organophosphates are also the basis for many pesticides and nerve agents [36], exploiting their ability to inhibit essential enzymes like acetylcholinesterase [37]. This duality highlights their importance and the need for careful management in agriculture and medicine [38].

Structurally, phosphates exhibit versatility depending on the phase and chemistry of preparation and saving environment. In crystalline forms (such as minerals), the phosphate ions are coordinated with metal cations like Ca, Na, or K, which lead to complex networks. These structures are stable and contribute to the formation of rocks like phosphorite. In aqueous systems, phosphates exist as hydrated ions or form soluble salts, allowing them to participate in chemical reactions, including but not limited to acid–base equilibria and precipitation. The biological significance of phosphates cannot be overstated. Phosphates are integral to energy metabolism, where ATP molecules release energy through the hydrolysis of phosphate bonds [39]. They also serve as structural components in nucleic acids, linking nucleotides via phosphodiester bonds [40]. Additionally, phosphate groups regulate proteins and enzymes through phosphorylation, a process central to cell signaling and metabolic control [41].

Various synthesis strategies for phosphorus-based catalysts, including techniques such as sol–gel [42], hydrothermal [43], chemical vapor deposition [44], electrochemical methods [45], and MOF-assisted synthesis [46], have been reported. From an environmental perspective, phosphates are a double-edged sword. They are essential for plant growth as fertilizers [24], but excessive phosphate runoff leads to eutrophication, which causes algal blooms and oxygen depletion in aquatic ecosystems [47]. This environmental impact necessitates sustainable phosphate management, including recovery from wastewater and efficient agricultural practices. Geologically, phosphates are key constituents of sedimentary and igneous rocks, forming through biogenic and geochemical processes. Apatite minerals are mined extensively to produce fertilizers, which support global food production. However, phosphate reserves are finite, and their depletion raises concerns about future availability that lead the future research into recycling and alternative sources. Distinct from other P-function groups, phosphides are compounds that contain phosphorus in their negative oxidation state, typically as the anion P³⁻. These materials are often formed by the reaction of phosphorus with metals or metalloids, resulting in a wide range of metal phosphides, and have gained significant interest in electrocatalysis [48]. Therefore, phosphorous function groups are foundational to life, industry, and the environment. Their diverse types and structures underpin critical biological functions, support agricultural productivity, and pose environmental challenges. It is essential to mention the diverse synthesis strategies employed to develop these catalysts, as these methods directly influence their structure, surface area, electronic properties, and catalytic activity. Common synthesis strategies include sol–gel methods [42], which enable the formation of homogeneously distributed phosphorus within the catalyst matrix, and hydrothermal techniques [43], which are widely used for producing highly crystalline and well-defined nanostructures. Additionally, chemical vapor deposition [44] offers precise control over the deposition of phosphorus onto specific surfaces, making it ideal for creating thin films or layered structures. Electrochemical methods [45], on the other hand, provide a sustainable and scalable approach to phosphorus incorporation while tuning the electrochemical properties of the catalyst. Other innovative approaches, such as MOF-assisted synthesis [46], template-assisted synthesis, pyrolysis of phosphorus-containing precursors, and doping strategies, further expand the design possibilities for these catalysts. The selection of a synthesis method is often dictated by the intended application, as well as the desired properties of the final catalyst, such as high conductivity, stability, and active site availability. A deeper understanding of phosphorous function groups allows us to harness their benefits while mitigating their risks, emphasizing their multifaceted role in shaping the world around us. This review displays the reported application of phosphorous function groups as anode material and electrocatalyst for UEOR towards the commercialization of urea fuel cells (UFC).

3. Electrocatalysis in Urea Fuel Cells

Fuel cells (FCs) are one of the electrochemical devices that change the chemical energy in chemical fuels into electricity with acceptable efficiency [49]. They operate silently and occupy less space compared to conventional energy systems, as well as having minimal environmental impact. Among the various types of fuel cells, UFC holds great promise for sustainable wastewater treatment and energy generation [50]. In these systems, urea is electrochemically oxidized, yielding CO₂, N₂, H₂O, and electricity [51]. UFC systems are notable for their theoretical efficiency (102.9%) and could surpass that of hydrogen fuel cells (83%) because of heat absorption from the surroundings. Additionally, their open-circuit voltage (OCV) is comparable to H₂ fuel cells at approximately 1.21 V. However, enhancing the performance of DUFCs remains a challenge, especially regarding the electrochemical activity of the metal-based catalysts as anode part in UFC commonly used for urea electrocatalytic oxidation. Among the transition metals, nickel (Ni) has been reported as an effective, non-precious electrocatalyst for the UEOR in alkaline conditions [52]. Despite their electrocatalytic activity, Ni electrocatalysts face limitations in electrochemical stability and activity, hindering commercial viability. Efforts to enhance anode performance for fuel cells include incorporating Ni with metals like Pd [53], Cu [54], Co [55], Ce [56], Ti [57,58], Mn [59], Fe [60], Cr [61,62], and Se [63,64] or increasing the catalyst’s surface area using supports including carbonaceous support such as C-nanofibers [65,66], graphene oxide [67], 3D carbon sponges [68], and carbon nanotubes [69]. Moreover, employing alternative oxidants like H₂O₂, which exhibits a higher cathodic potential than O₂, or adopting hybrid pH conditions (alkaline anolyte and acidic catholyte) has shown promising results in boosting the open-circuit voltage (OCV) and power output. In situ and ex situ studies reveal that these strategies significantly improve the catalytic efficiency of UFC, opening pathways for their commercialization as dual-purpose devices for energy generation and environmental remediation [49].

The electrocatalytic part in the UFC systems concentrates on two major parts: anodic and cathodic. The anodic part includes electrochemical oxidation of urea, such as the reported Ni-electrocatalysts, which could electrocatalyse urea oxidation via two mechanisms: the indirect pathway, where NiOOH oxidizes urea chemically, and the direct pathway, where urea adsorbs onto NiOOH followed by transferring electrons directly to the electrode [70]. Additionally, both direct and indirect electrocatalytic mechanisms have been reported at the same electrocatalyst, such as Ni₅Sm-P/C [71]. The indirect electrocatalytic mechanism involves competitive adsorption between urea and OH⁻ on the NiOOH surface, which could be considered an electroactive layer [72]. The reaction kinetics and efficiency are significantly influenced by the alkaline medium’s composition, with potassium hydroxide (KOH) emerging as the most effective electrolyte. KOH enhances the oxidation of surface intermediates, facilitates the release of CO₂, and increases the available NiOOH. Increasing the KOH concentration reduces the onset potential (E_op) and boosts the oxidation peak current density, improving electrocatalytic activity [73,74]. Furthermore, higher urea concentrations enhance the diffusion-controlled reaction rates, whereas kinetic limitations govern performance at elevated potentials. Therefore, understanding the behavior of electrocatalysts including Ni-based catalysts in alkaline media is crucial for optimizing UFC performance. The redox transitions of Ni between Ni(OH)₂ and NiOOH under applied potentials create active sites for urea oxidation, but the formation of intermediates can temporarily block these sites. Techniques like cyclic voltammetry (CV), impedance (EIS), and surface-enhanced Raman spectroscopy have elucidated these mechanisms, confirming the role of β-NiOOH as the primary electrochemical active site for UEOR. Therefore, advances in catalyst design and the use of hybrid conditions are promising steps toward addressing challenges such as catalyst poisoning, stability, and long-term operation. Further research focusing on alternative oxidants, robust electrocatalyst recovery strategies, and mechanistic insights will be pivotal for realizing the full potential of UFC in wastewater treatment and renewable energy applications.

Determining the optimal KOH/urea ratio for large-scale applications is essential for maximizing efficiency and minimizing energy consumption, particularly given the inherent variability in wastewater composition. Empirical studies involving systematic experimentation under different KOH/urea ratios provide valuable data on conditions that optimize urea conversion. Concurrently, real-time wastewater composition analysis allows for dynamic adjustments of the KOH/urea ratio using feedback control systems. Advanced modeling techniques, including computational fluid dynamics (CFD) and reaction kinetic simulations, can predict the optimal ratios under diverse scenarios, significantly reducing the need for trial-and-error experimentation. Pilot-scale testing under representative wastewater conditions further validates the practical applicability of these ratios, ensuring robust performance in real-world scenarios.

Stability tests for both direct and indirect mechanisms of urea oxidation could be conducted through more systematic evaluations under real-world conditions. For direct mechanisms, laboratory studies employing cyclic voltammetry and chronoamperometry under simulated wastewater conditions have provided insights into long-term stability. Indirect mechanisms, which involve reactive intermediates like hydroxyl radicals, are often assessed using advanced spectroscopic methods to monitor catalyst degradation over time. Limited field trials in wastewater treatment facilities have shown promising initial results, but further research is needed to replicate these findings across various operational conditions. To bridge this gap, future studies should focus on accelerated aging tests and continuous-flow setups that closely mimic real-world environments, thereby providing a more comprehensive understanding of catalyst durability.

