NiFe-layered Double Hydroxide Nanosheets Research Articles

Tribological Behavior of GTL Base Oil Improved by Ni-Fe Layered Double Hydroxide Nanosheets

The layered double hydroxide (LDH) has been practically applied in the field of tribology and materials science due to its unique physicochemical properties, weak bonding, flexible structural composition, and adjustable interlayer space. In this work, a series of ultrathin and flexible composition of Ni-Fe LDH samples were prepared via a cost-effective room-temperature co-precipitation process. Then, they were mechanically dispersed into GTL base oil and their lubricating performance were tested by a four-ball tribometer. It is found that the variation of Ni-Fe ratio of Ni-Fe LDH has a great influence on the improvement of lubricating performance of GTL base oil. At the same concentration (0.3 mg/mL), the Ni-Fe LDH with Ni/Fe ratio of 6 was demonstrated to exhibit the best lubricating performance and the AFC, WSD, the wear volume, surface roughness and average wear scar depth decreased 51.3%, 30.8%, 78.4%, 6.7% and 50.0%, respectively. SEM-EDS and X-ray photoelectron spectra illustrated that the tribo-chemical film consisting of iron oxides and NiO with better mechanical properties formed and slowly replaced the physical film, which resists scuffing and protect solid surface from severe collisions.

Dual efficiency enhancement in overall water splitting with defect-rich and Ru atom-doped NiFe LDH nanosheets on NiCo2O4 nanowires

The development of layered double hydroxides (LDHs) as bifunctional catalysts for efficient overall water splitting by controlling the composition and structure of the material remains challenging. In this study, defect-rich and Ru-doped NiFe LDH nanosheet shells on NiCo2O4 (NCO) nanowires were successfully synthesized using a hydrothermal method, followed by alkaline etching. The resulting def-Ru-NiFe LDH/NCO (def = defects) catalyst exhibited remarkable catalytic activity with overpotentials of 122 and 225 mV for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, at a current density of 10 mA cm−2. Density functional theory calculations demonstrated that atomic vacancy defects and the rational incorporation of Ru single atoms in def-Ru-NiFe LDH/NCO induced the regulation of the electronic structure and optimized the adsorption energies, thus enhancing the OER and HER performances. Particularly, an electrolyzer utilizing def-Ru-NiFe LDH/NCO exhibited exceptional performance, achieving cell voltages of 1.58 and 1.46 V in 1 M KOH at 25 and 75 °C, respectively. Moreover, the catalyst exhibited outstanding stability during 60 h of operation at a high applied voltage and temperature (75 °C). These results highlight the potential of def-Ru-NiFe LDH/NCO for hydrogen production.

Rapid and facile preparation of NiFe-layered double hydroxide nanosheets as self-supported electrode for glucose detection in drink sample

Layered double hydroxides have been widely used for electrochemical glucose detection due to their layered structure with more active sites, but they suffer from lower electrical conductivity and long-time hydrothermal preparation. In this paper, NiFe-layered double hydroxide nanosheets supported on nickel foam (NiFe-LDH NSs/NF) was prepared using an ultrafast and facile method via in-situ corroding foam nickel in FeCl3 solution under room temperature, and the whole synthetic process can be accomplished within several minutes. The as-fabricated NiFe-LDH NSs/NF shows significant catalytic activity in the glucose oxidation, showing its great promise in glucose detection. As a self-supported electrode, NiFe-LDH NSs/NF is favorable for glucose detection, with a sensitivity of 9.79 and 3.29 mA mM−1 cm−2 within the linear range of 0.001 to 1.16 mM and 1.16 to 4.67 mM, respectively. Moreover, NiFe-LDH NSs/NF is also selective and reliable towards glucose detection in drink sample.

