Trace F-doped Co₃O₄ nanoneedles for enhanced acidic water oxidation activity via promoting OH coverage†

Genyan Hao‡ ^a, Tao Zhao‡^bc, Qiang Fang‡^bc, Yunzhen Jia^bc, Dandan Li^d, Dazhong Zhong*^bc, Jinping Li^bc and Qiang Zhao*^bc
aShanxi College of Technology, Shuozhou 036000, Shanxi, P.R. China
^bCollege of Chemical Engineering and Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P.R. China. E-mail: zhaoqiang@tyut.edu.cn; zhongdazhong@tyut.edu.cn
^cShanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, Shanxi, P.R. China
^dShandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, Shandong, P.R. China

First published on 15th August 2024

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1. Introduction

Energy and environmental issues are becoming increasingly severe, and the development of new renewable energy is imperative. Hydrogen energy, as an essential energy carrier, can be used to store various unsustainable energy sources by electrolysis of water.^1–4 Compared with alkaline electrolysis water technology, proton exchange membrane (PEM) electrolyzers operating in acidic media have fast dynamic response speed, high working current density, and can adapt to fluctuations in renewable energy power generation, which has broad application prospects.^5–7 However, the operation of catalysts in strongly acidic and oxidizing environments poses severe challenges to their activity and stability. Currently, electrocatalysts in acidic media are mainly limited by precious metal-based materials, such as RuO_x and IrO_x.^8–12 Their scarcity and high cost limit the widespread application of PEM technology. Low-cost transition metals and their oxides are gradually receiving attention in acidic electrolytes.^13–16

In recent years, Co-based catalysts have been widely studied in the alkaline oxygen evolution reaction (OER) as an inexpensive alternative to precious metal electrocatalysts.^17–19 However, research on Co-based catalysts in acidic media is relatively limited due to their conductivity and stability factors.^20–24 Therefore, developing Co-based catalysts that can provide comparable or even higher performance than precious metals in the acidic OER is of great significance in promoting practical application. Introducing heteroatoms in spinel Co₃O₄ to regulate the electronic properties of Co is an essential strategy for improving its acidic OER properties, for example, Co₂TiO₄, Co₂MnO₄, P-Co₃O₄, Li-Co₃O₄, etc.^25–28 However, the activity of these catalysts is not comparable to that of precious metals.

Furthermore, the lack of OH in an acidic medium leads to slow OER reaction kinetics of intermediates on the catalyst surface, which is also an important factor limiting activity.²⁹ The generation from *O to *OOH is considered the rate determining step (RDS) of the reaction.³⁰ Designing a catalyst surface containing high-density electrophilic oxygen can accelerate the enrichment of OH, which promotes the transfer of intermediates and reduces the RDS energy barrier, thus accelerating O–O coupling. The preparation of self-supported nanoarray catalysts can further overcome the problem of slow electron transport to optimize catalytic performance.^31,32

In addition to metal doping in Co₃O₄, there are reports that introducing non-oxidizing anions at oxygen sites can generate many interesting properties. For instance, Song et al. found that NiCo oxide doped with F has strong synergistic effects and exhibits efficient water splitting activity.³³ Tondello et al. reported that F can greatly improve the performance of Co₃O₄ photocatalytic hydrogen production.³⁴ However, the effect of F-doped Co₃O₄ on the OER performance under acidic conditions has not been reported so far. Moreover, the mechanism of action of F in the acidic OER remains unclear.

In this work, F-modified Co₃O₄ nanoneedles were prepared by simple solvothermal and pyrolysis methods (marked as F-Co₃O₄/CP). F-Co₃O₄/CP has an overpotential of 350 mV at 10 mA cm⁻², which is better than that of pure Co₃O₄/CP (402 mV). F-Co₃O₄/CP also shows long-term structural stability in acidic media. Significantly, the membrane electrode assembly (MEA) test shows no significant deactivation at high current density (300 mA cm⁻²), which may have practical application potential in acidic media. A detailed study reveals that F doping promotes the coverage of OH to avoid acid corrosion and can inhibit the formation of high valence Co to prevent dissolution, providing excellent activity and stability. In addition, density functional theory (DFT) confirms that the introduction of F enhances the charge transfer capability and reduces the energy barrier for the formation of *OOH, resulting in excellent OER activity. This study provides a simple method for preparing transition metal OER acidic catalysts.

