Phosphorus doped two-dimensional CoFe₂O₄ nanobelts decorated with Ru nanoclusters and Co–Fe hydroxide as efficient electrocatalysts toward hydrogen generation

Abstract

Developing efficient and durable hydrogen evolution reaction (HER) electrocatalysts has attracted considerable attention for large-scale hydrogen generation. In this work, phosphorus doped two-dimensional (2D) CoFe₂O₄ nanobelts decorated with Ru and CoFe hydroxide clusters on iron foam (IF) (CoFeO_xH_y–Ru/P–CoFe₂O₄/IF) are synthesized via a solvothermal method followed by phosphorization procedures. The specific nanobelt morphology favors the exposure of active sites, advantageous electron transportation and intimate contact between the electrolyte and electrocatalyst. Interestingly, abundant metal hydroxide nanosheets are in situ generated on the nanobelt after the phosphorized sample is immersed in an alkaline electrolyte. Benefiting from the above merits, the developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF presents excellent electrocatalytic performances for the HER with low overpotentials of 38.6 mV, 43.2 mV, and 30.1 mV to attain 50 mA cm⁻² and 54.4 mV, 71.5 mV, and 69.3 mV to attain 100 mA cm⁻² in 1 M KOH, 1 M KOH seawater and 1 M PBS, respectively. As for overall water splitting, a cell voltage as low as 1.49 V is necessary to attain 10 mA cm⁻² and the developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF also exhibits excellent long-term stability. This work provides a new avenue for designing efficient HER catalysts with high exposure of precious metals.

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1. Introduction

It is of importance to exploit sustainable and green energies to alleviate the ever-increasing environmental contamination and energy crisis.^1–4 Among them, hydrogen, a high energy density and eco-friendly energy carrier, is identified as a promising energy storage medium to solve the issues of intermittent sustainable energies, such as solar, wind and ocean energies.^5–9 Electrocatalytic overall water-splitting has been considered the most eco-friendly and desired means to produce hydrogen without the generation of pollutant gases in comparison with the traditional synthetic technologies.^10–13 Therefore, it is crucial to utilize catalysts to expedite the HER to achieve high electrocatalytic performance.^14–19 So far, Pt-based nanomaterials are deemed to be the most efficient electrocatalysts for the HER with the most favorable reaction kinetics.^20,21 Nevertheless, their scarcity and high cost limited their further large-scale implementation for hydrogen generation.^22–24 Thus, tremendous efforts have been devoted to settle the challenges of exploring efficient and Pt-free catalysts for the HER.^25–28

More recently, Ru-based electrocatalysts have been investigated extensively to substitute Pt for the HER due to their low cost, excellent electrocatalytic performance and remarkable stability.^29,30 For example, Liu et al. reported phosphatized Ru–Fe bimetallic nanoclusters (Ru_xFe_yP-NCs) dispersed on graphitized carbon nanofiber (CNF) supports with low overpotentials of 65.8 and 16.0 mV at 10 mA cm⁻² in acidic and alkaline media with an excellent stability of 100 h for the HER.³¹ In general, a high content of Ru is necessary to drive the excellent electrocatalytic performance which then increases the cost of hydrogen energy. It is widely recognized that decreasing the particle size is a valid strategy to improve the electrocatalytic activity by exposing abundant active sites.^32–34 Moreover, the morphology of the designed electrocatalysts also plays a pivotal role in regulating the electrocatalytic performance.^35,36 Among the investigated nanostructures, the developed nanobelts possess particular merits, such as intimate contact between the catalyst and electrolyte, favorable electron and electrolyte transportation and the high exposure of active sites.³⁷ For instance, Zeng et al. synthesized silver doped cobalt selenide (Ag–CoSe₂) via a partial cation exchange strategy and the obtained electrocatalyst presented excellent electrocatalytic activity toward the OER.³⁸ Except the above-mentioned nanostructure regulation, heteroatom doping is also considered as a promising avenue to promote the electrocatalytic activity. Recently, Yu and co-authors demonstrated that the introduced N atom plays a significant role in improving the Ni–Mo based sulfide for the HER with small overpotentials of 68, 250, and 322 mV to attain 10, 500 and 1000 mA cm⁻², respectively.³⁹ Then, the combination of morphology regulation and heteroatom doping would be an attractive approach to optimize the electrocatalytic activity.

