One-step preparation of amorphous citrate-chelated CoNiFe trimetallic hydroxides for the oxygen evolution reaction

Abstract

Reasonable morphology regulation and electronic structure modulation enhance the oxygen evolution reaction (OER) performance of the catalyst. In this study, amorphous citrate-chelated CoNiFe trimetallic hydroxide nanoparticles were synthesized in one step via coprecipitation at room temperature. The choice of solvents controlled the hydrolysis rate of metal cations, allowing for the regulation of the product morphology. An alcohol–water system as the solvent facilitated the formation of more uniform and well-dispersed nanoparticles. Citrate was employed as a chelating agent, and its strong interaction with metal cations improved the stability of the amorphous materials, regulated the particle size, and increased the electrochemically active surface area. Furthermore, varying amounts of Ni ions were doped to modulate the electronic structure, exerting tri-metallic synergistic effects, which enhanced the OER performance. The results demonstrated that, in 1.0 M KOH, the optimized Co : Ni : Fe molar ratio of 2 : 1 : 1 achieved the highest OER activity, with an overpotential of 287 mV and a Tafel slope of 56.3 mV dec⁻¹, delivering a current density of 10 mA cm⁻², and maintaining stable performance over 24 hours with only a minor increase in the overpotential.

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1. Introduction

With the advancement of modern industry, ecological and environmental issues have become increasingly severe, and the rapid consumption of fossil fuels has heightened concerns about both energy and environmental sustainability.^1,2 The development of environmentally friendly renewable energy systems is critical, with the oxygen evolution reaction (OER) emerging as a focal point of research due to its pivotal role in advancing renewable energy technologies, such as water splitting and rechargeable metal–air batteries. However, the complexity of the four-electron transfer steps involved in the OER (4OH⁻ → O₂ + 2H₂O + 4e⁻ in alkaline media and 2H₂O → O₂ + 4H⁺ + 4e⁻ in acidic media), coupled with the limited overall efficiency of water splitting due to suboptimal OER performance, underscores the need for designing efficient OER electrocatalysts to reduce the reaction overpotential, enhance the reaction kinetics, and ultimately improve the efficiency of water splitting.³ To date, certain precious metal catalysts (such as IrO₂ and RuO₂) are recognized as the most effective OER catalysts under alkaline conditions due to their high electronic conductivity, which significantly enhances the OER kinetics and demonstrates excellent catalytic activity.^4,5 However, the high cost of these precious metals impedes their widespread commercialization. Consequently, there is an urgent need to develop more cost-effective non-precious metal OER catalysts.

In recent years, sulfides,^6–8 nitrides,^9,10 phosphides^11,12 and selenides^13,14 based on non-precious metal transition metals (Co, Ni, Fe, and Mn) have been extensively investigated, although the synthesis of these high-purity materials remains both time-consuming and technically challenging. However, these materials are prone to oxidation into their corresponding metal oxides or hydroxides. Under alkaline OER conditions, the hydroxide phase of these metals acts as the actual catalytic site.¹⁵ Consequently, transition metal hydroxides hold significant potential as OER electrocatalysts.^16,17

To enhance the electrocatalytic activity of cost-effective transition metal hydroxides, numerous constructive strategies have been proposed, including morphology control, metal doping, interlayer distance adjustment, defect engineering, phase engineering, and interface engineering. Zhou et al.¹⁸ prepared ultra-thin NiFe-layered double hydroxides (LDHs) enriched with oxygen vacancies (V_O) using a coprecipitation method in an alcohol–water system at room temperature. They investigated how different alcohol-to-water ratios affected the product structure and catalytic performance. N. K. Shrestha et al.¹⁹ performed a simple immersion chemical etching of foam nickel (NF) in the ethanol FeCl₃ solution to generate a microporous nickel (Ni) framework decorated with hierarchically structured metallic Fe doped Ni–Fe-hydroxide nanoparticles. The synergistic interaction between uniformly distributed micropores and metal iron doped iron hydroxide nanocatalysts, as well as the local amorphization in the lattice, promotes the transfer of mass and charge, thereby demonstrating excellent electrocatalytic performance and stability, and can operate for 80 hours at a current density of 400 mA cm⁻² without evidence of voltage decay. Citrate is frequently employed as an intercalation agent to aid in the synthesis of NiFe-LDHs,^20,21 and it also serves as a chelating agent.²² Citrate enhances the synergistic effect between Ni and Fe.²⁰ Additionally, citrate salts can effectively reduce and control the nanoparticle size.^23,24 Xu et al.²⁰ utilized trisodium citrate as an intercalating agent to produce a series of partially delaminated NiFe-LDHs by varying citrate contents. They examined the impact of the citrate concentration on interlayer spacing, morphology, surface elemental composition, surface metal electronic states, and electrochemical properties. Metal ion doping can modify the electronic structure and optimize adsorption/desorption properties for reactants and intermediates.^25,26 Yang et al.²⁷ synthesized a range of CoFeM-LDH nanorods doped with various inexpensive metals using a one-step hydrothermal method, designated as CoFeM-x (M = Cu, K, Al, Zn, and Cr; x = 5, 10, 15, and 20). CoFeK-15 exhibited the best OER performance, attributed to K ion-induced lattice distortion that optimized the electronic structure and increased oxygen vacancies. Chen et al.²⁸ developed hierarchical trimetallic NiFeCe-LDH with intercalated citrate via a coprecipitation method, and the introduction of Ce ions enhanced intrinsic activity by modifying the electronic structure through 3d–4f electron interactions. Currently, most of the studied OER catalysts are crystalline. Amorphous materials, with their abundance of coordination-unsaturated atoms, often demonstrate superior OER electrocatalytic activity compared to crystalline materials. Additionally, the defects and inherent structural disorder in amorphous materials facilitate charge transfer and ion diffusion.^29–31 N. K. Shrestha et al.³² induce site-selective disordering of the crystalline structure of MIL88B(Ni) frameworks via Ce-doping, resulting in crystalline/amorphous heterostructures. DFT calculations support the experimentally observed Ce³⁺ ion doping effect inducing site selective crystal disorder on the MIL-88B(Ni) framework structure, thereby enhancing OER electrocatalytic activity. However, amorphous materials with metastable structures can show instability during measurements in acidic or alkaline solutions.^33,34 Poor stability will seriously affect the practical application prospects of catalysts.

