Electrochemically Activated Tetra-Metallic Phosphate (Mn–Fe–Co–Ni) Electrocatalyst for the Oxygen Evolution Reaction (OER) in Alkaline Media

Abstract

The development of highly active and stable oxygen evolution reaction (OER) catalysts based on earth-abundant and low-cost metals remains a major challenge in the field of electrochemical water splitting. Previous studies have shown that metallic and bimetallic phosphates exhibit promising catalytic activity but often suffer from limited long-term stability. In this work, tetrametallic phosphate CoNiFeMn­(PO₄) microparticles were synthesized using a simple and cost-effective precipitation method with thermal annealing. These microflakes exhibit an average length and width of 4.14 and 1.21 μm, respectively. An electrochemical activation of the catalyst was done by cyclic voltammetry between 1.05 and 1.50 V vs RHE at a scan rate of 10 mV/s in 0.1 M KOH. X-ray photoelectron spectroscopy (XPS) analysis after activation revealed a decrease in the atomic percentage of P from 7.3% to 2.7% and a concomitant increase in O from 74.8% to 78.0%, while the formation of cobalt species with coexisting Co²⁺ and Co³⁺ oxidation states, consistent with Co₂O₃, was observed at a lower scan rate (10 mV s^–1). The activated CoNiFeMn­(PO₄) microflakes demonstrated enhanced OER catalytic activity compared to the RuO₂ reference catalyst, achieving an overpotential of 429 mV at 10 mA cm^–2 in 0.1 M KOH and 360 mV at 10 mA cm^–2 in 1.0 M KOH, with a Tafel slope of 40 mV dec^–1, corresponding to the lowest overpotential among the investigated catalysts and 17 mV lower than that of RuO₂. In addition, the long-term stability, evaluated by chronopotentiometry on a catalyst-modified GC-RDE at 1600 rpm and at constant current density of 10 mA cm^–2 in 0.1 M KOH, showed that RuO₂ exhibited a continuous increase in overpotential, exceeding that of CoNiFeMn­(PO₄) by 235 mV at 9 h, reflecting poor stability due to catalyst dissolution and structural degradation. CoNiFeMn­(PO₄) maintained a stable overpotential of ∼400 mV over 24 h with only a 7.4 mV variation, confirming its superior durability. The synergistic effect among the four transition metals plays a key role in achieving enhanced OER performance relative to single-metal phosphate catalysts.

Introduction

The need for new sources of clean, renewable energy to replace traditional fossil fuels has captured the attention of the scientific community. − A promising strategy to mitigate current energy and environmental challenges is water electrolysis. , This process consists of two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which generate hydrogen as a clean fuel and oxygen as an environmentally benign byproduct. The HER is a kinetically favorable reaction because it involves a 2e^–/2H⁺ charge transfer, whereas the OER requires a 4e^–/4H⁺ charge transfer, making it the bottleneck of this electrochemical process. Moreover, the OER thermodynamically requires an additional overpotential above 1.23 V vs RHE to achieve an acceptable current density. An electrocatalyst for the OER is a material that enhances the reaction by reducing the activation energy and improving the reaction kinetics. The OER, which involves water oxidation and the generation of multiple intermediates, is inherently complex. Electrocatalysts are therefore crucial, as they promote efficient electron transfer and facilitate the formation and cleavage of chemical bonds. Ruthenium oxide (RuO₂) and iridium oxide (IrO₂), along with their derivatives, have traditionally been regarded as the most effective benchmark electrocatalysts for the OER. , Nevertheless, their scarcity and high cost severely limit their large-scale implementation, thereby driving the search for alternative, earth-abundant materials. −

In recent years, other benchmark electrocatalysts based on the layered double hydroxide (LDH) platform have emerged. Advanced NiFe-LDHsthrough strategies such as trace noble metal doping, heterojunction formation, and 3D architectural designcan achieve overpotentials at 10 mA cm^–2 (η₁₀) of ≲180–210 mV and long-term stability at ≥200 mA cm^–2, surpassing the classical NiFe-LDH benchmark. Nevertheless, despite their outstanding activity, NiFe-LDH electrocatalysts still face practical challenges. Self-stacking of nanosheets limits active surface exposure, and their intrinsically low electrical conductivity requires conductive supports or hybrid structures. , Uncontrolled surface reconstruction during operation complicates structure–activity relationships, and long-term stability under industrially relevant conditions remains modest Additionally, the synthesis of advanced heterostructured or multidoped NiFe-LDHs can involve complex, nonscalable procedures.

In parallel, recent studies on perovskite and double-perovskite oxides have further expanded the landscape of OER electrocatalysis. Phase engineering in cobalt-based perovskites has been shown to markedly improve intrinsic OER activity by modifying the crystal structure, cobalt valence state, and oxygen vacancy concentration, thereby optimizing the adsorption and evolution of oxygenated intermediates. , Similarly, rational compositional control in B-site-ordered double perovskites, achieved through codoping strategies, can increase the density of exposed reactive sites and tune the surface chemical environment, leading to significantly reduced overpotentials and excellent electrochemical stability. These examples highlight the continued importance of multication design and structural regulation as effective strategies for advancing OER electrocatalysts in alkaline media.