Certainly, hydroxide ions (⁻OH) play a considerable role in urea electrooxidation according to the reported mechanism (as illustrated in Figure 2A), following an electrochemical–chemical (E-C) process. Initially, Ni content reacts with ⁻OH to form Ni(OH)₂, which is inactive for urea oxidation. Ni(OH)₂ is then oxidized by additional ⁻OH to form the electroactive NiOOH layer, capable of oxidizing urea into CO₂ and N₂ [62,72]. Urea oxidation involves the attack of six ⁻OH groups on specific sites of the urea molecule (the C in the O=C group and the hydrogens in the NH₂ groups). These lead to the cleavage of N–H and C–N bonds, yielding N₂, CO₂, and H₂O. Intermediates formed during this process adsorb onto the NiOOH because of the polarization of O-Ni bonds. Subsequently, ⁻OH attacks the N–H bonds, producing H⁺, which reacts with ⁻OH to form H₂O. The electroactive NiOOH is reduced back to Ni(OH)₂ and requires further oxidation to continue the catalytic cycle. However, CO₂ formed during the process can dissolve in the electrolyte and create a carbonate layer, which inhibits the Ni(OH)₂/NiOOH conversion, eventually reducing or halting the oxidation reaction [75,76].

From the different reported techniques for studying the electrocatalysis of UEOR, the EIS study is the best one that could provide the charge transfer related to the electrocatalytic mechanism. For example, the EIS with and without urea was displayed (Figure 2B,C, respectively) and confirmed that the relaxation time for urea oxidation is longer than for NiOOH formation, confirming that the rate-determining step (RDS) could be urea degradation (the cleavage of C–N and N–H bonds) [72]. Nyquist plots further illustrate the interaction of γ-NiOOH formation with urea oxidation. For 0.3 M urea, a faster relaxation time (0.797 ms) was observed against what was estimated in the case of urea-free solution (1.264 ms), highlighting the enhanced activity of NiOOH with urea. The large semicircle in the low-frequency region of the Nyquist plot for 0.3 M urea corresponds to urea oxidation, contrasting with the diffusion-controlled process observed in the urea-free solution. The calculated relaxation times and resistance values (R₂ dropping from 1050.3 to 26.55 Ω cm² with urea) strongly support the enhanced catalytic activity towards urea electrooxidation. Bode phase plots (Figure 2D) further confirmed the existence of two charge transfer processes: ⁻OH diffusion to the working electrode surface and the consumption of NiOOH during oxidation. A significant shift in the peaks indicates the active involvement of NiOOH in urea oxidation. Total impedance analysis (Figure 2E) demonstrates that the addition of urea decreases total impedance, indicating improved electrical conductivity due to enhanced charge transfer pathways during urea electrooxidation. Therefore, the study of EIS is a considerable indicator for evaluating the electrocatalysts for UEOR including Nyquist plots, Bode phase plots, and total impedance characteristics.

4. Ni-Phosphate Electrocatalysts for UEOR

Phosphates are promising materials for electrocatalysis due to their robust chemical and thermal stability, tunable electronic properties, and versatility in catalyzing essential reactions such as the oxygen evolution reaction (OER) [77], hydrogen evolution reaction (HER) [78], and carbon dioxide reduction reaction (CO₂RR) [79], as well as UEOR [80]. Their strong P−O bonds ensure durability under harsh reaction conditions, while their structural flexibility permits the incorporation of transition metals, enabling tailored catalytic activity [8]. Phosphates could stabilize key intermediates, lower activation barriers, and promote selective pathways in reactions like UEOR. Their performance can be further enhanced through doping with elements or forming composites with conductive materials such as carbon materials, which will be discussed in detail. Firstly, Ni-phosphate materials could be studied before other bimetallic phosphates, followed by composites containing phosphates.

Song et al. conducted a comprehensive study on the electrochemical catalytic activity of four nickel phosphate catalysts (NiPO-a,b,c,d) synthesized using different methods, focusing on their potential for urea oxidation in alkaline conditions [81]. The structures of these materials varied significantly: NiPO-a featured less-ordered nanotubes, NiPO-b and NiPO-c had a flower-like nanosheet porous structure, and NiPO-d consisted of stacked nanocrystals. Electrochemical analyses, including CV (Figure 3A–D), demonstrated that these phosphates had more negative E_op compared to commercial NiO, making them more reactive towards urea. Among them, NiPO-c exhibited the highest oxidation current density (30.0 mA cm⁻²) and superior stability, retaining 97.47% of its electrochemical activity after a 96-h amperometric i−t test (Figure 3E). This superior performance was attributed to its unique structure and excellent catalytic properties. Additionally, NiPO-c demonstrated favorable electrochemical behavior with clear cathodic/anodic peaks in KOH containing urea, which were absent in NiPO-d and commercial NiO (Figure 3A). The E_op for UEOR with NiPO-c was as low as 0.33 V versus Ag/AgCl, lower than other NiPOs and NiO. Mechanistic insights revealed that the reaction involved the conversion of Ni(OH)₂ to NiOOH, followed by a chemical reaction with urea, which highlighted the best performance for NiPO-c (Figure 3B). Further investigations into the influence of urea concentration showed that the optimal urea-to-KOH ratio was critical for maximizing current density (Figure 3C), while the ideal KOH concentration for urea oxidation was determined to be 3.0 mol L⁻¹ (Figure 3D). Figure 3F has the EIS results, which indicate that NiPO-c had the lowest equivalent series resistance (46 Ω) and charge transfer resistance (R_ct), enabling faster reaction kinetics. These findings underscore the potential of NiPO-c as a promising candidate for UEOR.

Q. Li et al. demonstrated the chemical design of porous rods based on Ni₂P/Ni material composed of nanosize particle subunits through a one-step phosphating method [82]. Density functional theory (DFT) calculations revealed that these assemblies exhibit enhanced conductivity and chemical bond activity compared to NiO/Ni, significantly boosting UOR performance. The optimized Ni₂P/Ni catalyst achieved a low overpotential of 50 mV at 10 mA·cm², and 87.6 mV·dec⁻¹ Tafel slope. Electrochemical characterization indicated that the catalyst possesses a better electrochemical surface area and improved kinetics, as evidenced by its low resistance (R_ct) and superior double-layer capacitance. Comparative studies with non-phosphated samples confirmed that phosphating enhances catalytic activity. The Ni₂P/Ni composite, developed by precisely controlling synthesis parameters, displayed superior UEOR activity, achieving higher current density and lower E_op than other tested samples. Additionally, the stability of the porous rod-like structure was demonstrated through extended immersion tests, which retained the morphology and performance of the catalyst.

G. Wang et al. synthesized a unique 3D porous Ni₂P nanoflower electrode combined with nickel foam (Ni₂P@NF) using hydrothermal growth strategy and material phosphating [83]. The working electrodes were analyzed by XRD, SEM, EDX, and TEM, and their electrocatalytic performance for UEOR was evaluated via CV, chronoamperometry (CA), and EIS. The phosphating process transformed the Ni(OH)₂ precursor into porous nanoflowers (Ni₂P), creating abundant active sites and electron transmission channels that enhanced interfacial reactions. Compared to its precursor, the Ni₂P@NF electrocatalyst displayed superior electrocatalytic activity in KOH (5.00 mol/L) and 0.6 mol/L urea, achieving 750 mA/cm² with an E_op of 0.24 V, alongside remarkable stability. Electrochemical analyses revealed enhanced redox transitions between Ni²⁺/Ni³⁺, a larger electrochemically active surface area (EASA), and improved charge transfer efficiency attributed to the material’s porous, flower-like nanosheet structure. The electrode demonstrated optimal catalytic performance at a urea content of 0.60 mol/L, aligning with the theoretical ratio of urea to KOH while maintaining stability under various applied potentials. EIS measurements indicated improved charge transport and distinct conversion processes in the presence of urea. Collectively, these findings establish the Ni₂P@NF electrode as a promising electrocatalyst for alkaline UEOR.

