Metal phosphides as efficient catalysts for methanol oxidation reaction

Abstract

The methanol oxidation reaction (MOR) is a key anodic process in sustainable energy conversion and the synthesis of value-added chemicals. While noble metals such as Pt and Pd exhibit high catalytic activity, their practical use is limited by high cost, poor stability in alkaline media, and the generation of pollutant gaseous byproducts. In contrast, transition metal phosphides (TMPs) have emerged as promising alternatives due to their tunable composition, strong resistance to catalyst poisoning, chemical robustness under alkaline conditions, and high selectivity for liquid formate, a valuable chemical with a current market price exceeding that of methanol. Although TMPs were initially studied as promoters for noble metal catalysts, recent research has shifted toward evaluating their performance as standalone catalysts for MOR. These systems typically require higher operating potentials (above 1.4 V vs. RHE) compared to noble-metal-based catalysts (0.8–1.1 V vs. RHE) but offer the advantage of avoiding CO and CO₂ emissions while maintaining high selectivity toward formate. A central challenge in comparing TMPs to noble metal systems stems from inconsistent data normalization practices in the literature: activity is often reported per mass of noble metal, rather than per total catalyst mass. When normalized to the total mass, TMP-based systems may exhibit comparable or even superior mass activity, suggesting their viability despite the higher required potential. Research to date has focused largely on Ni- and Co-based TMPs, while Fe- and Mo-based variants, although efficient in combination with noble metals, remain underexplored as independent catalytic materials and warrant further investigation.

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Dyuti Bandyopadhyay

Dr Dyuti Bandyopadhyay is currently a Postdoctoral Researcher at Ben-Gurion University of the Negev, Israel. She earned her Bachelor of Science (B.Sc.) degree from Bethune College, University of Calcutta, and her Master of Science (M.Sc.) degree from the National Institute of Technology, Durgapur, India. She later pursued a research career in Israel, obtaining a second M.Sc. in Hydrology and Water Quality, followed by a PhD in Chemistry from Ben-Gurion University of the Negev. Her research currently focuses on the synthesis of transition metal phosphide nanoparticles and their various electrochemical applications.

Maya Bar Sadan

Prof. Maya Bar-Sadan is currently the Chair of the Department of Chemistry at Ben-Gurion University of the Negev, Israel. She earned her Ph.D. in Chemistry and Materials Science from the Weizmann Institute of Science in 2007. Following her doctoral studies, she conducted postdoctoral research at the Institute of Solid-State Research and the Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons in Jülich, Germany. Prof. Bar-Sadan's research focuses on elucidating the correlations between macroscopic properties and the atomic-scale structure of materials. Her recent work involves two-dimensional materials and transition metal phosphides. She develops synthetic methodologies for bimetallic systems, aiming to design and optimize novel electrocatalysts and photocatalysts for sustainable energy applications.

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1. Introduction

The growing concerns over environmental degradation and the depletion of fossil fuel resources have intensified the demand for efficient and sustainable energy conversion technologies. Electrochemistry has emerged as a promising, sustainable, and environmentally friendly method for the production of chemicals and fuels.^1,2 In most electrochemical systems, cathodic reduction reactions such as hydrogen evolution, oxygen reduction, CO₂ electroreduction, and organic electrocatalytic hydrogenation, are coupled with the anodic oxygen evolution reaction (OER).^3–5 However, O₂ generated by OER holds minimal economic value while consuming approximately 90% of the total electricity input. Consequently, improving anodic reactions remains a significant challenge, spurring interest in alternative electrocatalytic pathways to overcome the sluggish kinetics of OER.^6–8

Among the alternatives, various organic compounds such as methanol, ethanol, urea, and glycerol have been proposed to replace OER at the anode. These reactions offer lower onset potentials, reduced energy requirements, and the co-generation of value-added products.^9–14 Alcohols are particularly attractive due to their high energy density, ease of production from biomass or organic waste, and convenient storage and handling.^15,16 Nevertheless, complete oxidation of alcohols to CO₂ or partial oxidation to CO still releases harmful pollutants into the environment. To fully exploit the potential of alcohol oxidation, research focuses on designing catalysts that selectively suppress complete oxidation to CO₂ while producing liquid chemicals such as formate or acetate, even at the cost of extracting less energy during oxidation. This strategy simultaneously mitigates CO₂ emissions and generates liquid products, eliminating the need for energy-intensive membrane separation between cell compartments.^17,18 Despite these advantages, developing efficient, cost-effective, and selective catalysts that avoid forming mixed products remains a challenge.^18,19

The methanol oxidation reaction (MOR) represents the simplest alcohol electrooxidation pathway, proceeding through a kinetically sluggish six-electron process that serves as the rate-limiting step in direct methanol fuel cells. The reaction includes the oxidation of methanol to formaldehyde and formic acid, with CO₂ as the final product of full oxidation.^20–22 The primary intermediates, i.e., adsorbed methoxy (CH₃O*), hydroxyl (OH*), and formate (HCOO*), play essential roles in facilitating the reaction steps.

Noble metals exhibit high intrinsic electrocatalytic activity for MOR; however, their industrial application is restricted by high cost, limited availability, and recycling challenges. Additionally, noble metals tend to produce a wide range of products, most of which are gaseous.^23–27 Transition metal phosphides (TMPs) have emerged as promising alternatives for anodic reactions such as OER and MOR, owing to their distinctive properties.^28–30 TMPs possess a unique electronic structure that enhances electrocatalytic performance through high conductivity and robust stability across a broad pH range. These features facilitate efficient catalysis of both OER and MOR. Moreover, TMPs exhibit notable poison tolerance, particularly in hetero-interface configurations, enabling sustained catalytic activity.¹⁶ Their surface reactivity supports the generation of key intermediate species, such as hydroxyl radicals (OH*), which play a pivotal role in the electrochemical steps of these reactions.^16,30–36 TMP versatility further allows tailored optimization of catalytic properties for specific reactions, underscoring their potential in sustainable energy conversion.^37,38