Herein, we synthesized CoFe hydroxide particles containing carbonate ions in a single step using the coprecipitation method at room temperature, and selected various solvents to control the product morphology. Ammonium citrate was used as a chelating agent to control the particle size and increase the number of exposed active sites, thereby enhancing the catalyst's performance. Research indicates that the addition of citrate not only improves the OER performance of the catalyst but also significantly enhances its stability during the OER process. Furthermore, Ni ions were doped to modify the electronic structure and examine the effect of doping concentration on the morphology and catalytic performance of the product. Finally, we synthesized amorphous citrate-chelated CoNiFe trimetal hydroxide nanoparticles using an alcohol–water solvent in a single step at room temperature. Under 1.0 M KOH conditions, the catalyst achieved a current density of 10 mA cm⁻² with an overpotential of only 287 mV and maintained stable performance over 24 hours with a minimal increase in overpotential.

2. Experimental section

2.1. Materials

All chemicals were procured from commercial suppliers and utilized without additional purification. Cobalt chloride hexahydrate (CoCl₂·6H₂O), nickel chloride hexahydrate (NiCl₂·6H₂O), and iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) were acquired from Sinopharm Chemical Reagent Co., Ltd. Ammonium bicarbonate (NH₄HCO₃) and ammonium citrate (C₆H₅O₇(NH₄)₃) were obtained from Shanghai Aladdin Biochemical Technology. Carbon paper was sourced from Toray Industries Co., Ltd.

2.2. Preparation of catalysts

2.2.1. Preparation of Co₃Fe-W, Co₃Fe-E and Co₃Fe-W/E. The synthesis was conducted at room temperature in a beaker. Cobalt chloride hexahydrate (0.75 mmol) and iron nitrate nonahydrate (0.25 mmol) were dissolved in 80 mL of water (W), ethanol (E), or an alcohol–water solution (W/E) with a volume ratio of V_H2O/V_H2O+EtOH = 0.2. Subsequently, ammonium bicarbonate (6 mmol) was introduced into the beaker as a pH regulator and carbon source. Following 12 hours of stirring with a magnetic stirrer, a precipitate was collected via centrifugation. The precipitate was washed three times with water and ethanol and then dried overnight in a 70 °C oven.

2.2.2. Preparation of Co₃Fe-CA, Co₂NiFe-CA, Co_1.5Ni_1.5Fe-CA and CoNi₂Fe-CA. The synthesis was conducted at room temperature in a beaker. Cobalt chloride hexahydrate (0.75 − x mmol), nickel chloride hexahydrate (x mmol), and iron nitrate nonahydrate (0.25 mmol) were sequentially dissolved in 80 mL of an alcohol–water solution with a volume ratio of V_H2O/V_EtOH = 0.2, where x = 0, 0.25, 0.375, or 0.5. Ammonium citrate (CA) (0.32 mmol) was added as a chelating agent and the solution was sonicated to ensure uniform dispersion. Subsequently, ammonium bicarbonate (6 mmol) was introduced into the beaker as a pH regulator and carbon source. Following 12 hours of stirring with a magnetic stirrer, the precipitate was collected via centrifugation. The precipitate was washed three times with water and ethanol and then dried overnight in a 70 °C oven.

2.2.3. Preparation of Co₂NiFe-CS. The synthesis of crystalline (CS) Co₂NiFe hydroxide particles was conducted at room temperature in a beaker. Cobalt chloride hexahydrate (0.5 mmol), nickel chloride hexahydrate (0.25 mmol) and iron nitrate nonahydrate (0.25 mmol) were dissolved in 80 mL of water. Subsequently, ammonium bicarbonate (6 mmol) was introduced into the beaker as a pH regulator and carbon source. Following 12 hours of stirring with a magnetic stirrer, the precipitate was collected via centrifugation. The precipitate was washed three times with water and ethanol and then dried overnight in a 70 °C oven.

2.2.4. Physical characterization. X-ray powder diffraction (XRD) measurements were performed using an Ultima IV X-ray diffractometer with Cu Kα radiation. A field-emission scanning electron microscope (SEM, Ultra Plus), coupled with an energy dispersive X-ray spectroscope (EDX), was employed at 5 kV to obtain morphological and compositional information. Transmission electron microscopy (TEM) data were acquired using a Talos F200X field-emission high-resolution transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific K-Alpha with Al Kα radiation (hν = 1486.6 eV) at 12 kV. The binding energy scale was calibrated by referencing the C 1s peak at 284.80 eV. Fourier transform infrared spectroscopy (FT-IR) spectra were acquired using an FTS 3000 spectrometer from the BIO-RAD Excalibur series, employing a KBr matrix.