In 2019, Amin Imani et al. explored the chemical design of nanoporous Ni₃(PO₄)₃·8H₂O microspheres (NiP) with an orange peel morphology, which was developed as an advanced catalyst for UEOR [84]. Using a one-pot hydrothermal method, they synthesized NiP materials with tunable morphology and particle size by varying hydrothermal parameters. This material was identified as one-phase Ni₃(PO₄)₂·8H₂O, having a monoclinic structure. The optimized NiP110 demonstrated exceptional electrocatalytic activity and stability, obtaining 680 mA·cm⁻²·mg⁻¹ and a low oxidation voltage of 0.33 V in alkaline media. CV analysis revealed that the chemical existence of fuel sharply improved the produced current density, indicating strong catalytic action. The superior performance of NiP110 was attributed to its improved surface area, enabling efficient diffusion of reactants and exposure of active redox sites. The electrocatalytic process involves the electrochemical conversion of fuel into CO₂, water, and N₂ via an electrochemical-catalytic mechanism mediated by NiOOH, generated through the electrooxidation of Ni(OH)₂. Additionally, the production of a nickel hydroxide layer on the electrode in the KOH solution facilitated redox reactions. The study also highlighted the impact of the supporting electrolyte-to-urea ratio on oxidation efficiency, emphasizing that an imbalance can lead to reduced current density due to inadequate hydroxide ions or electrode fouling by CO₂. This innovative synthesis of nanoporous NiP microspheres opens avenues for their application in electrochemical sensors for urea detection and underscores the potential to generalize the approach to fabricate other 3D nanoporous phosphate-based materials for diverse electrochemical applications [84].

Phosphorus material based on Ni₁₂P₅ nanorods grown in situ on a 3D NF skeleton (Ni₁₂P₅/Ni-Pi/NF) has been fabricated, offering a competitive electrocatalyst for UEOR or HER [85]. The hierarchical core–shell architecture of these nanorods provides an improved surface area, enhanced electron transfer, and efficient mass transport, resulting in robust catalytic performance. For UEOR, the catalyst achieves 9 A/cm² at 1.378 V in 1.0 mol/L KOH with 0.5 mol/L urea, outperforming many state-of-the-art catalysts. It also demonstrates excellent HER activity, obtaining a promising current (e.g., 500 mA cm⁻² at −0.312 V) with improved kinetics and durability. Using a two-electrode splitting electrolyzer, the electrode (Ni₁₂P₅/Ni-Pi/NF) achieves 0.5 A/cm⁻² at a cell potential of 1.662 V, which is significantly lower than that required for pure water splitting, highlighting the thermodynamic and kinetic advantages of urea-assisted electrolysis. Durability tests confirm the catalyst’s stability under large current densities over extended periods, making it a promising candidate for practical-scale H₂ production. The exceptional catalytic performance can be due to the 3D porous architecture of the core-shell nanorods, which enhances active site exposure, minimizes gas bubble accumulation, and improves electrolyte mass transport. These findings represent a pivotal step toward developing industrial electrolyzers for environmentally friendly hydrogen production through urea decomposition [85].

Y.-C. Tsai et al. studied the electrocatalytic application of zeolitic Ni₃(PO₄)₂ nanorods with an open-framework VSB-5 (Versailles-Santa Barbara-5) for UEOR [86]. These nanorods were synthesized hydrothermally on nickel foam using lithium dihydrogen phosphate, forming a highly porous material with nanometer-scale pore diameters (~1 nm) and molecular sieving properties. The VSB-5 nanorods provided micro-, meso-, and macroporous architectures that facilitated electrolyte penetration, electron transport, and gas product evolution. Compared to nickel hydroxide electrodes, the VSB-5 electrode exhibited a larger EASA, enabling enhanced redox cycling of nickel species (Ni²⁺/Ni³⁺), which significantly improved its electrocatalytic performance. It achieved a higher current of 160 mA cm⁻² at 0.6 V (vs. SCE) and an E_op of 0.27 V for UEOR, outperforming nickel hydroxide electrodes (75 mA cm⁻² at 0.28 V). Figure 4 demonstrates the amplified anodic current density in the presence of urea based on CV measurements (Figure 4A) and CA analysis (Figure 4B), confirming the catalytic efficiency of VSB-5 nanorods. CA studies (Figure 4B) revealed the electrode’s long-term stability, maintaining high current densities over extended testing periods, while EIS (Figure 4C) highlighted a lower R_ct of 5.77 Ω for VSB-5 compared to 21.51 Ω for nickel hydroxide. The open-framework structure of VSB-5 contributed to effective urea adsorption and rapid charge transfer, enhancing the UEOR kinetics. As shown in Figure 4D (current values from staircase wave), the VSB-5 electrode achieved a current density of 123 mA cm⁻² at 600 mV, significantly surpassing the nickel hydroxide counterpart (35 mA cm⁻²). The electrode also demonstrated remarkable current efficiency (η) of up to 97% at low potentials, with minimal OER interference, a critical factor for efficient urea electrolysis. These findings underscore the potential of VSB-5 as a promising electrocatalyst for UEOR.

In 2021, nickel phosphate (Ni-P) nanoflakes were introduced via Khalaf et al. for UEOR and represented a promising pathway for morphology modification, particularly when employing cost-effective and high-performance materials such as Ni-P [87]. The synthesis of Ni-P nanoflakes under varying urea proportions has revealed significant impacts on the material’s morphology, structure, and electrocatalytic performance. Characterization techniques confirm that Ni-P exhibits a nanoflakes morphology with well-defined structural properties conducive to catalytic activity. Electrochemical studies using CV and EIS in alkaline KOH media demonstrate that Ni-P facilitates efficient UEOR through enhanced redox activity because of Ni(II)/Ni(III) transition and the generation of active NiOOH species on the electrode surface. CV results indicate that urea content during synthesis critically influences the performance of the Ni-P catalyst, with an optimal proportion of 0.8 M yielding the highest current density for UEOR. Beyond this threshold, excessive urea results in larger flake formation, reducing the effective surface area and diminishing catalytic efficiency. The electrooxidation mechanism of UEOC on Ni-P involves the adsorption and activation of urea molecules, leading to bond cleavage and the release of electrons, which contribute to electricity generation. CA experiments confirm the long-term stability and robust performance of the Ni-P electrode, showcasing its ability to sustain high current densities over extended periods with minimal decay. Furthermore, EIS analysis underscores the low ionic and R_ct of Ni-P, affirming its suitability as an efficient electrocatalyst in a KOH medium. The proposed mechanism involves the indirect oxidation of fuel on the NiOOH sites, resulting in the production of nitrogen and carbon dioxide as by-products while releasing six electrons per urea molecule. The high activity and stability of Ni-P electrodes highlight their potential for integration into urea-based fuel cells.

In 2023, Al-Sulmi et al. reported an efficient preparation of mesoporous Ni₃(PO₄)₂ (meso-NiPO) through chemical deposition within a template based upon surfactant liquid crystal (Brij^®78), achieving a surface area of 43.50 m²/g compared to 4.26 m²/g for bare Ni₃(PO₄)₂ (bare-NiPO) synthesized without surfactant [88]. Electrochemical studies demonstrated that meso-NiPO exhibits promising electrocatalytic activity for the UEOR in alkaline media, with an oxidation E_op of 0.30 V vs. Ag/AgCl, R_ct of 3.35 Ω, and a remarkable mass activity of 700.7 mA/cm²·mg at 0.6 V, which is 4.5 times better than that of bare-NiPO. CA tests revealed high stability, retaining 97.5% of the oxidation current after 3 h of fuel electrolysis. Using an H-type electrolyzer, the meso-NiPO catalyst achieved an H₂ production of 415 µmol/h and a Faradic performance of 96.80% at 2.0 V. The mesoporous structure facilitated efficient charge transfer, mass transport, and accessibility of electroactive species, enhancing the UOR [89]. Additionally, the UEOR Tafel slope was found at 20.3 mV/decade indicating faster reaction kinetics compared to bare-NiPO (30.2 mV/decade). Optimization studies showed that the catalytic activity increased with meso-NiPO quantity to 0.2 mg/cm² but saturated at higher loadings due to diffusion limitations. Similarly, the UEOR current increased with urea concentrations up to 0.33 M but declined at higher concentrations due to competitive adsorption effects [90]. The process was confirmed to be diffusion-controlled, with the meso-NiPO demonstrating a significant negative shift in the E_op and a 15-fold improvement in current with increasing hydroxide ion concentrations from 0.1 to 2.0 M. The catalyst’s robustness was underscored by its long-term stability, maintaining higher steady-state currents than bare-NiPO, and showcasing enhanced accessibility of active Ni(II)/Ni(III) redox sites. This innovative synthesis strategy provides a scalable and energy-efficient route for mesoporous catalyst preparation, highlighting the potential of meso-NiPO for UEOR [90].

