Efficient Electrocatalyst For Oxygen Evolution Reaction Research Articles

Cobalt and vanadium dual-doping engineering of NiS/Ni7S6 with heterogeneous interface as efficient electrocatalyst for the oxygen evolution reaction

Developing cost-efficient and high-activity nonprecious metal-based oxygen evolution reaction (OER) electrocatalysts is essential to water electrolysis for hydrogen production. Herein, cobalt and vanadium co-doped NiS/Ni7S6 with a heterogeneous interface supported on nickel foam (CoV-NiSx/NF) through a simple hydrothermal and vulcanization method. The heterogeneous interfaces formed by NiS and Ni7S6 can generate more active centers and modulate electronic distribution of the two. The dual doping of Co and V can provide larger electrical conductivity and optimize the electron structure, achieving robust kinetics for OER reaction. Remarkably, CoV-NiSx/NF exhibits excellent OER catalytic performance with an overpotential of 261 mV for driving 100 mA cm−2. In addition, in-situ Raman indicates the NiOOH converted from NiS/Ni7S6 is the actual OER catalytic active component. The assembled electrolyzer with Pt/C/NF and CoV-NiSx/NF as the cathode and anode for overall water splitting only needs a low cell voltage of 1.41 V to yield 10 mA cm−2, outperforming the majority of published OER electrocatalysts based on non-precious metal This study provides a feasible solution using dual-metal doping with a heterogeneous interface design for developing efficient OER electrocatalysts for water electrolysis.

Structurally Robust Honeycomb Layered Strontium Iridate as an Oxygen Evolution Electrocatalyst in Acid

The poor catalyst stability, stemming from serious cation dissolution and uncontrollable amorphization, remains an outstanding problem in the acidic oxygen evolution reaction (OER). Herein, we report the synthesis and characterization of honeycomb layered strontium iridate (SrIr2O6) and demonstrate its potential as an efficient OER electrocatalyst with robust structural stability in acid. In contrast to the vast majority of iridate catalysts, SrIr2O6 can keep both bulk and surface structures crystalline during OER, as opposed to forming an amorphous active phase. The edges of SrIr2O6 are highly catalytically active for OER by following the adsorbate evolution mechanism, but the basal planes are catalytically inert. SrIr2O6 exhibits ∼10-fold higher intrinsic activity than the benchmark catalyst IrO2, affords an extremely low total iridium leaching level (∼0.03%) during OER, and retains its catalytic activity for more than 300 h.

A simple postsynthetic in-situ cation exchange strategy to construct Fe-doped Ni3Se4 nanoarray for enhanced oxygen evolution reaction

For industrial high-purity green hydrogen production, it is necessary to develop efficient, economical and stable non-precious metal-based alkaline media oxygen evolution reaction (OER) electrocatalysts. In this study, a simple stepwise synthesis strategy was proposed to construct a hierarchical MOF-based Prussian blue analogue by Ni-BPDC and derived from it to grow Fe-doped Ni3Se4 in situ on nickel foam (Fe-Ni3Se4/NF) as an efficient and stable alkaline OER electrocatalyst. Fe doping enables better modulation of the electronic structure of Ni3Se4 and increases the number of active sites and electrochemical surface area to improve the OER activity. Fe-Ni3Se4/NF exhibits remarkable electrocatalytic performance in 1 M KOH with low overpotential (211 mV at 10 mA cm−2) and small Tafel slope (34.44 mV dec−1), which is superior to that of commercial RuO2 catalysts. Its electrochemical characteristics are stable at 10 mA cm−2 for more than 24 h, making it a viable non-precious metal-based electrocatalyst for alkaline OER. This synthetic approach could open up new possibilities for the fabrication of MOF-based hierarchical Prussian blue analogs as well as derivatives to be oriented for applications in various fields.