Transition metal phosphates have emerged as highly promising oxygen evolution reaction (OER) electrocatalysts due to their earth abundance, cost-effectiveness, and the active role of PO₄ ligands in modulating catalytic behavior. , Unlike simple oxides, the phosphate group actively participates in catalysis by facilitating the oxidation of transition-metal centers, modulating their local coordination geometry, and enhancing adsorption and stabilization of OER intermediates such as OH* and OOH*. Early studies reported cobalt phosphate as an efficient OER catalyst in neutral media, while nickel and iron phosphates demonstrated superior performance compared to their corresponding oxides due to the stabilization of active metal centers by the phosphate ligand. The synergistic interaction between transition metals within the phosphate framework further enhances OER performance. For instance, LiNi_0.75Fe_0.25PO₄ anchored on a reduced graphene oxide framework exhibited low overpotential (295 mV at 10 mA cm^–2), low Tafel slope (47 mV dec^–1), and excellent stability, highlighting the beneficial effect of Ni–Fe coupling and the olivine structure on electrocatalysis. Similarly, Fe-doped LiCoPO₄ showed enhanced activity at high potentials via synergistic interactions between Co²⁺ and Fe²⁺, with the rate-determining step involving *OOH formation. More recently, Mn-containing phosphates have attracted interest because Mn can stabilize high-valence states and modulate electronic structure, thereby improving OER activity. Taken together, these findings indicate that multimetallic phosphate architectures, especially tetrametallic phosphates incorporating Ni, Co, Fe, and Mn, are particularly promising OER electrocatalysts, as they combine the electronic modulation and structural stabilization provided by PO₄ ligands with the cooperative catalytic effects of multiple transition-metal centers. Based on this background, the present work proposes the development of a novel tetrametallic phosphate catalyst synthesized through a simple, practical, and environmentally friendly method that avoids the use of high temperature or pressure, while aiming to deliver high OER activity.

Results and Discussion

Morphological and Compositional Characterization of Phosphate Particles

The morphology of the synthesized tetrametallic phosphate particles was analyzed using scanning electron microscopy (SEM). Figure a shows tetrametallic phosphate microflakes uniformly distributed in a sheet-like arrangement. These microflakes exhibit the average length and width of 4.14 and 1.21 μm, respectively, as shown in Figure b.

1.

Scanning electron microscopy images of tetrametallic phosphates: (a) at x2000 and atomic percentage composition from EDS analysis, (b) at x5000 and particle size distribution, and (c) EDS elemental mapping images for O, C, P, Co, Ni, Mn, and Fe.

This type of structure has been associated with an increased number of active sites available for reaction, as well as enhanced electron transport during electrochemical processes. This morphology was initially obtained in previously synthesized cobalt phosphate microparticles (see Figure S1) and is considered characteristic of this compound. Cobalt phosphate tends to grow along specific crystallographic planes, resulting in plate- or flake-like structures, even in the absence of surfactants. ,

In the case of the synthesized tetrametallic phosphate, a high cobalt content was observed in the structure, with an atomic percentage of 9.9%, which may explain the preservation of the flake-like morphology typical of cobalt phosphate in the tetrametallic sample. Moreover, cobalt has been reported to promote higher crystallinity, structural order, and stability in mixed-metal systems. In contrast, the other transition metals incorporated (excluding nickel) tend to result in moderate crystallinity but help stabilize the crystal lattice and promote anisotropic nanoflake growth. ,

Multiple precursor ratios were screened (e.g., 1Mn:1Fe:1Ni:1Co, 1Mn:1Fe:1Ni:2Co, 0.5Mn:2Fe:1Ni:1Co), with 1Mn:1.5Fe:3.5Ni:4Co selected for optimal OER activity due to its Co/Ni-rich matrix featuring the highest Co content (4 equiv) (see Figure S7). This dominant Co proportion preserves the characteristic microflake morphology for enhanced ion/electron transport, enables Ni^III–OOH activation via Co charge transfer, and maximizes Fe/Mn synergistic contributions within the conductive architecture.

Finally, Figure c confirms the presence of all constituent elements in the sample with a homogeneous distribution. It is worth noting that the detected carbon originates from the conductive adhesive tape used during measurement.

The presence of phosphate groups was analyzed by FT-IR spectroscopy in the region from 2000 to 500 cm^–1 (see Table ). All the samples exhibited the characteristic asymmetric (ν_as(P–O) and symmetric (ν_s(P–O) stretching bands, as well as the asymmetric bending band δ _as (O–P–O), which are typical of phosphate groups in inorganic phosphate salts.

1. Vibrational Modes in Phosphate Groups.

	Wavenumber/cm–1
Vibrational modes	νas (P–O)	νs (P–O)	δas (O–P–O)
CoNiFeMn(PO 4 )	1033,971,936 (triple split band)	848	533
CoPO 4	1033,971,936 (triple split band)	836	533
NiPO 4	1037,998	883	515
MnPO 4	1089,1064,998,941 (split band)	858	563
FePO 4	1011	-	423

In the spectra of NiPO₄ and FePO₄ (Figure ), a broad and less defined ν_as(P–O) band was observed, which is associated with amorphous regions. In contrast, the remaining samples displayed more intense and well-defined bands in the same spectral region, indicating a higher degree of crystallinity. Band splitting was attributed to lattice symmetry constraints. The similarity observed between the FT-IR spectrum of CoNiFeMn­(PO₄) and that of CoPO₄ may be related to the high atomic percentage of cobalt in the sample, as well as to the similar morphology of the synthesized microparticles.

2.

FT-IR spectra of CoNiFeMn­(PO₄), CoPO₄, NiPO₄, MnPO₄, and FePO₄ oxygen evolution reaction catalyst materials.

