A Co-doped Ni–Fe mixed oxide mesoporous nanosheet array with low overpotential and high stability towards overall water splitting

Abstract

The construction of bifunctional water splitting electrocatalysts with new and optimized chemical compositions and structures for promoting the efficiencies of both half-reactions of the water splitting process, that is, the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), represents a great advancement. Herein, a Co-doped NiO/NiFe₂O₄ mixed oxide mesoporous nanosheet array on Ni foam with controllable composition and morphology is constructed as a bifunctional electrocatalyst by a facile solvothermal synthesis method using methanol as a solvent and a reactant, followed by annealing in air. The well-connected mesoporous nanosheet array provides a large number of catalytically active sites and buffers the large volume changes that occur during the electrochemical process. Moreover, NiFe₂O₄ has a catalytic active phase that plays a significant role in improving the catalytic activity, and Co²⁺ doping in NiO/NiFe₂O₄ can increase the conductivity and activate the Ni sites at lower overpotentials via charge transfer effects. As a result, the optimized Co-doped NiO/NiFe₂O₄ mixed oxide mesoporous nanosheet array Co_6.25Fe_18.75Ni₇₅O_x exhibits excellent OER activity with a small overpotential of 186 mV at 10 mA cm⁻² and a low Tafel slope of 38.5 mV dec⁻¹. It also presents outstanding HER activity, only requiring a small overpotential of 84 mV to deliver 10 mA cm⁻², along with a low Tafel slope of 53.6 mV dec⁻¹. Moreover, this Co_6.25Fe_18.75Ni₇₅O_x electrode remains stable for 1000 cyclic voltammogram cycles and 12 hours of galvanostatic measurements in both OER and HER. A two-electrode electrolyzer using bifunctional Co_6.25Fe_18.75Ni₇₅O_x as both the anode and cathode demonstrates active and stable overall water splitting performance with a cell voltage of only 1.583 V at 10 mA cm⁻². This study explores multimetal Ni–Fe mixed oxide as an efficient bifunctional electrocatalyst for overall water electrolysis.

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1. Introduction

Hydrogen energy is one of the most promising alternative energies to diminishing fossil fuels. Water electrolysis is a simple approach to engender high-purity hydrogen via electrochemical splitting of water into hydrogen and oxygen. An effective electrocatalyst should optimize the efficiency of both the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) of the water splitting process. Noble metal-based materials, such as Ir- or Ru-based oxides for OER^1,2 and Pt-based materials for HER,^3–5 are considered to be the benchmark due to their outstanding catalytic performance. However, the rarity and high cost of these noble-metal-based materials hamper their practical applications. It is highly desirable to construct low-cost and high-efficiency alternative electrocatalysts for water splitting. Among various promising electrocatalysts available for OER, earth-abundant, low-cost 3d transition metal oxides are considered to be attractive alternatives because active electrocatalytic performance can be acquired through precise control of their compositions and structures.