A highly efficient and durable self-standing iron-cobalt-nickel trimetallic phosphide electrode for oxygen evolution reaction

The development of cost-effective, highly efficient, and durable oxygen evolution reaction (OER) electrocatalysts is both a necessity and challenge for large-scale water electrolyzers, which is crucial for future sustainable and clean energy systems. Here we report a self-standing trimetallic phosphide (FeCoNi-P/NF) electrode prepared by the electroreduction of Fe2O3 and Co3O4 on the Ni foam in molten salts and the subsequent phosphorization process. In the electroreduction process, the Ni source provided by Ni foam participates in the alloying process as a solvent phase, which leads to the strong bonding between the substrate and the active material. In addition, the alloys grown by co-reduction and alloying on the three-dimensional skeleton of the Ni foam can expose more active sites by increasing the surface areas through the phosphorization process. Benefiting from the synergistic effects and strong electronic coupling from the Fe, Co, and Ni trinary phosphides, the FeCoNi-P/NF shows excellent OER activity and stability with an overpotential of only 239.5 mV at 10 mA cm−2 and a long-term stability of more than 200 h at 50–300 mA cm−2 in 1.0 M KOH. Overall, this paper offers a straightforward method to prepare low-cost and durable muti-component transition metal-based self-standing OER electrodes.

Rapid fabrication of NiFe(OH)x/Fe0.2Co-Se complexes for oxygen evolution reaction electrocatalysis

Exploiting the inexpensive and efficient oxygen evolution reaction (OER) electrocatalysts is crucial for improving the efficiency of electrocatalytic water splitting (EWS), since the slow 4-electron OER process is a rate-limiting step in EWS. Herein, we prepared the NiFe(OH)x/Fe0.2Co-Se complexes on carbon paper (CP) via an easy two-step electrochemical deposition strategy. By varying the doping amount of Fe and the deposition time of Fe0.2Co-Se, the OER property of NiFe(OH)x/Fe0.2Co-Se/CP was optimized to the best: only requiring low overpotentials of 243 and 277 mV at the current density of 20 and 100 mA/cm2 (j20 and j100) and a small Tafel slope of 36.8 mV/dec, along with a terrific long-term durability in 1 M KOH. According to the experimental results, the rich exposed active sites, improved intrinsic site activity and enhanced electrical conductivity lead to the superior OER performance. Our work will provide some inspiration for designing other metal selenide/hydroxide heterostructures for other energy conversion or storage applications.

Fe-based amorphous alloy wire as highly efficient and stable electrocatalyst for oxygen evolution reaction of water splitting

The slow kinetics of the oxygen evolution reaction (OER) leads to low energy conversion efficiency during water electrolysis, thus it is very important to develop efficient and economical electrocatalysts. With high surface energy and rich active centers, amorphous alloys show excellent potential as self-supporting oxygen evolution catalysts for water splitting. In this work, (Fe0.8Ni0.2)71Mo5P12C10B2 amorphous alloy wire was successfully prepared, and after 5 h immersed in 0.5 mol/L HNO3, the wire surface was covered by the block metallic oxide. At a current density of 10 mA/cm2 in an alkaline electrolyte, the electrochemical results show that the overpotential is only 254 mV and the Tafel slope is 49 mV/dec. After the 194 h stability test, its oxygen evolution performance is even comparable to the commercial RuO2, which has been proven to have excellent electrochemical stability and low preparation cost. The highly efficient catalytic performance mainly originated from the synergistic effect of multi-metal elements and the unique crack surface characteristics which expose more active sites. This work provides new insight into low-metal-cost materials for efficient and durable OER electrocatalysts and their industrial applications in the future.

Heterogeneous engineering of nickel-iron sulfide with cerium-promoted reconstruction for enhanced oxygen evolution

The development of efficient oxygen evolution reaction (OER) electrocatalysts is critical for electrochemical water splitting. Herein, a cerium-doped Ni3S2/NiS/FeS heterogeneous structure nanoparticle supported on nickel foam (Ce-NiSx/FeS/NF) is prepared as a precatalyst for OER reaction in alkaline solution. The heterointerfaces of NiSx/FeS adjust the electronic structure and expose more active sites, accelerating the OER process. The Ce incorporation promotes the in situ reconstruction and oxidation of the sulfides to the heterogeneous NiOOH/FeOOH, which act as the actual active sites for the OER reaction. Thus, the Ce-NiSx/FeS/NF demonstrates excellent OER performance with a low overpotential of 125 mV at 10 mA cm−2 and long-term robustness. Additionally, a water splitting device is constructed using Ce-NiSx/FeS/NF and commercial Pt/C/NF as the anode and cathode, respectively. This device achieves a current density of 10 mA cm−2 at 1.38 V with 50 h of reliable continuous operation at 1.78 V. Density functional theory (DFT) calculation confirms that the active sites of Ce-NiOOH/FeOOH optimize intermediates adsorption energy (higher OH− adsorption energy and lower O2 desorption energy) and provide faster OER kinetics, conducive to the OER process. This work presents the concept of rare-earth-doping promotes reconstruction of heterogeneous transition-metal-based compounds for the development of high-performing OER electrocatalysts.