Electrocatalytic Performance

The electrocatalytic activity of all the synthesized phosphates for OER, including the commercial RuO₂ benchmark, was evaluated using Linear Sweep Voltammetry in 0.1 M KOH with a glassy carbon rotating disk electrode (GC-RDE). As shown in Figure a, NiPO₄ exhibits an anodic peak at 1.53 V vs RHE associated with the Ni²⁺/Ni³⁺ oxidation process, whereas no oxidation peaks are observed in this potential region for the other phosphates. However, for CoNiFeMn­(PO₄), the current density gradually increases prior to the onset of the OER, which is primarily attributed to nonfaradaic processes such as the charging and discharging of the electrical double layer at the electrode–electrolyte interface. The polarization curve of CoNiFeMn­(PO₄) in the LSV exhibits a higher current density and a lower onset potential in the range of 1.0–1.8 V compared to the individual metal phosphates and RuO₂. In the case of CoNiFeMn­(PO₄), the current density increases sharply at approximately 1.52 V vs RHE, enabling the achievement of 10 mA cm^–2 at lower potential values. The overpotential values for all catalysts are presented in Figure a. CoNiFeMn­(PO₄) displays an overpotential of 429 mV, the lowest among all catalysts investigated and 17 mV lower than that of RuO₂.

3.

(a) Linear sweep voltammograms (LSVs) for the oxygen evolution reaction (OER) by a glassy carbon-rotating disk electrode (GC-RDE) modified by RuO₂, CoNiFeMnPO₄, FePO₄, CoPO₄, MnPO₄, and NiPO₄ in 0.1 M KOH at a scan rate of 10 mV/s. (b) Tafel plots of GC-RDE/RuO₂, CoNiFeMn­(PO₄), FePO₄, CoPO₄, MnPO₄, and NiPO₄. (c) Activation cyclic voltammetry of CoNiFeMn­(PO₄) catalyst and (d) overpotential in mV at 10 mV/cm² for RuO₂, CoNiFeMn­(PO₄), CoPO₄, NiPO₄, FePO₄, and MnPO₄.

Tafel plots were constructed to evaluate the OER kinetics, as shown in Figure b. The Tafel slope for CoNiFeMn­(PO₄) (40.5 mV dec^–1) was the smallest among the phosphate-based catalysts and the benchmark, suggesting that the OER requires a smaller increase in overpotential to reach high current densities, indicative of faster reaction kinetics and superior catalytic efficiency. A Tafel slope of ∼40 mV dec^–1 has been linked to a rate-determining step (RDS) involving the formation of *OOH species via the reaction between an OH^– anion and an adsorbed *O intermediate. For Ni–Fe catalysts showing a similar slope, this behavior has been attributed to the reversible discharge of OH^– followed by a rate-limiting electron transfer, yielding a physically adsorbed *OOH intermediate. , Likewise, Lyons et al. reported that the reduction of the Tafel slope to ∼40 mV dec^–1 for Fe-based catalysts at low overpotentials can be explained by assuming *OOH formation as the RDS. Based on these findings, for Fe-doped LiCoPO₄ (LCPF₄), the most probable rate-determining step in OER is the formation of *OOH species from adsorbed *O intermediates. Given the compositional similarity between LCFP₄ and our CoNiFeMn­(PO₄) catalyst, both systems are likely to follow a similar reaction pathway. Accordingly, part of the remarkable activity of CoNiFeMn­(PO₄) can be attributed to a favorable interaction between Co and Fe. This interaction leads to synergistic coupling effects that promote an optimal surface electronic structure of Co, enhanced charge-transfer capability, increased formation of highly oxidizing oxygen species (O₂ ^2–/O^–), and facilitated O₂ desorption.

The enhanced OER performance of the CoNiFeMn­(PO₄) arises from a cooperative multimetal effect rather than from the contribution of a single cation. Co and Ni form a conductive and structurally robust matrix that facilitates charge transfer and promotes the formation of Co₂O₃/Ni­(III)-OOH-like environments, while Fe provides highly active sites that optimize the adsorption of OER intermediates. − Mn contributes to structural stabilization and the fine-tuning of surface hydroxyl binding, thereby improving the balance between activity and durability. From an electronic perspective, this cooperative behavior is reflected in the modulation of intermediate adsorption energies through d-band/O 2p hybridization, which governs the interaction strength between the catalyst surface and oxygenated species. This is consistent with established OER descriptors based on the balance of ΔGO∗ and ΔGOH*, where an optimal adsorption/desorption equilibrium is essential for efficient oxygen evolution. Accordingly, the observed catalytic activity is best understood as the result of electronic modulation, structural reinforcement, and dynamic surface reconstruction within the multimetal framework. This synergistic effect is further supported by the limited OER activity observed for individual phosphate catalysts, highlighting the crucial role of metal–metal interactions. The OER catalytic activity follows the order CoNiFeMn­(PO₄) > RuO₂ > CoPO₄ > NiPO₄ > FePO₄ > MnPO₄, consistent with the corresponding overpotential and Tafel slope values.

Although CoNiFeMn­(PO₄) exhibits improved activity relative to RuO₂ under our testing conditions, its overpotential of ca. 429 mV at 10 mA cm^–2 is higher than that reported for some advanced NiFe-based (oxy)­hydroxide, interface-engineered, and multisite OER catalysts. However, such comparisons must be made cautiously, as OER metrics are strongly dependent on the testing conditions, particularly electrolyte concentration, catalyst loading, and electrode configuration. In this work, the measurements were carried out in 0.1 M KOH, whereas many of the literature benchmarks were obtained in 1 or 6 M KOH. Since higher KOH concentrations generally improve hydroxide availability and ionic conductivity and reduce ohmic losses, the present overpotential should be interpreted in the context of a milder alkaline medium. −

A cyclic voltammetry (CV) activation test was performed at 10 mV s^–1 for 10 cycles before any electrochemical measurement, as shown in Figure c. To verify compositional changes in the catalysts during activation, X-ray photoelectron spectroscopy (XPS) measurements were carried out before and after the activation process, as shown in Figure a.

4.

X-ray photoelectron spectroscopy of the CoNiFeMn­(PO₄) catalyst: (a) before and after cyclic voltammetry activation in modified GC-RDE, (b) P 2p, (c) Co 2p, and (d) O 1s binding energy regions with different scan rate activation (10–50 mV/s).