In this work, we designed P doped CoFe₂O₄ nanobelts decorated with Ru nanoclusters and Co–Fe hydroxides through hydrothermal and annealing treatment, followed by hydrolysis in an aqueous solution of potassium hydroxide (Scheme 1). The obtained specific nanobelt structure favors the exposure of active sites, provides rich catalytic sites and facilitates mass transport during the electrocatalytic process. The Ru species was introduced by dripping onto the substrate after the solvothermal process and then it can contact the electrolyte directly. Moreover, the Co–Fe hydroxide species were in situ generated on the nanobelts and played a significant role in promoting the electrocatalytic performance by tuning the adsorption/desorption of the intermediates. Owing to the above merits, the as-developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF presented remarkable electrocatalytic activities and long-term stability in various electrolytes.

Scheme 1 Schematic illustration of the synthesis of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF.

2. Experimental section

2.1. Synthesis of Ru/P–CoFe₂O₄/IF

The IF (2.0 cm × 1.0 cm) was added into anhydrous ethanol, acetone, and anhydrous ethanol for 30 min under ultrasonic treatment to clean the surface impurities and then placed in a vacuum oven to dry overnight. After that, Co(NO₃)₂·6H₂O (0.3 g) and Fe(NO₃)₂·9H₂O (0.08 g) were dissolved in 15 mL H₂O under stirring to obtain a homogeneous solution. Then the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave with a piece of IF and heated at 120 °C for 7 h. After that, the black product was taken out and washed with absolute ethanol and deionized water to remove the unreacted materials. The standby solution was obtained by dissolving ruthenium chloride in anhydrous ethanol (0.116 M) and then dropping it onto the solvothermal product. Finally, 0.5 g of sodium hypophosphite and the reactant were placed upstream and downstream of a tube furnace for 2 h at 350 °C with a heating rate of 5 °C min⁻¹.

2.2. Synthesis of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF

The synthesized precursor of Ru/P–CoFe₂O₄/IF was immersed in 1.0 M KOH for 3 h under constant stirring. The resulting electrode was denoted as CoFeO_xH_y–Ru/P–CoFe₂O₄/IF which could directly serve as the working electrode for subsequent electrochemical measurements. For comparison, CoFeO_xH_y/P–CoFe₂O₄ was prepared without ruthenium chloride addition. Other samples prepared with different amounts (50 mg and 100 mg) of Fe(NO₃)₂·9H₂O were named 50 mg–Fe(NO₃)₂·9H₂O and 100 mg–Fe(NO₃)₂·9H₂O, respectively.

2.3. Synthesis of RuO₂/CoFe₂O₄

RuO₂/CoFe₂O₄ was obtained using a similar synthetic procedure to that of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF, except pyrolysis in a muffle furnace at 380 °C for 2 h.

2.4. Physical characterization

The morphologies of the as-designed electrocatalysts were obtained by scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM) (FEI Tecni G20, 200 kV) analysis. Energy-dispersive X-ray spectroscopy (EDX) (FEI Tecni G20, 200 kV) was used to analyze the composition and distribution of elements in the samples. The crystal information was obtained by X-ray diffraction (XRD) (Rigaku D/max-2500pc device with Cu Kα radiation (λ = 1.54 Å)). The information about valence states of samples was obtained by X-ray photoelectron spectroscopy (XPS) carried out in a Thermo Fisher Scientific II spectrometer with an Al Kα source (1486.6 eV). The contents of Fe, Co and P were measured by inductively coupled plasma emission spectroscopy (ICP-OES) using an Agilent 720 instrument.

2.5. Electrochemical characterization

Electrochemical tests were performed via an electrochemical workstation (Gamry Reference 3000) with a typical three-electrode setup in solution (1.0 M KOH, seawater (1 M KOH), and 1 M PBS) at room temperature. The electrochemical measurements were carried out with a saturated calomel electrode (SCE) and graphite rod which were used as the reference electrode and counter electrode, respectively. All final potentials were given by E(RHE) = E(SCE) + E°(SCE) + 0.059 pH (E°(SCE) + 0.243 V). Linear sweep voltammetry (LSV) curves for the HER were determined with a scanning rate of 1 mV s⁻¹. EIS Nyquist plots for the obtained catalysts were obtained in a frequency range of 100 kHz–0.1 Hz at a potential of −1.15 V (vs. SCE) in 1 M KOH for the HER. Equivalent electrical circuit: R_s (R₁): solution resistance, CPE: constant phase element, R_ct (R₂): charge transfer resistance. Multiple-step chronopotentiometry was used to measure the long-term stability of all samples.