2.2.5. Electrochemical measurements. Electrochemical measurements were performed using an electrochemical workstation (CHI 660E) equipped with a standard three-electrode system. A total of 3 mg of powder was uniformly dispersed in 0.34 mL of a mixed solution consisting of 300 μL ethanol and 40 μL Nafion dispersion (5 wt%). Forty microliters of the prepared ink were applied to a carbon paper with a working area of 1 cm × 1 cm. The catalyst loading for all catalysts in this study was standardized to approximately 0.353 mg cm⁻². A Hg/HgO electrode (with 1.0 M KOH as the internal electrolyte) and a carbon rod were used as the reference and counter electrodes, respectively. All potentials were referenced to this Hg/HgO electrode (0.098 V versus the normal hydrogen electrode). All measured potentials relative to the Hg/HgO electrode were converted to potentials relative to the reversible hydrogen electrode (RHE) using the following equation:

E_RHE = E_(Hg/HgO) + 0.098 V + pH × 0.059

Prior to each electrochemical test, the working electrode was activated by cycling the potential from 0 to 1.6 V versus Hg/HgO for 20 cycles at a scan rate of 100 mV s⁻¹. Linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5 mV s⁻¹ with 90% iR compensation. Double-layer capacitance (C_dl), which is linearly proportional to the electrochemical surface area (ECSA), was estimated from cyclic voltammetry (CV) curves within the non-faradaic potential window of 0.25 to 0.35 V versus Hg/HgO, using incremental scan rates of 20, 40, 60, 80, and 100 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) data were collected over a frequency range of 100 000 Hz to 0.1 Hz with an applied voltage of 0.25 V versus Hg/HgO.

3. Results and discussion

3.1. Synthesis and characterization

Cobalt–iron hydroxide particles containing carbonate ions were synthesized via co-precipitation at room temperature. The morphology of the product was optimized by varying the solvent to mitigate agglomeration and enhance electrochemical activity. However, the product is an amorphous substance, which often exhibits poor stability. To address this issue, ammonium citrate was introduced as a chelating agent. Ammonium citrate not only regulates the particle size of the product and exposes additional active sites to enhance catalytic performance,^20,23 but also significantly improves the stability of the catalyst. This enhancement is attributed to the stronger interaction between citrate ions and metal ions compared to carbonate ions, which reduces the leaching and aggregation of metal ions during the catalytic reaction and provides better support for the material.³⁵ Additionally, Ni ion doping was employed to optimize the electronic structure and enhance the synergistic effects among the three metals.^33,36 The effect of varying doping amounts on the morphology and catalytic performance of the products was investigated. Moreover, crystalline cobalt–nickel–iron trimetallic hydroxides were prepared for comparison with the above catalysts. Fig. 1 illustrates a schematic representation of a straightforward one-pot co-precipitation synthesis process for metal hydroxide particles containing carbonate/citrate ions at room temperature. At room temperature, cobalt nitrate, iron nitrate, and ammonium bicarbonate were added to beakers containing various solvents (water, ethanol, or an alcohol–water solution with V_H2O/V_H2O+EtOH = 0.2) and mixed using magnetic stirring. NH₄HCO₃ decomposed to generate carbonate ions, and the released NH₃ raised the local pH of the solution, leading to the hydrolysis of Fe³⁺ and Co²⁺ due to the low solubility constants of their hydroxide products. The formation of CoFe hydroxide particles is ultimately dependent on the diffusion of iron ions from β-FeOOH to the cobalt hydroxide phase.¹⁸ By employing different solvents, the hydrolysis rate of metal cations can be controlled, thereby altering the morphology of the products. Among these solvents, pure water results in the highest hydrolysis rate. The introduction of ammonium citrate as a chelating agent and the doping of Ni ions were both performed via in situ addition.

Fig. 1 Schematic illustration of the synthesis of Co₃Fe-W/E and Co₂NiFe-CA.

