Oxygen Evolution Reaction Overpotential Research Articles

Additive Manufacturing of Multiscale NiFeMn Multi-Principal Element Alloys with Tailored Composition

Abstract Nanostructured multi-principal element alloys (MPEAs) have been explored as next-generation engineering materials due to unique mechanical and functional properties which have significant advantages over traditional dilute alloys. However, the practical applications of nanostructured MPEAs are still limited due to the lack of scalable processing approaches to prepare a large quantity of nanostructured MPEAs, as well as lack of an efficient pathway for high-throughput discovery of better functional nanostructured MPEAs within their vast compositional space. Here we tackle these challenges by presenting an integrated approach by combining direct-ink-writing-based additive manufacturing, solid-state sintering, and chemical dealloying to manufacture hierarchically porous MPEAs. The hierarchical structure is comprised of macro- and micro-scale pores introduced via extrusion printing and polymer decomposition during sintering, as well as nanoscale pores formed via chemical dealloying. The macro- and micro-scale pores allow efficient dealloying of a large mass of material as the diffusion length that the corroding medium must penetrate remains at the scale of the ligaments formed after sintering (~10 μm), despite the large volume of the 3D-printed samples. In addition, this integrated approach enables versatile control of the alloy composition via precisely tuning the ratio of elemental powders in the starting ink, thus offering a pathway for high-throughput discovery of novel functional MPEAs. As a case study, multiscale macro/micro/nanoporous NiFeMn MPEAs with three different compositions were investigated as catalysts to reduce the overpotential of oxygen evolution reaction (OER), where NiFeMn-based electrocatalysts display composition-dependent performance such that the overpotential measured at a current of 0.5 A/g for OER increases in the order of Ni58Fe29Mn13 ≤ Ni64Fe26Mn10 < Ni76Fe18Mn6. This introduced manufacturing process offers new opportunities for scalable fabrication and rapid screening of nanostructured multi-component complex alloys. 

Screening of agroindustry residues for their usage as oxygen evolution reaction catalysts

The Oxygen Evolution Reaction (OER) is the limiting step of the water splitting process for the attainment of valuable hydrogen. A green and economic catalyst is hereby synthesized to reduce the OER overpotential. For the first time, a set of representative agroindustry residues were used and subjected to pyrolytic and hydrothermal treatments, fitting them into circular economy. Hydrothermally treated potato peels (PP-HC), particularly, reached 10, 50 and 100 mA/cm2 at overpotentials of 353, 408 and 431 mV, respectively. The novel material showed good electron-transfer and small Tafel slope (52.2±0.2 mV/dec) that is explained by the presence of electronic rich bonds as confirmed by physical-chemical and electrochemical characterization. PP-HC showed an astonishingly high stability for 90 h or 8000 cyclic voltammetries, with the hydrochar suffering only slight modifications, as measured by FTIR and XPS. This innovative study opens a path to efficient and environmentally friendly green hydrogen.

Nanostructured Fe-Doped Ni3S2 Electrocatalyst for the Oxygen Evolution Reaction with High Stability at an Industrially-Relevant Current Density.

A novel oxygen evolution reaction (OER) electrocatalyst was prepared by a synthesis strategy consisting of the solvothermal growth of Ni3S2 nanostructures on Ni foam, followed by hydrothermal incorporation of Fe species (Fe-Ni3S2/Ni foam). This electrocatalyst displayed a low OER overpotential of 230 mV at 100 mA·cm-2, a low Tafel slope of 43 mV·dec-1, and constant performance at an industrially relevant current density (500 mA·cm-2) over 100 h in a 1.0 M KOH electrolyte, despite a minor loss of Fe in the process. Based on a detailed characterization by (in situ) Raman spectroscopy, (quasi-in situ) XPS, SEM, TEM, XRD, ICP-AES, EIS, and Cdl analysis, the high OER activity and stability of Fe-Ni3S2/Ni foam were attributed to the nanostructuring of the surface in the form of stable nanosheets and to the combination of Ni3S2 granting suitable electrical conductivity with newly formed NiFe-based (oxy)hydroxides at the surface of the material providing the active sites for OER.