Recent studies have found that 3d transition metal oxides with complex metal compositions, such as Fe–Ni, Co–Ni and Fe–Ni–Co oxides, possess higher catalytic activities for OER and/or HER than single iron, nickel or cobalt oxides. For example, NiFeO_x film presented almost 10 times higher activity in 1 M NaOH than CoO_x, CoPi, CoFeO_x, NiO_x, NiCeO_x, NiCoO_x, NiCuO_x and NiLaO_x operating at ∼3 mA cm⁻².⁶ Ni–Fe oxide film containing 40% Fe showed OER current densities three orders of magnitude higher than that of Fe oxide film and two orders of magnitude higher than that of freshly deposited Ni oxide film in 0.1 M KOH.⁷ NiO/NiFe₂O₄ mixed oxide powder with 10 mol% Fe exhibited much higher activity in 1 M KOH than either of the pure oxides, with a Tafel slope of 40 mV dec⁻¹.⁸ Ni_0.9Fe_0.1O_x thin film with ∼2 to 3 nm thickness showed the most active OER performance compared with NiO_x, CoO_x, Ni_yCo_1−yO_x, IrO_x, MnO_x, and FeO_x films, requiring an overpotential of 336 mV to drive 10 mA cm⁻² with a Tafel slope of 30 mV dec⁻¹ in 1 M KOH.⁹ An amorphous Fe₄₀Ni₆₀O_x film exhibited the best catalytic parameters among amorphous metal oxide films, including iron, cobalt and nickel, requiring an overpotential of 250 ± 30 mV at 1 mA cm⁻² with a Tafel slope of 34 ± 8 mV dec⁻¹ in 0.1 M KOH.¹⁰ Mesoporous Ni_1−xFe_xO_y nanorods with 33 at% Fe presented a small overpotential of 302 mV at 10 mA cm⁻² with a low Tafel slope of ∼42 mV dec⁻¹ in 1 M KOH.¹¹ Amorphous Ni_0.69Fe_0.31O_x nanoparticles/C exhibited a 280 mV overpotential at 10 mA cm⁻² and a Tafel slope of 30 mV dec⁻¹ in 1.0 M KOH solution.¹² Mesoporous amorphous NiFe hydroxide nanosheets required an overpotential of 200 mV to initiate the reaction; they could deliver high current densities of 500 and 1000 mA cm⁻² at overpotentials of 240 and 270 mV, respectively, in 10 M KOH for OER.¹³ NiFe₂O₄–NiOOH nanosheet arrays could deliver an OER current density of 30 mA cm⁻² with a small overpotential of 240 mV; they required an overpotential of only 410 mV to achieve a remarkably high current density of 3000 mA cm⁻².¹⁴ Oxygen vacancy-rich NiCo₂O₄ ultrathin nanosheets provided a large current density of 285 mA cm⁻² at a small overpotential of 320 mV for OER in 1 M KOH.¹⁵ The oxygen vacancy-rich NiCo₂O₄ nanowire arrays exhibited a large current density of 360 mA cm⁻² with an overpotential of only 317 mV for HER in 1.0 M KOH.¹⁶ Nanoporous NiCo₂O₄ nanowires with cobalt–nickel layered oxide nanosheets produced a current density of 10 mA cm⁻² at an overpotential of 340 mV for OER in 0.1 M KOH and a current density of 10 mA cm⁻² at an overpotential of 52 mV for HER in 0.5 M H₂SO₄.¹⁷ Amorphous FeCoNi ternary oxide nanosheets obtained by anodization of an alloy plate and subsequent low-temperature annealing showed superior electrocatalytic activity toward the oxygen evolution reaction, with an overpotential of 170 mV at 5 mA cm⁻² and a low Tafel slope of 37 mV dec⁻¹ in 0.1 M KOH.¹⁸ Amorphous FeCoNi hydroxide nanosheets prepared by electrodeposition on nickel foam exhibited a high current density of 300 mA cm⁻² at an overpotential of 370 mV in 10 M KOH.¹⁹ Despite outstanding OER performance in alkaline electrolyte, the CoFeNi ternary oxide that can effectively catalyze hydrogen evolution in alkaline media has rarely been reported. The use of a CoFeNi ternary oxide as a state-of-the-art electrocatalyst for overall water splitting would represent a great advancement in this field. Optimizing the compositions and structures of transition metal compound catalysts is a promising method to further promote their OER and HER performances.^16,20 Constructing freestanding 3D porous nanostructure catalysts on conductive substrates and directly employing them as electrodes can provide abundant exposed active sites and cushion volume changes during reversible redox reactions, thus promoting their electrocatalytic activity and stability. More interestingly, the incorporation of Co²⁺ ions into Ni–Fe mixed oxide can manipulate its electronic structure, thereby greatly improving its electrocatalytic performance. Herein, a Co-doped NiO/NiFe₂O₄ mixed oxide mesoporous nanosheet array on Ni foam with controllable composition and morphology was constructed by a facile solvothermal approach using methanol as solvent and reactant, followed by an annealing process. The well-connected mesoporous nanosheet array provides a large number of catalytically active sites and cushions the large volume changes that occur during the electrochemical process. Moreover, the catalytic active phase of NiFe₂O₄ plays a crucial role in improving the catalytic activity, while Co²⁺ doping increases conductivity and activates the Ni sites at a lower overpotential via charge transfer effects. As a result, the optimal Co-doped NiO/NiFe₂O₄ mesoporous nanosheet array, Co_6.25Fe_18.75Ni₇₅O_x, exhibits excellent OER activity, with small overpotentials of 186, 219 and 228 mV at 10, 50 and 100 mA cm⁻², respectively, and can achieve 220.7 mA cm⁻² at a small overpotential of 235 mV. Interestingly, this Co_6.25Fe_18.75Ni₇₅O_x electrode also demonstrates outstanding HER activity, with small overpotentials of 84, 155 and 192 mV to reach 10, 50 and 100 mA cm⁻², respectively. A two-electrode alkaline electrolyzer using Co_6.25Fe_18.75Ni₇₅O_x as both anode and cathode requires a cell voltage of only 1.583 V to drive 10 mA cm⁻², along with good durability. This work furthers the development of multimetal Co–Ni–Fe mixed oxides for enhanced OER and HER electrocatalysis.

2. Experimental

Synthesis of Co-doped Ni–Fe mixed oxide mesoporous nanosheet array on Ni foam

Ni foam with dimensions of 2 × 3 cm was soaked in 6 M hydrochloric acid for 15 min and washed with deionized water and absolute ethanol. 3 mmol of Ni(NO₃)₂·6H₂O, 0.25 mmol of CoCl₂·6H₂O, 0.75 mmol of Fe(NO₃)₃·9H₂O and 1.5 mmol of CO(NH₂)₂ were dissolved in 40 mL of methanol; then, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The cleaned Ni foam was immersed in the solution, which was then heated at 120 °C for 6 h. The precipitate deposited on Ni foam was rinsed with deionized water and absolute ethanol three times, then dried in vacuum at 50 °C for 6 h. Afterward, the Ni foam covered by the precursor was annealed at 350 °C in air for 2 h to acquire Co-doped Ni–Fe mixed oxide on Ni foam (Co_yFe_zNi_100−y−zO_x). For other Co and Fe contents, the amounts of CoCl₂·6H₂O and Fe(NO₃)₃·9H₂O were changed to 0.5 and 0.5 mmol and 0.75 and 0.25 mmol, respectively. Co-doped NiO (Co_yNi_100−yO_x), Ni–Fe mixed oxide (Fe_yNi_100−yO_x), and pure NiO on Ni foam was prepared by the same method used for Co_yFe_zNi_100−y−zO_x except with the addition of only Co source, only Fe source, or no Co and Fe source, respectively. The elemental ratios of the metals in Co_yFe_zNi_100−y−zO_x, Co_yNi_100−yO_x and Fe_yNi_100−yO_x were calculated according to their energy-dispersive X-ray spectra; the ratios were found to be similar to the feeding ratios. Therefore, the samples were designated with their feeding ratios.