2. Experimental section

2.1 Chemicals and materials

Ammonium fluoride (NH₄F), ethanol, sulfuric acid (H₂SO₄), hydrochloric acid (HCl) and nitric acid (HNO₃) were bought from Sinopharm (Shanxi, China). Cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O) and urea (CH₄N₂O) were purchased from Aladdin (Shanghai, China). Ruthenium oxide (RuO₂) and platinum/carbon (Pt/C) were purchased commercially. Nafion® D-520 was provided by Alfa Aesar (Beijing, China). All chemical reagents used were not further purified.

2.2 Synthesis of F-Co₃O₄/CP and Co₃O₄/CP

F-Co₃O₄/CP was synthesized by mixing a solution of Co(NO₃)₂·6H₂O (0.9720 g), NH₄F (0.4940 g), CH₄N₂O (1.001 g), and ultrapure water (60 mL) in a 100 mL Teflon-lined stainless steel autoclave. The mixture was stirred for 10 min to form a uniform solution and put in carbon paper (CP (3 cm × 5 cm) was washed with 2 M HCl, ultrapure water, and ethanol under ultrasound), which was then sealed and placed in an oven at 120 °C for 6 h. After the reaction, the autoclave was cooled to room temperature. Subsequently, the CP was rinsed with ultrapure water and ethanol and dried at 80 °C overnight to obtain Co(OH)F/CP (the precipitate was centrifuged several times with ultrapure water and ethanol to obtain Co(OH)F powder). Finally, Co(OH)F/CP was heated to 300 °C at a ramp rate of 5 °C min⁻¹ under air and was maintained at that temperature for 3 h to prepare F-Co₃O₄/CP. The preparation of Co₃O₄/CP was similar to that of F-Co₃O₄/CP, except that NH₄F was missing. The Co(OH)F powder was subjected to the same heat treatment to obtain F-Co₃O₄ powder and Co₃O₄ powder.

2.3 Loading commercial RuO₂ (Pt/C) on CP

5 mg catalyst was dissolved in a solution with 490 μl ethanol, 490 μl ultrapure water and 20 μl Nafion suspension, then a homogeneous solution was obtained by ultrasound for 30 min, and finally, 200 μl solution was dropped on CP.

2.4 Instrument

The crystal structure of the catalyst was tested by X-ray diffraction (XRD) with Cu-Kα radiation (λ = 1.54 Å). The catalyst was characterized using a transmission electron microscope (TEM) with a 200 kV field emission gun (Japan) and a scanning electron microscope (SEM) (Hitachi, SU8010, Tokyo, Japan) at 15 kV. X-ray photoelectron spectroscopy (XPS) was recorded using a WSCAL-ab 220i-XL spectrometer and a monochromatic Al-Kα radiation source. The element distribution and content of the catalyst were tested using energy-dispersive X-ray spectrometry (EDS) with an IXRF SDD 2610 EDS system and an inductively coupled plasma (ICP) spectrometer on an Avio 200 (PerkinElmer). Raman spectra were obtained on an InVia 1WU072 Raman spectrometer with λ = 532 nm. The contact angle (CA) instrument (DSA100) was used to study the wettability of the samples.

2.5 Electrochemical measurements

All the electrochemical tests were implemented on the Princeton electrochemical workstation (PARSTAT MC, Princeton, USA) with a standard three-electrode system for overall water splitting in 0.5 M H₂SO₄ (pH = 0). The glassy carbon (GC; after polishing, the GC was ultrasonically cleaned with HNO₃, ultrapure water and ethanol, respectively. 5 mg catalyst was dissolved in a solution with 490 μl ethanol, 490 μl ultrapure water and 20 μl Nafion suspension, then a homogeneous solution was obtained by ultrasound for 30 min, and finally, 10 μl solution was dropped on GC) or CP-based catalysts (1 cm × 1 cm), Hg/Hg₂SO₄ and carbon rod acted as the working electrode, reference electrode, and auxiliary electrode, respectively. Linear sweep voltammetry (LSV) polarization curves were conducted with a 2 mV s⁻¹ scan rate. Electrochemical impedance spectroscopy (EIS) curves were obtained in the frequency range from 100 kHz to 0.05 Hz at 1.55 V. The electrochemically active surface areas (ECSAs) were calculated using the double-layer capacitance (C_dl). The reversible hydrogen electrode (RHE) and Tafel slope were calculated according to the following formula: E_RHE = E_Hg/Hg2SO4 + 0.652 + 0.059 pH; η = a + b × logj, where η, b and j represent overpotential, Tafel slope and current density, respectively. All electrochemical data were iR compensated and converted to RHE.

2.6 In situ Raman spectroscopy test

In situ Raman spectroscopy tests were performed in a custom-built Teflon cell. The electrode configuration was consistent with the electrochemical measurements (0.5 M H₂SO₄). The Raman spectra were collected in the potential range from open circuit potential (OCP) to 1.6 V vs. RHE at 532 nm laser power.