The main change observed in the XPS binding energy spectrum is a decrease in the intensity of the P 2s and P 2p peaks. Elemental analysis obtained from XPS indicates that the atomic percentage of phosphorus decreases from 7.3% to 2.7%, while that of oxygen increases from 74.8% to 78%. This behavior was also confirmed by EDS elemental analysis, performed using carbon paper to avoid damaging the modified surface, as shown in Figure S5.

Different potential scan rates (10–50 mV s^–1) were employed to evaluate surface modifications of the catalyst by XPS analysis. The XPS spectra corresponding to Co 2p, O 1s, and P 2p binding energy regions exhibited more pronounced variations as the scan rate decreased (Figure a–c). In the Co 2p binding energy region, the Co 2p₃/₂ satellite peak at 785.5 eV, characteristic of Co²⁺ species (Figure S3a), gradually decreased in intensity and eventually disappeared when the scan rate was reduced from 50 to 10 mV s^–1. This behavior is typical of compounds such as Co₂O₃, where Co²⁺ and Co³⁺ oxidation states coexist. A shift toward lower binding energies was also observed for the Co 2p₁/₂ and Co 2p₃/₂ peaks, from 781.89 and 797.80 eV to 779.91 and 794.90 eV, respectively, which is likewise consistent with the presence of Co₂O₃. The P 2p₃/₂ binding energy region showed a peak at 133.22 eV, indicating the presence of P in the form of (PO₄)^3–. The intensity of this signal decreased, confirming the reduction in the phosphorus content of the catalyst after the previously mentioned activation process. Additionally, the P 2p₃/₂ peak shows a slight shift toward lower binding energies (≈132.5 eV) as the scan rate decreases. Although this signal still corresponds to phosphorus in the +5 oxidation state, the observed binding energy is not characteristic of pure inorganic phosphate and typically indicates a modification of the phosphate species by its local environment. A similar behavior was reported in a recent study, where the P 2p₃/₂ signal of phosphate-loaded GR2-MOF (80) shifted to lower binding energies in the presence of a basic medium (NaOH). In the O 1s binding energy region, a broadening of the main peak is observed, resulting from the appearance of a shoulder around 529.52 eV as the scan rate decreases to 10 mV s^–1. This change is associated with the formation of the metal–O bond. On the other hand, no shift is observed in the peaks located at 530.76 and 532.51 eV, which correspond to the O 1s core levels of phosphate species and hydrated oxides, respectively, indicating the presence of hydroxyl (OH) groups, as shown in Figure S4c.

XPS analysis suggests partial phosphate leaching and the formation of oxide/oxyhydroxide species after electrochemical activation, indicating surface reconstruction under OER conditions. In this context, the phosphate phase primarily serves as a structural precursor and stabilizing scaffold. In this context, the phosphate phase primarily acts as a structural precursor and stabilizing scaffold, whereas the catalytically active surface is likely a reconstructed metal (oxy)­hydroxide layer formed in situ. The phosphate framework may still contribute indirectly by preserving structural integrity and modulating the local environment of the active sites; , however, the reconstructed near-surface oxyhydroxide layer is considered to be the true catalytically active phase under OER conditions, in agreement with previous studies on reconstructed transition-metal electrocatalysts. −

XPS confirms Co₂O₃ formation from high Co content (Figure c), enhancing OER conductivity alongside Ni­(III)-OOH activation via Co charge transfer. However, optimal activity requires synergistic FeOOH/MnOx contributions within this conductive Co/Ni matrix, as evidenced by superior performance compared to that of single compositions.

Post-OER SEM analysis was also performed (see Figure S8). Although the overall microflake morphology was largely preserved, a more detailed inspection revealed a decrease in particle dimensions, along with localized edge truncation and partial flake fragmentation. These changes are consistent with electrochemically driven surface reconstruction under anodic OER conditions, likely involving outer-layer oxidation, cation leaching and redistribution, and localized mechanical degradation induced by continuous O₂ evolution and bubble detachment. − Collectively, these processes may lead to progressive erosion of the flake boundaries and a reduction in the effective particle size, while retaining the characteristic flake-like framework.

Among the criteria used to evaluate OER electrocatalysts, the determination of specific activity (SA) is particularly important. The specific activity is calculated by normalizing the geometric current density at η = 0.35 V by the roughness factor (RF). The roughness factor is obtained by dividing the electrochemically active surface area (ECSA) by the geometric area of the rotating disk electrode (RDE, 0.196 cm²). The ECSA is calculated using the relation ECSA = C_dl/C_s, where the double-layer capacitance (C_dl) is determined from the slope of the linear fit obtained by plotting the capacitive current as a function of scan rate (Figure S6). The specific capacitance (C_s) represents the capacitance of a flat electrode surface of the catalytic material and is conventionally assumed to be 0.040 mF cm^–2 in alkaline media. ,

It is important to emphasize that although C_dl and ECSA provide complementary information, they should not be considered as decisive evaluation parameters for defining the performance of an OER catalyst. This is because the meaning of ECSA varies depending on the application, and although C_dl values do not significantly affect Faradaic processes, the method used for their calculation has a pronounced influence on the catalytic activity profile.

C_dl obtained for RuO₂ (1.94 mF cm^–2) was higher than that of the other catalysts studied, as shown in Figure a. This trend has been previously reported in the literature, where it is established that C_dl values for dimensionally stable anodes, such as IrO₂ and RuO₂, are higher than those of transition metal oxides. ,

5.

(a) C_dl (mF·cm^–2) values of the different catalysts (CoNiFeMnPO₄, CoPO₄, NiPO₄, FePO₄, and MnPO₄). Inset in (a): ECSA-normalized LSVs. (b) Specific activity for the OER of the catalysts at η = 0.35 V.