Reduced graphene oxide composite Ni3S2 microspheres grown directly on nickel foam as an efficient electrocatalyst for OER

The development of cheap, high-efficiency, and stable oxygen evolution reaction (OER) electrocatalysts is a current research hotspot. In this work, reduced graphene oxide (rGO) composite Ni3S2 microspheres grown directly on nickel foam (Ni3S2-rGO/NF) were prepared by tube furnace calcination and hydrothermal method. The Ni3S2-rGO/NF had excellent OER catalytic activity and stability with an overpotential of 303 mV at the current density of 100 mA cm−2, which was 100 mV lower than that of Ni3S2/NF, and its Tafel slope was as low as 23 mV·dec−1. The main reason for enhancing OER activity of the Ni3S2-rGO/NF is due to synergistic effect of Ni3S2 microspheres and rGO, which inhibited the production of NiS and refined the micron size of Ni3S2. This work offers a new method for developing stable and efficient OER catalysts.

Highly Stable and Efficient Oxygen Evolution Electrocatalyst Based on Co Oxides Decorated with Ultrafine Ru Nanoclusters.

Exploring highly active and durable electrocatalysts for oxygen evolution reaction (OER) is significant to achieve efficient anion exchange membrane (AEM) water electrolysis. Herein, hollow Co-based N-doped porous carbon spheres decorated with ultrafine Ru nanoclusters (HS-RuCo/NC) are reported as efficient OER electrocatalysts via the pyrolysis of carboxylate-terminated polystyrene-templated bimetallic zeolite imidazolate frameworks accommodating Ru (III) ions. The unique hollow structure with hierarchically porous characteristics contributes to the electrolyte penetration for fast mass transport and the exposure of more metal sites. Theoretical and experimental studies reveal the synergistic effect between the in situ formed RuO2 and Co3 O4 as another critical factor for the high OER performance, where the coupling of RuO2 with Co3 O4 can optimize the electronic configuration of RuO2 /Co3 O4 heterostructure and decrease the energy barrier during OER. Meanwhile, the presence of Co3 O4 can efficiently suppress the over-oxidation of RuO2 , endowing the catalysts with high stability. As expected, when the resultant HS-RuCo/NCwas integrated into an AEM water electrolyzer, the obtained electrolyzer exhibits a cell voltage of 2.07 V to launch the current density of 1 A cm-2 and excellent long-term stability at 500 mA cm-2 under room temperature in alkaline solution, outperforming the commercial RuO2 -based AEM water electrolyzer (2.19 V).

Solution combustion synthesis of hierarchical porous CoNiFeCu0.1 medium-entropy alloy: A highly efficient and robust electrocatalyst for water oxidation

The development of efficient and stable oxygen evolution reaction (OER) electrocatalysts with nonprecious materials is still a great challenge for the hydrogen-based energy industry. Here, we report the rational design and effective synthesis of hierarchical porous medium-entropy alloys (MEAs) as promising electrocatalysts for OER via a solution combustion method. Owing to the compositional advantages and unique hierarchical porous structure, the as-prepared CoNiFeCu0.1 MEA on nickel foam (CoNiFeCu0.1 @NF) shows superior electrocatalytic performance for OER with an overpotential of about 255 mV at 10 mA cm−2 and a small Tafel slope of 42.7 mV dec−1 in 1.0 M KOH solution. Moreover, it is revealed that the alloying of element Cu in CoNiFe MEA can significantly enhance the charge transfer ability for OER electrocatalysis. The present work shows the hierarchical porous MEAs prepared by solution combustion method have great potential as efficient and robust electrocatalysts for water oxidation in alkaline medium.