Beyond these bulk advantages, the surface chemistry of TMPs under anodic potentials plays a critical role in their catalytic function. During electrocatalysis, TMPs transform at anodic potentials in alkaline environments into (oxy)hydroxides that serve as active sites. A representative example is the Ni(OH)₂/NiOOH system, which undergoes reversible redox cycling involving Ni³⁺ and adjacent oxygen species that promote methanol oxidation. The suggested steps for selective methanol oxidation on Ni-based surfaces are:^17,39,40

Ni²⁺ + OH⁻ → Ni(OH)₂ (surface) (1)

Ni(OH)₂ + OH⁻ ⇌ NiOOH + H₂O + e⁻ (fast) (2)

RCH₂OH_sol ⇌ RCH₂OH_ads (rate limiting step) (3)

RCH₂OH_ads + NiOOH → RCHOH_ads + Ni(OH)₂ + e⁻ + H⁺ (4)

RCHOH_ads + H₂O + 3NiOOH → RCOOH_sol + 3Ni(OH)₂ (5)

Notably, the surface transformations described for TMPs are not unique to phosphides. Such transformations are also observed on other transition-metal-based materials, including nitrides, borides, and chalcogenides. The reaction mechanism varies based on the catalyst, particularly in the bond-breaking sequence of C–H and O–H.^41–43 Although TMPs are often considered neutral pre-catalysts, this intrinsic formation of a core–shell structure deems the crystallographic structure of the core critical as it influences the growth of the (oxy)hydroxide layer and enhances conductivity for oxidized species in the outer layer.^37,44

While their catalytic versatility is promising, the structural complexity of TMPs poses significant challenges for synthesis and characterization. The TMP phase diagram is highly complex, containing multiple stoichiometric phases. This complexity complicates controlled synthesis, as many catalysts are obtained through partially controlled processes, leading to mixed phases and facets that obscure clear understanding of structure–function relationships. Here, we review prominent examples of TMPs used for MOR, highlighting the performance benefits achieved and the research directions that have shaped the field in recent years.

2. Ni and Co-based TMPs

Nickel and cobalt phosphides represent some of the most extensively studied TMP systems for anodic alcohol oxidation. Therefore, this section highlights their design principles, synthesis strategies, and catalytic performance in methanol oxidation, using selected examples to illustrate key concepts. While numerous comprehensive reviews detail the full scope of TMP synthesis routes,^36,45,46 here we emphasize approaches relevant to MOR activity.

2.1 Synthetic strategies for Ni and Co TMPs

TMP synthesis strategies encompass a broad range of methodologies that vary in their ability to control morphology, crystallographic phase, and elemental distribution.⁴⁷ Gas–solid routes to transition metal phosphides have traditionally relied on temperature-programmed reduction of metal and phosphorus precursors at elevated temperatures.⁴⁸ Such methods provide access to a wide range of binary and ternary phases, yet they inherently limit morphological control because nucleation and growth occur under harsh conditions that promote particle sintering, broad size distributions, and surface heterogeneities. Moreover, excess phosphorus species and phase impurities are common, reflecting the difficulty of maintaining stoichiometric balance at high temperatures. These factors constrain the ability to isolate the role of composition from that of morphology or crystallinity when evaluating catalytic properties. Despite these drawbacks, gas–solid methods benefit from relative simplicity and scalability, which makes them useful for bulk catalyst preparation. For nickel phosphides, Ni-rich phases such as Ni₂P and Ni₁₂P₅ are preferred for MOR applications, as Ni atoms are typically considered the primary active sites.^31,49 Nickel phosphides are often prepared via calcination of nickel precursors with phosphorus sources under an inert atmosphere, where reaction temperature governs the final crystallographic phase.¹⁶ During calcination, phosphorus precursors decompose to toxic PH₃ in the gas phase, necessitating large precursor quantities, most of which remain unused in the reaction.

In contrast, solution-based syntheses have enabled substantial advances in the controlled preparation of transition metal phosphides.^49–51 The use of molecular or colloidal precursors, frequently metal–phosphine complexes with tunable decomposition temperatures, allows the generation of nanoparticles under comparatively mild conditions. These approaches provide uniform structural features and reproducible morphologies, including spherical, hollow, or rod-like architectures depending on ligand environment and reaction temperature. Arrested precipitation techniques and coreduction–phosphidation strategies have been particularly effective for incorporating multiple metals, enabling control over elemental ratios within ternary systems. Such methods therefore offer clear advantages in decoupling composition from morphology, a prerequisite for establishing precise composition–activity correlations. The trade-off, however, is that air-free handling is often required, and the phosphorus precursors can evolve pyrophoric or toxic species, complicating laboratory safety and scalability.

Two-step strategies are also common, where phosphorization follows the synthesis of an initial metal-based template, such as metal–organic frameworks (MOFs). For instance, Ni₂P can be derived from Ni-MOF precursors through phosphorization with white phosphorus or sodium hypophosphite hydrate (NaH₂PO₂·H₂O).³¹ Another template-based approach uses metal thin films or hydroxides (e.g., layered double hydroxides, LDHs).³⁵ For example, Ni₂P with a flower-like architecture has been synthesized using NaH₂PO₂·H₂O as the phosphorus source.^13,35 This strategy allows incorporation of additional transition metals by co-precipitating their hydroxides into the initial film. Similarly, free-standing TMP particles can be obtained via solvothermal synthesis followed by phosphorization, producing hollow spheres.³²

A major advantage of rational liquid phase low-temperature solution syntheses is their capacity to preserve the morphology and crystalline phase of parent nanoparticles while systematically varying composition. This precise synthetic control enables researchers to attribute variations in catalytic behavior directly to compositional differences, without convolution from particle size or structural changes. In terms of oxidation stability, solution-derived nanoparticles often require protective supports or capping ligands to mitigate degradation under ambient or electrochemical conditions, but their well-defined surfaces and tunable ligand shells provide strategies to address these vulnerabilities. Thus, while they may be less inherently stable than bulk products of gas–solid synthesis, they offer unparalleled control over nanoscale architecture. For instance, Ni phosphides have been synthesized by reacting Ni(acac)₂ with tri-octylphosphine (TOP) in oleylamine and octadecene (ODE).¹⁷ This route facilitates the preparation of homogeneous single-phase materials, such as Ni₁₂P₅, Ni₂P, and Ni₅P₄, enabling direct correlation between phase structure and catalytic properties.