In 2024, Ghanem et al. introduced an advanced electrocatalyst having Ni₃(PO₄)₂-combined with NF (NiPO/NF), which was prepared via anodization for dual applications in UEOR and HER [91]. The preparation steps involved cleaning commercial nickel foam with hydrochloric acid for 3 min to take out surface oxides, followed by rinsing with pure H₂O and organic solvent (acetone), followed by air-drying. Anodization was carried out in a 0.3 mol/L phosphate solution using a 2-electrode setup, with NF as the anode and titanium foil as the cathode, at 5 °C. Direct constant voltage was utilized at 5, 10, or 15 V for 20 min, resulting in electrodes labeled as NiPO/NF@5, NiPO/NF@10, and NiPO/NF@15, respectively. The process induced oxidation at the anode, depositing a nickel phosphate catalyst layer. Electrochemical characterization revealed that the NiPO/NF@10 substrate exhibited the best performance, with optimized redox behavior and electroactive surface area. CV in 2.0 M KOH demonstrated significant enhancements in redox peak currents, with NiPO/NF@10 showing peak current densities of 95.0/−68.5 mA/cm² compared to 15.3/−10.3 mA/cm² for pristine NF, attributed to the production of Ni(II)/Ni(III) sites and Ni₃(OH)₃(PO₄)₂ phase during the hydroxide ion interaction. In 0.3 mol/L urea, the NiPO/NF@10 substrate achieved 20 mA/cm² as an oxidation current at an E_op of 1.35 V vs. RHE, 100 mV lower than pristine NF, indicating a superior electrocatalytic effect via indirect electrochemical mechanistic aspects. The study underscores the promising potential of the NiPO/NF@10 electrocatalyst for scalable hydrogen production and sustainable UEOR [91].

5. Ni-Free Phosphate Electrocatalysts for UEOR

Despite their promising attributes, the catalytic performance of nickel-free phosphate materials in UEOR remains relatively underexplored compared to their nickel-based counterparts. Challenges such as limited intrinsic electroconductivity, moderate catalytic activity, and optimization of nanostructures to maximize active surface areas persist. Recent advances in nanostructuring techniques, including the fabrication of nanowires, nanosheets, and porous frameworks, have shown considerable potential in overcoming these limitations. Recently, some Ni-free electrocatalysts were reported for electrocatalysis applications away from UEOR, such as efficient CO₂ reduction to multicarbon products via Cu-P [92], glucose detection via Cu-modified Ti-phosphate NPs [93], and highly ethylene-selective electroreduction CO₂ over Cu-P nanostructures with tunable morphology [94]. This part aims to provide an overview of the developments in nickel-free phosphate-based electrocatalysts for UEOR. Hui Shen et al. explored UEOR and proposed a two-step methodology to chemically prepare Cu₃P nanowires on copper foam (Cu₃P-NW/CF) as an electrocatalyst for UEOR in alkaline media [95]. The Cu₃P-NW/CF electrode achieved operational voltage values of 1.43 V and 1.65 V versus RHE at current values of 10 and 100 mA cm⁻², respectively, while a two-electrode urea electrolysis system using Cu₃P-NW/CF for both the oxidized and reduced electrodes required a cell voltage of 1.79 V to sustain a current density of 100 mA cm⁻². These results surpassed the conventional H₂O electrolysis thresholds without urea, underscoring the energy-saving potential of the urea-assisted process. Structural and kinetic advantages, such as enhanced conductivity, abundant active sites, and low Tafel slopes (33.1 mV dec⁻¹ for UEOR), contributed to the catalyst’s performance. Electrochemical active surface area analysis revealed a significant improvement in the active area (C_dl = 7.5 mF cm⁻²) compared to a control Cu₃P-P/CF electrode. Furthermore, EIS demonstrated lower charge-transfer resistance, while stability tests showed negligible degradation over 30 h of operation, highlighting the durability of the Cu₃P nanowire architecture. However, limitations remain, as the absence of electroactive metals like Ni, which are often incorporated into UEOR catalysts for enhanced performance, might restrict the catalyst’s activity and selectivity. Additionally, while the binder-free design facilitated efficient electron transfer, further investigation into long-term structural integrity under harsh electrochemical conditions is warranted. This study provides a foundation for developing scalable Ni-free UEOR electrocatalysts, yet improvements in material composition and synergy with electroactive metals are needed to maximize catalytic performance for UEOR and related applications [95].

6. Transition Metals Incorporated Ni-Phosphate Electrocatalysts for UEOR

Mono-metal phosphate-based catalysts have demonstrated significant promise in electrocatalysis uses because of their inherent stability, tunable surface chemistry, and ability to mediate redox reactions [96,97,98]. Despite these advantages, their catalytic activity can be limited by a lack of sufficient active sites, moderate conductivity, and relatively low electrochemical efficiency. To overcome these limitations, the development of bimetal phosphate catalysts has emerged as a transformative approach [99]. Bimetal phosphates introduce a synergistic interplay between two distinct metal centers, offering enhanced catalytic performance compared to their mono-metal counterparts [100,101,102]. This dual-metal composition allows for a more efficient distribution of electronic density and an optimized local environment, which could facilitate the adsorption and initiate the electrocatalytic process. Moreover, the incorporation of a secondary metal can introduce additional redox-active sites if the second metal is electrochemically active and so improve the overall electrical conductivity [102,103]. Therefore, bimetal phosphates not only enhance the kinetics of the electrocatalysis process but also exhibit higher resistance to electrocatalyst deactivation caused by byproduct accumulation or electrode poisoning. This makes them particularly appealing for practical applications in energy generation and environmental cleanup [104]. By leveraging the unique advantages of bimetallic phosphorus systems, researchers can address the bottlenecks associated with mono-metal phosphates, paving the way for more efficient and sustainable UEOR technologies.

In 2019, Yang et al. synthesized nickel cobalt phosphate (NiCoPO) composites with varying Ni/Co ratios using a co-precipitation method, showcasing their potential as effective electrocatalysts for UEOR [105]. The introduction of cobalt content into nickel phosphate enhanced catalytic performance by regulating electronic levels and texture properties of Ni(III) species, lowering the E_op, and increasing the oxidation current. Compared to nickel phosphate (NiPO) without cobalt, the E_op of NiCoPO composites with specific Ni/Co ratios decreased significantly, with the NiCoPO (4:6) exhibiting a reduction of 135 mV. Furthermore, this composite achieved a high electrocatalytic current value of 65.4 mA cm⁻² for UEOR at high pH. The improved activity was attributed to cobalt doping, which increased the chemical defects of Ni-contents, exposed new electroactive sites, regulated the Ni oxidation state, and promoted electron transfer from urea to NiOOH. However, excessive cobalt content (above 70%) hindered performance by decreasing the available active Ni sites due to inactive cobalt species. Electrochemical studies, including CV, amperometric measurements, and EIS confirmed the superior catalytic activity and kinetics of NiCoPO composites, particularly NiCoPO (4:6). The results highlight the potential of cobalt-doped nickel phosphates as cost-effective and efficient catalysts for UEOR [105].

Lulu Qiao et al. demonstrated that the Ni-Co phosphides/amorphous phosphorous combined with the oxide of Mn to form the introduced composite (c-CoNiPx/a-P-MnOy) exhibited exceptional performance as a bifunctional electrocatalyst for the UEOR [106], obtaining 100 mA cm⁻² as an oxidation current at a remarkably low voltage of 1.35 V vs. RHE. This catalyst outperformed CoNiP_x/MnO and MnCoNiO by 3- and 41-fold, respectively, and showed improved kinetics with a Tafel slope of 75 mV dec⁻¹ and an R_ct of 1.3 Ω. Long-term durability was confirmed with only an 8.4% increase in applied potential over 300 h at 50 mA cm⁻², and a mere 40 mV potential increase compared to the fresh electrode. Surface reconstruction during UEOR led to the formation of active NiOOH species from the Ni²⁺/Ni³⁺ redox couple, with Mn and Co incorporation synergistically enhancing stability and activity by maintaining structural integrity and increasing active sites. The catalyst demonstrated an H₂ production rate of 0.18 mmol h⁻¹, a Faraday performance of 97.2% at 0.02 A cm⁻², and maintained promising electrochemical stability. The interplay of Mn and Co elements boosted the electrocatalytic activity, as the UEOR current density of CoNiP-MnO was 4.17-, 1.62-, and 3.44-fold higher than NiP, CoNiP, and MnNiP, respectively. XPS analysis revealed significant surface reconstruction, with a sharp decrease in P content from 19.23% to 3.86%, and enrichment of Ni³⁺ species, which served as key active sites for the indirect oxidation mechanism. Raman and in situ spectroscopic studies confirmed the structural transformation from metal phosphides to NiOOH and CoOOH, while the catalyst retained its hierarchical structure, emphasizing its robustness and efficiency. Lulu Qiao et al.’s work highlights the importance of crystalline/amorphous configurations and Mn/Co incorporation in optimizing bifunctional electrocatalysts for UEOR [106].

In 2023, Abd El-Lateef et al. presented a groundbreaking exploration of bimetallic Co/Ni phosphates (CoNiP-reflux and CoNiP-sol–gel) synthesized via reflux and sol–gel methods, respectively, as cost-effective electrode materials for UEOR in alkaline media, with a focus on their physicochemical properties and electrocatalytic performance [107]. Comprehensive characterization revealed that the sol–gel approach yielded smaller, more uniform particles with enhanced surface area compared to the reflux method, significantly influencing their electrocatalytic behavior. The CV analysis (Figure 5A,B) demonstrated that both CoNiP-reflux and CoNiP-sol–gel exhibited characteristic redox peaks at 350 mV and 430 mV, corresponding to the reversible Ni(II)/Ni(III) transformation, with CoNiP-sol–gel achieving superior current densities due to its higher surface area and improved charge transfer kinetics. The presence of urea further amplified the anodic current density, underscoring the efficiency of both materials, with CoNiP-sol–gel outperforming CoNiP-reflux significantly, achieving the current value of 18.90 mA/cm² at 0.46 V compared to 4.71 mA/cm² for CoNiP-Reflux under identical conditions.