3 Results and discussion

3.1 Structural characterization

The fluorine-doped Co₃O₄ supported on carbon paper (F-Co₃O₄/CP) was prepared via the calcination of Co(OH)F/CP (Fig. S1–S4†). Scanning electron microscopy (SEM) results showed that the flower-shaped morphology composed of nanoneedles covered the surface of the CP, indicating that temperature did not cause damage to the morphology of F-Co₃O₄ (Fig. S5†). The nanoneedle structure has been mentioned to facilitate the gas bubble release and promote the performance of the gas formation reaction.^35,36 The contact angle (CA) tests confirmed better bubble release ability of the F-Co₃O₄/CP (Fig. S6†). We noted that pure Co₃O₄ exhibits nanosheet morphology, demonstrating that the introduction of F contributes to the formation of nanoneedles with superior gas bubble release ability (Fig. S7†). The transmission electron microscopy (TEM) results further confirm that F-Co₃O₄/CP maintains the morphology of nanoneedles (Fig. 1a). And the EDS mapping confirms that Co, O, and F are uniformly dispersed throughout the nanoneedles, indicating the successful introduction of F (Fig. 1d). In addition, the SAED pattern of F-Co₃O₄/CP shows lattice spacings of 0.461 nm, 0.250 nm, and 0.169 nm, which are consistent with the (111), (311), and (422) crystal planes of standard Co₃O₄ (Fig. 1b). Similarly, HRTEM indicates that the different interplanar distances (0.462 nm and 0.201 nm) of F-Co₃O₄/CP correspond to the crystal planes (111) and (400) of Co₃O₄ (Fig. 1c).

The crystal structure of the prepared catalyst was further studied in detail. The XRD results indicate that the diffraction peaks of F-Co₃O₄/CP and Co₃O₄/CP are consistent with those of standard Co₃O₄ (PDF#42-1467) without any impurity (26° belongs to the peak of CP), suggesting that the introduction of F into Co₃O₄ does not change the crystal structure (Fig. 2a). To accurately evaluate the crystal structure and exclude the presence of other phases, Raman spectroscopy was used to verify the phase of the catalyst. All peaks displayed by Raman spectroscopy belong to the typical structure of Co₃O₄, verifying a single Co₃O₄ structure (Fig. 2b).³⁷ Among them, the F_2g (509 cm⁻¹ and 604 cm⁻¹) modes related to oxygen sites showed an apparent blue shift, which was associated with the introduction of F. In addition, the successful introduction of F into the Co₃O₄ lattice was verified by comparing the XPS spectra of F-Co₃O₄/CP and Co₃O₄/CP (Fig. S8†).³⁸ The above results demonstrate the successful introduction of F into Co₃O₄ without altering the original bulk structure.

3.2 Electrochemical performance

The performance of the prepared catalyst was evaluated in 0.5 M H₂SO₄ with a standard three-electrode system. According to Fig. 3a, the F-Co₃O₄/CP exhibited excellent acidic OER activity with a low overpotential of 350 mV at 10 mA cm⁻², which is lower than those of blank CP and Co₃O₄/CP (403 mV). In addition, the performance of F-Co₃O₄/CP is comparable to that of commercial RuO₂. Powders F-Co₃O₄ and Co₃O₄ were also prepared to understand the intrinsic OER performance without the effect of the CP substrate (Fig. S9†). Fig. S10† demonstrates that F doping plays a vital role in improving the performance of Co₃O₄. More importantly, the activity of F-Co₃O₄/CP is superior to that of recently reported Co-based catalysts (Fig. 3g and Table S3†). These results emphasize the critical role of F in Co₃O₄ for accelerating charge transfer and improving catalytic activity.

The Tafel slope reflects the catalyst kinetics (Fig. 3b). The Tafel slope of F-Co₃O₄/CP (67.8 mV dec⁻¹) is lower than those of Co₃O₄/CP (86.4 mV dec⁻¹) and commercial RuO₂/CP (205.2 mV dec⁻¹), indicating that the OER reaction kinetics of F-Co₃O₄/CP is more favorable. Moreover, EIS was also performed to analyze the reaction kinetics of catalysts. According to the equivalent circuit diagram, the R_ct value of F-Co₃O₄/CP (1.141 Ω) was lower than that of Co₃O₄/CP (1.631 Ω) at 1.55 V vs. RHE (Fig. 3c, Tables S1 and S2†). Therefore, F-Co₃O₄/CP has the fastest electron transfer rate in the acidic OER, which accelerates the reaction rate.

