Crystal Facet Regulation and Ru Incorporation of Co₃O₄ for Acidic Oxygen Evolution Reaction Electrocatalysis

Abstract

Acidic oxygen evolution reaction (OER) has long been the bottleneck of proton exchange membrane water electrolysis. Ru- and Ir-based oxides are currently state-of-the-art electrocatalysts for acidic OER, but their high cost limits their widespread application. Co₃O₄ is a promising alternative, yet the performance requires further improvement. Crystal facet engineering can effectively regulate the kinetics of surface electrochemistry and thus enhance the OER performance. However, the facet-dependent OER activity and corrosion behavior of Co₃O₄ have not been thoroughly studied. In this study, we systematically investigated the OER performance and crystal facet dependency of Co₃O₄. The results demonstrate that Co₃O₄ with mixed {111} and {110} facets exhibits better OER activity and stability than Co₃O₄ with {111} or {100} facets. The surface Co³⁺ species are responsible for the high OER activity, but its transformation to CoO₂ is also the root cause of the dissolution, leading to an activity–stability trade-off effect. The possible approach to addressing this issue would be to increase the Co³⁺ contents by nanostructure engineering. To further improve the performance, Ru is introduced to the best-performing Co₃O₄. The resulting Co₃O₄/RuO₂ heterostructure exhibits an overpotential of 257 mV at 10 mA cm^–2 and can stably catalyze the OER for 100 h.

1. Introduction

Proton exchange membrane (PEM) water electrolysis is recognized a sustainable avenue for hydrogen production.¹ Within this process, the oxygen evolution reaction (OER) is the kinetically slower yet crucial step that significantly influences the overall energy efficiency. The corrosive nature of the acidic environment of PEM electrolyzers presents additional challenges in catalyst selection and design.^2,3 Currently noble-metal-based compounds such as IrO₂, RuO₂, and their derivates are state-of-the-art OER catalysts, which account for approximately 25% of the total cost of a PEM electrolyzer.^4,5 Consequently, there is a pressing need to reduce or even completely phase out noble metal usage while preserving high OER activity.^6,7 Co₃O₄ stands out as a promising alternative due to its affordability, relative stability, and considerable catalytic activity.⁸⁻¹⁰

Co₃O₄ is characterized by its spinel structure (AB₂O₄), which features adjustable mixed-valence states of Co³⁺ and Co²⁺.¹¹⁻¹³ The dynamic between Co²⁺ and Co³⁺ within this framework as well as their spatial arrangement on various crystal facets plays a crucial role in its catalytic efficacy. This opens up new avenues for the development of high-performance cobalt-based catalysts. Koel et al. demonstrated that Co³⁺ on (111) facets exhibits superior OER activity compared to (100) facets in alkaline electrolytes.¹⁴ Ding et al. also pointed out that the OER performance of Co₃O₄ {112} and {110} facets is much better than that of {100}.¹⁵ These findings underscore that the ratio and distribution of Co²⁺ and Co³⁺ across crystal facets are directly correlated with the OER activity. Despite these progresses, there is still limited research on the OER performance of different Co₃O₄ crystal facets in acidic environments. It should be noted that the facets with high reactivity may also promote corrosion in acidic media, leading to inferior stability. However, the facet-dependent OER activity and Co₃O₄ corrosion have not been thoroughly studied.

To further enhance the performance of cobalt-based catalysts, researchers have explored various modification strategies, including defect engineering,¹⁶ morphology control,¹⁷ and elemental doping.⁹ Among these, doping with transition metals such as ruthenium (Ru) has proven to be an effective approach to enhance the OER performance.^18,19 Ru doping not only optimizes the electronic structure and increases the number of active sites but also improves charge transfer efficiency, enhances atom utilization, and boosts the OER performance of the catalyst. Our previous study demonstrated that through cation exchange, Ru³⁺ can replace the octahedral Co³⁺ sites in Co₃O₄, significantly enhancing both the OER activity and stability.²⁰