In summary, gas–solid syntheses are robust and relatively straightforward but limited in precision, while solution methods are highly tunable yet more complex and demanding. Morphology is largely uncontrolled in high-temperature solid-state routes but can be precisely engineered in liquid-phase systems. Composition can drift under harsh solid-state conditions, whereas solution based approaches allow independent tuning of elemental ratios. Finally, while both classes of materials may suffer oxidative degradation, solution-synthesized phosphides offer better opportunities for stabilization through ligand engineering and support dispersion. Fig. 1 presents representative morphological and structural characteristics of Ni- and Co-based TMP catalysts prepared via the synthetic strategies outlined in this section. Specifically, Fig. 1f exemplifies the phosphorization of a metal film, yielding a characteristic petal-like architecture, whereas the remaining panels display a selection of morphologies commonly achieved through liquid-phase synthesis approaches. Together, these complementary methods form a toolkit in which gas–solid syntheses are suited for scalable production, and solution routes provide the precision necessary to unravel fundamental structure-composition-property relationships in transition metal phosphides.

Fig. 1 Examples of various particle morphologies and structures of transition metal phosphide materials used as catalysts. Scanning electron microscopy (SEM) images of: (a) Ni₂P-high crystallinity and (b) Ni₂P-low crystallinity prepared by reacting Ni-based metal–organic frameworks and P₄ solvothermally (adapted with permission from ref. 31. Copyright 2021 American Chemical Society). (c) Low-magnification and (d) high-magnification images of Ni₁Co₂P_x synthesized via a template-free solvothermal approach and subsequently annealing and phosphating treatments, and (e) HAADF-STEM image with the overlayed EDS profile alongside elemental mapping of a Ni₁Co₂P_x particle (adapted with permission from ref. 32. Copyright 2020 American Chemical Society). (f) SEM image of bimetallic Ni₂P–CoP junctions on commercial nickel foam prepared via a facile electrodeposition–phosphidation approach. Adapted with permission from ref. 35. Copyright 2018, Wiley-VCH. TEM images of different phases of nickel phosphide: Ni₁₂P₅ (g) and Ni₂P (h), produced by colloidal chemistry. Adapted from ref. 17 with permission from the Royal Society of Chemistry.

2.2 Challenges in performance comparison

Although many studies report Ni, Co, and bimetallic Ni–Co phosphides as active for MOR, direct comparison of catalytic performance is difficult because different studies use varying experimental parameters (see Table 1 and Fig. 2). To facilitate meaningful comparison across the diverse studies reviewed, we standardized the reported potentials to the reversible hydrogen electrode (RHE) scale wherever possible. In addition, we estimated mass activity values by dividing the reported current densities (in mA cm⁻²) by the catalyst loading (in mg cm⁻²), when sufficient data were available. In some cases, when the authors included it, we also included specific activity values, normalized by the effective surface area, either based on BET-derived surface area or electrochemically active surface area (ECSA), depending on the methodology used in the original report. Calculated or re-derived values are marked with asterisks in the tables to clearly indicate instances where we performed conversions or estimations.

Fig. 2 Electrocatalytic activity of TMP catalysts for the methanol oxidation reaction (MOR). (a) Cyclic voltammograms (CVs) and linear sweep voltammograms (LSVs, inset) of Ni₁Co₂P_x in 1M KOH + 1M CH₃OH. Adapted with permission from ref. 32. Copyright 2020 American Chemical Society. (b) LSV curves of hollow CoP nano-octahedra (orange) and porous CoP nanosheets (olive) in 1.0 M KOH + 1.0 M CH₃OH (solid lines) and in 1.0 M KOH (dotted lines). Adapted from ref. 33 with permission from the Royal Society of Chemistry. (c) LSV curves for Ni₂P–CoP/NF, Ni₂P/NF, and CoP/NF in 1M KOH + 0.5M CH₃OH. Adapted with permission from ref. 35. Copyright 2018 Wiley-VCH. (d) CV curves of Cu–O, Cu₃P, and Cu–O|Cu₃P in 1M KOH + 1M CH₃OH. Adapted from ref. 52 with permission from MDPI. (e) CV curves of Ni₂P (upper panel) and Ni₁₂P₅ (lower panel) in 1M NaOH and in 1M NaOH + 1M CH₃OH. Adapted from ref. 17 with permission from the Royal Society of Chemistry.

Table 1 Summary of reported TMP-based catalysts for the methanol oxidation reaction (MOR), including activity metrics, faradaic efficiency (FE), and experimental conditions. Mass activities were recalculated where applicable