LSV curves tend to converge when normalized with respect to ECSA due to its direct proportionality to C_dl values (see Figure a, inset). Changes in the LSV curves were observed before (see Figure a) and after normalization with respect to ECSA. For all individual phosphates and RuO₂, the ECSA-normalized current density (J_ECSA) decreased within the applied potential window, with RuO₂ showing particularly low values, reaching below 1 mA cm^–2. In the case of CoNiFeMn­(PO₄), no significant change in the LSV curve was observed, with only a slight increase of approximately 10 mV in the overpotential at a current density of 10 mA cm^–2.

The specific activity (SA) values obtained at η = 0.35 V for all catalysts are shown in Figure b. Consistent with the ECSA-normalized LSVs discussed above, CoNiFeMn­(PO₄) exhibited the highest SA value (9.26 mA cm^–2), which is approximately 46 times higher than that of the benchmark RuO₂ catalyst. Similar results were reported by Zhu et al. for LiCo_0.8Fe_0.2O₂ when compared with IrO₂ in OER applications.

The SA was determined by normalizing the current density to the ECSA of the catalyst. Since ECSA is related to the number of surface atoms and, thus, to the actual number of active sites, SA can be regarded as a practical approximation of the turnover frequency (TOF) and a measure of the intrinsic reaction activity.

Although CoNiFeMn­(PO₄) exhibited a low ECSA, it delivered a high SA, confirming its excellent catalytic performance toward the OER, in agreement with its low overpotential at 10 mA·cm^–2 and low Tafel slope. In some catalysts, reduced ECSA values arise from particle agglomeration, larger particle size, or limited dispersion; however, this drawback can be offset by a higher intrinsic efficiency of the active sites, resulting in enhanced SA. These findings indicate that the superior catalytic performance primarily originates from high specific activity rather than a large ECSA.

To further assess the OER performance of CoNiFeMn­(PO₄) under more concentrated alkaline conditions, additional measurements were carried out in 1.0 M KOH (see Figure a). Upon increasing the KOH concentration from 0.1 to 1.0 M, the overpotential required to deliver 10 mA cm^–2 decreases from 429 mV to 360 mV, indicating a substantial improvement in apparent activity. In 1.0 M KOH, CoNiFeMn­(PO₄) reaches a high current density of 460 mA cm^–2 within the investigated potential window and exhibits an overpotential of 430 mV at 100 mA cm^–2, highlighting its robust performance at technologically relevant current densities. For comparison, RuO₂ also benefits from the increased electrolyte concentration, showing a reduced overpotential at 10 mA cm^–2 in 1.0 M KOH; however, its activity remains lower than that of CoNiFeMn­(PO₄), consistent with the trends observed in 0.1 M KOH. The improved OER performance at higher KOH concentrations can be attributed to enhanced ionic conductivity and a higher local OH^– availability at the electrode/electrolyte interface, which together mitigate ohmic and concentration polarization and facilitate the interfacial reaction kinetics. , These observations agree with previous reports demonstrating that electrolyte composition and concentration strongly affect OER kinetics by modifying the interfacial environment, including the concentration of active sites and the entropy/enthalpy of activation, thereby influencing the formation and stabilization of oxygen-evolution intermediates on transition-metal (oxy)­hydroxide surfaces.

6.

(a) Linear sweep voltammograms (LSVs) for the oxygen evolution reaction (OER) by a glassy carbon-rotating disk electrode (GC-RDE) modified by RuO₂ and CoNiFeMn­(PO₄), 1 M KOH at a scan rate of 10 mV/s. Inset: Linear sweep voltammograms of CoNiFeMn­(PO₄) in 0.1 M KOH and 1 M KOH. (b) Nyquist plots for RuO₂ and CoNiFeMn­(PO₄) at open-circuit voltage. (c, d) Equivalent circuits.

Electrochemical impedance spectroscopy (EIS) measurements were performed to gain further insight into the interfacial charge-transfer properties of RuO₂ and CoNiFeMn­(PO₄). The spectra were recorded in the frequency range from 100 kHz to 0.01 Hz using an AC perturbation of 10 mV under open-circuit conditions. The Nyquist plots obtained before and after electrochemical activation by cyclic voltammetry are shown in Figure b, together with the equivalent circuits employed to fit the data in Figure (c,d). For RuO₂ and the as-prepared CoNiFeMn­(PO₄), the impedance response consists of a single depressed semicircle, which can be described by a single time constant. In these cases, a simplified R_s–(CPE∥R_ct) circuit was used, where R_s represents the solution resistance, R_ct the charge-transfer resistance associated with the OER, and CPE represents a constant phase element accounting for the nonideal double-layer/pseudocapacitive behavior. , After electrochemical activation, the Nyquist plots of CoNiFeMn­(PO₄) evolve into two distinguishable semicircles, revealing the presence of two relaxation processes. Accordingly, the activated electrode was modeled with a R_s–(CPE₁∥R₁)–(CPE₂∥R_ct) equivalent circuit, in which the high-frequency CPE₂-R_ct branch is assigned to the charge-transfer process for OER at the reconstructed (oxy)­hydroxide surface, while the additional low-frequency CPE₁-R₁ branch is attributed to a slower process, likely related to ion/electron transport within the activated porous layer and/or the adsorption–desorption of surface redox intermediates. , This evolution in the impedance response supports the formation of a more complex and highly active surface upon activation.