Local hydroxyl enhancement design of NiFe sulfide electrocatalyst toward efficient oxygen evolution reaction

Herein, a local OH- reactant concentration enhancement design is proposed towards efficient oxygen evolution reaction (OER) electrocatalyst. In this strategy, N-doped carbon nanotubes (NCNT), serving as conductive OH- concentration enhancement matrix, are introduced near the adjacent Fe-NiS2 catalyst to accelerate OER reaction by promoting the local mass transfer process. This OH- enhancement effect is originated from the electropositive OH- absorption sites of NCNT and local-electronic field induced by conductive three-dimensional NCNT network that synergistically works with adjacent Fe-NiS2 nanocatalyst. Accordingly, the Fe-NiS2/NCNT catalyst only requires low overpotential of 247 mV at 100 mA cm−2, outperforms most of recently reported OER catalysts. The anion-exchange membrane water electrolyzer reaches 500 mA cm−2 at 1.70 Vcell. Analysis also reveals that, this enhanced local concentration design promotes the conversion of Fe-NiS2 into S, Fe co-doped NiOOH, which is the real OER active component. This work provides an alternative strategy to develop high-efficiency OER electrocatalysts.

Synergistic effect customized for cobalt pentlandite (Co,Ni,Fe)9S8 accelerated charge transfer toward highly efficient oxygen evolution reaction

The oxygen evolution reaction (OER) is one of the core reaction modules of important renewable energy technologies with applications in water splitting, metal-air (oxygen) cells and regenerative fuel cells. However, OER catalysts are forced to operate with a significant intrinsic overpotential and inefficient charge transfer due to the adsorption energy scaling balance between the reaction intermediates. It is critical to develop an understanding of structure-activity relationship of catalysts, determine the speed control steps and improve catalytic activity. In this work, cobalt pentlandite (Co,Ni,Fe)9S8 (CNFS) electrocatalysts were successfully prepared by combining the earth-abundant transition metal elements Co, Ni and Fe into a single structure. Benefiting from the structure, nanoflower morphology, and the synergistic effect among the three elements, the optimized sample requires an overpotential of only 335 mV to drive a current density of 50 mA cm–2 under alkaline conditions. The experimental results establish a composition-structure-function relationship, and the density functional theory results further indicate that the Fe sites bind more moderately to the intermediates and the electron synergistic effect occurs dynamically during the OER cycle reaction. This work not only provides a very promising catalyst candidate for efficient OER processes but also highlights the superiority of the cobalt pentlandite structure in the rational design of efficient OER electrocatalysts.

Screening of transition metal-based MOF as highly efficient bifunctional electrocatalysts for oxygen reduction and oxygen evolution

Designing efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is momentous for energy storage and conversion, as well as sustainable development. Herein, based on the recent prepared of two novel two-dimensional MOF materials, namely Ni–2D–SA and Ni–O–2D–SA, the replacement of Ni atoms in these two MOFs with 3d, 4d, and 5d transition metals is comprehensively investigated using density functional theory methods to screen out electrocatalysts with high bifunctional activity and stability for ORR and OER. Stability analysis indicates that some catalysts are thermodynamically and electrochemically stable. By calculating the overpotentials (ηORR and ηOER) and potential gap (∆E), it is found that Ir–2D–SA and Co–O–2D–SA possess the smallest ∆E values with 0.61 V (ηORR = 0.28 V, ηOER = 0.33 V) and 0.53 V (ηORR = 0.29 V, ηOER = 0.24 V), respectively, which are expected to be the most promising bifunctional electrocatalysts. The descriptor φ2 has proven that the binding strength for reaction intermediates is limited to active metal and environment around the active metal. Finally, all excellent catalysts have good resistance to poisoning. This work provides theoretical guidance for preparing efficient ORR and OER electrocatalysts.

Lamella-heterostructured nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride electrodes as stable catalysts for oxygen evolution

Developing robust nonprecious-metal electrocatalysts with high activity towards sluggish oxygen-evolution reaction is paramount for large-scale hydrogen production via electrochemical water splitting. Here we report that self-supported laminate composite electrodes composed of alternating nanoporous bimetallic iron-cobalt alloy/oxyhydroxide and cerium oxynitride (FeCo/CeO2−xNx) heterolamellas hold great promise as highly efficient electrocatalysts for alkaline oxygen-evolution reaction. By virtue of three-dimensional nanoporous architecture to offer abundant and accessible electroactive CoFeOOH/CeO2−xNx heterostructure interfaces through facilitating electron transfer and mass transport, nanoporous FeCo/CeO2−xNx composite electrodes exhibit superior oxygen-evolution electrocatalysis in 1 M KOH, with ultralow Tafel slope of ~33 mV dec−1. At overpotential of as low as 360 mV, they reach >3900 mA cm−2 and retain exceptional stability at ~1900 mA cm−2 for >1000 h, outperforming commercial RuO2 and some representative oxygen-evolution-reaction catalysts recently reported. These electrochemical properties make them attractive candidates as oxygen-evolution-reaction electrocatalysts in electrolysis of water for large-scale hydrogen generation.