The morphology and nanostructure of the product were studied using SEM and TEM. The SEM images of Co₂NiFe-CA are shown in Fig. 2a and b. Co₂NiFe-CA consists of nanoparticles. To investigate the effects of different conditions on the product morphology, SEM images of other samples were also taken (Fig. S1–S8, ESI†). Co₃Fe-W consists of irregular particles and flakes that aggregate, which can be attributed to the faster hydrolysis rate leading to a more rapid precipitation growth and larger particle sizes. Co₃Fe-E particles are smaller but exhibit a low yield across multiple experiments. This is because NH₄HCO₃ generates fewer hydroxide ions in pure ethanol, leading to insufficient precipitation with metal ions. Co₃Fe-W/E particles have an average diameter of approximately 95 nm and are more uniform compared to those in the pure ethanol system. This is attributed to the presence of water, which increases the concentration of hydroxide ions and facilitates particle growth. The addition of ammonium citrate resulted in Co₃Fe-CA particles with a smaller average diameter of approximately 72 nm. This indicates that ammonium citrate effectively regulates the particle size, resulting in a higher specific surface area. This is because citrate ions complex with metal cations, competing with hydroxide ion precipitation and slowing down the nucleation rate during the reaction.^21,37 Following the addition of a small amount of Ni ions, there was no significant change in the product (Fig. S5 and S6, ESI†). However, when the Co/Ni molar ratio reached 1 : 2 (Fig. S7, ESI†), a noticeable increase in the particle size and agglomeration occurred, which may be due to structural collapse from the in situ synthesis of cobalt nickel iron hydroxide. This phenomenon was also observed by Wu et al. during their preparation of amorphous/crystalline CoNiFe-LDH hollow nanocages.³³ Crystalline Co₂NiFe-CS is composed of aggregated irregular flakes. The particle size of the aggregates is significantly larger than that of Co₂NiFe-CA. The particle size distributions of the different catalysts (except for irregular Co₃Fe-E and Co₂NiFe-CS) were obtained based on SEM maps (Fig. S9, ESI†), which shows that the particle sizes are homogeneous. In addition, EDX was employed to determine the elemental ratio of the sample, revealing that the molar ratio of Co : Ni : Fe matched that of the raw material for Co₂NiFe-CA, as shown in Fig. S1–S8 (ESI†). The TEM images of Co₂NiFe-CA are shown in Fig. 2c and d. The absence of lattice fringes in the TEM images confirms that Co₂NiFe-CA has an amorphous structure. The crystal composition of all the prepared products was analyzed using XRD (Fig. S10, ESI†). Fig. 3a shows that Co₃Fe-W and Co₂NiFe-CS are crystalline products. Co₃Fe-W exhibits characteristic peaks of hydrotalcite, corresponding to Co_5.84Fe_2.16-LDH (JCPDS No. 50-0235). The four characteristic peaks are located at 11.5°, 23.3°, 34°, and 59.3°, corresponding to the (003), (006), (012), and (110) planes, respectively, which align with the layered structure observed in the SEM image of Co₃Fe-W. Co₂NiFe-CS also exhibits hydrotalcite characteristic peaks. When using ethanol or alcohol–water system as solvents, the characteristic hydrotalcite peaks disappeared (Fig. 3b), and the XRD pattern of the product exhibited a completely amorphous state. This is likely due to NH₄HCO₃ generating fewer hydroxide ions in ethanol-rich solvent systems, preventing the formation of a layered hydrotalcite structure. Fig. 3c presents the XRD pattern of pure carbon paper and Co₂NiFe-CA after CP testing at a current density of 10 mA cm⁻². Following CP testing, XRD tests were carried out on Co₂NiFe-CA stuck on carbon paper and Co₂NiFe-CA scraped off from carbon paper, respectively. It can be seen that the XRD patterns of pure carbon paper and Co₂NiFe-CA stuck on carbon paper are almost identical because the signal of amorphous products is masked by the signal of carbon paper. And Co₂NiFe-CA scraped off from carbon paper also exhibits amorphous characteristics. This indicates that after testing, Co₂NiFe-CA still has an amorphous structure and no new crystalline material has been generated.

Fig. 2 (a) and (b) SEM images. (c) and (d) HRTEM images. (e) Elemental mapping images of Co₂NiFe-CA.

Fig. 3 (a) XRD patterns of Co₃Fe-W and Co₂NiFe-CS. (b) XRD patterns of Co₃Fe-W/E, Co₃Fe-CA and Co₂NiFe-CA. (c) XRD patterns of carbon paper and Co₂NiFe-CA after CP. (d) FT-IR spectra of all catalysts.

FT-IR characterization was performed to verify the presence of citrate anions in Co₃Fe-CA, Co₂NiFe-CA, Co_1.5Ni_1.5Fe-CA, CoNi₂Fe-CA and Co₂NiFe-CS (Fig. 3d). In the spectrum of Co₃Fe-W and Co₂NiFe-CS, bands at around 1364 cm⁻¹ and 1490 cm⁻¹ are associated with CO₃²⁻ vibrations, while the band at around 3450 cm⁻¹ is characteristic of O–H stretching vibrations.^38–40 For Co₃Fe-E and Co₃Fe-W/E, the CO₃²⁻ vibrational bands remain at 1384 cm⁻¹ and 1490 cm⁻¹. For Co₃Fe-CA, Co₂NiFe-CA, Co_1.5Ni_1.5Fe-CA, and CoNi₂Fe-CA, new bands at around 1399 cm⁻¹ and 1573 cm⁻¹ are observed, assigned to the RCOO⁻ symmetric and asymmetric stretches.⁴⁰ Additionally, signals at 1252 cm⁻¹ and 1073 cm⁻¹ correspond to C–O stretching, while the signal at 905 cm⁻¹ is attributed to C–H bending.⁴¹ These results confirm the presence of citrate anions in Co₃Fe-CA, Co₂NiFe-CA, Co_1.5Ni_1.5Fe-CA, and CoNi₂Fe-CA.

