With the world's energy crisis and environmental pollutions, it is urgent to develop renewable energy conversion and storage technologies, such as water splitting [1–3], fuel cells [4, 5], lithium-ion batteries [6, 7], rechargeable metal-air batteries [8–10], and CO₂ conversion and utilization [11, 12]. Oxygen evolution reaction (OER) is the unavoidable half-reaction in water splitting, electrochemical CO₂ reduction, and rechargeable metal-air batteries, whereas the sluggish kinetics of OER and the high cost of the current commercial catalysts (IrO₂ and RuO₂) severely limit the commercialization of these technologies [13, 14]. Hence, it is necessary to develop OER electrocatalysts derived from based on earth-abundant elements with high efficiency and low-cost.

Current research on non-precious metal OER catalysts focuses on the first-row transition metal compounds [15–19]. Generally, transition metal oxides/(oxy)hydroxides show higher activity than their sulfides [16, 20, 21], selenides [22, 23], and phosphides [24–26]. Moreover, recent studies have shown that metal sulfides, selenides, and phosphides can be converted into oxides/(oxy)hydroxides (which act as the active species for OER) in the process of OER electrocatalysis [9, 21, 27]. Therefore, the structure engineering of transition metal oxides/(oxy)hydroxides tends to be an attractive way for the design of high performance OER electrocatalysts [17, 19, 28–31]. Transition metal-based layered double hydroxides (LDHs) are a group of excellent OER electrocatalysts due to their high specific surface areas, abundant active sites as well as the synergistic catalytic effect of bimetallic species [18, 32, 33]. Among the various LDHs, NiFe-LDHs have demonstrated outstanding OER activity attributable to their highly active Fe sites with modulated 3d electronic structure [15, 34, 35]. The possible partial-charge transfer activation effect between Fe and Ni and the lattice distortion of Fe site during OER are suggested to reduce the energy barriers for the formation of the OER intermediates, thus result in reduced OER overpotential [15, 36]. To further improve the OER performance of NiFe-LDHs, defects and other metal atoms have been introduced [37, 38]. However, the incorporation of new non-metallic atoms is rarely investigated in the OER electrocatalysis of NiFe-LDHs. Our recent work shows that the surface S residues in NiFe (oxy)hydroxide derived from NiFe disulfides could adjust the adsorption energy of OH* and O* on the Fe sites, leading to enhanced OER activity [34]. In this work, we synthesize and systematically study the OER activity of S doped NiFe-LDHs and demonstrate that the proper introduction of S can significantly improve the OER performance of NiFe-LDHs.

The Ni_4/5Fe_1/5-LDHs catalyst was fabricated by the hydrothermal method [18]. Briefly, 0.18 mmol nickel acetate tetrahydrate (99.9%, Aladdin) and 0.03 mmol iron nitrate nonahydrate (AR, Aladdin) were dissolved in 16 ml deionized water and 8 ml DMF (99.5%, Aladdin). The mixture was transferred into a Teflon container and heated at 120 ℃ for 12 h. After cooling down to room temperature naturally, the obtained mixed solution containing light yellow precipitation is centrifuged at 9000 rpm and washed with deionized water and ethanol. The precipitate is then freeze-dried by a lyophilizer, follow by grinding to obtain the final product.

To synthesize S-doped Ni_4/5Fe_1/5-LDHs, a certain amount of thiourea (99%, Aladdin) was added into the mixture as the S source and other procedures were kept the same. To study the effect of S content, 0.005, 0.01, 0.02, 0.04 and 0.08 mmol thiourea were added, and the resulting products were named Ni_4/5Fe_1/5-LDHs-S-1, Ni_4/5Fe_1/5-LDHs-S-2, Ni_4/5Fe_1/5-LDHs-S-3, Ni_4/5Fe_1/5-LDHs-S-4 and Ni_4/5Fe_1/5-LDHs-S-5, respectively.

The X-ray diffraction (XRD, Rigaku, DMAX-2400) was used to analyze the crystal structure of the catalysts, and the 2θ range from 10° to 80°. Scanning electron microscope (SEM, FEI, Sirion 200) images and transmission electron microscopy (TEM, FEI, Tecnai G2 20) images were collected to observe the morphology directly. X-ray photoelectron spectroscopy (XPS, Shimadzu, AXIS-ULTRA DLD-600W) was acquired to understand the catalyst's surface composition and the surface energy state distribution, and the C 1s hydrocarbon peek at 284.6 eV was used to revise the binding energy. X-ray fluorescence (XRF, EDAX, EAGLE Ⅲ) was carried out to detect the type and content of various elements in the sample.