Dahlia ball shape bimetallic phosphide-tungsten phosphide composite for anion-exchange membrane water electrolysis

Revolutionizing Water Splitting: Introducing a Breakthrough Single Heterostructure Catalyst for Highly Efficient Hydrogen and Oxygen Evolution Reactions. In this study, we present an innovative method for water splitting that introduces a remarkable single heterostructure catalyst capable of efficiently catalyzing both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Our research utilizes a highly effective hydrothermal synthesis technique to create a unique nanosphere composed of iron cobalt phosphide and tungsten phosphide (FCPWP). This FCPWP nanosphere exhibits finely tuned electronic structures and incorporates multiple active sites, resulting in significantly reduced overpotentials. When operating at current densities of 50 and 100 mA cm−2, the HER overpotentials are as low as 220 mV and 280 mV, respectively, while the OER overpotentials are only 270 mV and 350 mV. By employing the FCPWP nanosphere in a two-electrode electrolyzer configuration, we achieve exceptional results with minimal power requirements. At an operating current density of 10 mA cm−2, the electrolyzer functions at an impressively low voltage of 1.44 V, and at 1.85 V, it attains a high current density of 425 mA cm−2, showcasing an outstanding cell efficiency of 71.63% in an anion exchange membrane electrolyzer. These experimental findings are further supported by computational study. The FCPWP nanosphere emerges as an extraordinary electrocatalyst with dual-functionality for water splitting, offering great promise in the efficient production of hydrogen through electrochemical means. Our research not only introduces a fresh perspective but also paves the way for significant advancements in renewable energy technologies.

Unlocking the Electrocatalytic Behavior of Cu2MnS2 Nanoflake-Anchored rGO for the Oxygen Evolution Reaction in an Alkaline Medium.

A catalyst of the oxygen evolution reaction (OER) that is viable, affordable, and active for effective water-splitting applications is critical. A variety of electrocatalysts have been discovered to replace noble metal-based catalysts. Of these, transition metal-based sulfides are essential for incorporating carbonaceous materials to improve electrical conductivity, resulting in better electrocatalytic performance. Our study illustrates the synthesis of Cu2MnS2 (CMS) nanoflakes and their different rGO composites (10 to 40 wt %) via a hydrothermal technique for an effective water oxidation reaction. The X-ray diffraction pattern reveals that the prepared Cu2MnS2 nanoflakes exhibit a cubic crystal structure. The high-resolution scanning electron microscopy and the high resolution transmission electron microscopy images corroborate the formation of the nanoflake-like morphology of Cu2MnS2 with the strong interaction of rGO. The selected area electron diffraction analysis pattern reveals a polycrystalline nature. The Fourier transform infrared spectrum shows the existence of a metal sulfur vibrational band at 590 cm-1, and Raman analysis infers the presence of rGO. The X-ray photoelectron spectroscopy spectra reveal the oxidation states of the elements present in the samples. Using Brunauer-Emmett-Teller analysis, the surface area of CMS-20 is found to be 117.04 m2/g. The measured OER overpotentials using linear sweep volammetry and the values are 380, 370, 340, 376, and 400 mV at 10 mA/cm2 for CMS, CMS-10, CMS-20, CMS-30, and CMS-40, respectively, and the corresponding Tafel slope values are 179, 158, 149, 206, and 240 mV/decade, respectively. The electrochemical active surface area is estimated using cyclic voltammetry for all of the catalysts, where CMS-20 showed a larger surface area. Also, the same catalyst exhibits good stability for ∼24 h at a constant potential, which is confirmed via chronoamperometry. Thus, combining transition metal-based sulfides with carbonaceous materials indicates improved catalytic behavior for the preparation of high-performance OER electrocatalysts. Overall, the prepared CMS-20 performed as an efficient OER electrocatalyst and can be utilized for practical applications in energy conversion.

Chemical coupling engineered exceptional overall water splitting of autogenic NiS/FeS/NiFeOOH heterostructure

The commercial application of hydrogen production is heavily subjected to the expensive and complex fabrication of required bifunctional catalysts integrated with good oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) activity. Herein, a simple one-spot hydrothermal technique is advocated to simplify the synthesis of NiS/FeS/NiFeOOH heterostructure (denoted as NiFeS) and achieve exceptional bifunctional electrocatalyst for water splitting. Such NiFeS heterostructure reveals multi-synergistic heterointerfaces with chemical coupling to provide more accessible active sites, regulate electronic structure of the active center and enhance interfacial charge transfer, enabling a significant contribution in improving the catalysis. Therefore, the OER overpotential of NiFeS catalyst is only 249 mV to supply 100 mA cm−2 current density, and the overpotential for HER reaches to 153 mV at 10 mA cm−2. When constructed into a lab-made electrode system, it confirms a remarkable cell voltage of only 1.51 V at 10 mA cm−2 for water splitting, along with an exceptional durability up to 100 h at 100 mA cm−2. This study gives a novel insight into heterostructure engineering for the construction of bifunctional electrocatalysts.