3. Results and discussion

The crystal structure of the as-designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF is composed of CoFe₂O₄ (PDF#00-002-1045) without other impurities. The microstructure of the synthesized electrocatalysts was investigated by SEM and TEM. As shown in Fig. S1,† relative to the smooth surface of iron foam without specific decorated nanomaterials, smooth nanobelts were generated after the solvothermal reaction (Fig. S2 and S3†). Obviously, the nanobelt nanostructure was maintained well after decoration with Ru and the phosphorization process (Fig. 1b and Fig. S4†). Interestingly, the smooth surface of the nanobelt was covered with numerous nanosheets after immersion in 1 M KOH (Fig. 1c). In order to verify the species of the nanosheets, Raman spectra were recorded before and after immersion in 1 M KOH. Evidently, new peaks, ascribed to metal hydroxide,⁴⁰ appeared after treatment with an alkaline solution, demonstrating the formation of Co–Fe hydroxide (Fig. S5†). On this basis, the electrocatalytic performances were recorded every 15 minutes (Fig. S6†). The catalytic activities improved with increasing soaking time and were well maintained after prolonged times, confirming that the generation of metal hydroxide plays a key role in regulating the catalytic performance. However, the smooth surface was retained well after soaking in deionized water, indicating that the hydroxide ion plays a pivotal role in forming metal hydroxide (Fig. S7†). The ICP-OES results (Table S1†) proved that the metal ions dissolved in the solution after soaking in 1 M KOH and then formed metal hydroxides. The nanosheets can be clearly observed by TEM and they are generated on the nanobelt homogeneously (Fig. 1d). As illustrated in Fig. 1e and f, Ru nanoclusters were decorated on the nanobelt homogenously with a lattice spacing of 0.206 nm of the (101) plane. Moreover, it can be observed in Fig. 1f that the interplanar spacings of 0.296 nm and 0.253 nm corresponded to the (200) and (311) facet of CoFe₂O₄, respectively. The elemental mappings confirmed that Co, Fe, P, O and Ru elements existed in the designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF (Fig. 1g and S8†). Moreover, for CoFeO_xH_y–Ru/P–CoFe₂O₄/IF, the BET surface area was 12.69 m² g⁻¹ (Fig. S9†) with pore size distribution centers at 3.4 and 8.9 nm. The result confirms the mesoporous structure of the prepared catalysts which favours mass transport and exposure of abundant active sites during the electrocatalytic process.

Fig. 1 (a) XRD patterns of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF. SEM images of Ru/P–CoFe₂O₄/IF (b) and CoFeO_xH_y–Ru/P–CoFe₂O₄/IF (c). (d and e) TEM images of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF. (f) HRTEM image of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF. (g) Corresponding element mappings of Co, Fe, P, O, and Ru in CoFeO_xH_y–Ru/P–CoFe₂O₄/IF.

XPS measurement was conducted to investigate the valence and composition of the as-prepared CoFeO_xH_y–Ru/P–CoFe₂O₄/IF. Obviously, P, O, Fe, Co, and Ru elements were detected from the XPS survey spectrum, which is in line with the EDS results. For high-resolution Co 2p (Fig. 2b), the peaks located at 782.3 eV and 780.7 eV are ascribed to Co²⁺ and Co³⁺, respectively.^41–43 As illustrated in Fig. 2c, Fe²⁺ (710.5 eV) and Fe³⁺ (712.8 eV) are detected in the as-designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF.^44,45 For Ru 3p, the peak at a binding energy of 484.3 eV is assigned to Ru 3p_1/2 and the adjacent peak at 462.2 eV corresponds to Ru 3p_3/2 in the high-resolution Ru 3p XPS spectrum which confirmed that Ru was successfully incorporated.^31,46 The metal–oxide bond (531.6 eV, 530.8 eV and 529.7 eV) and the characteristic peaks of oxygen vacancies (530.8 eV) were obviously observed in the O 1s spectrum.⁴⁴ The P 2p in Fig. 2f demonstrates that P was doped into the prepared electrocatalyst successfully.^31,47–49 We investigated the XPS spectrum of Ru/P–CoFe₂O₄/IF and CoFeO_xH_y–Ru/P–CoFe₂O₄/IF to study the electronic interactions (Fig. S10†). For Co 2p, the peaks of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF shift to a lower binding energy after the formation of CoFe–OH demonstrating that the electronic interactions play a pivotal role in promoting the electrocatalytic performance. Besides, different peaks of Ru 3p shifted to a higher binding energy of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF compared to that of Ru/P–CoFe₂O₄/IF further indicating electron transfer in the catalyst. It has been widely recognized in some research studies that the introduction of hydroxide can effectively improve the electrocatalytic activity.⁵⁰ Consequently, the formation of CoFeO_xH_y will act as an electronic regulator leading to the redistribution of electrons, and finally the exhibition of outstanding HER activity by the sample.