Notably, the activated CoNiFeMn­(PO₄) electrode displays the smallest diameter of the high-frequency semicircle among all samples, indicating the most efficient charge-transfer process. The corresponding R_ct value for activated CoNiFeMn­(PO₄) (7.7 Ω) is significantly lower than that of the as-prepared CoNiFeMn­(PO₄) (13.13 Ω) and RuO₂ (28.58 Ω). A small R_ct favors fast electron transfer and thus improves electrocatalytic kinetics at the catalyst/electrolyte interface during the OER. These EIS results are consistent with the lower overpotential and smaller Tafel slope observed for CoNiFeMn­(PO₄) and further confirm that the activated CoNiFeMn­(PO₄) enables more efficient charge transfer than RuO₂ under OER conditions.

Catalyst stability is another key factor in evaluating OER performance. Chronopotentiometric measurements were conducted at a constant current density of 10 mA cm^–2 to compare the stability of the catalysts (see Figure a). In the case of RuO₂, the catalyst operated for only 9 h before exhibiting an exponential increase in potential vs RHE, reaching values close to 2 V. During this period, RuO₂ did not show stable behavior, but rather a continuous increase in overpotential, as shown in Figure b.

7.

Chronopotentiometry responses of the CoNiFeMn­(PO₄) (red) and RuO₂ (black) catalysts for 9 h on the GC-RDE (1600 rpm) at a constant current density of 10 mA·cm^–2 in 0.1 M KOH. Inset in (a): Chronopotentiometry response of CoNiFeMn­(PO₄) for 24 h under the same conditions. (b) Overpotential (mV) vs time (h) plot for the CoNiFeMn­(PO₄) and RuO₂ catalysts.

The poor stability of RuO₂ has been extensively investigated under acidic conditions and has been primarily attributed to the formation of oxygen vacancies and the leaching of active Ru species, which promote the collapse of the crystalline structure. − However, this behavior persists under neutral and alkaline conditions, where it is characterized by significant catalyst dissolution and consequent degradation of catalytic performance. , In contrast, CoNiFeMn­(PO₄) maintained stable behavior throughout the 24 h experiment, sustaining an overpotential of approximately 400 mV with only a minor variation of 7.4 mV. At 9 h, the overpotential of RuO₂ exceeded that of CoNiFeMn­(PO₄) by 186.1 mV, further demonstrating the superior stability of the catalyst compared to that of RuO₂.

The long-term electrochemical stability of CoNiFeMn­(PO₄) and RuO₂ under OER conditions was further assessed by stability cycling experiments. Cyclic voltammetry was performed at a scan rate of 10 mV s^–1 for 100 consecutive cycles in the OER potential region (see Figure ). For CoNiFeMn­(PO₄), the overpotential at 10 mA cm^–2 decreases slightly by 11 mV after 100 cycles, suggesting that the catalyst undergoes a mild activation process upon cycling and maintains a highly stable OER response. In contrast, RuO₂ exhibits an increase in the overpotential to 492 mV after 100 cycles, indicating a loss of catalytic activity and inferior stability under identical conditions. These results are in good agreement with the chronopotentiometric stability measurements and jointly demonstrate that CoNiFeMn­(PO₄) possesses superior long-term electrochemical stability toward the OER compared to RuO₂.

8.

Stability cycling experiments at a scan rate of 10 mV s^–1 for 100 cycles. (a) Cyclic voltammetry of CoNiFeMn­(PO₄). Inset in (a): LSV of CoNiFeMn­(PO₄) at the 1st cycle and 100th cycles. (b) Cyclic voltammetry of RuO₂. Inset in (b): LSV of RuO₂ at the 1st cycle and 100th cycle. (c) LSV of CoNiFeMn­(PO₄) (red) and RuO₂ (black) at the 100th cycle.

Beyond its low overpotential of 360 mV at 10 mA cm² in 1 M KOH, CoNiFeMn­(PO₄) presents several practical advantages for OER applications. The catalyst exhibits excellent long-term stability, which is especially important because in electrocatalysis, a durable material may be more valuable than a more active but rapidly degrading one. Additionally, the material is synthesized through a simple aqueous coprecipitation method under mild conditions, thereby avoiding high pressure, high temperatures, and complex postsynthetic processing. , The use of abundant metal acetate precursors and phosphate salts also supports a low-cost and accessible preparation strategy, particularly compared with noble-metal-based catalysts that require much more expensive precursors, such as Ru compounds. Moreover, the phosphate phase transforms under OER conditions into catalytically active metal oxyhydroxides, indicating a favorable in situ reconstruction process. Taken together, these characteristics make CoNiFeMn­(PO₄) attractive not only for its activity but also for its durability, scalability, and practical synthetic simplicity.

As summarized in Table , most state-of-the-art Ni/Co/Fe-based OER catalysts with similar transition-metal compositions achieve very low overpotentials at 10 mA cm^–2 only by relying on highly sophisticated architectures, such as ultrathin LDH nanosheets, multistep core–shell heterostructures, or ordered mesoporous oxides, often grown on specific 3D metallic supports and optimized for 1.0 M KOH. These delicate structures typically require complex synthetic protocols (e.g., hydrothermal growth, nanocasting, multistage conversions, or solvothermal and coordination-based routes), frequently involving controlled atmospheres, organic ligands, solvent exchange steps, or postsynthetic treatments, which increase both time and cost while making them difficult to reproduce and scale. In addition, some systems depend on electrodeposition under tightly controlled electrochemical conditions and specific substrates, further adding to the synthetic complexity. As a result, their reported stability is often restricted to moderate current densities and relatively short operation times. In contrast, the CoNiFeMnPO₄ catalyst developed in this work is obtained via a straightforward one-step coprecipitation route, yet it combines an overpotential of 360 mV at 10 mA cm^–2 in 1.0 M KOH with the ability to sustain very high current densities, around 100–450 mA cm^–2 with an overpotential of ∼430 mV at 100 mA cm^–2. Additionally, preliminary durability tests in 0.1 M KOH reveal an initial activation period, in which the overpotential slightly decreases during the first ∼9 h, followed by a stable regime, suggesting a beneficial surface reconstruction rather than continuous degradation. Taken together, these features indicate that CoNiFeMnPO₄ offers a more favorable balance between activity, robustness, and synthetic simplicity than many structurally more complex Ni/Co/Fe-based benchmark catalysts and thus appears particularly attractive for practical scale-up.