Material	Products	FE	Activity (mA cm^−2)	Mass activity (mA mg^−1)	Conditions	Ref.
a Calculated by the following equations using the pH according to the reported conditions: E (vs. RHE) = E (vs. SCE) + 0.241 V + 0.059 × pH; E(vs. RHE) = E(vs. Ag/AgCl) + 0.197 + 0.059 × pH; E(vs. RHE) = E(vs. Hg/HgO) + 0.927 V. b Calculated by multiplying the mass activity per cm^2 with the actual surface area of the glassy carbon electrode (0.07068 cm^2).
Ni2P-L (low crystallinity) nanospheres	Not reported	Not reported	Not reported	427 mA mg^−1 at 1.74 V vs. RHE	1 M KOH + 0.5M CH3OH	31
Ni2P-H (high crystallinity) nanospheres	Not reported	Not reported	Not reported	244 mA mg^−1 at 1.74 V vs. RHE	1 M KOH + 0.5M CH3OH	31
Ni12P5 nanoparticles	75% formate, 3% unreacted material, and unidentified gaseous products.	Not reported	210 mA cm^−2 at 1.72 V vs. RHE	4236 mA mg^−1 per 1 cm^2 at 1.72 vs. RHE, equal to 299 mA mg^−1^b	1 M KOH + 1 M CH3OH	17
Ni2P nanoparticles	Not reported	Not reported	158 mA cm^−2 at 1.72 V vs. RHE	2230 mA mg^−1 per 1 cm^2 at 1.72 V vs. RHE equal to 157 mA mg^−1	1 M KOH + 1 M CH3OH	17
Hollow cobalt phosphide octahedra	Not reported	Not reported	Specific activity: 1.53 mA cm^−2 at 0.4 V vs. SCE (or 1.47 V vs. RHE^a)	206.2 mA mg^−1 at 0.4 vs. SCE (or 1.47 V vs. RHE^a)	1 M KOH + 1 M CH3OH	33
Ni2P–CoP on nickel foam	Formate	>99.8	10 mA cm^−2 at 1.16 V vs. RHE	Not reported. Catalyst loading not reported.	1 M KOH + 0.5 M CH3OH	35
			100 mA cm^−2 at 1.30 V vs. RHE
Co–Ni–P nanoneedles	Formate	98.3	100 mA cm^−2 at 1.33 V vs. RHE	10 mA mg^−1 at 1.33 V vs. RHE (calculated using the loading ∼10 mg cm^−2)	1 M KOH + 0.5M CH3OH	30
3D hollow heterogeneous nickel–cobalt phosphides (Ni1Co2Px)	Not reported	Not reported	Specific activity: 155 mA cm^−2 at 1.7 V vs. RHE	436.9 at 1.7 V vs. RHE	1 M KOH + 1 M CH3OH	32
Urchin-like Ni–Co–P–O nanocomposite	Not reported	Not reported	39.90 mA cm^−2 at 0.8 V vs. SCE (or 1.85 V vs. RHE^a)	1567 mA mg^−1 at 0.8 V vs. SCE (or 1.85 V vs. RHE^a)	0.5 M KOH + 1 M CH3OH	16
Ni1.2Cr0.8P	Formate detected but not quantified	Not reported	10 mA at 1.14V	10.5 mA mg^−1 (potential not reported).	1 M KOH + 3 M CH3OH	53
3.4 Mo-doped Ni2P on Ni foam	Not reported	Not reported	652.1 mA cm^−2 at 0.75 V vs. Ag/AgCl (or 1.77 V vs. RHE^a)	Not reported. Catalyst loading not reported.	1 M KOH + 1 M CH3OH	54
NiMoPx@Ni5P4	Formate	Nearly 100%	500 mA cm^−2 at 1.49 V vs. RHE	156.3 mA mg^−1 at 1.49 V vs. RHE	1 M KOH + 1 M CH3OH	55
			1000 mA cm^−2 at 1.58 V vs. RHE
			Specific activity: ≈59 mA cm^−2 (potential not reported)	312.5 mA mg^−1 at 1.58 V vs. RHE
CuO/Cu3P	Not reported	Not reported	232.5 mA cm^−2 at 0.65 V vs. Hg/HgO (or 1.577 V vs. RHE^a)	Catalyst loading not reported.	1 M KOH + 1 M CH3OH	52
Cu–Co phosphides nanoflowers	Not reported	Not reported	95.8 mA cm^−2 at 0.6 V vs. SCE (or 1.67 V vs. RHE^a)	287.4 mA mg^−1 at 0.6 V vs. SCE (or 1.67 V vs. RHE^a)	1 M KOH + 1 M CH3OH	56

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To note, noble-metal-based MOR studies reported mass activity, defined as the oxidative peak current in the forward scan normalized to catalyst mass loading, i.e., by normalizing to the mass of the noble metal content.⁵⁷ For composite catalysts of TMPs and noble metals, the mass activity was normalized only to the mass of the noble metal content, without referring to the TMPS. In addition, much of the relatively older literature often omitted product analysis. In contrast, current research on utilizing TMPs for MOR aims at selective production of liquid products, specifically formate (HCOO⁻), rather than full oxidation to CO₂. Therefore, reporting mass activity alongside faradaic efficiency (FE) is essential for assessing selectivity and avoiding overestimation caused by competing processes, such as CO/CO₂ formation, OER, or oxidative corrosion of the electrode. Where FE measurements of TMP catalysts were performed, selectivity toward formate approached 100%,^30,35,55 confirming that reported activities reflect the desired partial methanol oxidation.

2.3 Nickel phosphides: phase-dependent activity and mechanism

Nickel phosphides generally exhibit higher overpotentials for MOR compared to noble metals.⁵⁷ Surface studies indicate that under MOR conditions, Ni phosphides are partially oxidized to (oxy)hydroxide phases, yet OH⁻ adsorption remains relatively weak, making this step rate-limiting and contributing to the observed high overpotentials.³¹ However, low-crystallinity surfaces display OH* adsorption energies closer to the thermodynamic optimum, enhancing kinetics and lowering Tafel slopes, resulting in a strong influence of the crystallinity and phase on the catalytic activity.

In one study, Ni phosphide nanospheres with different crystallinities were synthesized from Ni-based MOFs using a solvothermal route with P₄.³¹ Among these, low-crystallinity Ni₂P exhibited the most favorable ΔG_OH* (highest of the sample set, indicating a stronger OH* binding to the surface), placing it near the top of the volcano plot and resulting in a mass activity of 427 mA mg⁻¹ at 1.74 V vs. RHE.³¹ Another investigation compared three distinct Ni–P phases.¹⁷ Mechanistic insights revealed that methanol adsorption served as the rate-determining step. Among the tested phases, Ni₁₂P₅ demonstrated the best performance, delivering a peak current density of 210 mA cm⁻² at 1.72 V vs. RHE and a mass activity of 4236 mA mg⁻¹ per 1 cm², corresponding to 299 mA mg⁻¹. Its superior activity was linked to the formation of a thin Ni(OH)₂/NiOOH layer, unlike the thicker oxide layers formed during OER. The selectivity for partially oxidized products was attributed to abundant adsorption sites that stabilize bidentate configurations of the intermediates and facilitate O–H bond activation.¹⁷

