The transition to a low-carbon energy system has placed hydrogen at the center of many national decarbonization strategies, particularly as a clean fuel and feedstock for ammonia, steel, and chemical production. Conventional hydrogen production is still dominated by steam methane reforming and coal gasification, which are energy intensive and CO₂-intensive. In contrast, water electrolysis powered by renewable electricity offers a pathway to “green” hydrogen with near-zero direct emissions [1].

 However, the overall efficiency of water electrolysis is limited by the anode. The OER is a complex four-electron, four-proton process involving multiple oxygenated intermediates and strong O-O bond formation, which leads to large kinetic barriers and high over potentials even on state-of-the-art Ir- or Ru-based catalysts. In practical alkaline or acidic electrolyzes, cell voltages of 1.8-2.0 V are typically required at 200-500 mA cm^–2, far exceeding the thermodynamic minimum of 1.23 V at 25 °^C [1,2]. These energy losses translate directly into higher electricity consumption and operating costs.

 To overcome this bottleneck, there has been growing interest in replacing OER with thermodynamically more favorable anodic organic oxidation reactions that can, in principle, lower the anode potential while co-producing valuable chemicals or remediating pollutants. Representative examples include the oxidation of small organics (glycerol, ethanol, ethylene glycol), hydrazine oxidation (HzOR), and the urea oxidation reaction (UOR) [3]. Among these, UOR is particularly appealing because urea is cheap, widely available, and present at high concentrations in industrial and municipal waste streams, making it both a pollutant and a potential resource.

The overall UOR in alkaline media can be expressed as:

CO(NH₂)₂ + 6OH^– → N₂ + CO₂ + 5H₂O + 6e^–                                     (Eq. 1)

with CO₂ rapidly converted to carbonate/bicarbonate under alkaline conditions. When coupled with cathodic hydrogen evolution, urea electrolysis has long been cited as having a theoretical equilibrium cell voltage of ~0.37 V, much smaller than 1.23 V for conventional water splitting [4,5]. Recent thermodynamic work by Protsenko revisited this value and argued that, for the typical reaction pathway considered in urea-assisted hydrogen production, the open-circuit voltage is closer to 0.07 V, further strengthening the case for UOR-assisted hydrogen generation from an energy perspective [6,7]. Regardless of the exact value, there is broad consensus that UOR is significantly less energy-demanding than OER and thus can reduce the required cell voltage and electricity consumption.

Beyond energy savings, UOR-based systems offer two additional advantages. First, they enable the simultaneous removal of urea from wastewater and urine, mitigating eutrophication and nitrogen pollution. Second, urea is a relatively safe, non-toxic, and non-volatile compound that can act as a hydrogen carrier, as emphasized in early perspective work on “urea as a hydrogen carrier” [8]. These attributes make urea-assisted electrolysis attractive for decentralized hydrogen production, integrated wastewater treatment, and even closed-loop sanitary systems.

Several comprehensive reviews have appeared recently on urea electro catalysis and catalytic urea oxidation for energy and environmental applications [9-11]. Compared with these broader treatments, the present article narrows the focus explicitly to UOR as an anodic process for energy-efficient hydrogen generation. We emphasize (i) how thermodynamics and reaction pathways translate into potential energy savings at the device level, (ii) how electro catalyst structure and composition influence UOR activity and stability at practical current densities, and (iii) what challenges must be overcome to realize industrially relevant urea-assisted hydrogen systems.

In modern electrochemical research, experimental characterization alone is often insufficient for resolving the complex interplay between surface chemistry, reaction energetics, mass transport, and degradation pathways. As a result, computational and simulation-based methodologies have become indispensable tools for deepening mechanistic insight and guiding rational materials design. Depending on the system under investigation, approaches such as density functional theory (DFT), molecular dynamics (MD), Monte Carlo simulations, finite-element modeling (FEM), and polymer-physics simulations enable quantitative evaluation of adsorption energetics, charge distribution, molecular interactions, inhibitor–surface affinity, ion transport, and current distribution. Recent studies across diverse corrosion-, coating-, polymer-, and protection-related engineering fields have demonstrated how the integration of simulation with experimental work yields more accurate predictions, clarifies rate-determining steps, and enhances structure–property understanding [12-20]. The collective message from these works is clear: high-fidelity simulation frameworks are essential complements to experiment, particularly in electrochemical systems where multiple coupled phenomena govern performance. A simplified schematic of a two-electrode urea assisted electrolyzer, illustrating the UORHER coupling and the resulting low-voltage hydrogen production pathway, is presented in Figure 1.