The electronic structure and surface elemental composition of metal sites are closely related to the catalytic activity of catalysts.^38,39,42 X-ray photoelectron spectroscopy (XPS) was employed to investigate the electronic structure of Co₃Fe-W/E, Co₃Fe-CA, and Co₂NiFe-CA. The XPS survey scan shown in Fig. 4a reveals the presence of Co, Ni, and Fe in Co₂NiFe-CA. All XPS data in this study were corrected for C 1s spectral contributions. Co²⁺ and Co³⁺ are represented by two peaks in the Co 2p XPS spectrum of Co₃Fe-W/E (Fig. 4b), with Co³⁺ at 781.4 and 797.2 eV, and Co²⁺ at 783.7 and 799.5 eV. The remaining four peaks correspond to the satellite (Sat.) peaks of Co 2p_3/2 and Co 2p_1/2. Additionally, shake-up satellite peaks were observed at 786.0 eV and 802.5 eV, characteristic of cobalt oxide.^36,43 Upon the addition of ammonium citrate (Co₃Fe-CA), the peak position of Co³⁺ shifted to a lower binding energy (−0.3 eV) compared to Co₃Fe-W/E, and the Co³⁺/Co²⁺ molar ratio decreased from 2.72 to 1.94. However, with additional nickel doping (Co₂NiFe-CA), the Co³⁺/Co²⁺ molar ratio increased to 2.10 but did not exceed the initial 2.72. It has been reported in the literature that high valence cobalt enhances electrochemical activity,⁴⁴ suggesting that changes in cobalt's electronic structure are not the direct cause of performance improvement. In the Fe 2p spectrum of Co₃Fe-W/E (Fig. 3b), peaks at 712.5 and 725.3 eV correspond to Fe 2p_3/2 and Fe 2p_1/2 of Fe³⁺, while peaks at 710.7 and 723.2 eV correspond to Fe 2p_3/2 and Fe 2p_1/2 of Fe²⁺.²⁷ Upon comparison, it is observed that adding ammonium citrate (Co₃Fe-CA) shifts the peak position of Fe³⁺ to higher binding energy by 0.4 eV compared to Co₃Fe-W/E, and the molar ratio of Fe³⁺/Fe²⁺ significantly increases from 1.09 to 1.49. Xu et al.²⁰ reported a similar phenomenon in their study of citrate-intercalated NiFe-LDH, where the molar ratio of Fe³⁺/Fe²⁺ increased further with additional citrate. Research generally indicates that high-valence iron enhances oxygen evolution reaction (OER) activity.⁴⁵ After further nickel doping (Co₂NiFe-CA), the peak position of Fe³⁺ shifts to a lower binding energy by 0.3 eV compared to Co₃Fe-CA, and the Fe³⁺/Fe²⁺ molar ratio decreases to 1.12. This shift may be attributed to electron transfer from Ni to Fe, as explained by Yao et al.⁴⁶ through electron coupling. In the Ni 2p spectrum of Co₂NiFe-CA (Fig. 3c), the peaks at 857.4 and 875.3 eV correspond to Ni 2p_3/2 and Ni 2p_1/2 of Ni³⁺, respectively, while the peaks at 856.0 and 873.5 eV are assigned to Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺. The remaining two peaks are attributed to the double satellite (Sat.) peaks of Ni 2p_3/2 and Ni 2p_1/2, located at 861.5 and 879.5 eV, respectively.⁴⁶ The molar ratio of Ni³⁺/Ni²⁺ is 0.48. It is widely accepted that catalysts with a higher proportion of high-valence Ni components can more readily form Ni⁴⁺-OO or Ni³⁺-OO, which serve as active centers for the oxygen evolution reaction (OER).⁴⁷ Based on the XPS spectral analysis, after adding ammonium citrate, the Co³⁺/Co²⁺ molar ratio decreases while the Fe³⁺/Fe²⁺ molar ratio increases. This suggests that electrons may have transferred from iron to cobalt, leading to an increase in high-valence iron, which enhances the oxygen evolution reaction (OER). Subsequent doping with Ni ions results in a slight increase in the Co³⁺/Co²⁺ molar ratio, while the Fe³⁺/Fe²⁺ molar ratio decreases significantly. This indicates that electrons may have transferred from Ni to Fe, resulting in the formation of more Ni³⁺, which enhances the OER through a synergistic effect of the three metals, optimizing electron transfer and the electronic structure.^36,46

Fig. 4 (a) Overall XPS surveys of Co₃Fe-W/E, Co₃Fe-CA and Co₂NiFe-CA. High resolution XPS spectra of (b) Co 2p, (c) Fe 2p and (d) Ni 2p.

3.2. OER performance

To investigate the effects of various solvents used during synthesis, the introduction of ammonium citrate, and the doping of Ni ions on electrocatalysis, we assessed the oxygen evolution reaction (OER) performance of all prepared samples in a 1.0 M KOH electrolyte. Prior to each electrochemical test, the working electrode was activated by cycling the potential from 0 to 1.6 V versus Hg/HgO, 20 times at a scan rate of 100 mV s⁻¹. Fig. S11 (ESI†) presents the cyclic voltammetry (CV) plot (90% iR-corrected) of this activation process. It is evident that before the addition of ammonium citrate (Fig. S11a and b, ESI†), the voltage decay was more pronounced, which can be attributed to the poor stability of the amorphous materials. Following the addition of ammonium citrate as a chelating agent (Figure S11c and d, ESI†), the voltage attenuation was markedly reduced, and the stability of the material was significantly improved. This improvement is due to the strong interaction between ammonium citrate and metal ions, which reduces the leaching and aggregation of metal ions during the reaction process, thereby providing enhanced support for the material. The linear sweep voltammetry (LSV) curves with 90% iR correction shown in Fig. 5a reveal that, prior to the addition of ammonium citrate (indicated by the dashed line), the product synthesized using water as the solvent exhibited the highest overpotential at 10 mA cm⁻², while the product synthesized using an alcohol–water mixture as the solvent displayed the lowest overpotential. This indicates that an alcohol–water solvent not only results in a more uniform morphology but also enhances OER performance. Upon introducing ammonium citrate into the alcohol–water system (as indicated by the solid line), a significant decrease in overpotential was observed, demonstrating that the addition of citrate effectively improves catalytic performance. Among the catalysts studied, Co₂NiFe-CA exhibits the highest OER activity, with a minimum overpotential of 287 mV at a current density of 10 mA cm⁻², outperforming Co₃Fe-CA, which lacks Ni ion doping and has an overpotential of 328 mV. This enhanced performance is attributed to the optimization of the electronic structure through Ni ion doping, which creates a synergistic effect among the three metals. As the amount of Ni ion doping increases, the electrocatalytic activity gradually decreases, potentially due to structural collapse associated with excessive Ni ion doping. Deka et al.⁴⁸ also observed that doping Zn and Co into the NiFe-layered double hydroxide (LDH) matrix deteriorates its OER performance. The reduced activity has been attributed to changes in charge localization and disruptions in the charge transfer chain within the Ni–O–Fe–O–Zn and Ni–O–Fe–O–Co moieties. However, their experimental Co : Ni : Fe molar ratio of 1 : 3 : 1 does not contradict the improved OER performance observed with small amounts of Ni ion doping in this study. Instead, it highlights the decline in performance associated with excessive Ni ion doping. In addition, comparing the amorphous Co₂NiFe-CA with crystalline Co₂NiFe-CS, it can be seen that Co₂NiFe-CA has a significant advantage in oxygen evolution performance. Fig. 5a illustrates that the introduction of Ni ion doping results in an anodic peak corresponding to the Ni²⁺/Ni³⁺ transition. The trend of the oxidation peak aligns with the change in activity observed with varying Ni ion contents. Additionally, Fig. S12 (ESI†) confirms that the OER activity of carbon paper is negligible. The Tafel slope not only serves as an important parameter for assessing OER activity but also provides information about the OER kinetics of the catalyst. A relatively low Tafel slope reveals more advantageous kinetics and higher catalytic activity.⁴⁹ The Tafel slope of Co₂NiFe-CA is approximately 56.3 mV dec⁻¹ (Fig. 5b), which is lower than that of all other products. This indicates that the optimal content of Ni ions and the addition of ammonium citrate enhance the catalytic kinetics.