Fig. 2 XPS survey spectrum (a) and high-resolution XPS spectra of Co 2p (b), Fe 2p (c), Ru 3p (d), O 1s (e) and P 2p (f) of the designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF.

The electrocatalytic HER performances of the as-designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF and reference samples were investigated in 1 M KOH via a typical three-electrode setup. Fig. 3a and b show the linear sweep voltammetry of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF, CoFeO_xH_y–CoFe₂O₄/IF, Pt/C and IF. Remarkably, the designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF displays excellent HER activity with small overpotentials of 38.6 mV and 54.4 mV to attain 50 mA cm⁻² and 100 mA cm⁻², respectively, compared with CoFeO_xH_y/P–CoFe₂O₄/IF (156 mV@10 mA cm⁻², 182 mV@100 mA cm⁻²), indicating that the introduced Ru nanoclusters play key roles in improving the electrocatalytic performance. Ru clusters anchored on oxides with high conductivity play a key role in electron traps, and rapidly occupy electron and reduce H⁺ to H₂. The intrinsic activity of the catalyst is significantly increased due to the synergistic effects of electron coupling between Ru and CoFeO_xH_y.⁵¹ Besides, the Tafel plots demonstrated that CoFeO_xH_y–Ru/P–CoFe₂O₄/IF possesses a smaller value (43.2 mV dec⁻¹) than CoFeO_xH_y–CoFe₂O₄/IF (85.6 mV dec⁻¹), verifying its favorable reaction kinetics (Fig. 3c). The catalytic activity of the as-synthesized CoFeO_xH_y–Ru/P–CoFe₂O₄/IF surpasses most of the reported values (Fig. 3d and g and Tables S2 and S3†). As shown in Fig. S11,† CoFeO_xH_y–Ru/P–CoFe₂O₄/IF displays the smallest Nyquist semicircle diameter, demonstrating the lowest resistance of charge transfer during the electrocatalytic HER process.

Fig. 3 Electrochemical measurements: (a) HER polarization curves. (b) Overpotential comparison at 50 mA cm⁻² and 100 mA cm⁻². (c) Tafel slopes. (d) Comparison of the overpotentials and Tafel slopes with the references. (e and f) Stability tests of the obtained catalysts in 1 M KOH. (g) Comparison of the overpotentials (E_j100) with the reported electrocatalysts.

In addition, the prepared composite catalyst contains dense and tiny hydroxide nanosheets relative to Ru/P–CoFe₂O₄/IF, which plays a pivotal role in boosting the catalytic process, and the specific nanostructure benefits the transport of the electrolyte and weakens the O–H bond in water. Despite the remarkable electrocatalytic performance, the developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF also possesses excellent long-term stability. As illustrated in Fig. 3e and f, the current densities increased when the applied potentials were increased, and the current densities were well maintained during the reverse process, indicating its outstanding stability. For an optimal performance, we investigated the role of the mass content of Fe(NO₃)₂·9H₂O in regulating the electrocatalytic activity for the HER (Fig. S12†). The results show that the electrocatalyst with 80 mg Fe(NO₃)₂·9H₂O exhibits the lowest overpotential and the smallest Tafel slope.

As mentioned above, the developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF exhibits excellent HER performance coupled with long-term stability. Then, we investigated its properties after the stability test by SEM and XPS measurements. As shown in Fig. 4a and b, the CoFeO_xH_y–Ru/P–CoFe₂O₄/IF maintained the nanobelt morphology covered with numerous Co–Fe hydroxides, demonstrating its robust structure. The XPS results (Fig. 4c–f and Fig. S13†) verify the elemental composition and the chemical valence shows negligible changes, verifying its stable property. Thus, all the tests confirmed the great potential of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF as an effective electrocatalyst for practical hydrogen production.