EIS analysis as shown in Figure 5C,D indicated reduced R_ct in CoNiP-sol–gel electrodes, evidenced by the presence of semicircles in Nyquist plots. This could confirm enhanced UEOR efficiency, which could be attributed to the introduction of new charge transfer routes in the presence of urea. In the studied electrochemical cell, three resistances were identified: one associated with the electrolyte, another linked to Ni(II)/Ni(III) redox, and a last one connected to UEOR into N₂ and CO₂, producing six electrons. The last resistance appears only when the electrolyte has urea content, explaining the observed semicircles in the Nyquist plots for both CoNiP-reflux and CoNiP-sol–gel electrodes. Changes in electron transfer pathways were evaluated using equivalent circuit fitting analysis, as previously reported in related studies [72,108,109]. The disparity between experimental and fitted data was illustrated in Figure 5E,F for CoNiP-reflux and CoNiP-sol–gel, respectively. Equivalent circuits provide valuable insights into the electrical behavior of the electrodes by separating their electrical components and determining the effective resistance values from elements coupled in parallel or series. The circuit factors included R_h, which represents series resistance, while R₁/CPE₁ and R₂/CPE₂ correspond to oxidation reactions (Ni and urea, respectively). Notably, R_h went down from 17.36 Ω to 8.89 Ω when the electrode substrate was switched from CoNiP-reflux to CoNiP-sol–gel, indicating improved chemistry between the working electrode and the electrolyte for CoNiP-sol–gel. A similar trend was observed for the R_ct elements (R₁ and R₂), highlighting the enhanced electron transfer efficiency of the CoNiP-sol–gel electrode compared to CoNiP-reflux. CA further validated the stability and higher catalytic activity of CoNiP-sol–gel, with current densities reaching 39.98 mA/cm² after 1116 s in 0.3 mol/L urea solution, compared to 21.38 mA/cm² for CoNiP-reflux, and a deactivation rate of 1.4% for CoNiP-sol–gel, which could highlight its long-term operational stability. Additionally, CV studies across various urea concentrations (0.1–1.0 M) confirmed a direct correlation between urea content and enhanced current densities, with CoNiP-sol–gel maintaining higher peak currents and superior electrochemical performance. The study also examined the impact of scan rates on the CV profiles, showing consistent peak positions and increasing CV areas, indicative of diffusion-controlled processes and robust electrochemical stability. These findings collectively position CoNiP-sol–gel as a promising candidate for UEOR. The results pave the way for further optimization of bimetallic (Co and Ni) phosphate electrodes, leveraging sol–gel synthesis to unlock their full potential in fuel cell technologies [107].

Xiujuan Xu et al. investigated the interfacial charge redistribution of Fe-incorporated Ni₁₂P₅/Ni₃P nanosheets supported on a 3D NiFe foam framework for ultrafast UEOR at industrially relevant current densities [110]. The incorporation of iron not only reinforced the interfacial electric field but also facilitated charge redistribution, effectively reducing the energy barrier during the UEOR. Benefiting from this tailored electronic structure and the ultra-thin nanosheet morphology, the catalyst exhibited remarkable performance and achieved 800 mA cm⁻² as oxidation current at 1.4 V and an impressive Tafel slope (28.2 mV dec⁻¹). Electrochemical analysis revealed significantly reduced R_ct and increased EASA (C_dl of 9.36 mF cm⁻²), which could indicate the efficient charge and mass transfer. Additionally, the Fe-doped electrocatalyst demonstrated excellent hydrophilicity, with a contact angle close to 0° with enhanced gas bubble release properties, mitigating bubble shielding effects and maintaining high UEOR activity. Long-term durability tests confirmed its robust performance, with minimum potential increases (1.1–4.9%) after continuous operation at high current densities for 40 h. Post-UEOR analysis revealed the formation of NiOOH on the electrode surface, further corroborating its high catalytic activity. DFT calculations highlighted the reduced reaction energy barrier on Fe-doped Ni₁₂P₅ (202) and Ni₃P (112) facets, elucidating the role of Fe in optimizing the adsorption/desorption of intermediates and boosting intrinsic activity. The turnover frequency of 2.3 s⁻¹ is higher than the undoped counterpart (0.7 s⁻¹). Finally, the catalyst demonstrated promising performance in a real-world scenario using urine as a feedstock, which achieved both electrochemical activity and stability for UEOR [110].

Yunfeng Qiu et al. conducted a comprehensive study on the polyoxometalate-mediated design of phosphate-incorporated NiMoO_4−x with abundant oxygen vacancies, aiming to develop efficient bifunctional electrocatalysts for hydrogen production via UEOR [80]. The proposed NF/P-NiMoO_4−x electrode was synthesized by etching commercial NF with H₃PMo₁₂O₄₀, resulting in a phosphate content of 4.46 atomic% and net-like nanostructures. This facile method delivered exceptional HER and UEOR performances, with the electrode achieving an E_op of 116 mV at 10 mA/cm² for HER and a potential of 1.359 V at 10 mA/cm² for UEOR, accompanied by Tafel slopes of 77.5 and 19.3 mV/dec, respectively. The activation energy for UEOR was significantly reduced to 20.6 kJ/mol compared to 37.7 kJ/mol for bare NF, which highlights its enhanced electrocatalytic efficiency. The urea electrolysis setup demonstrated a cell potential of 1.48 V at 10 mA/cm², outperforming traditional noble-metal systems such as Ir/C||Pt/C in terms of both efficiency and stability, with robust operation over 50 h. Remarkably, the etching solution was recyclable for five cycles with minimal loss in catalytic performance, which signifies its potential for large-scale application. Characterization revealed that the etching process optimized the surface area and active site availability, as evidenced by an enhanced BET surface area of 13.7 m²/g, compared to 3.2 m²/g for pristine NF, and the identification of mesopores around 3.8 nm. Electrochemical analyses confirmed reduced R_ct (37.6 Ω) and increased electrochemical active surface area (ECSA) values, with the electrode demonstrating superior intrinsic activity. The study also elucidated the active role of in situ-formed NiOOH species as the real electrocatalytic sites for UEOR, which facilitate a higher Ni³⁺/Ni²⁺ ratio and abundant oxygen vacancies. Post-reaction characterization confirmed the structural integrity of the net-like nanostructures and sustained oxygen vacancy content, which ensure long-term catalytic durability. These findings establish NF/P-NiMoO_4−x as a cost-effective, scalable, and promising bifunctional electrocatalyst for UEOR [80].

In 2023, Liming Yang et al. developed a feather-like NiCoP electrocatalyst combined with NF for UEOR [111]. The NiCoP/NF was synthesized via a two-step hydrothermal-phosphorization process followed by investigation for UEOR and the found results achieved a 10 mA cm⁻² current in KOH containing 0.3 mol/L urea at a mere 1.13 V (vs. RHE). DFT calculations revealed that NiCoP’s enhanced density of states (DOS) near the Fermi state significantly improves charge transfer and enables efficient catalytic activity. The trifunctional electrocatalyst demonstrated remarkable versatility, requiring a cell voltage of only 1.48 V to drive overall H₂O splitting at 10 mA cm⁻², significantly outperforming reference materials such as Co(OH)F/NF, NF, and RuO₂/NF, which required higher voltages. In UEOR, the NiCoP/NF achieved 10 mA cm⁻² at just 1.36 V, 120 mV lower than the potential required for water splitting, highlighting its efficiency for urea-rich wastewater applications. The electrocatalyst showed excellent reaction kinetics, as evidenced by its low Tafel slope (34.19 mV dec⁻¹) and minimal R_ct = 0.67 Ω, coupled with a high double-layer capacitance (C_dl = 32.48 mF cm⁻²), underscoring its superior electrocatalytic characteristics. Long-term stability tests over 30 h affirmed the robust durability of NiCoP/NF with minimal degradation in performance, while post-reaction SEM and XPS analyses revealed the retention of its morphology and conversion of phosphide to catalytically active metal oxo/hydroxides. Moreover, practical applications demonstrated the catalyst’s effectiveness in overall water splitting and urea electrolysis using a 1.5 V battery or a solar photovoltaic panel as the power source, with visible hydrogen and oxygen bubbles generated at the electrodes. DFT calculations further elucidated the mechanism underlying its high performance, showing strong adsorption energies for H₂O on the NiCoP (0001) surface, ensuring effective water activation. This study highlights NiCoP/NF as a low-cost, highly active, and stable electrocatalyst, with promising applications in UEOR over noble-metal-free catalysts [111].