Based on these findings, we synthesized spinel Co₃O₄ catalysts with different exposed crystal facets to investigate the impact of cobalt geometric sites on the electrocatalytic activity and stability. The results demonstrated that h-Co₃O₄, with mixed {111} and {110} crystal facets, exhibited a higher OER activity due to its higher Co³⁺ content. However, the abundance of Co³⁺ also led to deactivation of the catalyst over time due to the generation of soluble CoO₂. To address this issue, we further incorporated Ru into the best-performing Co₃O₄ to modify the electronic structure and thus the electrochemical behavior. The resulting Co₃O₄/RuO₂ heterostructure exhibits an overpotential of 257 mV at 10 mA cm^–2 and can stably catalyze the OER for 100 h, representing an order of magnitude improvement in stability compared to the unmodified catalyst.

2. Materials and Methods

2.1. Materials

Cobalt nitrate [Co(NO₃)₂·6H₂O], cobalt acetate [Co(CH₃COO)₂·4H₂O], perchloric acid (HClO₄), sulfuric acid (H₂SO₄), ethyl alcohol (C₂H₅OH), isopropyl alcohol (CH₃CHOHCH₃), acetone (CH₃OCH₃), and nitric acid (HNO₃) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ruthenium chloride hydrate (RuCl₃·xH₂O, 37% Ru basis) and commercial ruthenium dioxide (RuO₂) were purchased from Shanghai Aladdin Biochemical Co., Ltd. Carbon paper (thickness: 2 mm) and 5 wt % Nafion were purchased from Suzhou Sinero Technology Co. All chemicals (analytical grade) were used without further purification.

2.2. Preparation of Co₃O₄ with Different Crystal Facets

2.2.1. Synthesis of Co₃O₄ Nanocubes (c-Co₃O₄)

80 mmol of Co(NO₃)₂·6H₂O and 20 mmol of NaOH were dissolved in 80 mL of deionized water. The above mixture was then transferred to a Teflon stainless steel hydrothermal reactor, which was then sealed and placed in a 180 °C blast oven for 5 h. After the reaction stops, the product was centrifuged, washed twice with deionized water and once with ethanol, and then dried overnight in a 60 °C oven. Finally, the dried black powder was calcined for 4 h in a tube furnace or Muffle furnace under an air atmosphere at a calcination temperature of 500 °C and a heating rate of 5 °C min^–1 to obtain c-Co₃O₄.

2.2.2. Synthesis of Co₃O₄ Octahedra (o-Co₃O₄)

The process is similar to that of c-Co₃O₄, except that the concentrations of Co(NO₃)₂·6H₂O and NaOH were adjusted to 400 and 20 mmol, respectively.

2.2.3. Synthesis of Co₃O₄ Hexagonal Nanoplates (h-Co₃O₄)

0.25g CH₃COO)₂Co·4H₂O was dissolved in 18 mL of glycol and then transferred into a 25 mL Teflon stainless steel hydrothermal reactor. After sealing, it was placed in a blast oven at 200 °C for 12 h. After the reaction, the product was centrifuged, washed using deionized water, and ethanol, and then dried overnight in a 60 °C oven. Finally, the dried pink powder was calcined for 3 h in a tube furnace under an air atmosphere; the calcination temperature was 350 °C, and the heating rate was 5 °C min^–1.

2.3. Preparation of Co₃O₄/RuO₂

The Co₃O₄/RuO₂ was synthesized by cation exchange followed by thermal calcination. Specifically, the h-Co₃O₄ grown on a carbon cloth substrate was soaked in a RuCl₃ aqueous solution under 60 °C, and then rinsed with deionized water, dried, and finally calcined at 350 °C for 3 h under an air atmosphere. The heating rate was 5 °C min^–1. The contents of Ru and Co were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES) at 0.55 and 2.64 mg cm^–2, respectively.