2.4 Cobalt phosphides: morphology-dependent performance

Cobalt-based phosphides also exhibit high MOR activity. For instance, hollow CoP octahedra significantly outperformed porous CoP nanosheets, achieving a mass activity of 206.2 mA mg⁻¹ at 0.4 V vs. SCE, approximately three times higher than nanosheets (69.7 mA mg⁻¹). Specific activity normalized by ECSA reached 1.53 mA cm⁻² compared to 0.87 mA cm⁻² for nanosheets, underscoring the intrinsic superiority of hollow architectures.³³

2.5 Ni–Co bimetallic phosphides: synergistic enhancement

Combining Ni and Co to form bimetallic structures is a widely adopted strategy.^16,30,32,35 For example, segregated Ni₂P-CoP heterojunctions grown on nickel foam (Ni₂P-CoP/NF) via electrodeposition and phosphidation exhibited strong interfacial electron interactions confirmed by XPS.³⁵ These heterostructures delivered higher MOR activity than monometallic counterparts, requiring only 1.15 V and 1.24 V vs. RHE to achieve 10 and 100 mA cm⁻², respectively, with FE for formate reaching 99.8%. Similarly, a CoP/Ni₂P nanoneedle array on Ni foam achieved 100 mA cm⁻² at 1.33 V vs. RHE with 98.3% FE.³⁰ Other studies synthesized hollow porous heterostructures via template-free solvothermal methods followed by annealing and phosphidation, enabling phase composition tuning. Optimized Ni₁Co₂P_x nanoparticles comprising multiple phases (NiCoP/CoP/CoP₂) delivered specific activity of 155 mA cm⁻² at 1.7 V vs. RHE and mass activity of 436.9 mA mg⁻¹.³² Another work investigated Ni–Co–P–O hybrids prepared by one-pot solvothermal synthesis, yielding an urchin-like architecture with nanoscaled phosphide/phosphate domains at a Co²⁺/Ni²⁺ ratio of 0.18.¹⁶ This composition achieved 1567 mA mg⁻¹ and 39.90 mA cm⁻² at 0.8 V vs. SCE. These results demonstrate that crystallographic phase, metal ratio, and phosphorus speciation (phosphide vs. phosphate) critically influence MOR activity.

2.6 Other bimetallic TMP structures based on Ni

As in catalyst design for OER, doping TMPs with secondary metals has been explored to improve MOR performance.^45,58,59 Enhancements were reported for Ni₂P doped with Mo and Cr.^53,54 For instance, Cr-doped Ni₂P nanorods (Ni_1.2Cr_0.8P) achieved 10 mA cm⁻² at only 1.14 V vs. RHE, although the overall mass activity remained modest compared to state-of-the-art catalysts (see Table 1). DFT calculations indicated that Cr incorporation induces local structural distortions that modify the electronic environment of Ni, lowering the Gibbs free energy for *HOOCH and favoring selective oxidation toward formate.⁵³ A similar mechanism was proposed for Mo doping, which enabled activity as high as 652.1 mA cm⁻² at 0.75 V vs. Ag/AgCl.⁵⁴ A recent report introduced a core–shell NiMoP_x@Ni₅P₄ heterostructure enabling MOR via a direct pathway without forming Ni(OH)₂/NiOOH, as confirmed by operando spectroscopy.⁵⁵ This catalyst achieved exceptional performance, delivering 500 mA cm⁻² at 1.49 V vs. RHE with nearly 100% FE for formate and excellent durability under industrial conditions.

2.7 Cu-based TMPs and other emerging systems

Studies on Cu-based TMPs for MOR are relatively scarce compared to Ni- and Co-based systems. One investigation examined a CuO/Cu₃P composite surface prepared via a two-furnace atmospheric pressure chemical vapor deposition method. The results indicated that while Cu₃P alone outperforms CuO as a catalyst for MOR, the combination of both phases significantly enhanced catalytic activity and stability.⁵² Furthermore, incorporating Co into Cu structures was shown to improve performance beyond that of monometallic CoP. For example, a Co–Cu-based TMP achieved a current density of 95.8 mA cm⁻² at 0.6 V vs. SCE and a mass activity of 287.4 mA mg⁻¹.⁵⁶ These findings highlight the potential of heterostructure engineering and elemental synergy in advancing MOR catalysis.

Other TMP systems, such as Fe-based and Mo-based phosphides, have been extensively studied for water-splitting reactions, particularly OER, but remain largely unexplored for MOR applications.

3. Noble metals phosphides and noble metals – TMP composites

Although this review primarily focuses on TMPs, several studies have explored noble metals phosphides or noble metals integrated with TMPs to enhance methanol oxidation performance. Incorporating phosphorus into noble metals markedly improves their catalytic activity for alcohol oxidation by modifying surface electronic properties and adsorption energetics.

3.1 Noble metal phosphides

One representative study synthesized three palladium phosphide catalysts (PdP₂, PdP₂–Pd, and Pd₅P₂–Pd) by phosphidating commercial Pd/C at progressively higher temperatures.¹⁴ This approach yielded catalysts with distinct compositions and morphologies, among which the PdP₂–Pd/C heterostructure exhibited the highest catalytic performance. Testing across a range of alcohol substrates demonstrated that, for MOR, PdP₂–Pd/C achieved a current density of 36.3 mA cm⁻², surpassing all other formulations, including commercial Pd/C. The enhanced activity was attributed to a bifunctional mechanism wherein PdP₂ promotes hydroxyl adsorption while accelerating CO intermediate oxidation on adjacent Pd sites.