Modulating the Electronic Structure of Co in Co–Co6Mo6C2 for Effective Oxygen Evolution Reaction

Tuning the electronic structure of the materials to an optimal state has been regarded as an attractive strategy for designing efficient oxygen evolution reaction (OER) electrocatalysts. In this study, ultrasmall Co–Co6Mo6C2 nanoparticles (2–6 nm) evenly dispersed on the nitrogen-doped carbon (CMC@NC) were constructed by pyrolysis of the precursor of hollow structured Co3O4 encapsulated in Mo7O246– polyaniline. The electronic structure of Co was controllably adjusted by regulating the ratio of Co/Mo in the Co–Co6Mo6C2 system. The optimized electrocatalyst exhibits an excellent OER performance with an overpotential of 266 mV at 10 mA cm–2 and an outstanding stability over a 70 h OER process. Based on the characterization results of before- or post-OER materials, it can be found that CMC@NC-120-750 has an optimized electronic structure for OER via drawing electrons from 3d metal Co by non-3d metal Mo6+, ultimately resulting in the highest oxidation peak and the highest amount of Co3+ (the real OER active sites). This study provides novel insight into the construction of efficient transition metal-based catalysts.

Te-doped NiFe2O4 stabilized by amorphous carbon layers derived from one-step topological transitions of NiFe LDHs with significantly enhanced oxygen evolution reaction

Constructing efficient oxygen evolution reaction (OER) electrocatalysts that can stably operate under high temperatures and strong alkaline conditions still faces great challenges for industrial hydrogen production from water electrolysis. Herein, a self-supported tellurium (Te)-doped spinel NiFe2O4 with a carbon-stabilized nanosheet structure anodic catalyst on nickel foam (NF) (Te-NiFe2O4@C/NF) is successfully synthesized by calcination of a layered double hydroxide precursor (NiFe LDH). Benefiting from Te doping as well as coupling with the amorphous carbon layer and the unique superhydrophilic/aerophobic surface property, which provides efficient mass transport channels and rapid electron transport, the optimized Te-NiFe2O4@C/NF requires an overpotential of 220 mV to achieve a current density of 10 mA cm−2 for the OER in 1 M KOH at 25 ℃. Furthermore, an extremely low overpotential of 180 mV is required to achieve 500 mA cm−2 in 6 M KOH at 80 ℃ and maintains superior stability over 60 h under such harsh quasi-industrial conditions. Density functional theory calculations further reveal that Te doping can effectively modulate the local electronic environment of the Ni and Fe sites, which optimizes the intermediate (*OH) adsorption and facilitates the OER kinetics. This work may open up opportunities to explore efficient electrocatalysts for scalable hydrogen production.

BN/Cu/CNT nanoparticles as an efficient tri-functional electrocatalyst for ORR and OER

Preparing economic, stable, and highly efficient non-precious-metal materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great significance to the development of storage technologies and energy conversion. In this manuscript, we report a novel Cu based nano-catalyst as an electrode material fabricated by complexation-reduction method. Benefit from the stable of h-NB and the good electrical conductivity of CNT, the tri-functional electrocatalyst BN/Cu/CNT displays enhanced activity towards ORR and with the high current density of 0.95 mA/cm2, which is comparable to that of commercial Pt/C catalyst (cathodic peak current: 1.05 mA/cm2). The BN/Cu/CNT also demonstrates high catalyst activity for the OER. From the rotating disk electrode and rotating ring-disk electrode tests, the number of electrons exchanged during the ORR is revealed to be 3.92, suggesting a 4e− pathway of O2 reduction to H2O. The as prepared catalyst also has good stability and superior methanol tolerance to the commercial Pt/C catalyst in alkaline medium. Thus, it is anticipated that the BN/Cu/CNT catalyst is surely helpful for providing a new space to develop Pt-free catalyst.