Characterization

X-ray diffraction (XRD) patterns were acquired on a D8 advance diffraction system with high-intensity Cu Kα radiation. The field-emission scanning electron microscope (FESEM) images were acquired on a Hitachi S-4800 field-emission scanning electron microscope. The transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) images were acquired on a JEOL JEM-ARF200F instrument. Raman spectra were measured on a M-9836-3991-04-A Raman spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB MK II X-ray photoelectron spectrometer.

Electrochemical measurements

All electrochemical measurements were performed on a CHI 660D electrochemical working station (Chenhua Corp., China) at room temperature (25 °C). The catalysts were measured in 1.0 M KOH aqueous solution using a typical three-electrode configuration, in which Co_yFe_zNi_100−y−zO_x/NF, Co_yNi_100−yO_x/NF, Fe_yNi_100−yO_x/NF and NiO/NF (cut into 1 × 1 cm pieces with a loading of ∼4 mg cm⁻²) were used as the working electrode; platinum wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Cyclic voltammograms (CVs) were acquired at a scan rate of 50 mV s⁻¹. Linear sweep voltammetry (LSV) polarization curves were acquired at a scan rate of 5 mV s⁻¹ with 90% iR-compensation unless otherwise mentioned. The mass activity of the catalyst was calculated by the measured current density (mA cm⁻²) divided by the loading amount of catalyst (mg cm⁻²).²¹ The electrochemical impedance spectroscopy (EIS) tests were measured at the open-circuit potential in the frequency range of 100 kHz to 0.01 Hz. Chronoamperometric curves were recorded at a constant current density without iR-compensation. The electrochemical active surface areas (ECSAs) were determined from the capacitance measurements in the potential region of no faradic process at different scan rates of 2, 4, 6, 8, 10 and 12 mV s⁻¹. Commercial RuO₂ and Pt/C-loaded electrodes were fabricated as benchmarks for comparison of the OER and HER performance of the catalyst, respectively. 20 mg of RuO₂ or Pt/C was ultrasonically scattered in 0.2 mL of Nafion solution (0.5 wt%) and 0.8 mL of ethanol. The dispersion was transferred onto Ni foam by a controlled drop casting approach to obtain a loading of 4 mg cm⁻². Overall water splitting tests were performed in a two-electrode configuration with Co_6.25Fe_18.75Ni₇₅O_x as both anode and cathode by acquiring linear sweep voltammetry (LSV) polarization curves with 90% iR-compensation at a scan rate of 5 mV s⁻¹ and chronoamperometric curves at a constant current density without iR-compensation.

3. Results and discussion

X-ray diffraction (XRD) patterns were first applied to examine the effects of the Fe/Co ratio on the crystalline phases and compositions of the samples. As shown in Fig. 1, when the feeding ratio of Fe/Co was 1 : 3, the sample primarily showed the reflections of cubic NiO (JCPDS No. 78-0643). As the feeding ratio of Fe/Co was increased to 1 : 1, in addition to the reflections of cubic NiO, two peaks at 2θ about 30.3° and 35.7° appeared; these can be attributed to the diffraction peaks of the (220) and (311) planes of spinel NiFe₂O₄, respectively (JCPDS No. 74-2081), indicating the formation of NiO/NiFe₂O₄ mixed oxide. When the feeding ratio of Fe/Co was further increased to 3 : 1, the diffraction peaks at 30.3° and 35.7° were stronger, and another diffraction peak at 57.4° attributed to the (511) plane of NiFe₂O₄ appeared; this suggests that more NiFe₂O₄ phase was formed. Meanwhile, without addition of Fe and Co sources, the sample was pure NiO. Moreover, the diffraction peaks of the Co-doped NiO/NiFe₂O₄ samples showed no noticeable shifts.

Fig. 1 XRD patterns of Co-doped NiFe mixed oxides and pure NiO, acquired with powders ultrasonically detached from the Ni foam. From bottom to top: pure NiO; Co-doped NiFe mixed oxide samples fabricated with Fe/Co ratios of 1 : 3, 1 : 1 and 3 : 1, respectively.

To reveal the roles of the Co and Fe sources in determining the final phases of the samples, contrast experiments with addition of only Co or Fe source were carried out. When only Co source was introduced, the XRD patterns of the samples could be all readily indexed to cubic NiO (Fig. S1a†); no apparent peak shift was observed, demonstrating that the similar radii of Co and Ni atoms enable Co atoms to substitute Ni atoms in the lattice of NiO without noticeable changes in the lattice constants. When only Fe source was introduced, the reflections of the XRD patterns could be indexed to cubic NiO with the addition of 0.25 mmol Fe source (Fig. S1b†); the reflections showed a slight shift to lower 2θ angles, revealing that the lattice constants are shifted to higher values and implying substitutional incorporation of some Fe ions into the cubic NiO structure. The XRD patterns of the samples with addition of 0.5 and 0.75 mmol Fe source presented NiFe mixed oxides of cubic NiO and spinel NiFe₂O₄. It can be seen that the diffraction peaks of Co-doped NiO/NiFe₂O₄ samples with addition of 0.5 and 0.75 mmol Fe source showed no peak shifts compared with the corresponding NiO/NiFe₂O₄ samples, further suggesting that Co doping has no effect on the lattice constants of NiO/NiFe₂O₄.