2.4. Preparation of Electrodes

2 mg of catalyst (Co₃O₄ or RuO₂), 100 μL of isopropyl alcohol, and 10 μL of 5 wt % Nafion were dispersed in 100 μL of deionized water. After the catalyst was completely dispersed into a stable suspension by ultrasound for 40 min, it was evenly coated on the pretreated carbon paper and then dried using an infrared lamp.

2.5. Characterization

X-ray diffraction (XRD) patterns of Cu Kα radiation (λ = 1.5405 Å) in the 2 theta range of 10–70 were recorded by a Rigaku Ultima IV XRD diffractometer. Scanning electron microscopy (SEM) images were collected by using a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images were recorded on an FEI Tecnai F30 at an acceleration voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo Scientific ESCALAB 250Xi spectroscope with Al Kα radiation. Raman spectra were obtained using a Renishaw inVia confocal Raman microscope under the excitation of a 4.0 mW, 532 nm laser. The in situ Raman tests were conducted in 0.5 M H₂SO₄ using AgCl/Ag as the reference electrode, wherein the potentials were not corrected for iR_s compensation. The mass loading of materials and the rate of metal dissolution were measured by an Agilent 7700 ICP-MS spectrometer.

2.6. Electrochemical Test

OER electrocatalysis was performed at room temperature using a standard three-electrode system in 0.5 M H₂SO₄ using a CHI 660E workstation. The catalyst was used as the working electrode, a Pt sheet was used as the counter electrode, and Hg/Hg₂SO₄ was used as the reference electrode. Linear sweep voltammetry (LSV) curves were recorded at a scanning rate of 5 mV s^–1. All the potentials reported in this work were corrected by ohmic loss unless otherwise specified except for the in situ Raman spectra. R_s was obtained by electrochemical impedance spectroscopy (EIS) measurement. Chronopotentiometry measurements were conducted at a benchmark current density of 10 mA cm^–2. The metal dissolution in the electrolyte was monitored by ICP-MS.

3. Results and Discussion

We synthesized spinel Co₃O₄ nanocrystals with distinct morphologies, specifically nanocubes (c-Co₃O₄), octahedra (o-Co₃O₄), and hexagonal nanoplates (h-Co₃O₄), using reported methods.²¹ The morphology of the prepared samples was investigated by SEM and TEM. The result (Figure 1a–f) reveals that c-Co₃O₄ exhibits a well-defined cubic structure with six {100} facets, and o-Co₃O₄ consists of octahedra with eight {111} facets, while for the h-Co₃O₄, although the SEM image indicates its well-structured plate-like morphology, TEM observation clearly suggests that the nanoplates are in fact an assembly of many nanoparticles (Figure S1). The main exposed facets of these particles were further determined to be {111} and {220} facets. The structures of the three Co₃O₄ samples were further characterized using XRD. The result (Figure 1g) reveals that the diffraction patterns of all three samples match well with pure spinel Co₃O₄, and no impurities are observed. XPS spectra of Co 2p_3/2 can be fitted into Co³⁺ and Co²⁺ components, with binding energies at 779.4 and 780.8 eV, respectively (Figure 1h). Further analysis reveals that h-Co₃O₄ has the highest surface Co³⁺/Co²⁺ ratio, followed by o-Co₃O₄ and then o-Co₃O₄ (Table S1). This could be attributed to a surface atomic arrangement that ensures a high Co³⁺ density (Figure S2). We further conducted Raman analysis (Figure 1i). The vibrational modes at 198, 486, 523, and 693 cm^–1 correspond to the F_2g, E_g, and A_1g modes of spinel Co₃O₄, respectively, whereas the modes at 198 and 693 cm^–1 correspond to the stretching vibrations of Co²⁺-O in the tetrahedral CoO₄ units and Co³⁺-O in the octahedral CoO₆ units, respectively.^22,23 The intensity ratio of these peaks to the E_g peak reflects the relative surface content of Co²⁺ and Co³⁺ species. The result indicates that the Co³⁺/Co²⁺ ratio follows the order h-Co₃O₄ > o-Co₃O₄ > c-Co₃O₄ (Table S2), consistent with the XPS result. It has been reported that Co³⁺ is the primary active site for the OER,^12,13,24 suggesting that h-Co₃O₄ is likely to exhibit higher OER activity.