2. Recent NiFe/CoFe/NiFeCo-Based Catalysts for OER.

Catalyst	Electrolyte	η10 (mV)	Tafel (mV dec–1)	Reference
NiFeCo thin film (AEM)	0.1 M KOH	∼280–300	<60
NiFeCo oxyhydroxide (amorphous)	1 M KOH	234	∼45
NiFeCo-LDH@FeNi2S4/NF	1 M KOH	138	33
NiFe-LDH (0.1 M KOH)	0.1 M KOH	∼290	40–48
Supported NiFe-LDH (rGO, Ni foam)	1 M KOH	195–240	39
CoFe-LDH/oxides	1 M KOH	260–320	37.85
3D-a-NiFeOOH/N-CFP	1 M KOH	170	39
NiFe-BTC-GNPs MOF	1 M KOH	220	51
CoNiFeMnPO4	0.1 M KOH  1M KOH	420 360	40	This work

Conclusion

Using a simple precipitation synthesis method and thermal annealing, CoNiFeMn­(PO₄) microparticles were successfully prepared. These microflakes exhibit an average length and width of 4.14 and 1.21 μm, respectively. CoNiFeMn­(PO₄) displayed superior OER activity and stability compared to all evaluated catalysts, including individual phosphate catalysts and the OER RuO₂ benchmark. The catalyst, a phosphate containing four earth-abundant transition metals, exhibits an overpotential of 429 mV at a current density of 10 mA·cm^–2 in 0.1 M KOH and 360 mV at 10 mA·cm^–2 in 1.0 M KOH, with a Tafel slope of 40 mV·dec^–1. The enhanced catalytic activity of CoNiFeMn­(PO₄) relative to individual phosphates is attributed to synergistic coupling effects among the transition metals, as well as to the formation of oxyhydroxide species during electrochemical activation, which promotes favorable charge-transfer kinetics for the OER. XPS analysis enabled the identification of surface modifications occurring on the catalyst after electrochemical activation. The use of a lower scan rate (10 mV s^–1) was critical clearly evidence the formation of species in which Co²⁺ and Co³⁺ coexist (Co₂O₃), as well as to observe a decrease in the presence of P and a concomitant increase in O. The long-term stability of CoNiFeMn­(PO₄) during the OER in alkaline electrolyte (0.1 M KOH) over 24 h is associated with the presence of Mn in the system, as similar catalysts lacking Mn did not exhibit comparable stability over extended operation times.

Experimental Section

Chemical Reagents

The following reagents were used in the work: Potassium hydroxide (KOH, 90%), 5 wt % Nafion solution, Iron­(II) acetate (Fe­(CH₃COO)₂, ≥99.9%), Isopropyl alcohol (IPA, 99.9%), Ethyl alcohol (EtOH, ≥99.5%), and Ruthenium­(IV) oxide (RuO₂, 99.9%) from Sigma-Aldrich and Cobalt­(II) acetate tetrahydrate (Co­(CH₃COO)₂·4H₂O, 98%), Nickel­(II) acetate tetrahydrate (Ni­(CH₃COO)₂·4H₂O, ≥98%), Manganese­(II) acetate tetrahydrate (Mn­(CH₃COO)₂·4H₂O, ≥99%), Potassium Phosphate Monobasic (KH₂PO₄, ≥99.9%), and Potassium Phosphate Dibasic (K₂HPO₄, ≥98%) from Thermo Scientific. Solutions were prepared by using ultrapure deionized water (DI water, 18 MΩ-cm resistivity) supplied by an Ultrapure water purification system from Thermo Scientific MicroPure UV/UF-ST Barnstead MicroPure.

Synthesis of Individual Phosphate Particles

For the synthesis of the individual phosphate catalyst, 3.75 mmol of each metal acetate was dissolved in 10 mL of DI water. The mixture was sonicated 15 min until complete dissolution. One liter of a 0.1 M potassium phosphate buffer solution was prepared using a 2:3 ratio of monobasic (6.3 g) to dibasic (9.3 g) phosphate with a pH of 7. A simple spontaneous precipitation synthesis was performed under sonication: 10 mL of the metal acetate solution was placed in a vial, and 5 mL of the PO₄ buffer was then slowly added dropwise over the course of 10 min. The precipitate was then left to sonicate for another 20 min to prevent nanoparticle agglomeration. Once finished, the synthesized phosphate particles were separated by centrifugation. The product was washed with DI water and subsequently with ethanol. The particles collected in the centrifuge tube were dried in over 24 h at 60 °C. Finally, the dried product was crushed with an agate mortar and vialed. Individual particles of cobalt, nickel, iron, and manganese phosphate were synthesized.

Synthesis of Tetrametallic Phosphate Particles

For the synthesis of the tetrametallic phosphate catalyst, 0.9375 mmol of each metal acetate was dissolved in 10 mL of DI water. The tetrametallic precursor was made by mixing a volume ratio of 1Mn:1.5Fe:3.5Ni:4Co and sonicated for 15 min until complete dissolution. 1 L of a 0.1 M potassium phosphate buffer solution was prepared using a 2:3 ratio of monobasic (6.3 g) to dibasic (9.3 g) phosphate with a pH of 7. A simple spontaneous precipitation synthesis was performed under sonication, as described above. Finally, the tetrametallic phosphate particles were synthesized.