A similar strategy was reported for platinum-based systems. PtP nanocrystals supported on Fe₂O₃@FeP/C core–shell nanocubes were synthesized through a hydrothermal process followed by low-temperature phosphidation and NaBH₄ reduction.⁶⁰ The resulting PtP nanoparticles (∼7 nm) significantly outperformed Pt–Fe₂O₃ in acidic media for MOR. This improvement stemmed from both the dilution of Pt sited by the phosphorus incorporation, and the irregular morphology of PtP nanostructures that enhances surface reactivity. Control experiments revealed negligible MOR activity for Fe₂O₃@FeP alone and no activity for Fe₂O₃, although the authors refer to the activity within the relevant range for MOR on Pt and did not check the higher potentials.⁶⁰

3.2 Noble metal–TMP composite catalysts

An alternative approach involves anchoring noble metal nanoparticles onto TMP substrates, which act both as supports and co-catalysts. Representative examples of such structures are shown in Fig. 3, and a summary of their performances is available in Table 2. For instance, Pt combined with Fe₂P demonstrated superior MOR activity compared to Pt alone, with formate formation confirmed, though not quantitatively measured.⁶¹ Similarly, Pt–FeP composites exhibited improved activity relative to standalone Pt.⁶² A notable example is Pt/4% CoP supported on carbon nanotubes (Pt/CoP/CNTs), which delivered an outstanding MOR activity of 1600 mA mg⁻¹_Pt – approximately six times higher than Pt/CNTs.¹⁵ Structural analyses indicated that CoP incorporation significantly reduced Pt particle size and increased the electrochemical surface area through strong Pt–CoP interactions, accounting for the observed enhancement in activity and durability. Mo-based TMPs have also been utilized to optimize noble metal performance. Pure MoP exhibits excessively strong methanol adsorption on its (101) facet, impeding intermediate desorption. Conversely, Pt(111) alone binds methanol too weakly, limiting charge transfer and subsequent oxidation steps. Decorating MoP with Pt tunes the methanol adsorption energy to an intermediate value. Specifically, a monolayer Pt(111)/MoP(101) interface achieved the desired methanol binding strength, balancing adsorption and desorption for optimal kinetics.⁶³ Similarly, the enhanced performance of Ni₂P–Pt composites is largely attributed to electronic interactions between the two components, where electron transfer from Ni₂P to Pt modifies the electronic structure of Pt active sites and improves their catalytic activity.⁶⁴ Cu-based TMPs have also been explored as promoters for noble metals. Composite Pt/C–Cu₃P composites were synthesized with varying Cu₃P loadings for MOR in acidic media. Increasing the Cu₃P content consistently improved both catalytic activity and long-term stability (Figure 4).¹²

Fig. 3 TEM images of composites of transition metal phosphides with noble metals. (a) Pt–Ni₂P/C-30%. Adapted from ref. 64 with permission from the Royal Society of Chemistry. (b) Pt-WP. Adapted from ref. 65 with permission from the Royal Society of Chemistry. TEM (c) and HRTEM (d) images of Pt–CoP/C-30% with Pt loading of 20 wt%. Adapted from ref. 66 with permission from the Royal Society of Chemistry.

Fig. 4 Electrocatalytic activity of composites of TMP-based catalysts with noble metals for the MOR. (a) CV curves of Pt–Ni₂P/C-30%, Pt–Ni/C, Pt–P/C, Pt/C-JM, and Pt/C–H catalysts in 0.5M H₂SO₄ + 1.0 M CH₃OH. Adapted from ref. 64 with permission from the Royal Society of Chemistry. (b) CV curves of Pt–CoP/C-30%, Pt–Co/C, Pt–P/C, Pt/C-JM, and Pt/C–H catalysts in 0.5M H₂SO₄ + 1.0 M CH₃OH. Adapted from ref. 66 with permission from the Royal Society of Chemistry. (c) CV curves of 8 nm and 17 nm Pd–Ni–P/C catalysts in 0.5M KOH + 1.0 M CH₃OH. Adapted with permission from ref. 29. Copyright 2020 American Chemical Society.

Table 2 Summary of reported noble-metal phosphides and noble-metal/TMP composite catalysts for the methanol oxidation reaction (MOR), including activity metrics, faradaic efficiency (FE), and experimental conditions. Mass activities are normalized to the noble metal content where applicable