The catalyst ink for electrochemical testing were prepared by mixing the catalyst and KJ black carbon with the mass ratio of 4:1 in 800 μl isopropanol and 200 μl deionized water, then ultrasonication for one hour. The working electrode was prepared by dropping the ink on glass carbon with the catalyst loading of 0.2 mg cm^‒2. The carbon rod and the saturated calomel electrode were used as the counter and the reference electrodes, respectively. All the potentials were converted to RHE (Platinized platinum wire inserted into the electrolyte continuously bubbling with pure hydrogen) scale experimentally. All the electrochemical OER testing was carried out in 1.0 M KOH at room temperature on CHI 760D (Chenhua, China). The stability test was carried out at the current density of 10 mA cm^‒2 by chronopotentiometry. Electrochemical impedance spectroscopy (EIS) was also collected under the appropriate voltage.

The one-step hydrothermal syntheses of the Ni_4/5Fe_1/5-LDHs-S catalysts are illustrated in Fig. 1(a). The lamellar morphology of the Ni_4/5Fe_1/5-LDHs-S-2 catalysts can be clearly observed by the SEM (Fig. 1(b)) and TEM (Fig. 1(c)) images of the Ni_4/5Fe_1/5-LDHs-S-2 sample with thickness of tens of nanometers. Meanwhile, other Ni_4/5Fe_1/5-LDHs and Ni_4/5Fe_1/5-LDHs-S catalysts (Figs. S1 and S2) also reveal similar nanosheet structures, suggesting that the S doping amount plays an insignificant role in determining the catalyst morphology.

The XRD patterns of Ni_4/5Fe_1/5-LDHs and Ni_4/5Fe_1/5-LDHs-S-2 are shown in Fig. 1(d), and both of their patterns are consistent with the jamborite (JCPDF No. 89-1495), which is a hybrid of β-nickel hydroxide and γ-nickel (oxy)hydroxide and is hydrotalcite-like compound (HTLC, belong to LDH). Compared to the standard PDF card, the diffraction peaks of Ni_4/5Fe_1/5-LDHs and Ni_4/5Fe_1/5-LDHs-S-2 shift negatively, likely due to the increased lattice parameter by replacement of nickel with iron (oxy)hydroxide and [20]. Similarly, the XRD patterns of Ni_4/5Fe_1/5-LDHs-S-1, Ni_4/5Fe_1/5-LDHs-S-3, Ni_4/5Fe_1/5-LDHs-S-4, and Ni_4/5Fe_1/5-LDHs-S-5 (Fig. S3) can be well indexed to that of the jamborite, indicating that the trace S doping would not affect the crystal structure of Ni_4/5Fe_1/5-LDHs. Meanwhile, the XRF measurements manifest that the Ni/Fe ratios of all samples are close to 4:1.

The elemental composition and bonding configuration of the obtained samples are detected by XPS. All the binding energies are calibrated by the reference of the C 1s peak, whose position is at 284.6 eV. The peak of Ni 2p_3/2 of Ni_4/5Fe_1/5-LDHs (Fig. 2(a)) is located at 855.4 eV, which is regarded as the Ni (Ⅱ) that is bonded with OH. The Ni 2p_3/2 peak of Ni_4/5Fe_1/5-LDHs-S-2 (Fig. 2(b)) is at 855.5 eV, indicating that the binding energy of Ni is not significantly altered by S doping. In the contrast, the peaks of Fe 2p_3/2 for Ni_4/5Fe_1/5-LDHs (Fig. 2(c)) and Ni_4/5Fe_1/5-LDHs-S-2 (Fig. 2(d)) are at 712.96 and 712.44 eV, respectively, which can be assigned to Fe(Ⅲ). The decrease of binding energy suggests that the electron density on the surface of Fe atoms gets denser after S doping, attesting to the strong interaction between Fe and S. The S has lower electronegativity compared to O, which would decrease the valence state of Fe when O is partially replaced by S. It is also observed that the peak of S 2p_3/2 (Fig. S4) is at 167.39 eV, suggesting the interaction of S with O rather than only sulfide. Elemental analysis by XPS shows that the surface S contents of Ni_4/5Fe_1/5-LDHs-S-2, Ni_4/5Fe_1/5-LDHs-S-3, Ni_4/5Fe_1/5-LDHs-S-4, and Ni_4/5Fe_1/5-LDHs-S-5 is 0.43%, 1.37%, 2.30%, and 3.56%, respectively. S contents measured by XRF (1 at%‒4 at%) is also comparable with that from XPS.