Integration of surficial oxygen vacancies and interfacial two-dimensional NiFe-layered double hydroxide nanosheets onto bismuth vanadate photoanode for boosted photoelectrochemical water splitting

Considering its outstanding cost-to-efficiency ratio, N-type bismuth vanadate (BiVO4) is an ideal photoanode for photoelectrochemical (PEC) water splitting. Nevertheless, the widespread application of BiVO4 is being hindered commercially by innate challenges such as its poor carrier mobility, short hole diffusion length, and sluggish oxidation chemistry. Herein, we report the unique integration of surficial oxygen vacancies (Ovac) in conjunction with two-dimensional (2D) nickel-iron (NiFe) layered double hydroxide (LDH) nanosheets onto pristine BiVO4 photoanode to tweak the charge transfer kinetics during water splitting. The introduction of oxygen vacancies efficiently modulates band energetics, amplifies light absorption, and augments the carrier density. Concurrently, the 2D NiFe-LDH nanosheets play a dual role by facilitating interfacial hole transfer, thereby reducing excessive defect states, and serving as a protective layer against photocorrosion, ultimately resulting in a notable improvement in solar-driven water oxidation efficiency. The results of PEC water-splitting experiments demonstrate that the BiVO4 photoanode enriched with surficial oxygen vacancies and NiFe-LDH (BiVO4:Ovac/NiFe-LDH) achieved a photocurrent density of 2.92 mA cm−2 with ∼3-fold enhancement compared with a pristine BiVO4 photoanode. The engineered BiVO4 system yielded an impressive photoconversion efficiency of 1.29 %, charge separation efficiency (ηsep) > 40 %, and charge transport efficiency (ηtrans) > 35 %, clearly evidencing a boost in the charge dynamics of pristine BiVO4. Furthermore, during long-term stability testing, BiVO4:Ovac/NiFe-LDH retained 91 % of its initial photocurrent density for ∼20 h without any significant deterioration. This work sheds light on the role of integrating surface and interface engineering tactics for enhancing photo-induced charge transport in BiVO4, thereby providing efficient and stable PEC water splitting.

In situ construction of S-doped NiFe-layered double hydroxide nanoarrays on porous reduced graphene oxide as efficient oxygen evolution electrocatalysts for electrolytic cells

The application of NiFe-layered double hydroxide (NiFe-LDH) as electrocatalysts for oxygen evolution reaction (OER) is seriously limited owing to low conductivity, insufficient active sites and self-aggregation. The construction of anion-doped NiFe-LDH nanoarray structures with ideal electrochemical activity is an effective strategy. Herein, we report a facile solvothermal approach for the synthesis of S-doped NiFe-LDH nanosheets loaded in situ on porous reduced graphene oxide (rGO) to form a composite with nanosheet array structure (S-NiFe-LDH/rGO) for OER. The S-NiFe-LDH/rGO nanosheet array ensures a large specific surface area, abundant accessible active sites and high stability. The moderate S doping in NiFe-LDH can tune the local electronic structures of Ni and Fe to optimize the binding of metal sites with reaction intermediates, thus enhancing the OER activity. The S-NiFe-LDH/rGO electrocatalyst shows the small OER overpotential of 264 mV at 10 mA cm−2 and the low Tafel slope of 55 mV dec−1. Moreover, S-NiFe-LDH/rGO displays good durability and structural stability due to the strong anchoring effect. The assembled two-electrode S-NiFe-LDH/rGO||Pt electrolytic cell exhibits a low cell voltage of 1.53 V at 10 mA cm−2, which is superior to the RuO2||Pt benchmark cell (1.60 V), indicating the practical feasibility for water electrolysis. This design strategy provides a promising insight into the fabrication of advanced OER non-noble metal electrocatalysts.

Enhancing Water Oxidation Catalysis by Controlling Metal Cation Distribution in Layered Double Hydroxides

AbstractThe sluggish kinetics of the oxygen evolution reaction (OER), the limiting step of the electrochemical water splitting process, hinders the eventual commercialization of this important renewable energy strategy. Hence, the development of efficient electrocatalysts for this reaction is crucial. Multi‐metal‐based (hydr)oxides are promising OER electrocatalysts because the electronic interactions between multiple constituent metal cations can potentially enhance electrochemical performances. However, complex compositions may not always lead to positive synergistic effects. The appropriate distribution of the cations is also critical. However, the high dispersibility of cations in these hydroxides renders the control of their distribution challenging. Herein, an approach is reported to control the metal cation distribution in layered double hydroxides (LDHs) to improve their OER performances. Restacking of exfoliated NiFe and CoAl LDH nanosheets leads to electrochemical synergistic effects between different nanosheets. As far as it is known, the restacked LDH described herein exhibits the lowest overpotential (224 mV) and Tafel slope (34.26 mV dec−1) among reported powder‐type (hydr)oxide and alloy OER electrocatalysts with more than three different metal cations. Thus, a new design approach is suggested to enhance the electrochemical performances of LDHs.