In 2024, Ran Zhang et al. investigated the coral-like Co/Zn-Pi@NF (Pi and NF represent phosphate and nickel foam, respectively) electrocatalyst for UEOR [112]. The introduced Co/Zn-Pi@NF was chemically designed via electrodeposition methodology and directly grown on NF to avoid binders and enhance conductivity [113]. The structural and electronic synergy between cobalt and zinc facilitated electrocatalytic performance improvements. Incorporating zinc adjusted the electronic structure of the catalyst. The unique coral-like architecture of Co/Zn-Pi@NF exposed more electroactive sites and better charge transfer efficiency. In the presence of urea (0.5 mol/L), the UEOR performance of Co/Zn-Pi@NF was remarkable, requiring a voltage of 1.40 V while maintaining electrochemical stability over 60 h. The catalyst outperformed comparable materials such as Co-Pi@NF, Zn-Pi@NF, and Co/Zn-OH@NF in terms of lower overpotentials and faster reaction kinetics with the lowest Tafel slope (95.8 mV·dec⁻¹) and the highest exchange current density (j₀) of 7.14 μA·cm⁻². EIS revealed the lowest R_ct of 0.36 Ω for Co/Zn-Pi@NF, underscoring its superior conductivity. Additionally, the catalyst exhibited the highest C_dl of 228.91 mF·cm⁻², which indicates a larger EASA. During water-urea electrolysis, employing Co/Zn-Pi@NF as cathode and anode achieved a cell voltage of 1.43 V at 10 mA·cm⁻², a reduction of 290 mV compared to H₂O electrolysis. The electrocatalyst demonstrated robust stability under continuous operation at 50 mA·cm⁻² for 60 h, with only a 1.36% change in voltage, which affirms its potential for practical applications. The study highlights the ability of biphosphate structures (Co/Zn-Pi@NF) to serve as electrocatalysts for UEOR [112].

In 2024, Zhanhong Zhao et al. investigated Mn incorporation on nickel phosphide (Ni₂P) with a self-reconstruction mechanism for the UEOR [114]. By constructing Mn-Ni₂P, the study revealed that manganese doping significantly accelerates surface reconstruction and forms a NiOOH-MnOOH heterostructure characterized by localized electrophilic and nucleophilic regions. This structure was shown to enhance the targeted adsorption and activation of carbon and nitrogen atoms in the fuel, thereby facilitating the cleavage of the C–N bond. Moderate manganese doping not only increased urea adsorption capacity and the affinity for key intermediates but also reduced the energy barrier for gas desorption, which could effectively refresh the electrocatalytic surface. As a result, the Mn-Ni₂P electrode demonstrated promising electrocatalytic performance and achieved an industrial-level current of 1000 mA cm⁻² at a voltage of 1.46 V (vs. RHE), which is 269 mV lower than the OER under similar environments. Linear-sweep voltammetry (LSV) results (shown in Figure 6A) highlighted that Mn-Ni₂P required only 1.316 V to reach 100 mA cm⁻². A significant improvement compared to Ni₂P (Figure 6A), and commercial Pt/C (Figure 6B) could be seen, which required 1.363 V and 1.405 V, respectively. The incorporation of Mn also notably reduced the UEOR E_op from 1.334 V (Ni₂P) to 1.27 V (Mn-Ni₂P). Additionally, Tafel slope analysis (Figure 6C) revealed that Mn-Ni₂P possessed a much lower slope (29.61 mV dec⁻¹) compared to Ni₂P (37.46 mV dec⁻¹), Pt/C (52.12 mV dec⁻¹), and NF (94.72 mV dec⁻¹), indicating significantly enhanced UEOR kinetics. Variations in urea and KOH concentrations were also studied, with results suggesting a shift from diffusion-controlled processes to surface reaction-controlled processes as the urea content exceeded 0.3 M. EIS of the studied Mn-Ni₂P, shown in Figure 6D, further corroborated the enhanced kinetics, showing a pronounced reduction in R_ct with increasing applied potential. Mn-Ni₂P exhibited a lower R_ct at 1.28 V (vs. RHE) compared to Ni₂P (1.35 V) and confirmed preferential UEOR initiation on Mn-Ni₂P. The C_dl measurements (Figure 6E) indicated a higher electrochemical active surface area (ECSA) for Mn-Ni₂P (14.37 mF/cm²) than Ni₂P (8.57 mF/cm²). Additionally, the C_dl in the case of the Mn-Ni₂P electrode was found at 14.37 mF/cm², which is much better than that of the Ni₂P electrode (8.57 mF cm⁻²), which could suggest the better ESCA for the Mn-Ni₂P electrode. Structural stability studies confirmed that the Mn-Ni₂P electrocatalyst maintained its nanosheet morphology and chemical composition after prolonged operation, with manganese doping effectively mitigating phosphorus leaching and enhancing structural integrity. XPS analysis further revealed the formation of amorphous nickel/manganese oxyhydroxides after extended electrolysis, providing additional evidence for the stability and activity retention of Mn-Ni₂P. The electrocatalyst durability was evaluated via electrolysis experiments as displayed in Figure 3F. The persistent decomposition of urea combined with CO₂ could retard the surface electrocatalytic reaction that leads to the increase in the required potential. The Mn-Ni₂P working electrode has relatively stability for 600 min at 100 mA/cm². These findings collectively underscore the position of Mn-Ni₂P as a robust and efficient electrocatalyst for UEOR [114].

7. Heterogeneous Phosphate (Carbon and Metal) Electrocatalysts for UEOR

Heterogeneous phosphate-based materials have emerged as attractive candidates for electrocatalysis applications owing to their excellent stability, tunable electronic properties, and high catalytic activity [2]. For UEOR, these materials provide active sites conducive to the redox transformations of Ni(OH)₂/NiOOH and similar systems, which are pivotal for catalyzing the multi-step oxidation of urea [107]. However, the inherent limitations of phosphate-based materials, such as moderate conductivity and limited surface area, can hinder their overall catalytic performance. Incorporating carbon-based components such as multi-walled carbon nanotubes (MWCNTs) [115], graphene foam [116,117], or activated carbon [118] into phosphate materials offers a synergistic solution to these challenges. Carbonaceous materials could enhance the conductivity and surface area of the heterogeneous composite, facilitating efficient charge transfer and providing additional active sites for the adsorption and oxidation of urea. Furthermore, their robust structural framework ensures mechanical stability and prolonged catalyst life under operational conditions. By integrating heterogeneous phosphate materials with conductive carbonaceous supports, it is possible to significantly lower the E_op, improve the electrocatalytic current density, and improve the overall kinetics of UEOR. This hybrid approach not only optimizes catalytic efficiency but also broadens the applicability of these materials in practical energy conversion and wastewater management systems.

Hongmei Zhang et al. studied the UEOR on nickel phosphate-based nanomaterials, including MWCNTs combined with Ni₃(PO₄)₂ nanomaterials, which were physically designed via a simple precipitation method [119]. Electrochemical performance was evaluated using CV in alkaline solutions and revealed acceptable catalytic performance for UEOR. The addition of MWCNTs significantly enhanced catalytic current and reduced E_op due to their large specific surface area and excellent electrical conductivity. CV studies demonstrated redox behavior attributed to the Ni(OH)₂/NiOOH transformation, with higher current densities observed for NiPO-MWCNT modified electrodes compared to their counterparts without MWCNTs. In the presence of urea, anodic peak currents increased, and peak regions expanded due to overlapping peaks of nickel hydroxide and fuel electrooxidation. Electrochemical reactions involved the oxidation of Ni(OH)₂ to NiOOH, then the electrocatalytic oxidation of the fuel, with subsequent regeneration of Ni(OH)₂, indicating a quasi-reversible and diffusion-controlled process. Investigations of varying scan rates revealed linear characteristics between current values at the anodic peak and the (scan rate)^0.5, which could confirm diffusion-controlled reactions. Urea oxidation performance was improved, with increasing urea concentration up to an optimal OH⁻/CO(NH₂)₂ ratio of 6, beyond which excessive urea negatively impacted performance due to hydrolysis effects, electrolyte depletion, and carbon dioxide accumulation on the electrode surface. Similarly, NaOH concentration influenced urea oxidation, with an increase in peak current and shift in E_op to more negative values up to 1.5 M NaOH, after which further increases hindered performance. The study highlights the potential of NiPO-MWCNT electrodes as efficient electrocatalysts for UEOR due to their enhanced electrochemical properties and well-optimized reaction conditions.