Fig. 5 Electrocatalytic OER performance in 1.0 M KOH electrolyte. (a) LSV curves. (b) Tafel slopes. (c) Double-layer capacitances (C_dl). (d) Chronopotentiometric curves of Co₂NiFe-CA at 10 mA cm⁻².

The electrochemically active surface areas (ECSAs) of all catalysts were determined by electrochemical double layer capacitance measurements.⁵⁰ The double layer capacitance was estimated by plotting Δj(j_a − j_c) against the scan rate, where j_a and j_c are the anodic and cathodic current densities, respectively, at 0.1 V vs. Hg/HgO, as derived from the CV curves in 1 M KOH (Fig. S13, ESI†). The slope of the fitted linear curve, which is twice the double layer capacitance, was used to represent the ECSA.⁴⁷Fig. 5c shows that the C_dl values for Co₃Fe-W, Co₃Fe-E, Co₃Fe-W/E, and Co₃Fe-CA are estimated to be 1.1, 1.7, 2.4, and 7.9 mF cm⁻², respectively. The variation in C_dl corresponds to the activity order, suggesting that the increased ECSA, resulting from the use of an alcohol–water solvent and the addition of ammonium citrate, exposes more edge active sites. However, the C_dl for Co₂NiFe-CA, doped with a small amount of Ni ions, decreased to 4.9 mF cm⁻². As the doping amount increased, C_dl continued to decline, indicating that Ni ion doping can lead to structural collapse and reduced ECSA. Nevertheless, the doping of nickel ions optimizes the electronic structure, exerts a synergistic effect among the three metals, increases the activity per unit area, compensates for the reduction of ECSA, and enables Co₂NiFe-CA to exhibit optimal OER activity. The ECSA of crystalline Co₂NiFe-CS is 1.7 mF cm⁻², significantly lower than that of amorphous Co₂NiFe-CA.

To further investigate interfacial kinetics between the electrolyte and catalysts, electrochemical impedance spectroscopy (EIS) measurements were conducted (Fig. S14, ESI†). The R_ct values are provided in Fig. S15 (ESI†). Co₂NiFe-CA exhibited the optimized R_ct value, indicating reduced charge transfer resistance and faster electron transfer rate due to the combined effects of appropriate Ni ion doping and ammonium citrate addition. The R_ct value of Co₂NiFe-CA is 1.45 ohms, much lower than that of crystalline Co₂NiFe-CS (2.43 ohms). The durability of the Co₂NiFe-CA catalyst was assessed using chronopotentiometric measurements at a fixed current density of 10 mA cm⁻² and room temperature, as shown in Fig. 5d. The Co₂NiFe-CA catalyst exhibited minimal voltage attenuation during the 24-hour test, demonstrating its good durability. At the same time, the OER durability of all other catalysts was tested using chronopotentiometry at a current density of 10 mA cm⁻² (Fig. S16, ESI†), and it can be seen that the durability of the catalyst was significantly improved after adding citrate, while the durability was not affected after further doping with nickel ions. In addition, to investigate the durability of Co₂NiFe-CA at higher current densities, we conducted CP testing at a current density of 250 mA cm⁻² and performed XRD, XPS, and SEM tests on the tested samples. CP testing shows (Fig. S17, ESI†) that Co₂NiFe-CA can remain stable for over 5 hours at a current density of 250 mA cm⁻². The XRD pattern (Fig. S18, ESI†) shows that after CP testing, Co₂NiFe-CA still has an amorphous structure. By comparing the XPS (Fig. S19, ESI†) of Co₂NiFe-CA before and after the CP test, it can be found that the proportion of trivalent nickel and cobalt has increased significantly after the test. Meanwhile, the positions of the trivalent peaks of the three metals have all shifted positively, which is conducive to the oxygen evolution reaction. The SEM image of Co₂NiFe-CA coated on carbon paper after CP testing was taken (Fig. S20, ESI†), which shows that the morphology of the catalyst particles did not change and remained in a spherical structure.