Co-doped NiFe-LDH nanosheets arrays supported on nickel foam as an efficient oxygen evolution electrocatalysis

In the field of water electrolysis for hydrogen production, NiFe-layered double hydroxides (NiFe-LDH) is currently one of the most excellent oxygen evolution electrocatalyst. However, its application in this field is limited due to its low durability, low conductivity and few active sites. In this work, we present a Co-introduced fine-modulated NiFe-LDH performance strategy to construct Co-doped NiFe-LDH nanosheets arrays supported on nickel foam (Co-NiFe-LDH/NF). The optimized Co-NiFe-LDH/NF showed ultralow overpotentials at 211, 237, 261 and 281 mV at 50, 100, 200 and 300 mA cm−2 current densities, separately, with an extremely low Tafel slope (40 mV dec−1). The Co-NiFe-LDH/NF was highly stable during the constant current test, and the potential increased by only 10 and 13 mV after 100 h stability test at 200 and 300 mA cm−2. Notably, it also works stably at the 500 and 1000 mA cm−2 very high current densities, representing high potential of commercial application. The theoretical calculation showed that: (1) Introducing Co into NiFe-LDH can reduce the energy barrier of OH* conversion to O* and increase the activity of OER; (2) Considering the formation energy, Co is more likely to replace Fe site in NiFe-LDH than Ni site; (3) The energy barrier, differential charge and partial density of states analysis show that the best OER performance is obtained when the Fe-to-Co ratio of 2:1, and the theoretical calculation is consistent with the experimental verification. This work provides a simple and effective method for the design and construction of doping elements into NiFe-LDH electrocatalysts for efficient water oxidation, and promoted NiFe-LDH material application in commercial alkaline water electrolysis cell.

Carbon nanotubes supported Cs-doped NiFe-layered double hydroxide nanosheets as efficient catalyst for oxygen evolution reaction

The sluggish kinetics of oxygen evolution reaction (OER) becomes one of the big obstacles for large-scale electrolytic water splitting. It is urgent to develop low-cost and efficient OER catalyst with fast reaction kinetics and low energy barrier. Herein, a novel OER catalyst of carbon nanotubes (CNT) supported cesium-doped NiFe-layered double hydroxide nanosheets (Cs–NiFe-LDH) is synthesized via a facile one-pot solvothermal reaction. The porous Cs–NiFe-LDH@CNT catalyst delivers an ultralow overpotential of 240 mV @10 mA cm−2, a small Tafel slope of 56.22 mV dec−1, and excellent long-term stability. The outstanding catalytic performance is mainly attributed to the synergistic effects of Cs doping and CNT network: the modification of the electronic structure of NiFe-LDH by Cs doping and the conductivity enhancement by CNT network can facilitate the electron transfer and kinetics; the Cs doping and CNT introduction can generate rich active sites; the nanoporous architecture can facilitate the ion diffusion and O2 bubble release. This work provides a synergistic strategy to design and synthesize transition metal based OER catalysts with low-cost, fast kinetics and good stability for water splitting.

NiFe-LDH nanosheets with high activity in three dimensions on NiFe foam electrode for water oxidation

Water decomposition is the most promising system for developing renewable energy, but the sluggish kinetics of the oxygen evolution reaction (OER) is the key limiting factor. Recently, inorganic nanomaterials have been widely used as an effective catalyst for the stimulation of OER kinetics, for which the subsequent dispersion and the current collector are essential. We present a three-dimensional NiFe-layered double hydroxide (NiFe-LDH) synthesized by electrochemical corrosion, with increased electrochemical performance for electrocatalytic water splitting. Electrochemical measurements reveal that the NiFe-LDH has a lower Tafel slope of 46 mVˑdec−1 compared with the NiFe foam. The resulting electrode had a current density of 50 mAˑcm−2 and a remarkably low overpotential of 150 mV. Theoretical calculations and experimental observations showed and explained NiFe-LDH-90 electrode has high alkaline medium endurance, implying excellent OER electrocatalytic activity. The produced composite is expected to bring new insights into the design of foam electrocatalyst materials for OER.