Raman spectroscopy is an excellent complement to XRD because it can also detect small amounts of crystalline nanoparticles and amorphous phases that cannot be detected by XRD. Raman spectroscopy was utilized to investigate the as-prepared oxide phases. As shown in Fig. 2, a broad peak at 490 cm⁻¹ was present in the spectrum of the pure NiO sample, in agreement with the literature.²² After addition of Co and Fe sources, the samples exhibited three Raman bands at about 470, 550 and 680 cm⁻¹, indicative of spinel NiFe₂O₄ phase.⁸ Although the sample with the feeding ratio of Fe/Co 1 : 3 only showed the reflections of cubic NiO in its XRD pattern, Raman spectroscopy detected the presence of spinel NiFe₂O₄ phase. With the addition of only Co source, the peak attributed to NiO was shifted to higher wavenumbers at about 530 cm⁻¹ (Fig. S2a†). When only Fe source was added, the samples exhibited three Raman bands at about 480, 560 and 690 cm⁻¹ (Fig. S2b†), from which we can deduce that the bands of the Co-doped NiO/NiFe₂O₄ samples were negatively shifted; this demonstrates that Co doping can adjust the electronic structure of the samples.

Fig. 2 Raman spectra of the Co-doped NiFe mixed oxides and pure NiO. From bottom to top: pure NiO; Co-doped NiFe mixed oxide samples fabricated with Fe/Co ratios of 1 : 3, 1 : 1 and 3 : 1, respectively.

The morphologies of the Co-doped NiFe mixed oxide samples were investigated by scanning electron microscope (SEM). A pure NiO mesoporous nanosheet array on Ni foam without addition of Fe and Co sources was obtained (Fig. 3a and b) in which the average lateral size of the nanosheets was about 450 nm. Careful observation reveals that many mesopores are distributed on the surface of the nanosheets. After introduction of Fe and Co sources with a Fe/Co ratio of 1 : 3, the sample exhibited a denser mesoporous nanosheet array structure, with smaller lateral nanosheet size of about 300 nm (Fig. 3c and d). When the Fe/Co ratio was increased to 1 : 1, the sample showed much a denser mesoporous nanosheet array structure in which the lateral size of the nanosheets was about 250 nm (Fig. 3e and f). When the Fe/Co ratio was further increased to 3 : 1, a uniformly distributed mesoporous nanosheet array with the smallest nanosheets (about 180 nm) was formed (Fig. 3g and h). These results clearly indicate that the introduction of Fe and Co atoms can effectively modulate the density and size of Co-doped NiFe mixed oxide nanosheets on Ni foam. The EDX results (Fig. S3†) revealed that the chemical elements of Co, Fe, Ni and O were present in all the Co-doped NiFe mixed oxides; the atomic ratios of Co, Fe and Ni were calculated from these results and were basically consistent with the feeding ratios. When only Co source was introduced, the samples were all sparse mesoporous nanosheet arrays composed of large nanosheets (Fig. S4a–c†); as more Co atoms were introduced, the samples became more sparse. Meanwhile, when only Fe source was added, the samples were all dense nanosheet arrays with small nanosheets (Fig. S4d–f†); as more Fe atoms were introduced, the samples became denser. In addition, large cracks began to appear in the NiFe mixed oxides upon addition of 0.75 mmol Fe source (Fig. S4f†). Herein, methanol was selected as the solvent to control the hydrolysis of Ni²⁺ and Fe³⁺ and obtain well-constructed nanosheet array structure precursors. Taking Co_6.25Fe_18.75Ni₇₅O_x precursor as an example, in the contrast experiment in 40 mL of deionized water, some nonuniform nanosheets-assembled flower-like structures appeared on the nanosheet array (Fig. S5†). Therefore, 40 mL of methanol was selected as the solvent to obtain the desired nanosheet array precursors. The EDX results revealed that the atomic ratios of Co and Ni in the Co-doped NiO samples as well as Fe and Ni in the NiFe mixed oxide samples were all essentially consistent with their feeding ratios (Fig. S6†).

Fig. 3 Low- and high-magnification SEM images of pure NiO and the Co-doped NiFe mixed oxides. (a and b) Pure NiO. (c and d) Co_18.75Fe_6.25Ni₇₅O_x. (e and f) Co_12.5Fe_12.5Ni₇₅O_x. (g and h) Co_6.25Fe_18.75Ni₇₅O_x.

The TEM image of the Co_6.25Fe_18.75Ni₇₅O_x sample in Fig. 4a shows that it consisted of flexible and nearly transparent mesoporous nanosheets. The magnified TEM image of part of an individual nanosheet in Fig. 4b reveals that numerous mesopores with an average diameter of 2 nm were evenly distributed on the surface of the nanosheet. Therefore, the high number of exposed active edge sites could be anticipated to facilitate the dissociation of water. The HRTEM image of Co_6.25Fe_18.75Ni₇₅O_x in Fig. 4c revealed that one set of adjacent fringes was 0.25 nm, corresponding to the d-spacing of the (311) planes of NiFe₂O₄; another set of adjacent fringes was 0.21 nm, corresponding to the d-spacing of the (200) planes of NiO. Fig. 4d presents a high-angle annular dark-field STEM image of a Co_6.25Fe_18.75Ni₇₅O_x nanosheet, which further displays abundant mesoporosities on the nanosheet. The corresponding elemental mapping images suggested the homogeneous distribution of Co, Fe, Ni and O elements across the nanosheet.