The effect of F content on the catalytic performance of Co₃O₄ was systematically investigated. Fig. S11a and b† show the XRD patterns and Raman spectra of F-Co₃O₄/CP prepared at different calcination temperatures, which confirm the successful preparation of the Co₃O₄ structure. Additionally, the EDS data indicate that the F content in Co₃O₄ decreases with increasing temperature (Fig. S11c†), and the catalyst prepared at 300 °C has the best catalytic performance for the OER in acidic solution (Fig. S11d†).

The double-layer capacitance (C_dl) was used to measure the electrochemically active surface area (ECSA) of the catalyst (Fig. S12†). The C_dl value of F-Co₃O₄/CP is similar to that of Co₃O₄/CP (Fig. 3d), which highlights the higher intrinsic catalytic performance of F-Co₃O₄/CP (Fig. 3e). In addition to catalytic performance, electrocatalytic stability was also an indicator for evaluating the practical application of catalysts. As shown in Fig. 3f, Co₃O₄/CP was seriously deactivated after 40 h. In comparison, F-Co₃O₄/CP could work continuously for 80 h without a significant increase in overpotential, indicating that the stability of F-Co₃O₄/CP was better than that of Co₃O₄/CP. The practicality of F-Co₃O₄/CP was further verified by MEA, indicating its excellent water oxidation performance and long-term stability as an anode (Fig. S13†). Besides, XRD, SEM, and XPS of F-Co₃O₄/CP before and after the OER were compared. The morphology of F-Co₃O₄/CP did not change significantly, and it could still maintain the original structure (Fig. S14 and S15†). XPS spectra showed that F was still present, which is one of the main reasons for the high activity and stability of the catalyst (Fig. S16†). The above results confirm that F-Co₃O₄/CP has excellent stability in the process of acidic water oxidation.

3.3 Mechanism explanation

The effect of trace F doping on the electron transfer and adsorbed species of Co₃O₄ was studied by in situ EIS (Fig. S17†). The Bode modulus and the corresponding equivalent circuit results derived from EIS at different potentials are shown in Fig. 4a and b, Tables S1 and S2.† Clearly, the Bode diagrams show that the impedance of F-Co₃O₄/CP is lower than that of Co₃O₄/CP at the same potential, indicating that the reaction in F-Co₃O₄/CP involves rapid charge transfer. R₂ values at each potential demonstrate that F-Co₃O₄/CP has a smaller charge transfer resistance than Co₃O₄/CP (Fig. 4c).^39,40

In acidic media, the OER process requires the participation of OH obtained by hydrolysis, and the OER rate may be increased as more OH becomes available. Thus, the catalytic rate can be determined by evaluating the coverage of OH on the catalyst surface, while the coverage of OH adsorbed on the catalyst surface can be determined by calculating CPE₂.³ The CPE₂ of F-Co₃O₄/CP is higher than that of Co₃O₄/CP in the whole voltage range, and the CPE₂ of F-Co₃O₄/CP shows a more rapid increase with the increase of voltage, which indicates that F-Co₃O₄/CP has a higher coverage of *OH (Fig. 4d). The rapid accumulation of OH on the surface of F-Co₃O₄/CP is conducive to the catalytic reaction. Furthermore, methanol (CH₃OH) was used as a molecular probe to test the coverage of OH on the catalyst surface. CH₃OH oxidation tends to nucleophilically attack OH, so the oxidation activity of CH₃OH is better on the surface of catalysts with high coverage of OH.^41,42 Fig. 4e shows that F-Co₃O₄/CP has the highest CH₃OH oxidation activity, demonstrating high surface OH coverage. The O 1s spectra of Co₃O₄/CP and F-Co₃O₄/CP are also utilized to investigate the O species on the catalyst surface. The results of the XPS spectra show that F-Co₃O₄/CP has a higher content of OH and a small number of oxygen vacancies (O_v), confirming that F doping not only enhances the adsorption of OH on the surface but also promotes the exposure of the active sites (Fig. S18†). Thus, F-Co₃O₄/CP with high OH coverage has better OER performance in acidic media.