Material	Products	FE	Activity (mA cm^−2)	Mass activity (mA mg^−1)	Conditions	Ref.
a Estimated from the graph in the paper by a data extraction software. b Calculated by the following equations using the pH according to the reported conditions: E (vs. RHE) = E (vs. SCE) + 0.241 V + 0.059 × pH; E(vs. RHE) = E(vs. Ag/AgCl) + 0.197 + 0.059 × pH; E(vs. RHE) = E(vs. Hg/HgO) + 0.927 V.
PdP2–Pd	Not reported	Not reported	36.3 mA cm^−2 at −0.228 V vs. SCE^a (or 0.841 V vs. RHE^b)	∼2500 mA mg^−1Pd ^a at −0.228 V vs. SCE^a (or 0.841 V vs. RHE^b)	1 M KOH + 1 M CH3OH	14
			2.44 mA cm^−2 at 0.61 V vs. SCE (or 0.91 V vs. RHE^b)	∼170 mA mg^−1Pd ^a at 0.61 V vs. SCE^a (or 0.91 V vs. RHE^b) (total mass loading 0.128 mg cm^−2. When the activity is normalized to total mass loading: 283 mA mg^−1 in alkaline 19 mA mg^−1 in acid)	0.1M HClO4 + 0.5M CH3OH
Pd–Ni–P metallic glass nanoparticles	Not reported	Not reported	Specific activity: 0.58 mA cm^−2Pd at 0.91 V vs. RHE^a	∼355 mA mg^−1Pd ^a at 0.91 V vs. RHE Total mass loading 0.3 mg cm^−2.	0.5 M KOH and 1.0 M CH3OH	29
PtP decorating Fe2O3@FeP cubes (8 hours)	Not reported	Not reported	Specific activity: 1.51 mA cm^−2 at 0.626 V vs. SCE (or 0.881 V vs. RHE^b) in acidic coonditions	∼3215 mA mg^−1pt at 0.626 V vs. SCE (or 0.881 V vs. RHE^b)	0.5 M H2SO4 + 1 M CH3OH	60
				831 mA mg^−1pt −0.202 V vs. SCE (or 0.867 V vs. RHE^b) Total catalyst loading not reported.	1 M KOH + 1 M CH3OH
Pt–Ni2P/C (30% wt	Not reported	Not reported	Specific activity 4.05 mA cm^−2Pt at 0.591 V vs. SCE (or 0.85 V vs. RHE^b)	1432 mA mg^−1Pt at 0.591 V vs. SCE (or 0.85 V vs. RHE^b) Total catalyst loading not reported.	0.5 M H2SO4 + 1 M CH3OH	64
Pt–NiCoPx within mesoporous carbon	Not reported	Not reported	Specific activity 1.60 mA cm^−2Pt at 0.636 V vs. SCE^a (or 0.895 V vs. RHE^b)	867 mA mg^−1Pt at 0.636 V vs. SCE^a (or 0.895 V vs. RHE^b) Total catalyst loading not reported.	0.5 M H2SO4 + 1 M CH3OH	67
Pt–Fe2P	Formate detected but not quantified	Not reported	Not reported	521 mA mg^−1Pt −0.296 V vs. SCE (or 0.773 V vs. RHE^b) Total catalyst loading not reported.	1 M KOH + 1 M CH3OH	61
Pt–FeP nanosheet	Not reported	Not reported	Specific activity 0.994 mA cm^−2 at 0.871 V vs. SCE^a (or 1.13 V vs. RHE^b)	Total catalyst loading not reported	0.5 M H2SO4 + 0.5 M CH3OH	62
Pt-MoP	Not reported	Not reported	Not reported	681 mA mg^−1Pt at 0.598 V vs. SCE (or 0.857 V vs. RHE^b) Total catalyst loading not reported	1.0 M H2SO4 + 1.0 M CH3OH	68
Pt-MoP	Not reported	Not reported	Specific activity 3.29 mA cm^−2Pt at 0.646 V vs. SCE^a (or 0.905 V vs. RHE^b)	1860.7 mA mg^−1Pt at 0.646 V vs. SCE^a (or 0.905 V vs. RHE^b) Total catalyst loading not reported	0.5 M H2SO4 + 1 M CH3OH	63
Pt-WP	Not reported	Not reported	Specific activity 131 mA cm^−2 at 0.77 V vs. SCE (or 1.029 V vs. RHE^b)	2217.6 mA mg^−1Pt at 0.77 V vs. SCE (or 1.029 V vs. RHE^b)	0.5 M H2SO4 + 1 M CH3OH	65
Pt/4%CoP/CNTs	Not reported	Not reported	Specific activity 2.15 mA cm^−2Pt at 0.713 V vs. Ag/AgCl ^a (or 0.91 V vs. RHE^b)	1600 mA mg^−1Pt at 0.713 V vs. Ag/AgCl ^a (or 0.91 V vs. RHE^b)	1 M HClO4 + 1 M CH3OH	15
Pt–CoP	Partially oxidized species were observed but not identified and quantified	Not reported	Specific activity 1.96 mA cm^−2 at 0.6 V vs. SCE (or 0.859 V vs. RHE^b)	1609 mA mg^−1Pt at 0.6 V vs. SCE (or 0.859 V vs. RHE^b)	0.5 M H2SO4 + 1 M CH3OH	66
Pt–Co2P	Not reported	Not reported	Specific activity 1.81 mA cm^−2Pt 236.8 mA mg^−1Pt at 0.655 V vs. SCE^a (or 0.914 V vs. RHE^b)	1236.8 mA mg^−1Pt at 0.655 V vs. SCE^a (or 0.914 V vs. RHE^b)	0.5 M H2SO4 + 1 M CH3OH	69
Pt/6%Ni2P/CNTs	Not reported	Not reported	Not reported	1400 mA mg^−1Pt at 0.73 V vs. Ag/AgCl^a (or 0.927 V vs. RHE^b)	1 M HClO4 + 1 M CH3OH	70
Pt–Cu3P	Not reported	Not reported	Specific activity 2.02 mA cm^−2Pt at 0.646 V vs. SCE (or 0.905 V vs. RHE^b)	578 mA mg^−1Pt at 0.646 V vs. SCE (or 0.905 V vs. RHE^b)	0.5 M H2SO4 + 1 M CH3OH	12

---

4. Analysis of performance trends

Fig. 5 compares the mass activity versus potential of the TMP-based catalysts included in Tables 1 and 2. The catalysts are grouped and color-coded based on composition and normalization criteria: non-noble TMPs (black squares), noble metal-based phosphides in acid (pink circles), noble metal-based phosphides in alkaline media normalized by metal content (blue triangles), and a specific case where an additional normalization by total mass was possible (green squares). Noble metal-based phosphides tested in acidic media consistently exhibit the highest mass activities. Notably, PtP decorating Fe₂O₃@FeP cubes (8 hours) reaches an exceptional activity of ∼3300 mA mg⁻¹ at ∼0.85 V vs. RHE, significantly outperforming all other catalysts. High-performing TMPs that excelled through combination with noble metals include Pt-WP, Pt-MoP, and Pt–CoP, each delivering mass activities in the range of 1500–2200 mA mg⁻¹_Pt. The activity is generally higher in acid than in alkaline environments. Earlier studies, especially those incorporating noble metals, were typically performed in acidic media. However, the field is gradually transitioning toward alkaline, less corrosive environments, and recent investigations, particularly those involving TMPs, are now conducted almost exclusively under alkaline conditions.

Fig. 5 Mass activities of various TMP catalysts for methanol oxidation, plotted as a function of potential (vs. RHE), based on the data in Tables 1 and 2. Catalysts are grouped by composition and normalization criteria: TMPs (black squares), noble-metal based phosphides in acid normalized by noble metal content (pink circles), noble-metal based phosphides in alkaline media normalized by noble metal content (blue triangles), and cases where additional normalization by total catalyst mass was possible (green squares). Green circles mark geometric current density data points from which the green square values were calculated.