4. Conclusion

In summary, amorphous CoNiFe tri-metal hydroxide nanoparticles were synthesized in a single step via a co-precipitation method at room temperature, with citrate anions serving as chelating agents. The use of an alcohol–water solvent system enabled morphological adjustments, resulting in a higher electrochemically active surface area. Citrate anions reduce the particle size, expose additional active sites, and modify the electronic structure, leading to an increase in high-valent iron and improved catalytic activity. The presence of citric acid as a chelating agent significantly enhances the stability of the oxygen evolution reaction catalyst, due to its strong interactions with metal ions. Doping with a small amount of Ni optimizes the electronic structure, facilitates the synergistic effects of the three metals, and generates more high-valent Ni through electron transfer, which are conducive to enhancing OER performance. However, excessive Ni doping can lead to structural collapse, resulting in reduced electrochemical surface area and diminished OER performance. By optimizing the Ni doping concentration, a balanced system was achieved, where Co₂NiFe-CA nanoparticles with a Co : Ni : Fe molar ratio of 2 : 1 : 1 exhibited superior OER performance. Under 1.0 M KOH conditions, these nanoparticles achieved a current density of 10 mA cm⁻² with an overpotential of only 287 mV, maintaining stable performance with minimal overpotential increase over 24 hours.

Data availability

The data are available from the corresponding author on reasonable request.

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 acknowledge the financial support from the National Natural Science Foundation of China (52173158 and 32171725), the Natural Science Foundation of Jiangsu Province (BK20200107 and BK20230702), the Industrial Prospect and Key Technology Competition Projects in Jiangsu Province (BE2021081), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX24_0445), the SEU Innovation Capacity Enhancement Plan for Doctoral Students (CXJH_SEU 24169), the Foundation of the Jiangsu Higher Education Institutions (23KJB530006) and the Qinglan Project of Jiangsu Province.