Fig. 4 (a and b) Low- and high-magnification TEM images, (c) HRTEM image, (d) STEM image and corresponding elemental mapping images of Co_6.25Fe_18.75Ni₇₅O_x.

The surface chemical valence states on Co_6.25Fe_18.75Ni₇₅O_x were examined by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5a, the Ni 2P XPS spectrum showed two peaks located at 855.3 and 873.1 eV, which can be ascribed to Ni 2p_3/2 and Ni 2p_1/2, respectively, while the peaks at 861.7 and 880.2 eV can be assigned to the Ni satellite peaks of NiO.²³ The fitting peaks of the Fe 2p spectrum showed a peak at 705.2 eV (Fig. 5b), which is the Ni auger peak.²⁴ The peaks at 711.6 and 724.8 eV correspond to Fe 2p_3/2 and Fe 2p_1/2, respectively, which are typical for Fe³⁺.^8,22,25 The missing shakeup satellites at 719 and 732 eV excluded the presence of a separate Fe₂O₃ phase.⁸ As shown in Fig. 5c, the deconvolved peaks of Co 2p with binding energies at 781.3 and 796.4 eV can be ascribed to Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, respectively, and the peaks at 786.8 and 803.8 eV can be individually assigned to the satellite peaks of Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, suggesting the presence of Co²⁺.^26,27 The O 1s spectrum in Fig. 5d exhibited two binding energies of 530.2 and 531.7 eV, which correspond to the lattice oxygen and surface hydroxyl oxygen, respectively.^28,29 The XPS spectra of three Co-doped NiO samples presented the binding energies of Ni 2p without obvious shifts (Fig. S7a†); however, those of Co 2p (Fig. S7b†) and O 1s (Fig. S7c†) were negatively shifted with increasing Co content, indicating that Co doping can promote electron transfer from Ni atoms to Co atoms in Co-doped NiO. The XPS spectra of the three Co-doped NiO/NiFe₂O₄ samples showed the binding energies of Ni 2p (Fig. S8a†) and Fe 2p (Fig. S8b†) without noticeable shifts; however, those of Co 2p (Fig. S8c†) and O 1s (Fig. S8d†) were positively shifted with decreasing Co content. Moreover, the peak positions of Co 2p in Co-doped NiO/NiFe₂O₄ shifted slightly to higher binding energies compared with those in Co-doped NiO. These results indicate that electrons were transferred from Ni atoms to Co atoms and Fe atoms in Co-doped NiO/NiFe₂O₄.

Fig. 5 High-resolution XPS spectra of Ni 2p (a), Fe 2p (b), Co 2p (c), and O 1s (d) of Co_6.25Fe_18.75Ni₇₅O_x.

The effective electrochemical surface areas (ECSAs) of the Co-doped NiFe mixed oxide samples were estimated by measuring their double layer capacitances (C_dl) (without faradaic processes) by cyclic voltammetry at different scan rates (Fig. S9†). The C_dl increased from 3.48 mF cm⁻² for pure NiO to 3.92 mF cm⁻² for Co_18.75Fe_6.25Ni₇₅O_x and increased to 5.62 mF cm⁻² for Co_12.5Fe_12.5Ni₇₅O_x; then, the C_dl increased to a maximum of 8.39 mF cm⁻² for Co_6.25Fe_18.75Ni₇₅O_x. These results demonstrate that introduction of Co and Fe atoms is a feasible method to modulate the ECSAs of Co-doped NiFe mixed oxide samples.

The electrocatalytic activities of the Co-doped NiFe mixed oxides for OER were evaluated. The polarization curves in Fig. 6a showed that all the Co-doped NiFe mixed oxide mesoporous nanosheet arrays demonstrated higher OER activity than pure NiO; among the arrays, the Co_6.25Fe_18.75Ni₇₅O_x sample exhibited the highest OER activity, with the lowest overpotentials of 186, 219 and 228 mV at 10, 50 and 100 mA cm⁻², respectively, and could achieve 220.7 mA cm⁻² at a quite small overpotential of 235 mV. For RuO₂ catalyst, the required overpotentials to reach 10, 50 and 100 mA cm⁻² were 244, 294 and 323 mV, respectively. Remarkably, Co_6.25Fe_18.75Ni₇₅O_x exhibited better OER catalytic activity than RuO₂. The Ni foam substrate had poor OER activity and contributed only slightly to the catalytic performance of the Co-doped NiFe mixed oxide mesoporous nanosheet array. The corresponding OER LSV curves of Co_6.25Fe_18.75Ni₇₅O_x electrode with and without iR correction are shown in Fig. S10.† The OER mass activity of Co_6.25Fe_18.75Ni₇₅O_x (Fig. S11†) was 55.2 A g⁻¹ at a quite small overpotential of 235 mV, which is 84-fold higher than that of NiO. The variation of the OER catalytic activity for different Co-doped NiFe mixed oxides should be related to the different Fe and Co contents as well as the change of ECSAs. The superior OER activity of Co_6.25Fe_18.75Ni₇₅O_x was confirmed by its smallest Tafel slope of 38.5 mV dec⁻¹ (Fig. 6b), which is indicative of prominent electrocatalytic performance. The low overpotential and small Tafel slope of Co_6.25Fe_18.75Ni₇₅O_x demonstrated its outstanding catalytic activity for OER; these values are comparable to or better than those of other state-of-the-art Ni-based OER catalysts, as listed in Table S1.†

Fig. 6 (a) Polarization curves for the Co-doped NiFe mixed oxides, NiO, commercial RuO₂ on Ni foam and bare Ni foam for OER. (b) Corresponding Tafel plots. (c) Multi-step chronopotentiometric curve of Co_6.25Fe_18.75Ni₇₅O_x without iR compensation. (d) Polarization curves of Co_6.25Fe_18.75Ni₇₅O_x before and after 1000 CV cycles. (e) Galvanostatic curve of Co_6.25Fe_18.75Ni₇₅O_x at 20 mA cm⁻² without iR compensation.