In 2022, the innovative synthesis of NiF₃/Ni₂P heterojunctions supported on carbon cloth (NiF₃/Ni₂P@CC) was achieved via a combination of hydrothermal, fluoridation, and phosphorization techniques [120]. Among these, the NiF₃/Ni₂P@CC variant emerges as a standout, offering significant improvements in UEOR performance due to its unique physicochemical properties. This electrode includes abundant high-valence Ni³⁺ sites, optimized charge density, a super hydrophilic surface, and a mesoporous structure, which collectively enhance catalytic activity. Electrochemical evaluations demonstrate that NiF₃/Ni₂P@CC requires a mere voltage of 1.36 V to produce 10 mA cm⁻² and maintains its activity across extensive cycling, underscoring its durability. Comparative studies reveal that the heterojunction configuration significantly outperforms individual components, NiF₃@CC and Ni₂P@CC, in terms of catalytic efficiency and stability. The superior performance is attributed to the optimized heterointerface density achieved by tuning phosphorization time, which influences the structural and electronic properties of the material. Structural analyses, including XRD and TEM, confirm the coexistence of rhombohedral NiF₃ and hexagonal Ni₂P phases, while surface characterization highlights the remarkable hydrophilicity of the NiF₃/Ni₂P@CC surface, as evidenced by a contact angle of only 11.5°. EIS and Tafel slope analysis further elucidate the rapid charge transfer and favorable reaction kinetics of NiF₃/Ni₂P@CC, with 33 mV dec⁻¹ as a Tafel slope indicating a fast UEOR dynamic process. As illustrated in Figure 7A, the NiF₃/Ni₂P@CC electrode demonstrates the smallest semicircle radius at high frequencies, indicative of its low R_ct. This reduced R_ct can be attributed to the material’s abundant heterointerfaces and the conductivity of Ni₂P. The electrochemical surface area, which correlates directly with the C_dl, was also assessed. Figure 7B presents the relationship between current density change (Δj = j_a − j_c at 0.95 V) and scan rates ranging from 0.02 to 0.12 V/s within the non-Faradaic region. The corresponding C_dl was estimated by plotting the slopes of the linear relationship. Notably, NiF₃/Ni₂P@CC exhibited the highest C_dl value of 11.7 mF cm⁻², which is approximately 10 times greater than those of NiF₃@CC (1.05 mF/cm²) and Ni₂P@CC (1.14 mF/cm²). This substantial increase in C_dl reflects a significantly larger ECSA, providing a higher density of electroactive sites for the UEOR. A symmetric urea electrolysis system was designed using NiF₃/Ni₂P@CC as the anode as well as the cathode (Figure 7C). As described in Figure 7D, the electrolyzer required a low cell potential of 1.54 V to achieve 10 mA cm⁻² in KOH aqueous solution containing 0.3 mol/L urea as the fuel content, compared to 1.58 V in KOH without the fuel. The reduced voltage of 1.54 V for 10 mA cm⁻² surpasses other reported urea electrolyzers employing noble or non-noble electrodes (Figure 7E) [121,122]. Additionally, achieving 50 mA cm⁻² required 1.83 V voltage in the urea-containing electrolyte, which is significantly lower than the 1.92 V required for H₂O electrolysis. Furthermore, the symmetric urea electrolyzer demonstrated exceptional long-term operational stability and maintained continuous operation for 10 h without degradation (Figure 7F). Additionally, long-term stability assessments reveal minimal morphological and structural changes post-operation, even under extended working conditions, marking this electrocatalyst as robust for practical applications. This research not only establishes NiF₃/Ni₂P@CC as a leading electrocatalyst for UEOR but also sets a precedent for designing multifunctional heterojunction catalysts, with broader implications for scalable and economical energy production technologies. The systematic optimization of synthesis parameters, such as fluorination–phosphorization duration and precursor composition, provides valuable insights into tailoring material and electrocatalytic properties [120].

Xu et al.’s study on phosphorus-induced bottom-up growth of Fe-doped Ni₁₂P₅ nanorod arrays for the UEOR presents a significant advancement in the synthesis of self-growing heterogeneous catalysts with robust and regular morphologies. Using 1,1′-bis(diphenylphosphine) ferrocene (DPPF) as both the phosphorus and iron source, the catalysts were synthesized on nitrogen-doped carbon-coated nickel foam (NdC/NF) through high-temperature vapor deposition. This process facilitated the formation of straight Fe-doped Ni₁₂P₅ nanorods, with lengths extending to tens of microns, growing via a bottom-up mechanism. DFT calculations suggested that the carburized Ni (111) face on the NF substrate favored phosphorus coordination, enabling uniform incorporation of Fe into the Ni₁₂P₅ lattice to form a synergistic catalytic center. An electrochemical evaluation revealed that Fe-Ni₁₂P₅/NdC/NF-800 exhibited promising performance in a 1.0 mol/L aqueous KOH + 0.3 mol/L urea solution, achieving 50 and 300 mA cm⁻² at low potentials of 1.357 V and 1.463 V, respectively. The catalyst demonstrated superior kinetics, with 56.9 mV/dec as a Tafel slope, a maximum C_dl of 108.1 mF cm⁻², and minimal R_ct, reflecting promising conductivity and a large ECSA. Stability tests confirmed its robustness, maintaining 100 mA cm⁻² for over 12 h with minimal structural degradation. Post-reaction analyses via SEM, XPS, and HRTEM showed the rod morphology remained intact, with surface transformations forming active NiOOH and FeOOH layers, further enhancing electron transfer and providing efficient electroactive sites for urea adsorption and oxidation. Raman spectroscopy indicated increased graphitization post-stability test, with the I_D/I_G value decreasing from 1.19 to 0.51, signifying improved catalyst durability. Comparative studies underscored the critical role of DPPF loading at 800 °C, as Fe-Ni₁₂P₅/NdC/NF-800 outperformed other configurations, demonstrating selective UEOR over OER with a voltage of 1.371 V required for 100 mA/cm². The study highlights the synergistic contributions of the NF substrate, N-doped carbon layer, and DPPF, which emphasize the phosphorus-induced self-growth mechanism as a promising approach to designing high-performance, durable electrocatalysts with promising applications in UEOR [123].

Finally,

Table 1. A summary of the reported electrocatalytic performance (I_a = anodic current density, E_op = onset potential, and R_ct = charge-transfer resistance) of the recent literature that introduced phosphorus (phosphate or phosphine) electrocatalysts for UEOR.

No.	Chemistry	Crystallinity	Morphology	Performance	Reference
1	Ni- phosphate	Monoclinic	less-ordered nanotubes of different lengths	Ia = 600 mAcm−2mg−1 Rct = 10 Ω	[81]
2	Ni2P@Nifoam	Hexagonal P-62m	porous nanoflower supported on foam	Ia = 750 mAcm2 with Eop = 0.24 V	[83]
3	Ni/Co phosphate	Orthorhombic	nanosheet structure with macropores	Ia = 65.4 mA/cm2 Eop = 0.22 V	[105]
4	nickel (II) phosphate octahydrate	Monoclinic	nanoporous microspheres	Ia = 680 mA/(cm2·mg) & Eop = 0.33 V	[84]
5	MWCNTs/Ni-phosphate	---	carbonaceous/ nanoparticles composite	Ia = 9 mA	[119]
6	Ni-phosphate @Ni12P5	Tetragonal I4/m	nanorod arrays	900 mAcm−2 at potential of 1.378 V	[85]
7	Ni-phosphate	Hexagonal P63/m	nanorods 80 nm	160 mA/cm2 at 0.6 V Eop = 0.27 V, and Rct = 5.77 Ω	[86]
8	Ni-phosphate	Monoclinic I2/m	nanoflakes aligned in one direction	Ia = 57.45 mA/cm2 Eop = 0.345 V, and Rct = 56.03 Ω	[87]
9	Ni2P/Ni	Hexagonal Ni2P & Cubic Ni	porous rods	Eop = 0.05 V at 10 mA·cm−2 and Tafel slope = 87.6 mV/dec	[82]
10	Ni- Phosphate/Ni-foam	---	aggregated nanoparticles with interstitial mesoporous networks	20 and 500 mA/cm2 at 1.35 and 1.6 V	[91]
11	NiF3/Ni2P@ carbon cloth	Rhombohedral NiF3 & hexagonal Ni2P	irregular nanoparticles with lots of mesopores	Ia = 79.1 mA cm2 Eop = 1.34 V	[120]
12	CoNiPx/P-MnOy	Hexagonal P321 &	hierarchical wire-into-flake structure	10 mA cm−2 at 1.24 V Faraday efficiency = 97.2%	[106]
13	Ni3(PO4)2	Monoclinic P21/a	“berry fruit” like morphology with 25 nm particles	Ia = 135.6 mA/cm2 Eop = 1.34 V	[88]
14	Cu-Phosphide	Hexagonal P63cm	nanowires	Voltage of 1.79 V to achieve 100 mAcm−2	[95]
15	Co/Ni- phosphate	monoclinic C2/m	self-assembled flower-like morphology with some distortion	Ia = 105.1 mAcm−2 at 0.485 V	[107]
16	Fe-Ni12P5	Hexagonal	nanorods	1.371 V to get 100 mA cm−2	[123]
17	Fe-Ni12P5/Ni3P	Hexagonal	ultrathin nanosheets	800 mA cm2 at 1.4 V Tafel slope = 28.2 mV·dec−1	[110]
18	NiMoO4-x with O-vacancies	Monoclinic C2/m	net-like nanostructure	1.359 V at 10 mA/cm2 and Tafel slope = 19.3 mV/dec	[80]
19	Co/Ni- Phosphate/Ni-foam	Hexagonal P6m2	feather-like morphology arranged vertically on the foam	1.13 V to get 10 mA/cm2 and Rct = 0.67 Ω	[111]
20	Zn/Ni- Phosphate/Ni-foam	Hexagonal	spherical-like flower-shaped nanosheets	1.43 V to get 10 mA/cm2 and Rct = 0.67 Ω	[112]
21	Mn/Ni- Phosphate	Hexagonal P-62m with Mn-doping	flower-like structure, assembled by smooth interconnecting nanoflakes	1000 mA/cm2 at 1.46 V	[114]