To further verify the effect of F doping on the microstructure of Co₃O₄, the charge structure of Co was analyzed. The XPS spectrum of Co 2p is shown in Fig. S19;† the change of the Co 2p peak is not evident due to the low F content. Additionally, the change of the Co charge structure was accurately analyzed by X-ray absorption spectroscopy (XAS), and the intensity of the Co K-edge spectrum increased after F doping (Fig. 4f). This phenomenon means that electrons in the F-Co₃O₄/CP are transferred from Co to F. The strong electron-withdrawing properties of F contribute to generating more OH species on the F-Co₃O₄/CP surface, which may be critical for the activity and stability of the OER. Comparing Co 2p XPS before and after the catalytic OER, Co²⁺ and Co³⁺ are found to be present in all catalysts. It is easy to find that Co 2p_3/2 in F-Co₃O₄/CP is shifted towards a lower bonding energy (0.64 eV), and the content of Co³⁺ increases (from 50.31% to 61.14%), indicating that Co³⁺ is more readily generated in the OER to participate in water oxidation (Fig. S20†).³⁷

CV curves were used to determine the structure of the catalyst to understand its activity and stability. Notably, Co^IVO₂ is considered an active species of the OER in alkaline media,⁴³ while dissolution under acidic conditions reduces the stability of the OER.⁴⁴ Compared with F-Co₃O₄/CP, a pair of reversible Co^IVCo^IV redox reaction peaks were observed in Co₃O₄/CP. In contrast, the high Co^IIICo^III peak is significant in F-Co₃O₄/CP. This means that the overoxidation of Co in F-Co₃O₄/CP is significantly inhibited, which also results in excellent activity and stability (Fig. 5a). The superb stability of F-Co₃O₄/CP is also confirmed by in situ Raman spectroscopy. Besides the Raman peak of SO₄²⁻ in the electrolyte, the characteristic peak of F-Co₃O₄/CP did not change significantly as the potential increased, implying high structural stability (Fig. 5b and Fig. S21†).

Furthermore, DFT calculations were performed to reveal the factors that enhance OER activity through F doping. The calculation results indicate the formation energy of F doping at different positions in Co₃O₄ (Fig. S22†). The optimal model was selected for calculation, and the schematic diagram of the optimal model and oxygen intermediate is shown in Fig. 5g and S23.† Firstly, the OH adsorption energy of Co₃O₄/CP and F-Co₃O₄/CP was calculated by DFT (Fig. S24†). F-Co₃O₄/CP shows strong OH adsorption energy, which is conducive to OH adsorption.⁴⁵ The charge density distribution shows that F doping can effectively regulate the electron transfer between Co and O, which may affect the adsorption energy of the intermediate (Fig. 5d and e). And the d-band center of F-Co₃O₄/CP (−1.86 eV) is higher than that of Co₃O₄/CP (−1.99 eV), indicating that the introduction of F makes the d-band center of F-Co₃O₄/CP closer to the Fermi level, which is conducive to promoting electron transfer to achieve fast OER reaction kinetics (Fig. 5f).⁴⁶ Furthermore, the Gibbs free energies (ΔG) of different oxygen intermediates of F-Co₃O₄/CP and Co₃O₄/CP were calculated (Fig. 5c). The largest ΔG is considered to be the RDS, and the formation of *OOH (2.02 eV) is clearly the RDS for Co₃O₄/CP, consistent with the literature.⁴⁷ For F-Co₃O₄/CP, the formation of *O (1.56 eV) becomes the RDS, indicating that F doping promotes *OH coverage to optimize the RDS energy, thereby improving OER activity.

4. Conclusions

In summary, we propose a trace F doping strategy to simultaneously enhance the OER activity and stability of Co₃O₄ in acidic media. The presence of F can effectively promote OH enrichment and inhibit high-valence Co formation to slow down the dissolution. These features have a positive impact on OER activity and structural stability. The optimal F-Co₃O₄/CP requires an overpotential of only 350 mV to drive a current density of 10 mA cm⁻² in acidic media, with the corresponding Tafel slope of 67.8 mV dec⁻¹. In addition, MEA tests have shown that the catalyst can operate stably even at high current densities. DFT reveals that F doping accelerates electron transfer and optimizes the energy barrier of the RDS, thereby promoting performance. This work emphasizes the significance of trace non-metallic elements for preparing highly active transition metal acid electrocatalysts.

Author contributions

Genyan Hao: investigation, writing – original draft, visualization, and methodology. Tao Zhao: investigation, writing – original draft, writing – review & editing, and conceptualization. Qiang Fang: investigation and writing – original draft. Yunzhen Jia: investigation and methodology. Dandan Li: investigation. Dazhong Zhong: writing – review & editing, methodology, formal analysis, data curation, and supervision. Jinping Li: formal analysis and writing – review & editing. Qiang Zhao: formal analysis, writing – review & editing, data curation, and supervision.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 22308246, 21975175, and 21878202), the Central Government Guided Local Science and Technology Development Special Fund (No. YDZJSX20231A015) and the Fundamental Research Program of Shanxi Province (No. 202203021212266).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc01895h

‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2024