Figure 1. Schematic representation of a two-electrode electrolyzer coupling the urea oxidation reaction (UOR) at the anode with the hydrogen evolution reaction (HER) at the cathode. The assisted pathway lowers the required cell voltage and enables energy-efficient hydrogen production in alkaline media.

Fundamentals of the Urea Oxidation Reaction

Thermodynamic landscape and comparison with OER: The thermodynamic driving force for UOR can be assessed by combining standard Gibbs free energy data for urea, nitrogen, carbonate, and water under alkaline conditions. Classical analyses, treating the overall urea-assisted hydrogen production reaction

CO(NH₂)₂ + H₂O → N₂ + CO₂ + 3H₂                                                                    (Eq. 2)

yield a theoretical cell voltage of about 0.37 V at 298 K, corresponding to a standard potential of ~0.37 V vs. RHE for UOR when paired with HER [1,5]. This figure has been widely used to motivate urea-assisted water electrolysis as a low-voltage alternative to OER-based water splitting.

However, a more detailed thermodynamic re-evaluation by Protsenko showed that the commonly quoted 0.37 V value arises from inconsistent reaction stoichiometries and that, when the appropriate half-reactions and activities are used, the true open-circuit voltage of the relevant urea oxidation pathway is closer to 0.07 V [6,7]. While this correction does not change the qualitative conclusion UOR is far more favorable than OER it suggests that there is theoretically even more room for reducing the cell voltage than previously recognized.

In practical alkaline electrolytes (e.g., 0.33-1.0 m urea in 1.0 m KOH or NaOH), kinetic limitations, mass transport, and ohmic losses mean that measurable UOR currents are typically obtained only at potentials ≥1.3-1.4 V vs. RHE on conventional Ni-based catalysts, and industrially relevant hydrogen production rates usually require cell voltages in the range of 1.35-1.6 V rather than the theoretical limit [1,9]. Nevertheless, this still represents a significant reduction compared with OER-based water electrolysis at the same current density.

Thermodynamic and green-metrics analyses indicate that urea oxidation pathways leading to N₂ and carbonate with minimal side products exhibit particularly favorable “electricity economy” defined as the minimum electrical work required per mole of desired product. Protsenko’s green chemistry evaluation showed that the oxidation of urea to gaseous N₂ and carbonate ions has the most advantageous combination of low electricity consumption and acceptable atom economy among several possible urea oxidation routes [7]. These considerations strongly support targeting highly selective UOR pathways to maximize energy and environmental benefits.

Mechanistic pathways and intermediates: Despite its thermodynamic favorability, UOR is mechanistically complex. It involves six-electron transfer, multiple dehydrogenation steps on the urea molecule, C-N bond cleavage, N-N coupling, and C-O bond formation, occurring on dynamically reconstructed catalyst surfaces that often transform into high-valent (oxy)hydroxide phases under operating conditions [10,21]. Several mechanistic frameworks have been proposed to rationalize UOR kinetics:

ü  Adsorbate evolution mechanism (AEM): In analogy to OER, UOR is described in terms of sequential adsorption and dehydrogenation of urea-derived intermediates (CO(NH₂)₂*, CONH*, CO*, NH_x*) on active metal sites, coupled with OH⁻ adsorption and electron transfer. Rate-determining steps often involve C-N bond cleavage or the dehydrogenation of adsorbed amide species.

ü  Lattice oxygen-involved mechanisms (LOM): For certain oxides and ox hydroxides, lattice oxygen may actively participate in forming C-O bonds or in oxidizing nitrogen intermediates, with concomitant formation of oxygen vacancies and re-oxidation by OH^- from the electrolyte.

ü  Chemical electrochemical (CE) pathways: Some studies suggest that the initial dehydrogenation of urea can proceed via chemical steps on higher-valent NiOOH or CoOOH surfaces, followed by electrochemical regeneration of the oxidized surface states, akin to “redox mediator” behavior.