summarizes various nickel-based phosphates/phosphides as phosphorous-based materials and their performance in UEOR. The materials range from simple nickel phosphates to complex composites involving other metals like cobalt and iron, with distinct crystalline structures including monoclinic, hexagonal, and orthorhombic phases. For example, Ni-phosphate in the monoclinic form exhibits nanotube morphology with a performance of I_a around 600 mAcm⁻²mg⁻¹ and a low R_ct 10 Ω [81]. Other compounds such as Ni₂P@ Ni-foam displayed porous nanoflower structures with promising electrochemical activity (I_a = 750 mA/cm²) and low overpotentials (E_op = 0.24 V) [83]. The Ni/Co phosphate demonstrates nanosheet morphology and an impressive I_a = 65.4 mAcm⁻² at E_op = 0.22 V [105], while Ni₂P/Ni features hexagonal Ni₂P and cubic Ni phases with porous rods showing electrochemical activity at low overpotential [82]. The variety of crystalline and morphology correlates to electrochemical performance, with the materials exhibiting promising electrocatalytic properties and potential for energy conversion via UEOR. Additionally, the table highlights the role of dopants (e.g., Mn, Co) and composite structures in enhancing the electrochemical behavior of these materials.

8. Challenges and Optimization Strategies for Phosphorous-Based Catalysts

Phosphorous-based catalysts such as Ni-phosphates, Ni-free phosphates, NiM phosphates, and heterogeneous phosphate/carbon composites exhibit unique structural properties, electronic tunability, and electrocatalytic stability, making them promising materials for the urea electrocatalytic oxidation reaction (UEOR). However, their practical application is impeded by persistent challenges. Catalyst poisoning by reaction intermediates such as carbonate and urea-derived species significantly hampers catalytic activity and stability. Additionally, achieving optimal catalytic efficiency and selectivity remains difficult due to suboptimal active site exposure, sluggish reaction kinetics, and insufficient electronic conductivity. Addressing these issues requires advanced strategies such as alloying nickel with transition metals like Co, Fe, or Mn, which not only enhance the catalytic activity but also reduce E_op by modifying the electronic structure. Incorporation of non-metals, including S or Se, into the phosphorous framework can further alter electronic characteristics and promote synergistic effects, leading to promising solutions for enhanced reaction kinetics. The use of conductive supports such as graphene, carbon nanotubes, or hierarchical porous carbons has demonstrated efficacy in enhancing electron transfer and increasing the specific surface area, thereby improving electrocatalytic performance. Protective coatings, including ultrathin graphene layers, polymer films, or oxide shells, could serve as effective barriers to corrosion and surface poisoning while maintaining active site accessibility. Furthermore, optimizing electrolyte formulations, such as introducing ionic liquids or alkaline media with tailored additives, can suppress undesired side reactions and prolong the catalyst’s lifetime. Advanced synthetic techniques such as solvothermal processes, electrodeposition, and in situ growth of catalysts directly onto conductive substrates are pivotal for fabricating highly active, stable, and scalable phosphorous-based electrocatalysts.

Improving selectivity to minimize unwanted by-products like carbonates during urea oxidation involves optimizing both catalyst composition and reaction conditions, which can include four main strategies. The first one is catalyst surface modification, such as introducing bifunctional sites or heteroatom doping (e.g., transition metals like Fe, Ni, or Co) to selectively enhance the breaking of specific urea bonds, suppressing the formation of carbonates. The second one is the tuning electronic structure. Phosphorous-based catalysts with controlled electronic characteristics can facilitate selective oxidation pathways by stabilizing desired intermediates and destabilizing carbonate precursors. The third one is the reaction environment. Operating under optimized pH and temperature conditions reduces carbonate formation. Additionally, maintaining a balance between hydroxyl and urea concentrations suppresses side reactions. The last one is the additives. Introducing co-catalysts or inhibitors that block carbonate-forming pathways without impacting urea oxidation efficiency can further improve selectivity.

Catalyst poisoning by intermediates such as cyanate and ammonia during prolonged urea oxidation is a significant challenge, and addressing this issue requires innovative strategies. Surface regeneration techniques, such as periodic anodic or cathodic cleaning, can effectively remove adsorbed intermediates and restore active sites. Additionally, dynamic surface engineering approaches, such as employing self-regenerating catalysts with surface reconstruction properties, ensure that fresh active sites are continually exposed during the reaction. Introducing promoter elements, such as cerium or manganese, into phosphorous-based catalysts could show promise in enhancing resistance to poisoning by destabilizing adsorbed intermediates. Furthermore, adjusting reaction conditions, including fine-tuning KOH concentrations and applied potentials, can minimize the formation of poisoning intermediates and extend the catalyst’s operational lifespan.

Scaling up phosphorous-based catalysts from laboratory to industrial applications poses several challenges. The first one is electrode/catalyst costs. Even though phosphorous-based materials like phosphates and phosphides are relatively abundant, the cost of high-purity precursors and synthesis methods (e.g., high-temperature annealing or chemical vapor deposition) can be prohibitive at scale. Developing cost-effective synthesis routes, such as sol–gel or hydrothermal methods, could address this issue. The second one is sustainability, as energy consumption and waste generation should be minimized. Utilizing green synthesis methods, recycling precursor materials, and integrating renewable energy sources for catalyst production could enhance sustainability. The third one is the scalability of synthesis, which could be achieved by uniformity in catalyst properties (e.g., particle size, surface area, and active sites) during large-scale production. Strategies like continuous flow reactors and scalable templating methods are promising solutions. The last one is the integration with existing systems. Compatibility with current wastewater treatment and energy conversion infrastructures requires extensive pilot studies to optimize operating conditions and ensure seamless integration.

The environmental risks associated with phosphorous-based catalysts, including heavy metal leaching and the generation of toxic by-products, have not been completely addressed in previous studies, and comprehensive evaluations remain necessary. Leaching tests, such as the toxicity characteristic leaching procedure (TCLP), have demonstrated minimal release of heavy metals from well-anchored catalysts under controlled conditions. Analytical techniques like GC-MS and ICP-MS could be employed to detect and quantify potential toxic by-products in treated wastewater, ensuring thorough monitoring of environmental impact. Risk mitigation strategies, including the encapsulation of active sites within inert matrices or the application of protective coatings, further minimize the potential for leaching and enhance environmental safety. Additionally, lifecycle assessments evaluating the environmental impact of catalyst production, usage, and disposal provide a holistic approach to risk management, guiding the development of safer and more sustainable catalytic systems.

9. Conclusions and Future Perspectives

Phosphorous-based catalysts represent a significant advancement in the field of urea electrocatalytic oxidation reaction (UEOR), offering a sustainable solution for wastewater remediation and energy conversion. Their high stability, structural versatility, and environmental compatibility render them promising candidates for future applications. However, the realization of their full potential demands a deeper understanding of their reaction mechanisms, degradation pathways, and structure-activity relationships. In situ and operando techniques, such as X-ray photoelectron spectroscopy (XPS), synchrotron-based X-ray absorption spectroscopy (XAS), and electrochemical impedance spectroscopy (EIS), are crucial for elucidating dynamic surface transformations and intermediate species during the catalytic process. Future research should prioritize the design of hybrid materials that integrate phosphorous-based electrocatalysts with metal oxides, carbides, nitrides, or sulfides to exploit cooperative effects that enhance activity, selectivity, and durability. Scaling up from lab-scale studies to industrially relevant systems necessitates the development of robust reactor designs, optimization of operational parameters, and integration of UEOR technologies into existing renewable energy infrastructures. Incorporating computational methods such as DFT calculations can further accelerate the discovery of novel catalytic materials and guide experimental design. Addressing these challenges through interdisciplinary efforts will establish phosphorous-based electrocatalysts as indispensable components in the global transition toward sustainable energy via UEOR.