Regarding mass activity, as shown in Table 2, the reported values are normalized to the mass of the noble metal only, based on the assumption that TMP alone does not exhibit significant MOR activity under these conditions. Nevertheless, if a synergistic mechanism exists, where TMP actively participates in formate production, normalizing with respect to the total catalyst mass would be more appropriate. One illustrative example is the calculation of mass activity based on data provided in the paper. For Pd–PdP₂, the authors report the maximum geometric current densities under acidic (36.3 mA cm⁻²) and alkaline (2.44 mA cm⁻²) conditions, along with a total catalyst loading of 0.128 mg cm⁻².¹⁴ Notably, this is one of the few cases in which the activity in alkaline media surpasses that in acid. When calculating the mass activity by dividing the current density by the total mass loading, the resulting values are 283 mA mg⁻¹ in alkaline (instead of 2500 mA mg⁻¹_Pd) and 19 mA mg⁻¹ in acid (instead of 170 mA mg⁻¹_Pd), comparable to the performance of TMP-based catalysts that do not contain noble metals.

In addition to improving catalytic performance, the incorporation of TMP components also contributed to higher resistance against poisoning and lowered the onset potential. However, due to the lack of comprehensive product analysis in most studies, it remains unclear how TMP addition influences the overall reaction mechanism. It is plausible that the improved tolerance toward poisoning is primarily associated with the formation of formate rather than a pathway involving CO intermediates.

Standalone (non-noble) TMPs generally exhibit lower mass activities, typically below 500 mA mg⁻¹, and require higher overpotentials (≥1.4 V) for effective methanol oxidation. Most notably, their selectivity is high: where faradaic efficiency or selectivity was reported, these catalysts consistently favored the formate pathway, with minimal formation of gaseous byproducts. Under alkaline conditions, clear trends highlight the influence of crystallinity, doping, and morphology. Low-crystallinity Ni₂P outperforms its high-crystallinity counterpart, and alternative stoichiometries such as Ni₁₂P₅ also show improved activity over Ni₂P. Incorporation of secondary metals further enhances performance, as demonstrated by urchin-like Ni–Co–P–O (1567 mA mg⁻¹) and NiMoPx@Ni₅P₄ (up to 312.5 mA mg⁻¹). Hollow or hierarchical architectures, such as 3D Ni₁Co₂P_x, contribute additional improvements. These findings underscore the role of multimetallic design, defect engineering, and nanostructuring in optimizing TMP catalysts for methanol oxidation. Interestingly, transition metals phosphides such as Fe and Mo – highly effective in combination with noble metals – remain underexplored as standalone TMP catalysts for this reaction. Although standalone TMPs appear less active, their mass activities, when normalized by total catalyst mass, may in fact approach those of noble metal–TMP composites, which are often reported per noble metal content only.

5. Challenges and future perspectives

TMP-based catalysts enable the selective oxidation of methanol to value-added products such as formate. The economic value of formate may justify operation at higher potentials, thereby suppressing the formation of undesirable gaseous byproducts like CO and CO₂. Moreover, the absence of noble metals allows operation in alkaline, less corrosive environments, more compatible with industrial settings, and significantly reduces catalyst cost, while also avoiding issues associated with noble metals such as supply instability, ethical production concerns, and geopolitical constraints. In addition, the avoidance of CO-generating pathways and noble metal surfaces reduces or even eliminates catalyst poisoning.

Despite significant advances in the development of TMPs for MOR, several key challenges remain. First, the structural complexity of TMPs, including their multiple phases, variable stoichiometries, and dynamic surface transformations, continues to hinder the establishment of clear structure–activity relationships. Precise control over crystallinity, phase purity, and heterointerfaces during synthesis is therefore essential for rational catalyst design.

Second, current evaluation methods lack consistency, necessitating the urgent adoption of standardized protocols that include measurements of faradaic efficiency, partial current density toward desired products (e.g., formate), and long-term durability under industrially relevant conditions. Fair comparison requires that catalysts be evaluated over similar potential ranges, for example, 1.5–1.6 V vs. RHE for non-noble transition metal phosphides (TMPs), mirroring the effort to standardize measurements at 0.8–1.1 V vs. RHE for noble metal-containing catalysts. This imperative for standardization was recently echoed in the general field of alcohol oxidation, where recent reviews concluded that the current heterogeneity in electrochemical evaluation practices significantly complicates reliable comparisons of electrocatalyst performance and thereafter, their applicability in industrial processes.^71,72 To enhance comparability, a critical first step involves mandating the uniform reporting of all activity data on the Reversible Hydrogen Electrode (RHE) scale, which necessitates detailed documentation of both the procedures utilized for potential conversion and the methodology for iR compensation, particularly given that the choice of correction percentage is often poorly documented and empirically derived. Beyond reporting metrics, the physical experimental setup must also adhere to best practices; specifically, researchers are encouraged to employ compartmentalized setups, such as an H-type cell separated by an appropriate ion exchange membrane, to effectively isolate the counter electrode and mitigate issues arising from the back-reaction of products or interference from dissolving ions, thus ensuring the accuracy of the measured catalytic response. Furthermore, achieving truly robust performance evaluation requires a substantial improvement in the consistency and quality of product analysis, including postulating a uniform definition for faradaic efficiency when multiple products are present, and providing clear data on catalyst mass loading. Ultimately, to move beyond the limitations of single-technique quantification, combining complementary techniques like NMR analysis is highly recommended to yield a truly comprehensive and critically robust product analysis.

Moreover, the mechanistic understanding of MOR on TMPs remains incomplete. Although electronic effects and bifunctional mechanisms have been proposed, operando spectroscopic studies and computational modeling are required to identify active sites and clarify reaction pathways. Future research should also emphasize cost reduction by incorporating earth-abundant metals (e.g., Fe, Mo, W) and minimizing noble metal content without compromising catalytic performance or stability. Finally, integrating MOR with hydrogen evolution or CO₂ reduction in coupled devices presents a promising direction for sustainable chemical production with high energy efficiency.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This research was supported by the Israel Science Foundation (Grant No. 650/21).

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