In situ/operando Raman, FTIR, X-ray absorption spectroscopy (XAS), and differential electrochemical mass spectrometry (DEMS) have identified key intermediates such as adsorbed cyanate (CNO^-), carbamate, carbonate, and various NH_x* species [9,21]. While N₂ is often considered the desired nitrogen product, recent work has highlighted that competing oxidation paths can lead to NO_x^-/NO₃^- species, especially at high anodic potentials or on particular catalyst compositions. Highly selective UOR catalysts that favor N₂ formation over nitrate/nitrite are therefore crucial to minimize nitrogen pollution and maximize atom efficiency [9,11].

Kinetic descriptors and performance metrics: Typical electrochemical metrics used to evaluate UOR electro catalysts include:

ü  Onset potential and potential at fixed current densities (e.g., η at 10, 50, 100, or 500 mA cm^-2), often reported vs. RHE (three-electrode measurements) or as full-cell voltages in two-electrode configurations.

ü  Tafel slopes, reflecting the rate-determining step and overall kinetics.

ü  Turnover frequency (TOF) normalized by electrochemically active surface area (ECSA) or the number of active metal sites.

ü  Charge transfer resistance from electrochemical impedance spectroscopy (EIS).

ü  Long-term chronoamperometric or Chrono potentiometric stability, including tolerance to carbonate buildup and poisoning species.

ü  Faradaic efficiency for hydrogen at the cathode and for nitrogen-containing products at the anode.

Recent structure activity correlation studies have revealed that high UOR activity is typically associated with catalysts that can: (i) stabilize high-valent Ni³⁺/Ni⁴⁺ or analogous oxidized states at relatively low potentials; (ii) exhibit optimized adsorption energies for urea and key intermediates; and (iii) provide abundant oxyphilic sites and favorable proton-coupled electron transfer pathways [21,22]. Such descriptors guide rational catalyst design.

Electro catalyst Landscape for UOR-Assisted Hydrogen Generation

General design principles: Modern UOR electro catalyst design builds on three overarching ideas:

ü  Electronic structure engineering: Tuning the d-band center, oxidation state distribution, and metal oxygen covalence to favor the formation of highly active, high-valent species (e.g., γ-NiOOH-like phases) at lower potentials while maintaining sufficient conductivity [1, 11].

ü  Nano structuring and interface design: Creating 3D porous architectures, core shell morphologies, and heterostructures that maximize active surface area, expose edge/defect sites, and facilitate reactant/product transport, especially in concentrated urea/KOH mixtures.

ü  Support and hybridization strategies: Integrating active phases with conductive scaffolds such as Ni foam, carbon fibers, doped carbons, or MXenes to reduce charge transport resistance and improve mechanical robustness.

Within this framework, Ni-based catalysts remain the workhorse for UOR due to their favorable adsorption properties for urea and OH^-, suitable redox potentials for Ni²⁺/Ni³⁺/Ni⁴⁺, and moderate cost. However, a wide range of material classes has now been explored. Representative studies on self-supported Fe-Co-Ni nanostructures grown on copper foam for alkaline HER, hierarchical three-phase Ni/Co/Fe selenide networks anchored on 2D NiCo-MXene for seawater electrolysis and hydrazine oxidation, and ultrafast-synthesized NiCo-MXene nanocomposites for coupled HER and UOR in alkaline and seawater media collectively show that properly engineered multicomponent Ni/Co/Fe-based transition-metal architectures, especially when integrated with MXene supports, can deliver high activity and durability for low-voltage hydrogen production using only earth-abundant elements [23-25].

Ni-based hydroxides, ox hydroxides and LDHs: Pure Ni(OH)₂/NiOOH systems on Ni foam or Ni mesh are historically the earliest and most studied UOR catalysts. Under anodic polarization in alkaline urea solutions, Ni(OH)₂ is transformed into γ-NiOOH, which is widely regarded as the true active phase for UOR, analogous to its role in OER [4,10]. On such electrodes, UOR onset typically occurs near the Ni²⁺/Ni³⁺ redox potential (~1.4 V vs. RHE), with current densities of 10-50 mA cm^-2 reached around 1.45-1.50 V vs. RHE in 0.33-0.5 m urea +1.0 m KOH.