Fig. 4 (a and b) SEM images and XPS spectra of Co (c), Fe (d), Ru (e) and P (f) of the CoFeO_xH_y–Ru/P–CoFe₂O₄/IF after the long-term stability test.

The ocean covers nearly 71% of the Earth's surface area and seawater accounts for about 97% of the total water on the Earth. Seawater is inexhaustible for human beings, so the reasonable development of electrolytic seawater is very promising for hydrogen production.^52,53 As illustrated in Fig. 4a and b, the obtained electrocatalyst only needs low overpotentials of 43.2 mV and 71.5 mV to attain 50 mA cm⁻² and 100 mA cm⁻², respectively (Fig. 5a and b). Relatively, the reference sample CoFeO_xH_y–CoFe₂O₄/IF needs 162 mV and IF needs 377 mV to deliver 100 mA cm⁻², respectively. Significantly, the synthesized CoFeO_xH_y–Ru/P–CoFe₂O₄/IF presented the lowest Tafel slope of 52.9 mV dec⁻¹ compared with CoFeO_xH_y–CoFe₂O₄/IF (60.0 mV dec⁻¹) and IF (134.1 mV dec⁻¹), indicating its faster reaction kinetics for the HER (Fig. 5c). Fig. 5d shows the electrochemical impedance spectroscopy (EIS) results of the obtained catalysts in 1 M KOH seawater; the R_ct value of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF is 9.65 Ω, lower than those of CoFeO_xH_y–CoFe₂O₄/IF (15.31 Ω) and IF (135.6 Ω). In addition, we also studied the stability of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF in 1 M KOH seawater using an identical method in an alkaline medium. As shown in Fig. 5e and f, the durability measurement was confirmed by multiple-step chronopotentiometry with negligible decay as long as 30 h demonstrating its excellent stability in seawater.

Fig. 5 Electrochemical measurements: (a) HER polarization curves. (b) Overpotential comparison at 50 mA cm⁻² and 100 mA cm⁻². (c) Tafel slopes. (d) Nyquist plots. (e and f) Stability tests of the obtained catalysts in 1 M KOH seawater.

Encouraged by the remarkable HER catalytic performance in alkaline media, the catalytic activity of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF was further investigated in neutral medium (1 M PBS). As shown in Fig. 6a and b, the as-prepared CoFeO_xH_y–Ru/P–CoFe₂O₄/IF also possesses better catalytic activity than CoFeO_xH_y–CoFe₂O₄/IF and IF with the lowest overpotential of 30.1 mV and 69.3 mV to achieve current densities of 50 mA cm⁻² and 100 mA cm⁻². Moreover, the Tafel slope of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF (58.2 mV dec⁻¹) is lower than those of CoFeO_xH_y–CoFe₂O₄/IF (161.2 mV dec⁻¹) and IF (163.1 mV dec⁻¹) (Fig. 6c). The catalytic performance of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF is even better than those of most of the reported noble metal catalysts, such as CC@WO₃/Ru-450 (64 mV),⁵⁴ Ru_SA–N–Ti₃C₂T_x (81 mV),⁵⁵ Ru/OMSNNC (70 mV),⁵⁶ Rh₂P/NPC (119 mV),⁵⁷ Ru–Cr₂O₃/NG (53 mV),⁵⁸ Ru/MEOH/THF (83 mV),⁵⁹ Rh₂P (38 mV)⁶⁰ and RhP₂@NPC (57 mV)⁶¹ (Fig. 6d and Table S4†). As shown in Fig. 6e and f, the as-prepared electrocatalyst presents excellent long-term stability in 1 M PBS.

Fig. 6 Electrochemical measurements: (a) HER polarization curves. (b) Overpotential comparison at 50 mA cm⁻² and 100 mA cm⁻². (c) Tafel slopes. (d) Comparison of the overpotentials with the references. (e and f) Stability tests of the obtained catalysts in 1 M PBS.