Synergistic coupling of NiFe-layered double hydroxide nanosheets with Co-doped porous interconnecting carbon frameworks for efficient bifunctional electrocatalysis

The design of low-cost and durable electrocatalysts with high catalytic performance of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the development of sustainable energy technologies. NiFe-layered double hydroxide (NiFe-LDH) is a promising electrocatalyst for OER, but the poor ORR activity limits its large-scale application as a bifunctional electrocatalyst in energy storage and conversion devices. The rational design of hierarchical nanohybrids is an effective strategy to construct bifunctional OER/ORR electrocatalysts. Herein, a high-performance OER/ORR bifunctional non-noble metal electrocatalyst (NiFe-LDH/CoNC-PIN) was projected. The Co-doped carbon frameworks with porous interconnecting networks (CoNC-PIN) is prepared by pyrolysis of ZIF-8/67 via a salt template strategy, followed by the uniform in situ growth of ultrathin NiFe-LDH nanosheets on CoNC-PIN to construct the hierarchical NiFe-LDH/CoNC-PIN hybrid. The molten NaCl template in pyrolysis activates the surface of ZIF-8/67 and connects them into porous carbon networks to improve the surface area, porosity and electronic conductivity of catalysts. Due to the efficient electron transfer and strong coupling between CoNC-PIN and NiFe-LDH, the NiFe-LDH/CoNC-PIN exhibits a small OER overpotential of 249 mV at 10 mA/cm2, a low Tafel slope of 27.7 mV/dec, an ORR half-wave potential of 0.80 V, and excellent durability and structural stability. This strategy provides a novel insight to fabricate advanced OER/ORR bifunctional non-noble metal electrocatalysts.

“One stone three birds” plasma engraving strategy to boost the hydrogen evolution reaction activity of layered double hydroxides

Possessing large specific surface areas and rich metal redox sites, layered double hydroxides (LDHs) are potentially suitable oxygen evolution reaction catalysts. It is a pity that they usually show poor hydrogen evolution reaction (HER) activity on account of the limited conductivity and deficient active sites. Taking NiFe LDH nanosheets as an example, we develop a “one stone three birds” plasma engraving strategy to enhance the HER activity of NiFe LDH. The “three birds,” including the reduction of Ni2+ to Ni nanoparticles (Ni NPs), generation of more oxygen vacancies (Ov), and exfoliation of nanosheets into much thinner ones, can obviously improve the conductivity and active sites of NiFe LDH. The plasma processing can also enhance water adsorption and accelerate the Volmer step during HER. As expected, the plasma-engraved NiFe LDH (PEH) exhibits enhanced HER activity with a low overpotential of 22 mV at 10 mA cm−2 and a small Tafel slope of 38 mV dec−1 in 1 M KOH, much better than NiFe LDH (202 mV, 145 mV dec−1). By combining optical emission spectroscopy diagnosis and structural/electrochemical characterizations, the relationship among the electron excitation temperature (Texc) in plasma, the amount of Ni NPs and Ov in PEH, and the HER activity of PEH is established. Excitingly, the PEH also displays splendid HER activity in both alkaline real seawater and overall water splitting.

Cu2S Nanorods Decorated with NiFe-Layered Double Hydroxide Nanosheets as Bifunctional Electrocatalysts for Hydrogen Evolution in Alkaline Saline Water/Seawater

The progress of highly efficient and stable nonprecious metallic-based electrocatalysts for the hydrogen evolution reaction (HER) in saline water/seawater is of great importance for the mass-scale production of hydrogen energy. Herein, a nanostructured NiFe-LDH/Cu2S heterostructure is constructed by growing NiFe-LDH nanosheets onto Cu2S nanorods. This heterostructure possesses exceptional HER performances with η10 = 36 mV (η10 indicates the overpotential at a current density of 10 mA cm2) and worked stably over 100 h in alkaline saline water. Moreover, the heterojunction catalyst acts as a bifunctional electrocatalyst for the anode and cathode, achieving a cell voltage of η10 = 1.463 V, which is superior to many reported electrocatalysts. The improvement of catalytic performance of NiFe-LDH/Cu2S can be ascribed to the charge redistribution on the heterointerface that accelerates the dissociation process caused by the formation of Ni with low electron cloud density. Moreover, the obtained covalent S–O bonds are beneficial to the adsorption of H*. In addition, NiFe-LDH/Cu2S also exhibits considerable catalytic performances in seawater electrolytes, delivering a cell voltage of η10 = 1.483 V with a stability of 27 h. We present a promising bifunctional electrocatalyst for water electrolysis in alkaline saline water/seawater which may find wide applications in the area of hydrogen energy.