Bimetallic LDHs (e.g., NiFe-, NiCo-, NiMn-LDH) supported on Ni foam, carbon cloth, or other conductive substrates show systematically improved UOR performance compared with simple Ni(OH)₂ [9,11]. In these materials, the second metal cation (Fe, Co, Mn, etc.) modulates the electronic structure of the Ni centers, introduces cation disorder, and often enhances the density of active high-valent sites. Oxygen vacancies and defect-rich LDHs further facilitate urea adsorption and OH^- activation. For example, oxygen-deficient NiAl-LDH Nano sheets on Ni foam have been reported to achieve ~10 mA cm^-2 at ~1.42 V vs. RHE in 1 m KOH + 0.33 m urea, with small Tafel slopes indicative of favorable kinetics [26].

Recent reviews by Yang et al. and Ge et al. provide exhaustive catalogs of Ni-based LDHs and reconstructed NiOOH catalysts for UOR, outlining correlations between composition, defect chemistry, and activity trends across hundreds of compositions [10,27]. These works underline that subtle changes such as Fe doping level, Co/Ni ratio, or introduction of high-entropy multination LDHs can significantly shift UOR onset potentials and durability. To place the above trends on a more quantitative footing, several representative Ni-based UOR electro catalysts are compiled in Table 1. As seen from these data, defect‐rich Ni(OH)₂/NiOOH and NiFe- or NiCo-based LDHs grown on conductive three-dimensional substrates such as Ni foam or carbon cloth typically reach 10-50 mA cm^–2 at ≈1.36-1.45 V vs. RHE in alkaline urea electrolytes, in line with the general picture drawn above. Compositional modulation (e.g., incorporation of Fe, Co, Mn or high-valent dopants such as Hf) and the use of nitride or oxide–nitride pre-catalysts further lower the operating potential and enable high current densities at relatively small over potentials. Overall, the comparative performance in Table 1 reinforces that electronic-structure engineering combined with hierarchical 3D architectures is central to optimizing Ni-based hydroxides, ox hydroxides and LDHs as practical anodes for urea-assisted hydrogen generation.

Table 1. Representative Ni-based hydroxide/ox hydroxide and LDH electro catalysts for the urea oxidation reaction.

Nickel chalcogenides and nitrides: Nickel sulfides, selenides, and nitrides are often considered “pre-catalysts” that transform under anodic UOR conditions into Ni-(oxy)hydroxide shells on chalcogenide/nitride cores. This core–shell architecture can enhance activity by combining a highly conductive inner core with a catalytically active outer NiOOH layer.

NiS and Ni₃S₂ nanostructures on Ni foam or carbon supports, including honeycomb-like Ni-Mo-S architectures, have shown high functional activity for HER and UOR, enabling low overall cell voltages for urea electrolysis (e.g., ~1.47 V at 10 mA cm^-2) [33, 34]. Long-term stability, however, can be compromised by sulfur leaching and surface reconstruction.

Similarly, NiSe₂, Ni₃Se₂ and bimetallic CoNiSe or FeNiCoSe phases exhibit strong UOR activity, benefiting from improved conductivity and tuned electron density around Ni [11,35]. Under operating potentials, these selenides tend to develop Ni/Co ox hydroxide shells enriched in high-valent species. Incorporating urea-active selenides into hierarchical 3D architectures (e.g., Nano flowers, Nano sheets, or nanotubes) further improves mass transport and bubble removal.

Nickel nitrides (Ni₃N, Ni₂N) have also been investigated as UOR pre-catalysts; their higher intrinsic conductivity and favorable Ni-N covalence can facilitate electron transfer, while in situ oxidation produces NiOOH-like surfaces [9].

Phosphides and phosphate-based catalysts: Transition-metal phosphides (Ni₂P, Ni₅P₄, CoP, FeP, and bimetallic phosphides) represent another major class of UOR pre-catalysts. Their metallic or semi-metallic conductivity and tunable P-M covalence are advantageous for both HER and UOR, making them promising functional electrodes for overall urea electrolysis.