Figure 1.

Structural characterization of Co₃O₄ catalysts. (a–c) SEM and (d–f) HRTEM images (insets of d–f are the corresponding FFT images), (g) XRD patterns, (h) Co 2p_3/2 XPS spectra, and (i) Raman spectra of c-Co₃O₄, o-Co₃O₄, and h-Co₃O₄.

The OER performance of Co₃O₄ with different exposed crystal facets was then studied in 0.5 M H₂SO₄. LSV curves indicate that h-Co₃O₄ exhibits the best OER activity among the three samples, with an overpotential of 500 mV at 10 mA cm^–2, followed by o-Co₃O₄ (557 mV) and c-Co₃O₄ (565 mV). The Tafel plots (Figure 2b) further verify the distinct OER activity of these three catalysts, with h-Co₃O₄ exhibiting the lowest Tafel slope of 82.52 mV dec^–1. This result confirms that a higher surface Co³⁺ ratio would result in higher apparent OER activity. The stability of these catalysts was further evaluated using a chronoamperometric test (Figure 2f). Interestingly, the result reveals that h-Co₃O₄ also has a higher stability at a current density of 10 mA cm^–2. However, it is also noted that the potential of h-Co₃O₄ increased slowly but continuously in the initial phase before stabilizing, while the potentials of o-Co₃O₄ and c-Co₃O₄ stabilized quickly after a quick rise. The morphology of the three catalysts after the OER remains largely unchanged (Figure S3). The sharp decrease in catalytic activity without significant morphological changes may be attributed to the deactivation of active sites at the catalytic interface during repeated electrochemical cycles. This deactivation could result from the accumulation of adsorbed molecules, charge rearrangement, or local variations around the active centers.²⁵ According to the previous discussion, the content of Co³⁺ in h-Co₃O₄ is the highest; however, ICP-MS analysis of cobalt dissolution during the OER process reveals that h-Co₃O₄ also exhibits the fastest cobalt dissolution rate (Figure 2d), while o-Co₃O₄ and c-Co₃O₄ show relatively lower levels. XPS analysis further shows a significant decrease in the Co³⁺/Co²⁺ ratio for h-Co₃O₄ (Figure S4). This indicates that Co³⁺ at octahedral sites is more active for the OER, but a high Co³⁺ content also accelerates cobalt dissolution during the OER process (Figure 2e). This was further verified by the evolution of the Co³⁺/Co²⁺ ratio of the three catalysts based on in situ Raman analysis (Figure 2f). h-Co₃O₄ shows a constant decrease in the Co³⁺ ratio during the OER. Note that the ratio of Co³⁺/Co²⁺ determined by Raman is slightly higher than that determined by XPS, which might be because the former is bulk sensitive while the latter is mostly surface sensitive. The in situ Raman spectra collected during the OER further reveal that as the potential increases, a vibrational peak associated with CoO₂ appears at around 578 cm^–1.²⁶ For h-Co₃O₄, this peak of CoO₂ emerged at a relatively low potential of 1.4 V (Figure 2g) due to the high Co³⁺ content, and it disappeared at higher potentials as CoO₂ readily dissolves in acidic conditions. In contrast, o-Co₃O₄ exhibited the CoO₂ peak at a higher potential of 1.6 V, likely due to its lower surface Co³⁺ content (Figure 2h). Although no distinct peak was observed for c-Co₃O₄ at 1.6 V, the absence of the adjacent 620 cm^–1 peak indicates the formation of CoO₂ (Figure 2i). The above results indicate that the Co³⁺ species play important roles in determining both the OER activity and the stability. On the one hand, the presence of abundant Co³⁺ species promotes the generation of the highly active CoO₂ phase, contributing to the high OER activity. On the other hand, the generated CoO₂ would easily dissolve in acidic electrolytes, leading to the loss of active materials and consequently performance decay. These two factors would result in the activity–stability trade-off effect of the Co₃O₄ catalyst. However, it is surprising that although h-Co₃O₄ has the fastest Co dissolution rate, it shows the best durability (Figure 2c). This might be due to the high surface Co³⁺ density that could sustain a relatively long dissolution, although further investigation is required.