To reveal the roles of Co and Fe atoms in the Co-doped NiFe mixed oxide for the enhanced OER activity, comparative experiments with individual addition of Co or Fe source to the reaction system were carried out. When only Co source was added, the OER catalytic activity of Co-doped NiO was somewhat enhanced compared to that of pure NiO; the Co_14.3Ni_85.7O_x electrode exhibited the highest catalytic activity, with overpotentials of 270, 317 and 338 mV at 10, 50 and 100 mA cm⁻², respectively (Fig. S12a†). These values are obviously much larger than those of the Co_6.25Fe_18.75Ni₇₅O_x electrode. When only Fe source was added, the OER catalytic activity of NiFe mixed oxide was significantly improved compared to that of pure NiO; the Fe_14.3Ni_85.7O_x electrode showed the highest catalytic activity, with overpotentials of 204, 238 and 247 mV to afford 10, 50 and 100 mA cm⁻², respectively (Fig. S12b†). However, these values were also inferior to those of the Co_6.25Fe_18.75Ni₇₅O_x electrode. These results indicate that the Co and Fe atoms play a synergistic role in enhancing the OER catalytic activity of NiO. Fig. 6c presents a multi-step chronopotentiometric curve for the Co_6.25Fe_18.75Ni₇₅O_x electrode. The potential instantaneously levels off at 1.527 V at 50 mA cm⁻² and remains constant for the next 500 s; the other steps present similar results up to 500 mA cm⁻², demonstrating the outstanding mass transport properties and mechanical robustness of the Co_6.25Fe_18.75Ni₇₅O_x electrode. Stability is a crucial parameter to assess the quality of an electrocatalyst. Long-term potential cycling tests of the Co_6.25Fe_18.75Ni₇₅O_x electrode were conducted by acquiring continuous cyclic voltammograms between 1.0 and 1.6 V (vs. RHE) at a scanning rate of 50 mV s⁻¹ for 1000 cycles. The polarization curve of this catalyst displayed negligible loss of anodic current after 1000 cycles (Fig. 6d). The long-term durability of Co_6.25Fe_18.75Ni₇₅O_x for OER was assessed by continuous galvanostatic measurement for 12 h at 20 mA cm⁻² (Fig. 6e). The overpotential of this electrode slightly increased by about 10 mV during 12 h of continuous galvanostatic electrolysis, manifesting its outstanding stability for OER. The morphology and phase of the sample were well preserved after the long-term durability test, as indicated by SEM and TEM images, XRD patterns and XPS spectra (Fig. S13†).

The catalytic activity for hydrogen evolution displayed a similar trend on the Co-doped NiFe mixed oxide mesoporous nanosheet arrays. The polarization curves of the Co-doped NiFe mixed oxide electrodes all showed improved HER activity compared to the NiO electrode (Fig. 7a); the Co_6.25Fe_18.75Ni₇₅O_x electrode presented the highest HER activity, exhibiting a rapid increase in current density with applied potential. The overpotentials of the Co_6.25Fe_18.75Ni₇₅O_x electrode required to deliver cathodic current densities of 10, 50 and 100 mA cm⁻² were 84, 155 and 192 mV, respectively, which are much smaller than those for NiO (146, 251 and 291 mV at 10, 50 and 100 mA cm⁻², respectively). The corresponding HER LSV curves of Co_6.25Fe_18.75Ni₇₅O_x electrode with and without iR correction are shown in Fig. S14.† The HER mass activity of Co_6.25Fe_18.75Ni₇₅O_x was 6.8 A g⁻¹ at an overpotential of 125 mV (Fig. S15†), which is 3.5 times higher than that of NiO. Although Pt/C electrode manifested prominent activity for HER, the overpotentials of the Co_6.25Fe_18.75Ni₇₅O_x electrode were comparable to or smaller than those of other state-of-the-art Ni-based HER catalysts, as listed in Table S2,† indicating that the present Co_6.25Fe_18.75Ni₇₅O_x electrode also exhibits excellent catalytic performance for HER. The corresponding Tafel slopes, shown in Fig. 7b, reveal a value of 53.6 mV dec⁻¹ for Co_6.25Fe_18.75Ni₇₅O_x, which is smaller than that of the other Co-doped NiFe mixed oxides and NiO and comparable to or better than most Ni-based catalysts for HER (Table S2†). The Tafel slope is generally determined at a small overpotential where the apparent increase in current density begins. The current density of the Pt/C electrode increased more greatly than that of the Co_6.25Fe_18.75Ni₇₅O_x electrode in the overpotential range where the current density began to evidently increase, leading to a lower Tafel slope of the Pt/C electrode (30.0 mV dec⁻¹). However, the overpotential of the Pt/C electrode increased quickly at high current density because the glued Pt/C catalyst tends to readily detach from the substrate. Similar results for glued Pt/C electrodes were observed in other studies.^16,30 The comparative experiments with Co-doped NiO catalysts showed that their HER catalytic activities were slightly enhanced compared to NiO, and the Co doping content had no obvious effect on the HER activity (Fig. S16a†). Meanwhile, the HER catalytic activities of the NiFe mixed oxide catalysts were significantly improved; Fe_14.3Ni_85.7O_x manifested the highest activity, with overpotentials of 115, 216 and 262 mV at 10, 50 and 100 mA cm⁻², respectively (Fig. S16b†). However, these values are obviously inferior to those of the Co_6.25Fe_18.75Ni₇₅O_x electrode. These results indicate that the concurrent introduction of Co and Fe atoms can significantly enhance the HER catalytic performance of NiO.