Ink Formulation/Catalyst Thin Film Fabrication

The catalyst ink formulation consisted of 5 mg of catalyst mixed with 600 μL of DI water, 200 μL of IPA, and 10 μL of Nafion in a glass vial. The mixture was sonicated for 1 h. A glassy carbon (GC) rotating disk electrode (RDE) (5 mm in diameter, 0.196 cm²) was polished using an aluminum oxide polishing solution (0.05 μm) on microcloth polishing disks (73 mm OD) employing a NANO 1000S Metallurgical/Metallographic Grinder/Polisher. The GC-RDE was cleaned ultrasonically in DI water until achieving a mirror-like finish. A 10 μL aliquot of the catalytic ink, corresponding to a 0.0617 mg catalyst loading, was deposited onto the GC-RDE and allowed to dry for 30 min, obtaining a catalyst thin film. For the film surface characterization, the catalyst films were transferred onto double-sided carbon tape after the electrochemical measurements by gently pressing the GC electrodes onto the tape.

Electrochemical Cell Setup/Electrochemical Studies

A large RDE glass cell for standard three-electrode voltammetry with a water jacket (250 mL) was employed in all electrochemical measurements. A GC-RDE was used as the Working Electrode (WE), with a 4 M KCl Ag/AgCl Reference Electrode (RE) and a Pt wire as the Counter Electrode (CE). A 0.1 M KOH solution (pH = 13) was used as the electrolyte. To ensure minimal O₂ presence, the electrolyte solution was bubbled with ultrapure N₂ or Ar for 20 min at 600 cc before each measurement. An activation Cyclic Voltammetry (CV) was done before electrochemical measurements for all catalysts, with a potential window of 0.2 to 0.55 V vs Ag/AgCl (4 M KCl) at a 10 mV/s scan rate for 10, 30, and 50 cycles until a stable overlapping response was obtained. The Linear Sweep Voltammetry (LSV) was performed with a scan rate of 10 mV/s between 0.2 and 0.8 V vs Ag/AgCl (4 M KCl) at 1600 rpm to facilitate mass transport and O₂ removal from the reaction interface. The OER performance was also examined in 1.0 M KOH. The measurements were conducted in a Teflon cell suitable for GC-RDE using an alkaline-resistant Hg/HgO reference electrode (filled with 4.24 M KOH solution) and a Pt wire as the counter electrode. The catalytic stability was evaluated by chronopotentiometric measurements carried out at a constant current density of 10 mA·cm^–2 for 24 h in 0.1 M KOH. In addition, the long-term electrochemical stability under OER conditions was further assessed by stability cycling experiments, in which cyclic voltammetry was performed at a scan rate of 10 mV s^–1 for 100 consecutive cycles within the OER potential region. iR compensation was applied during all electrochemical measurements.

The Calculation of ECSA

The electrochemically active surface area (ECSA) was calculated using the following equation (eq ):

$$\mathrm{ECSA}=\frac{{\mathrm{C}}_{\mathrm{dl}}}{{\mathrm{C}}_{\mathrm{s}}}$$ 1

The double-layer capacitance (C_dl) was obtained by measuring the capacitive current associated with the solid–liquid interface within a non-Faradaic potential region. To this end, cyclic voltammetry (CV) measurements were performed at various scan rates (2, 5, 10, 25, 50, 75, 100, 125, and 150 mV s^–1) over a potential window ranging from 0.76 to 1.01 V vs RHE. A specific capacitance value (C_s) of 0.040 mF cm^–2 was used for the ECSA calculation.

The C_dl value was determined according to the following equation (eq ):

$${\mathrm{C}}_{\mathrm{dl}}=\frac{\mathrm{\Delta }i/2}{\nu }$$ 2

where Δi represents the difference between the anodic and cathodic currents (ia – ic) measured at 0.885 V vs RHE and ν is the scan rate.

Electrochemical impedance spectroscopy (EIS) was carried out to investigate the interfacial charge transfer properties of RuO₂ and CoNiFeMnPO₄. Impedance spectra were recorded over the frequency range from 100 kHz to 0.01 Hz by using an AC perturbation amplitude of 10 mV under open-circuit conditions.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c02353.

• SEM images at x2000 of a) cobalt phosphate and b) nickel phosphate; atomic percentages from EDS of c) cobalt phosphate and d) nickel phosphate (see Figure S1); SEM images at x2000 of e) iron phosphate and f) manganese phosphate; atomic percentage from EDS of g) iron phosphate and h) manganese phosphate (see Figure S2); XPS spectra of CoFeNiMn-PO₄: a) Co 2p, b) Ni 2p, and c) Mn 2p (see Figure S3); XPS spectra of CoFeNiMn-PO₄: a) Fe 2p, b) P 2p, and c) O 1s (see Figure S4); SEM images and EDS analyses of CoFeNiMn-PO₄ ink deposited on carbon paper; panels (a–c) correspond to the sample before cyclic voltammetry (CV), while panels (d–f) show the sample after CV (see Figure S5); cyclic voltammograms measured at different scan rates (2, 5, 10, 25, 50, 75, 100, 125, and 150 mV/s) of a) CoPO₄, b) NiPO₄, c) MnPO₄, d) FePO₄, e) RuO₂, and f) CoNiFeMn­(PO₄) (see Figure S6); overpotential in mV at 10 mV/cm for tetrametallic phosphates (see Figure S7) and post-OER SEM analysis CoFeNiMn­(PO₄) (see Figura S8) (PDF)

Y.C.C.: Writingoriginal draft, methodology, formal analysis, data curation, investigation. J.L.L. and A.P.-D.: Investigation, formal analysis, data curation. C.R.C.: Writingreview and editing, validation.

The authors declare no competing financial interest.