References

1. Z. Li, C. Yu, Y. Kang, X. Zhang, Y. Wen, Z.-K. Wang, C. Ma, C. Wang, K. Wang, X. Qu, M. He, Y.-W. Zhang and W. Song, Nat. Sci. Rev., 2021, 8, nwaa204.
2. Z. Li, M. Hu, P. Wang, J. Liu, J. Yao and C. Li, Coord. Chem. Rev., 2021, 439, 213953.
3. D. Zhou, P. Li, X. Lin, A. McKinley, Y. Kuang, W. Liu, W.-F. Lin, X. Sun and X. Duan, Chem. Soc. Rev., 2021, 50, 8790–8817.
4. J. Wang, H. Yang, F. Li, L. Li, J. Wu, S. Liu, T. Cheng, Y. Xu, Q. Shao and X. Huang, Sci. Adv., 2022, 8, eabl9271.
5. C. Guo, Y. Jiao, Y. Zheng, J. Luo, K. Davey and S.-Z. Qiao, Chem, 2019, 5, 2429–2441.
6. J. Zhou, P. Li, X. Xia, Y. Zhao, Z. Hu, Y. Xie, L. Yang, Y. Liu, Y. Du, Q. Zhou, L. Yu and Y. Yu, Appl. Catal., B, 2024, 359, 124461.
7. F. Li, H. Wu, S. Lv, Y. Ma, B. Wang, Y. Ren, C. Wang, Y. Shi, H. Ji, J. Gu, S. Tang and X. Meng, Small, 2024, 20, 2309025.
8. F. N. I. Sari, Y. Lai, Y. Huang, X. Wei, H. Pourzolfaghar, Y. Chang, M. Ghufron, Y. Li, Y. Su, O. Clemens and J. Ting, Adv. Funct. Mater., 2024, 34, 2310181.
9. D. Tian, S. R. Denny, K. Li, H. Wang, S. Kattel and J. G. Chen, Chem. Soc. Rev., 2021, 50, 12338–12376.
10. W. Ma, D. Li, L. Liao, H. Zhou, F. Zhang, X. Zhou, Y. Mo and F. Yu, Small, 2023, 19, 2207082.
11. D.-H. Park, M.-H. Kim, M. Kim, J.-H. Byeon, J.-S. Jang, J.-H. Kim, D.-M. Lim, S.-H. Park, Y.-H. Gu, J. Kim and K.-W. Park, Appl. Catal., B, 2023, 327, 122444.
12. H. Song, J. Yu, Z. Tang, B. Yang and S. Lu, Adv. Energy Mater., 2022, 12, 2102573.
13. Y. Shi, S. Zhou, J. Liu, X. Zhang, J. Yin, T. Zhan, Y. Yang, G. Li, J. Lai and L. Wang, Appl. Catal., B, 2024, 341, 123326.
14. J. Cao, K. Wang, J. Chen, C. Lei, B. Yang, Z. Li, L. Lei, Y. Hou and K. Ostrikov, Nano-Micro Lett., 2019, 11, 67.
15. P. Wang, S. Song, M. He, C. Li, W. Wang, H. Li, X. Yuan, Z. Fang, K. P. Rinel, W. Song, J. Li, D. G. Vlachos and Z. Li, Chem. Catal., 2023, 3, 100497.
16. D. K. Cho, H. W. Lim, A. Haryanto, B. Yan, C. W. Lee and J. Y. Kim, ACS Nano, 2024, 18, 20459–20467.
17. X. Zou, Y. Liu, G. Li, Y. Wu, D. Liu, W. Li, H. Li, D. Wang, Y. Zhang and X. Zou, Adv. Mater., 2017, 29, 1700404.
18. Y. Zhou, W. Zhang, J. Hu, D. Li, X. Yin and Q. Gao, ACS Sustainable Chem. Eng., 2021, 9, 7390–7399.
19. N. K. Shrestha, S. A. Patil, J. Han, S. Cho, A. I. Inamdar, H. Kim and H. Im, J. Mater. Chem. A, 2022, 10, 8989–9000.
20. H. Xu, W.-D. Zhang, J. Liu, Y. Yao, X. Yan and Z.-G. Gu, J. Colloid Interface Sci., 2022, 607, 1353–1361.
21. H. He, J. Gu, X. Liu, D. Yang, Y. Zhu, R. Yao, Q. Fan and R. Huang, Catalysts, 2020, 10, 431.
22. J. Peng, L. Zhang, M. Song, W. Zhang and K. Peng, Mater. Chem. Phys., 2021, 258, 123918.
23. P. K. Ngumbi, S. W. Mugo, J. M. Ngaruiya and C. K. King’ondu, Mater. Chem. Phys., 2019, 233, 263–266.
24. L. Qin, H. Ye, C. Lai, S. Liu, X. Zhou, F. Qin, D. Ma, B. Long, Y. Sun, L. Tang, M. Yan, W. Chen, W. Chen and L. Xiang, Appl. Catal., B, 2022, 317, 121704.
25. N. K. Shrestha, S. A. Patil, A. S. Salunke, A. I. Inamdar and H. Im, J. Mater. Chem. A, 2023, 11, 14870–14877.
26. N. K. Shrestha, A. I. Inamdar, H. Im and S. Cho, J. Mater. Chem. A, 2024, 12, 29978–29988.
27. L. Yang, Y. Song, F. Luo, L. Yang, X. Wu and X. Liu, Colloids Surf., A, 2024, 686, 133360.
28. S. Chen, Z. Zheng, Q. Li, H. Wan, G. Chen, N. Zhang, X. Liu and R. Ma, J. Mater. Chem. A, 2023, 11, 1944–1953.
29. Z.-L. Wang, K.-A. Wang, X. Xiao and H.-B. Zhu, Chem. Eng. J., 2024, 496, 154218.
30. Y. Duan, Z. Yu, S. Hu, X. Zheng, C. Zhang, H. Ding, B. Hu, Q. Fu, Z. Yu, X. Zheng, J. Zhu, M. Gao and S. Yu, Angew. Chem., 2019, 131, 15919–15924.
31. H. Yu, Y. Wang, C. Zhu, Y. Jing, Q. Song, C. Guan and C. Du, Adv. Mater. Interfaces, 2021, 8, 2001310.
32. N. K. Shrestha, S. A. Patil, J. H. Seok, A. S. Salunke, S. Cho, A. I. Inamdar, Y. Park, S. U. Lee, H. Kim and H. Im, Mater. Today Phys., 2023, 38, 101252.
33. J. Wu, T. Yang, R. Fu, M. Zhou, L. Xia, Z. Wang and Y. Zhao, Adv. Funct. Mater., 2023, 33, 2300808.
34. Z. Gong, R. Liu, H. Gong, G. Ye, J. Liu, J. Dong, J. Liao, M. Yan, J. Liu, K. Huang, L. Xing, J. Liang, Y. He and H. Fei, ACS Catal., 2021, 11, 12284–12292.
35. Y. Chen, Q. Li, Y. Lin, J. Liu, J. Pan, J. Hu and X. Xu, Nat. Commun., 2024, 15, 7278.
36. A. T. N. Nguyen, M. Kim and J. H. Shim, RSC Adv., 2022, 12, 12891–12901.
37. V. Singh, A. Tiwari and T. C. Nagaiah, J. Mater. Chem. A, 2018, 6, 22545–22554.
38. M. Luo, Z. Cai, C. Wang, Y. Bi, L. Qian, Y. Hao, L. Li, Y. Kuang, Y. Li, X. Lei, Z. Huo, W. Liu, H. Wang, X. Sun and X. Duan, Nano Res., 2017, 10, 1732–1739.
39. Y. Dong, S. Komarneni, F. Zhang, N. Wang, M. Terrones, W. Hu and W. Huang, Appl. Catal., B, 2020, 263, 118343.
40. S. Sasidharan, A. Jayasree, S. Fazal, M. Koyakutty, S. V. Nair and D. Menon, Biomater. Sci., 2013, 1, 294–305.
41. A. K. Thottoli and A. K. A. Unni, J. Nanostruct. Chem., 2013, 3, 56.
42. Y. Dong, S. Komarneni, N. Wang, W. Hu and W. Huang, J. Mater. Chem. A, 2019, 7, 6995–7005.
43. F. Li, Z. Sun, H. Jiang, Z. Ma, Q. Wang and F. Qu, Energy Fuels, 2020, 34, 11628–11636.
44. Y. Ding, X. Du and X. Zhang, Dalton Trans., 2020, 49, 15417–15424.
45. K. Zhu, H. Liu, M. Li, X. Li, J. Wang, X. Zhu and W. Yang, J. Mater. Chem. A, 2017, 5, 7753–7758.
46. J. Yao, D. Xu, X. Ma, J. Xiao, M. Zhang and H. Gao, J. Power Sources, 2022, 524, 231068.
47. R. D. L. Smith and C. P. Berlinguette, J. Am. Chem. Soc., 2016, 138, 1561–1567.
48. S. Deka, M. K. Jaiswal, P. Rajput and B. Choudhury, J. Mater. Chem. A, 2024, 12, 9532–9545.
49. W. Zhang, X. Jiang, Z. Dong, J. Wang, N. Zhang, J. Liu, G. Xu and L. Wang, Adv. Funct. Mater., 2021, 31, 2107181.
50. F. Song and X. Hu, Nat. Commun., 2014, 5, 4477.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04240a

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