Fig. 7 (a) Polarization curves for the Co-doped NiFe mixed oxides, NiO, commercial Pt/C on Ni foam and bare Ni foam for HER. (b) The corresponding Tafel plots. (c) The multi-step chronopotentiometric curve of Co_6.25Fe_18.75Ni₇₅O_x without iR compensation. (d) The polarization curves of Co_6.25Fe_18.75Ni₇₅O_x before and after 1000 CV cycles. (e) The galvanostatic curve of Co_6.25Fe_18.75Ni₇₅O_x at 20 mA cm⁻² without iR compensation.

Tafel slopes are commonly used to distinguish the predominant HER mechanism. In alkaline electrolyte, this can be either the Volmer–Heyrovsky or Volmer–Tafel pathway. When the rate-limiting step is the Volmer, Heyrovsky, or Tafel reaction, the Tafel slope will be ∼120, ∼40, or ∼30 mV dec⁻¹, respectively.³¹ Therefore, the HER over the Co-doped NiFe mixed oxide nanosheet array proceeded via a Volmer–Heyrovsky mechanism, where rapid discharge of a proton is followed by the rate-limiting electrochemical step of recombination with an additional proton.

A multi-step chronopotentiometric curve of the Co_6.25Fe_18.75Ni₇₅O_x electrode from 50 mA cm⁻² to 500 mA cm⁻² demonstrated good voltage response vs. current density (Fig. 7c), manifesting that this electrode also has outstanding mass transport properties and mechanical robustness for HER. To assess the HER stability of the Co_6.25Fe_18.75Ni₇₅O_x electrode, a deterioration experiment was conducted by applying potential sweeps from −0.3 to +0.3 V for 1000 cycles. The polarization curve after the sweeps showed only a slight increase in overpotential (Fig. 7d). The long-term electrochemical stability of this electrode was further evaluated by galvanostatic measurement at 20 mA cm⁻² for 12 h. The potential increased by only 9.7 mV at the end of 12 h of electrolysis (Fig. 7e), indicating the superior stability of the Co_6.25Fe_18.75Ni₇₅O_x electrode for HER in the long-term electrochemical process. The durability test presented no obvious effects on the morphology or phase of the catalyst (Fig. S17†), demonstrating that the Co_6.25Fe_18.75Ni₇₅O_x electrode is also cathodically stable in strongly alkaline media.

Because EIS provides information relating to the internal resistance of an electrode material and the resistance between the electrode and the electrolyte, EIS measurements were performed to further elucidate the superior electrochemical performance of the Co-doped NiFe mixed oxide mesoporous nanosheet array. Fig. 8 presents the Nyquist plots of the Co-doped NiFe mixed oxides and NiO electrodes; all the plots are composed of a depressed semicircle in the high-frequency region, which can be assigned to the charge transfer resistance of the electrode/electrolyte interface. It can be seen that the semicircles of the Co-doped NiFe mixed oxide electrodes are all much smaller than that of NiO; this indicates the greatly increased conductivity of the Co-doped NiFe mixed oxide electrode, which can improve the charge transfer kinetics and contribute to superior electrochemical performance in OER and HER. The EIS of the Co-doped NiO and NiFe mixed oxide electrodes all manifested greatly increased conductivity compared to NiO (Fig. S18†), in which the Co-doped NiO electrodes presented better electrical conductivity than the NiFe mixed oxide electrodes. The results also showed that the conductivities of the Co-doped NiFe mixed oxides were better than those of the NiFe mixed oxides, which contributed to their better electrochemical activities.

Fig. 8 Nyquist plots of the Co-doped NiFe mixed oxide electrodes. As a comparison, the plot of NiO is also presented.

The excellent electrocatalytic performance of the Co-doped NiFe mixed oxide nanosheet array electrode for OER and HER can be ascribed to its unique architecture, including the mesoporous nanosheet array structure, the introduction of NiFe₂O₄ active phase and Co²⁺ doping. The open spaces between the nanosheets and the mesopores within the nanosheets offer rich accessible electroactive sites, superior electron collection efficiency and buffering of the large volume changes during reversible redox reactions. The enhancement of the OER and HER activities of the NiFe mixed oxides in comparison to NiO can be attributed to the introduction of NiFe₂O₄ active phase.³² The further enhancement of the OER and HER performances of the Co-doped NiFe mixed oxides can be ascribed to conductivity and charge-transfer effects. The enhanced conductivity of Co-doped NiO/Ni₂FeO₄ elevates its charge transfer kinetics and contributes to the superior electrochemical performance of OER and HER. The Co doping in NiFe mixed oxides enables electron transfer from Ni atoms to Co atoms and Fe atoms. The electron-deficient Ni atoms can readily adsorb hydroxyl or oxyhydroxyl groups, which is beneficial for OER kinetics. The charge-transfer effects of Co promote oxidation of Ni to the more conductive NiOOH phase at lower overpotentials.³³ Meanwhile, electron-enriched Co atoms are prone to obtain electrons and react with adsorbed water molecules to produce hydrogen atoms, resulting in efficient HER activity. As the OER and HER kinetics are most favorable with moderate coverage of the intermediates, the appropriate electron transfer in Co_6.25Fe_18.75Ni₇₅O_x endows this catalyst with superior activity for both OER and HER.