Figure 2.

Electrochemical characterization of the three Co₃O₄ catalysts. (a) LSV curves, (b) Tafel plots, and (c) chronopotentiometry curves. (d) Cobalt dissolution during the stability test as monitored using ICP-MS. (e) Changes in the Co³⁺/Co²⁺ ratio as measured by XPS. (f) Changes in Co³⁺/Co²⁺ as measured by in situ Raman. (g, h) In situ Raman spectra under applied potentials of the h-Co₃O₄, c-Co₃O₄, and o-Co₃O₄.

The above result suggests that h-Co₃O₄ has better OER activity and stability against o-Co₃O₄ and c-Co₃O₄. We then employed h-Co₃O₄ as the support to incorporate Ru atoms to further enhance the OER performance (see the detailed synthesis procedure in the Supporting Information). The resulting catalyst inherits the hexagonal plate-like morphology (Figure 3a). The HAADF image and the corresponding elemental mapping further confirm the successful incorporation of Ru (Figure 4b), and the Ru content was determined to be only 2.42 wt %. The HRTEM image reveals an interface between the (220) facet of Co₃O₄ and the (110) facet of RuO₂ (Figure 4c), indicating the formation of a Co₃O₄/RuO₂ heterostructure. Indeed, the XRD patterns further confirm the presence of both Co₃O₄ and RuO₂ (Figure 4d). Note that the strong diffraction peak at around 26.5° is a characteristic peak of the carbon paper substrate (Figure S5). To identify the possible electronic interactions between the Co and Ru species, we conducted the XPS analysis. Compared with h-Co₃O₄, the Co³⁺/Co²⁺ ratio in Co₃O₄/RuO₂ decreases, indicating that some Ru atoms occupy Co³⁺ sites in h-Co₃O₄ (Figure 3e). The Ru 3d spectra shows that the binding energy of Ru 3p in Co₃O₄/RuO₂ is higher than that in RuO₂ but lower than in RuCl₃ (Figure 3f), suggesting that the oxidation state of Ru in the synthesized catalyst lies between +3 and +4.²⁷ This confirms that Ru doping in h-Co₃O₄ can modulate the electronic distribution and coordination environment.

Figure 3.

Structural characterization of Co₃O₄/RuO₂. (a) TEM image. (b) HAADF image and the corresponding elemental maps. (c) HRTEM image. (d) XRD patterns. (e) Co 2p_3/2 and (f) Ru 3d_3/2 XPS spectra.

Figure 4.

Acidic OER performance of h-Co₃O₄/RuO₂. (a) LSV curves. (b) Tafel plots. (c) EIS spectra. (d) Chronopotentiometry curve. (e) Calculated specific C_dl plots. (f) Metal dissolution during the stability test.