Ni₂P Nano flowers on Ni foam have been shown to drive UOR at low over potentials (e.g., <1.45 V vs. RHE at 10 mA cm^-2) while also serving as efficient HER catalysts, enabling full urea electrolyzes operating at ≲1.5 V for moderate current densities [9,11]. Recent work on cerium-doped Ni₅P₄ highlights how aliovalent doping can modulate the electronic structure, promote surface reconstruction to NiOOH-like phases, and enhance UOR kinetics at industrial current densities [11].

Complementary to phosphides, phosphate and biphosphate materials such as Ni, Co, Cu, or bimetallic Ni-Co phosphates have been explored as UOR catalysts. Their robust polynomic frameworks and strong metal-oxygen-phosphate interactions confer good structural stability and can tune the acidity/basicity of surface sites. A recent review by Abd El-Lateef and co-workers systematically summarized phosphorus-based catalysts for UOR, emphasizing their potential to couple urea electro oxidation with sustainable waste-to-energy conversion [11].

To provide a quantitative perspective on these materials, representative phosphide and phosphate-based UOR electro catalysts and their key performance metrics are compiled in Table 2.

Table 2. Summary of selected P-based UOR electro catalysts.

Non-nickel transition-metal catalysts: While Ni dominates UOR research, there is growing interest in Ni-free catalysts for specific applications or to mitigate potential Ni leaching. Co-based oxides and hydroxides, Fe-rich perovskites, and Mn-containing mixed oxides have been investigated as UOR anodes, sometimes in combination with Ni but also as stand-alone catalysts. A recent article on Co₃O₄-based materials for UOR, for example, demonstrated that appropriate Nano structuring and doping can yield respectable UOR activity even without Ni, although achieving the same performance as optimized Ni-based systems remains challenging [9].

Metal-oxide UOR catalysts more broadly including doped spinels, perovskites, and MOF-derived oxides have been reviewed by Phule et al., who highlight strategies such as heteroatom doping, defect creation, and integration with carbon supports for improving charge transport and urea adsorption. These Ni-free or Low-Ni systems may be particularly attractive in environments where Ni dissolution is problematic or where regulation restricts Ni usage.

MXene- and carbon-based hybrid catalysts: Two-dimensional carbides and nitrides (MXenes) provide highly conductive, hydrophilic, and surface-tunable supports for UOR catalysts. Their large surface area, abundant surface terminations (-O, -OH, -F), and flexible stacking behavior facilitate intimate contact with active nanoparticles or (oxy)hydroxide layers.

Recent studies have demonstrated that decorating MXenes such as Ti₃C₂T_x, Mo₂TiC₂T_x or V₄C₃T_x with Ni-based LDHs, chalcogenides, or phosphides yields hybrid electrodes with enhanced UOR activity and stability [42-45]. The MXene support enhances electron transport, suppresses particle agglomeration, and may also engage in synergistic interactions with the active phase, modulating adsorption energies of urea and intermediates.

For instance, NiCr-LDH grown on V₄C₃ MXene showed markedly improved UOR activity compared with NiCr-LDH alone, attributed to enhanced conductivity and optimized OH^-/urea adsorption at the LDH-MXene interface [42]. Similarly, hierarchical NiCo₂O₄-FeCo₂S₄/Mo₂TiC₂T_x composites leveraged triple junctions between spinel oxide, sulfide, and MXene to provide multiple synergistic active sites for both UOR and HER [43].

Carbon-based hybrids including N-doped carbon, carbon nanotubes, graphene, and 3D carbon foams play a broadly analogous role: they provide high conductivity, mechanical stability, and tunable surface chemistry for anchoring Ni-based nanoparticles or LDH Nano sheets. Numerous studies on Ni/NiO, Ni-B, Ni-C, and Ni-doped carbon dots supported on doped carbons or biomass-derived carbons have reported improved UOR kinetics and long-term stability relative to unsupported catalysts [9,34].

To highlight these MXene-supported UOR architectures in a more quantitative manner, representative MXene-based electro catalysts and their key performance figures are summarized in Table 3.

Table 3. Selected MXene-based electrodes for UOR / urea-assisted electrolysis.