Because the Co_6.25Fe_18.75Ni₇₅O_x mesoporous nanosheet array can be used as an active and durable catalyst for both OER and HER in 1.0 M KOH, a two-electrode electrolyzer was assembled with Co_6.25Fe_18.75Ni₇₅O_x as both the anode and cathode. The linear sweep voltammogram curve of the overall water electrolysis on Co_6.25Fe_18.75Ni₇₅O_x is presented in Fig. 9a. For comparison, the Pt/C‖RuO₂ couple was also investigated. As expected, the Pt/C‖RuO₂ couple produced an excellent catalytic system at low current density. However, the Pt/C‖RuO₂ couple tended to readily detach from the Ni foam substrate at high current density because Pt/C and RuO₂ were glued on the Ni foam. Therefore, the single-cell water electrolyzer tested with the Pt/C‖RuO₂ couple provided a current density of less than 150 mA cm⁻². The Co_6.25Fe_18.75Ni₇₅O_x‖Co_6.25Fe_18.75Ni₇₅O_x couple only required 1.583 V to reach 10 mA cm⁻². This excellent low potential is comparable to or lower than the values for recently reported state-of-the-art catalysts, such as 1.68 V for Ni(OH)₂ nanosheets/NF,³⁴ 1.66 V for NiSe₂ nanoparticle film/Ti,³⁵ 1.60 V for NiFe-LDH ultrathin sheets/NiCo₂O₄ nanowires,³⁶ 1.6 V for Co–Ni–Se/C/NF,³⁰ 1.63 V for NiSe nanowire film/NF,³⁷ 1.61 V for CoNi₂Se₄ on carbon fiber paper,³⁸ 1.62 V for Co_0.13Ni_0.87Se₂/Ti,³⁹ 1.59 V for NiCoP/rGO/CFP,⁴⁰ and 1.612 V for Ni₃Se₂ nanoforest/NF.⁴¹ A chronopotentiometric test at 20 mA cm⁻² for 20 h showed a slight overpotential loss of 12 mV (Fig. 9b), suggesting that the catalyst is very stable after long-term operation. Therefore, the Co-doped NiFe mixed oxide mesoporous nanosheet array is a promising low cost, highly active, and stable bifunctional electrocatalyst for overall water splitting.

Fig. 9 (a) LSV curves for overall water splitting with Co_6.25Fe_18.75Ni₇₅O_x‖Co_6.25Fe_18.75Ni₇₅O_x and RuO₂‖Pt/C couples in a two-electrode device. (b) The long-term chronopotentiometric curve for Co_6.25Fe_18.75Ni₇₅O_x ‖Co_6.25Fe_18.75Ni₇₅O_x couple at a current density of 20 mA cm⁻² in the two-electrode device.

The actual amounts of generated O₂ and H₂ were determined by gas chromatography in an H-type electrolytic cell with a separate anode and cathode. The amounts of experimentally quantified gases produced during electrolysis at 20 mA cm⁻² are presented in Fig. 10 and are roughly in agreement with theoretically calculated values, with faradic efficiencies over 92% for both OER and HER.

Fig. 10 Amounts of gas experimentally measured and theoretically calculated vs. time for Co_6.25Fe_18.75Ni₇₅O_x at 20 mA cm⁻².

4. Conclusions

A Co-doped NiFe mixed oxide mesoporous nanosheet array with controllable composition and morphology has been constructed on Ni foam by a simple solvothermal reaction in methanol followed by an annealing process. The optimized Co_6.25Fe_18.75Ni₇₅O_x can be used as a novel active and durable bifunctional electrocatalyst for both OER and HER. The small overpotentials of 186, 219, and 228 mV at 10, 50 and 100 mA cm⁻² for OER are lower than those of most reported Ni-based compounds, while the small overpotentials of 84, 155 and 192 mV at 10, 50 and 100 mA cm⁻² for HER manifest that Co_6.25Fe_18.75Ni₇₅O_x can also serve as an efficient HER electrode. As a result, a two-electrode water electrolyzer using this Co_6.25Fe_18.75Ni₇₅O_x electrode provides a current density of 10 mA cm⁻² at a cell voltage of 1.583 V and shows long durability. The mesoporous nanosheet array structure offers a large number of catalytically active sites and open channels for ion and electron transport. Moreover, NiFe₂O₄ is a catalytic active phase. In addition, Co²⁺ doping in NiO/NiFe₂O₄ can increase its conductivity and activate the Ni sites at lower overpotentials via charge transfer effects. This work may encourage the design and exploration of new cost-effective multimetal bifunctional electrodes for enhanced OER and HER electrocatalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support of the Natural Science Foundation of China (2157050547 and 21671006) is greatly acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta07956g

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