Deeply Reconstructed NiFe Layered Double Hydroxide Nanosheets for an Efficient Oxygen Evolution Reaction

Deeply Reconstructed NiFe Layered Double Hydroxide Nanosheets for an Efficient Oxygen Evolution Reaction

Cu2S@NiFe layered double hydroxides nanosheets hollow nanorod arrays self-supported oxygen evolution reaction electrode for efficient anion exchange membrane water electrolyzer

Herein, we prepared highly active self-supported Cu2S@NiFe layered double hydroxides nanosheets (LDHs) oxygen evolution reaction (OER) electrode (Cu2S@NiFe LDHs/Cu foam) with three-dimensional (3D) multilayer hollow nanorod arrays structure, which is composed of the outer layer (two-dimensional (2D) NiFe LDHs) and the inner layer (one-dimensional (1D) Cu2S hollow nanorod arrays). The unique structure of NiFe LDHs and Cu2S hollow nanorod composites can expose more active sites, and simultaneously promote electrolyte penetration and gas release during the water electrolysis process. Thus, the Cu2S@NiFe LDHs/Cu foam electrode exhibits a significant OER performance, with the overpotentials of 230 and 286 mV at 50 and 100 mA cm−2, respectively. Anion exchange membrane water electrolyzer (AEMWE) with the prepared electrode can attain a voltage of 1.56 V at the current density of 0.50 A cm−2, showing a good performance that is comparable to the-state-of-the-art AEMWE in 1 M KOH. In addition, the AEMWE can be run for 300 h at the current density of 0.50 A cm−2. The high performance and good stability of AEMWE are attributed to the special structure of the OER electrode, which can prevent the agglomeration of nanosheets and thus expose more active sites at the edge of the nanosheets.

NiFe-Layered Double Hydroxides/Lead-free Cs2AgBiBr6 Perovskite 2D/2D Heterojunction for Photocatalytic CO2 Conversion

Designing of heterojunction photocatalysts with appropriate interfacial contact plays crucial roles in enhancing the interfacial charge transfer/separation. A two-dimensional (2D)/2D face-to-face heterojunction is an ideal option since this architecture with a large contact area can provide abundant reactive centers and promote the interfacial charge transfer/separation between layers. Herein, a novel 2D/2D heterojunction of NiFe-layered double hydroxides (NiFe-LDH)/Cs2AgBiBr6 (CABB) was fabricated by electrostatic self-assembly of NiFe-LDH and CABB nanosheets. This unique 2D/2D architecture endowed NiFe-LDH/CABB with a large contact area and a short charge transport distance, assuring remarkable interfacial charge transfer/separation rates. As a result, the 2D/2D NiFe-LDH/CABB heterojunction exhibited significant improvement in photocatalytic CO2 reduction under visible light than the pristine counterparts. Based on density functional theory calculations and various characterizations, a step scheme charge-transfer mechanism was proposed. This investigation sheds light on the designing and manufacturing of highly efficient 2D/2D heterostructure photocatalysts for artificial photosynthesis.

Promoting Surface Reconstruction of NiFe Layered Double Hydroxide for Enhanced Oxygen Evolution

AbstractA dynamic surface reconstruction of oxide electrocatalysts in alkaline media is widely observed especially for layered double hydroxide (LDH), but little is known about how to promote the reconstruction toward desired surfaces for improved oxygen evolution reaction (OER). Here, surface reconstruction of NiFe LDH nanosheets is successfully induced to a higher degree via in situ sulfur doping than that by natural electrochemical activation. Theoretical calculations, operando Raman, and various ex situ characterizations reveal the S anion‐induced effect can lower the energy barrier and facilitate the phase transformation into highly active S‐doped oxyhydroxides. The generated S‐NixFeyOOH can optimize the intermediate adsorption and facilitate the OER kinetics. The reconstructed S‐oxyhydroxides catalyst presents superior OER activity and long‐term durability compared to undoped ones. This work provides a structure–composition–activity relationship during the in situ surface restructuring of NiFe LDH pre‐catalysts.