Encouraged by the excellent electrocatalytic activity toward the HER, a two-electrode water-splitting setup was built with the prepared CoFeO_xH_y–Ru/P–CoFe₂O₄/IF as the cathode and RuO₂/CoFe₂O₄ (Fig. S14–S17†) as the anode for overall water-splitting in 1 M KOH solution (Fig. 7a). As shown in Fig. 7b, the electrolyzer can attain 10 mA cm⁻² with a low voltage of 1.49 V which is lower than those of many other reported available catalysts (Fig. 7c and Table S5†).As solar energy is a sustainable and green energy, it attracts tremendous attention in hydrogen production. Thanks to the small voltage, the overall water-splitting setup is easily powered by solar energy then amounts of bubbles generated on the electrodes (Fig. 7d and Movie S1†). Importantly, the designed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF∥RuO₂/CoFe₂O₄ also exhibits excellent stability for overall water-splitting via multi-step measurements (Fig. 7e and f). As mentioned above, the developed electrocatalyst presents excellent catalytic activity which can be attributed to the following factors: the specific nanobelts favour the transport of the electrolyte; a trace amount of Ru nanoclusters plays a crucial role in improving the HER performance; and phosphorus can effectively optimize the density of electron clouds around metal atoms, resulting in an optimized hydrogen adsorption energy and favorable hydrogen release.⁶² P atoms facilitate proton adsorption and thus accelerate the reaction kinetics. In addition, the doped P can improve the electron mobility for attaining rapid electron transport, which can adjust the electron structure and play a role in regulating charge accumulation and consumption for an excellent HER.⁶³ Due to the different electronegativities of phosphorus, the catalytic activity of the composite components can be regulated, so that the obtained catalyst has good hydrogen evolution performance in 1 M KOH, 1 M KOH seawater and 1 M PBS; the in situ generated metal hydroxides play a key role in promoting the electrocatalytic performance by accelerating the reaction kinetics; the IF substrate provides abundant channels for the transport of the electrolyte and avoids the usage of a binding agent.

Fig. 7 (a) Schematic diagram of water splitting in a two-electrode setup. (b) LSV curves of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF∥RuO₂/CoFe₂O₄ and the catalyst after the stability test of 2000 cycles for overall water splitting in 1 M KOH (the inset is the photograph of the water splitting cell). (c) Solar cell driving overall water-splitting. (d) Comparison of the overpotentials (E_j10) of the synthesized electrocatalysts with the reported catalysts. (e and f) Stability tests of the obtained catalysts in 1 M KOH.

4. Conclusions

In summary, a hierarchical structure of CoFeO_xH_y–Ru/P–CoFe₂O₄/IF nanobelts decorated with dense and tiny CoFe double hydroxide nanosheets supported on iron foam is developed via hydrothermal and annealing treatment and further hydrolyzed in alkaline media. Owing to the combination of metal oxide species and a trace amount of Ru nanoclusters, in addition to the Co–Fe double hydroxide nanosheets during the hydrolysis process, the obtained composite nanomaterial serves as a remarkable electrocatalyst for boosting the HER activity in alkaline, seawater and neutral electrolytes. Accordingly, the obtained electrocatalyst exhibits excellent catalytic performances toward overall water-splitting with remarkable long-term stability in 1 M KOH, which can also be powered by sustainable solar energy. Additionally, the iron foam substrate possesses an intricate skeleton with rich channels which improves the electrical conductivity and timely release of the generated bubbles to expose the active substances. Ru clusters can accelerate the HER rate by converting into a faster electron transport paths and possess a moderate H* adsorption value in favour of the HER process. Consequently, the developed CoFeO_xH_y–Ru/P–CoFe₂O₄/IF presents superior performances to that of previously reported materials toward the HER and overall water-splitting. This work provides a valuable strategy to develop other specific nanostructures as efficient electrocatalysts for energy-related applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge funding support from the National Natural Science Foundation of China (22002068, 51772162, and 52072197), the Youth Innovation and Technology Foundation of Shandong Higher Education Institutions, China (2019KJC004), the Outstanding Youth Foundation of Shandong Province, China (ZR2019JQ14), the Taishan Scholar Young Talent Program (tsqn201909114), the Major Scientific and Technological Innovation Project (2019JZZY020405), the Major Basic Research Program of Natural Science Foundation of Shandong Province under Grant No. ZR2020ZD09, the Project funded by the China Postdoctoral Science Foundation (2021M691700), and the Natural Science Foundation of Shandong Province of China (ZR2019BB002 and ZR2018BB031).

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Footnote

† Electronic supplementary information (ESI) available: SEM images, XRD patterns, XPS spectra, electrochemical data. See DOI: 10.1039/d2qi00086e

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