Since the poor electron-conductivity of jamborite severely limits the OER performance of the catalysts, KJ black carbon is used as the conductive supports to mix with the catalysts. The OER activity of the developed catalysts are evaluated by linear sweep voltammetry (LSV) in 1.0 M KOH. Fig. 3(a) demonstrates that the overpotential of Ni_4/5Fe_1/5-LDHs-S-2, Ni_4/5Fe_1/5-LDHs, and IrO₂ at 10 mA cm^‒2 is 257, 325, and 450 mV, respectively. Similar with most literatures, Ni_4/5Fe_1/5-LDHs without S doping outperform IrO₂ in OER electrocatalysis. On the other hand, the S-doped Ni_4/5Fe_1/5-LDHs presents excellent OER activity with an overpotential of only 257 mV at 10 mA cm^‒2, which is superior to most of the reported OER electrocatalysts [39–41]. Importantly, the introduction of trace S significantly reduces the overpotential of Ni_4/5Fe_1/5-LDHs by 68 mV at 10 mA cm^‒2. The lowered Fe valence state in the Ni_4/5Fe_1/5-LDHs-S-2 catalyst, as evidenced in XPS, may relieve the energy barriers for the formation of the OER intermediates (OH* to O*) on Fe sites of Ni_4/5Fe_1/5-LDHs, which potentially favors the OER electrocatalysis [19]. The Tafel slope of Ni_4/5Fe_1/5-LDHs-S-2 is only 61.5 mV dec^‒1, far lower than the 86.1 mV dec^‒1 of Ni_4/5Fe_1/5-LDHs and 127.5 mV dec^‒1 of IrO₂. EIS (Fig. S7) is also measured to understand the effect of S doping. The charge transfer resistance of Ni_4/5Fe_1/5-LDHs-S-2 is 80 Ω at the overpotential of 260 mV, which is only half of the resistance of Ni_4/5Fe_1/5-LDHs at the same overpotential. The lower charge transfer resistance suggests the smaller reaction barrier on Ni_4/5Fe_1/5-LDHs-S-2, which also contributes to the OER electrocatalysis enhancement.

The influence of S doping amount on OER activity of Ni_4/5Fe_1/5-LDHs-S catalysts is also explored (Figs. 3(c) and 3(d)). Initially, with the increase of S content to 0.43 at% (Ni_4/5Fe_1/5-LDHs-S-2), the overpotential of Ni_4/5Fe_1/5-LDHs-S for OER significantly decreases. Further increasing S content to 3.56 at% leads to the reduced OER activity of the Ni_4/5Fe_1/5-LDHs-S catalysts, but still better than undoped Ni_4/5Fe_1/5-LDHs.

Stability under working conditions is another important requirement for electrocatalysts. To evaluate the stability of the Ni_4/5Fe_1/5-LDHs-S-2 catalyst, it is coated on hydrophilic carbon paper and tested at the constant 10 mA cm^‒2 for more than 30 h. Fig. 4 illustrates that that the OER overpotential change is less than 14 mV after 30 h continuous testing, demonstrating the outstanding durability of the developed Ni_4/5Fe_1/5-LDHs-S-2 catalyst. SEM image (Fig. S8) of the catalyst after OER show that Ni_4/5Fe_1/5-LDHs-S-2 sheets tend to stack together after OER test. The XRD pattern of the Ni_4/5Fe_1/5-LDHs-S-2 catalyst after OER test is demonstrated in Fig. S9. The weak diffraction peaks suggest that its structure become amorphous after OER test, which is consistent with literature reports and may be beneficial to the improvement of OER activity [42]. But the typical peaks for Ni-based (oxy)hydroxide can still be recognized from the XRD pattern. To further investigate the durability of the catalysts, potential cycling form 1.0 to 1.5 V vs. RHE is applied to the Ni_4/5Fe_1/5-LDHs-S-2 catalyst. After 2000 potential cycles, only slight degradation can be observed (Fig. S10), corroborating the excellent durability of the developed catalyst.

In conclusion, we report a facile and efficient strategy to improve the OER activity of NiFe-LDHs by S doping. It is found that trace S (~0.43 at%) introduced in Ni_4/5Fe_1/5-LDHs plays a vital role in boosting its catalytic activity towards OER. The best performing S doped Ni_4/5Fe_1/5-LDHs catalyst demonstrates excellent OER performance with an overpotential of 257 mV at 10 mA cm^‒2 and a Tafel slope of 61.5 mV dec^‒1 as well as outstanding stability. XPS measurements suggest that the interaction between surface S and Fe reduces the valence state of Fe, which may reduce the energy barriers for the formation of the OER intermediates on Fe sites and thus enhance the OER activity according to our previous research. This work offers an easy way to engineer the structure and electronic state of transition metal oxides by non-metallic elemental doping, which would be a promising method for the design of highly active OER electrocatalysts.

The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for carrying out the XRD and XPS measurements.