Chemical scission derived two-dimensional NiFe layered double hydroxide nanoswords for efficient water splitting

Advanced electocatalysts have been designed and reported for water electrolysis. However, significant challenge and gap remains for the design and fabrication of highly efficient, robust yet cost-effective multifunctional catalyst. One of the fundamentals of the effective catalysts lies in the effective active sites capable of lowering the adsorption energy required for the intermediate steps. In this work, for the first time, two-dimensional (2D) NiFe layered double hydroxide (NiFe-LDH) nanoswards is reported by chemical scission of NiFe layered double hydroxide nanosheets with silicon tetrachloride (SiCl4). Si is incorporated into the structure of NiFe-LDH nanoswords with the emergence of new bond of Si–O-M. In situ Raman together with theoretical calculation confirmed that Tailored-NiFe-LDH has a more active internal structure than NiFe-LDH nanosheet, favoring the formation of active intermediates. The new Si–O-M bond lowers the energy required for the adsorption process. Tailored-NiFe-LDH shows low η10 of 280 mV and Tafel slope of 52.11 mV dec−1 at 10 mA cm−2 (Oxygen evolution reaction). When used as water splitting catalyst in a full cell, Tailored-NiFe-LDH shows good activity and stability. We believe this simple and rapid synthesis route is proposed to prepare novel catalysts with good electrical conductivity, and opens new strategy to tailor 2D LDH materials and other similar hydroxyl-rich superstructures and composites.

MOF-derived Mo-CoP@NiFe LDH hierarchical nanosheets for high-performance hybrid supercapacitors

The well-designed hierarchical nanosheets composed of different components can enhance specific capacitances and cycling stabilities of electrode materials. Herein, [email protected] layered double hydroxide (LDH) hierarchical nanosheets were prepared by growing NiFe LDH nanosheets on MOF-derived ultrathin Mo-CoP nanosheets fabricated by etching Co-MOF with Mo ions. The hierarchical nanosheet structure can facilitate the transport of electrons/ions and provide numerous electroactive sites. Furthermore, the synergistic effects between the active materials provided more oxidation states for redox reactions. Owing to the hierarchical nanosheets and the synergistic effects between Mo-CoP and NiFe LDH, the [email protected] LDH electrode exhibited a large specific capacity of 1655 C g−1 (2796 F g−1) and good long-term stability with 79.6 % capacity retention after 8000 cycles. Additionally, the hybrid supercapacitor (HSC) delivered an energy density of up to 48.6 Wh kg−1 at 800 W kg−1 and outstanding cycling stability (90.6 % retention of the initial capacitance over 20,000 cycles).

NiFe Layered Double Hydroxide Nanosheets Anchored on Cobalt Nanocrystal Matrixes as Electrocatalysts for Oxygen Evolution

NiFe Layered Double Hydroxide Nanosheets Anchored on Cobalt Nanocrystal Matrixes as Electrocatalysts for Oxygen Evolution

Binary Layered Double Hydroxide Electrode Array Synthesized via Metal Alloy Corrosion for Oxygen Evolution Reaction

Developing highly efficient and cost-effective anodic electrodes is one of the major challenges in hydrogen evolution via water electrolysis technology. We demonstrate that the chloride corrosion can transform commercial Ni–Fe alloy (invar) into an electrode, revealing a high-efficiency oxygen evolution reaction (OER). The corroded electrode consisted of a dense vertical array of NiFe layered double hydroxide (LDH) nanosheets on the invar foil surface. We discovered that the corrosion rate difference between metallic components of the binary metal serves the growth of the NiFe LDH at the invar surface. This NiFe LDH electrode required an overpotential of 211 mV at a current density of 10 mA/cm2 in 1 M KOH, outperforming the commercial IrO2 catalyst by 47 mV. This electrode showed excellent activity against OER owing to their high intrinsic electrical conductivity of direct growth and high electrochemical surface area of NiFe LDH nanosheets. Furthermore, this electrode exhibited excellent electrochemical stability without decay in performance for 7 days at the current density of 50 mA/cm2. This study provides rationally designed electrocatalysts with high and stable catalytic activity to obtain a convenient route to the large-scale production of OER electrodes.