Subsequently, the OER performance of h-Co₃O₄/RuO₂ was measured in 0.5 M H₂SO₄, revealing a significant enhancement in the activity. The catalyst requires an overpotential of only 257 mV to drive a current density of 10 mA cm^–2 (Figure 4a), which is 243 mV lower than the pristine h-Co₃O₄ and superior to commercial RuO₂ (300 mV). Additionally, the OER kinetics are notably accelerated, as evidenced by a decrease in the Tafel slope from 82.52 to 69.82 mV dec^–1 (Figure 4b). EIS measurement demonstrates that h-Co₃O₄/RuO₂ has the lowest charge transfer resistance (Figure 4c), further corroborating its excellent electrocatalytic activity and efficient charge transfer properties. h-Co₃O₄/RuO₂ also exhibits excellent stability that can stably drive the current density of 10 mA cm^–2 for 100 h (Figure 4d). The potential increase is negligible compared to the initial potential, marking a significant stability improvement over that of pristine h-Co₃O₄ by an order of magnitude. With the incorporation of Ru, h-Co₃O₄/RuO₂ exhibits a significantly higher electrochemically active surface area (ECSA, Figure S6), soaring from 42.7 to 526.5 cm² (Figures 4e). At the same time, this remarkable increase not only underscores the success of the Ru doping strategy but also highlights its efficacy in increasing the number of active sites, thereby enhancing the catalytic efficiency. In strong acidic electrolytes, cobalt-based oxides exhibit a decline in activity during prolonged OER, with significant dissolution being a major cause of deactivation,¹⁰ as confirmed by our above characterizations. During the testing of h-Co₃O₄/RuO₂, it was observed that the incorporation of Ru not only significantly suppresses the dissolution of cobalt but also maintains the Ru dissolution rate at a low level (Figure 4f). This could be attributed to the interactions between Ru and Co that suppress the overoxidation of Ru/Co species. In addition, it should be noted that the required potential at 10 mA cm^–2 for h-Co₃O₄/RuO₂ (1.73 V vs RHE) is significantly lower than that for h-Co₃O₄ (1.48 V), which also contributes to the slower Co dissolution rate. We also synthesized o-Co₃O₄/RuO₂ and c-Co₃O₄/RuO₂ following the same recipe and measured the electrocatalytic OER performance (Figure S7). The result indicates that RuO₂ tends to accumulate on the surface rather than incorporate into the lattice, likely due to the dense surface of the Co₃O₄ octahedra and cubes. Although the as-obtained o-Co₃O₄/RuO₂ and c-Co₃O₄/RuO₂ both show improved OER activity and stability, the enhancement is much less significant compared to h-Co₃O₄/RuO₂. This result suggests that the incorporation of Ru³⁺ into the h-Co₃O₄ lattice contributes significantly to the high OER performance.

Post XRD analysis reveals that although the diffraction peaks of h-Co₃O₄ are no longer visible (Figure S8), the TEM investigation shows that the morphology remains almost unchanged (Figure S9). XPS analysis further indicates that the valence state of Ru exhibits negligible changes, suggesting the stability of RuO₂ in the h-Co₃O₄/RuO₂ heterostructure (Figure S10). The Co³⁺/Co²⁺ ratio in h-Co₃O₄/RuO₂ decreases slightly from 1.42 to 1.15 after the test due to the dissolution of Co during the stability process. This supports the idea that Co³⁺ in h-Co₃O₄ oxidizes to soluble CoO₂ in the heterojunction. The significant increase in oxygen vacancies after the test may indicate the coexistence of the adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM) during the OER process, and the LOM contributes to the formation of additional oxygen vacancies and leads to the irreversible dissolution of Co₃O₄.²⁸ These findings indicate that h-Co₃O₄/RuO₂ exhibits superior corrosion resistance in highly acidic environments, meeting the demands for long-term operation.

4. Conclusions

In summary, we systematically investigated the differences in the acidic OER performance of Co₃O₄ exposing three distinct crystal planes. Electrochemical tests indicate that h-Co₃O₄, exposing mixed {111} and {110} planes, exhibits the highest OER activity and stability, which could serve as an ideal support to load Ru. We further synthesized Co₃O₄/RuO₂ heterostructures, which require an overpotential of only 257 mV at an OER current density of 10 mA cm^–2 and could stably catalyze the OER over 100 h at this current density, achieving an order-of-magnitude improvement in stability compared to pure Co₃O₄. The significant enhancement in catalytic activity is attributed to Ru substituting part of the octahedral Co coordination, thereby inhibiting Co dissolution and providing high-activity sites. This Ru doping strategy might be extended to other transition metal oxide catalysts beyond cobalt oxides.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.4c00037.

• Additional SEM, TEM figures, and electrochemical data (PDF)

The authors declare no competing financial interest.

Special Issue

Published as part of ACS Nanoscience Auspecial issue “Advances in Energy Conversion and Storage at the Nanoscale”.