Spectroscopically unraveling high-valence Ni-Fe catalytic synergism in NiSe2/FeSe2 heterostructure

Heterostructured compounds with rationally engineered interfaces are widely reported as efficient oxygen evolution reaction (OER) electrocatalysts, but the mechanism of the intrinsic activities still unclear. Herein, a typical model catalyst of NiSe2/FeSe2 heterostructure with a low overpotential of 230 mV at 10 mA cm−2 and long-term durability is fabricated by selenizing Ni-Fe Prussian-blue analog (PBA) templates. In-situ/operando Raman spectroscopy and synchrotron-based X-ray absorption spectroscopy (XAS) are employed to reveal the real catalytic process. It is attested that the dynamic surface reconstruction into Ni-O-Fe configurations, and the high-valence NiIII-O-FeIII moieties in Ni–FeOOH is responsible for the high oxygen evolution. Density functional theory (DFT) simulations further confirm that the electron transfer driven by strong Ni-Fe synergistic effect in NiSe2/FeSe2 heterostructure not only improves the density of Ni/Fe active sites, but also modulates the nature active sites. This works may deepen our understanding of intrinsic activities of metal compound heterostructures and the correlation between dynamic surface structural evolution and catalytic activity.

Nanostructured NiFe (oxy)hydroxide fabricated on nickel foams by laser-induced water plasma for enhanced alkaline oxygen evolution reaction

Hydrogen production from water electrolysis is hindered by the sluggish reaction kinetics, especially the anodic oxygen evolution reaction (OER). Dynamic surface structures and high valence metal sites are usually required in the development of highly efficient OER electrocatalysts, which are hardly realized using conventional preparation method. Here, we propose a novel and facile method to fabricate nickel-based OER electrocatalysts with non-thermal laser-induced water plasma. Using nickel foams (NFs) as the initial substrates, the NFs or Fe-doped FeOx/NFs treated with laser-induced water plasma own abundant Ni(Fe) hydroxide and oxyhydroxide nanostructures,labeled as Ni(Fe) (oxy)hydroxide thereafter, formed on the skeleton of NFs, which are characterized in detail by SEM, TEM, XPS and Raman spectra. The as-prepared Ni(Fe) (oxy)hydroxide/NFs electrocatalysts show greatly enhanced OER activity and stability. The optimized NiFe (oxy)hydroxide/NFs bimetallic electrocatalysts achieve a current density of 10 mA cm−2 at the overpotential of only 268 mV with a small Tafel slope of 53.6 mV dec-1. During 28 h long term stability test at 10 mA cm−2, there is negligible increase in the required overpotential. These findings demonstrate that non-thermal laser-induced water plasma is a promising and efficient way to prepare highly efficient electrocatalysts with dynamic surface structures and high valence metal sites.

Polysulfide-Induced Synthesis of Hierarchical Ni3S2/NiCo2S4 Nanorods Supported on Nickel Foam for Boosted Oxygen Evolution Catalysis

Crystal structures and chemical states can be reconstructed by inducing the positioning growth of metal sulfide, which is an efficient strategy to enhance the oxygen evolution activity of an anode. In this paper, polysulfide (Poly(S-r-DIB))-induced S-doped hierarchical heterostructures Ni3S2/NiCo2S4 were successfully grown on nickel foam (Ni3S2/NiCo2S4/NF) by an in situ solvothermal sulfuration reaction using polysulfide as the sulfur source. The unique hierarchical heterostructure and strong electron interaction in Ni3S2@NiCo2S4/NF increased the catalytic activity area and provided more mass transfer channels for electron transfer to facilitate water splitting. As-prepared Ni3S2/NiCo2S4/NF exhibits excellent electrocatalytic activity for oxygen evolution reaction (OER) with an ultralow overpotential of 330 mV to reach 100 mA cm–2 and a small Tafel slope of 67.6 mV dec–1, which is superior to most reported nickel-based sulfide electrocatalysts. This study provides not only a new method for the design of hierarchical nanomaterials but also an efficient strategy to design efficient OER electrocatalysts by an in situ polysulfide confinement method.