Enhancing Iron Oxide Electrocatalysis for Efficient Overall Water Splitting: A Study of Tailored Synthesis for Advanced Energy Generation and Storage

ABSTRACTWater splitting, a critical milestone in the development of renewable energy, allows the production of pure hydrogen and oxygen. Iron oxide (Fe2O3), a fundamental component in electrochemical water splitting for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), offers potential because of its accessibility, low cost, and environmental safety. Herein, to analyze the impact of the methodology on its properties, Fe2O3 was produced in three different ways: freeze‐drying (aerogel) (Fe2O3‐AG), hydrothermal (Fe2O3‐HT), and microwave (Fe2O3‐MW). The Fe2O3‐AG outperformed Fe2O3‐HT and Fe2O3‐MW in most properties which showed improved current and overall water‐splitting efficiency. The resulting materials demonstrated good electrocatalytic performance for both HER and OER in alkaline media, with overpotentials for HER of 204, 235, and 255 mV and overpotentials for OER of 222, 288, and 292 mV for the Fe2O3‐AG, Fe2O3‐HT, and Fe2O3‐MW samples, respectively, at a current density of 10 mA/cm2. The freeze‐drying synthesis process has significant potential as a feasible method for the manufacture of Fe2O3‐based electrocatalysts for water‐splitting applications. This study provides important insights into the influence of electrocatalytic and energy storage properties of Fe2O3 based on the use of different methodologies, that is, hydrothermal, microwave‐assisted, and freeze‐drying. Through that, a more assertive analysis can be made concerning changes in morphology, conductivity, exposure of active area, and electrochemical stability which are crucial for the overall performance, hence providing valuable information and considerations for possible large‐scale applications for energy generation and energy storage.

The strong metal-support interaction at Ni–O–Pt interface facilitates rapid electrocatalytic hydrogen production

Electrocatalytic water splitting technology, as an essential method for storing and converting renewable energy, has garnered significant attention. However, traditional electrolytic water splitting is hampered by issues such as noble metal catalysts are expensive and unstable, limiting its widespread application. To address this challenge, this study proposes an innovative method that utilizes nickel metal-organic framework (Ni-MOF) as a support to firmly anchor platinum (Pt) nanoparticles on its surface. This approach not only overcomes the high cost and instability associated with traditional noble metal catalysts but also leverages the strong chelation effect of ethylenediaminetetraacetic acid disodium salt (EDTA·2Na) and the strong metal-support interaction (SMSI) at the Ni–O–Pt interface, prompting catalysts to possess excellent stability and catalytic activity. The catalyst exhibits excellent performance in promoting the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall electrolysis of water, maintaining stability throughout the entire electrochemical process. At a current density of 10 mA cm−2, the overpotentials for HER and OER with Pt1·5/Ni-MOF are only 29 mV and 234 mV, respectively. When Pt1·5/Ni-MOF serves as both the cathode and anode for overall water splitting, only a low voltage of 1.557 V is needed. This study offers fresh insights into the development of stable, efficient, and low-budget dual-functional catalysts for water electrolysis, with the potential to drive the commercialization of water electrolysis technology and make significant contributions to the advancement of clean energy.

Hierarchical design of self-standing FeNi-based metallic glass by in-situ surface amorphous oxide construction for durable and efficient oxygen evolution

As the driving force to accelerate water electrolysis in commercialization, electrocatalysts concurrently satisfying electrocatalytic activity, mechanical flexibility and low cost are essential in the design to lower the kinetic barriers of oxygen evolution reaction (OER). Herein, we present a strategy to in situ construct amorphous metal oxide nanolayers on different compositions of self-standing non-precious FexNiyP20 (at.%) metallic glasses (AMO-MG). With the mechanical flexibility, the processed FexNiyP20 MGs, particularly at Fe30Ni50P20, reaches low OER overpotentials of 238 at a current density of 10 mA cm−2 and a Tafel slope of 33 mV dec−1 in 1.0 M KOH solution while preserving a long-term durability exceeding 60 h. Further studies indicate these robust OER activities appear to be directly linked with the unique surface patterns of the in situ formed AMO nanolayer providing abundant active sites, low charge transfer resistance and superior structural-integration. Modelling analysis further shows that both Fe and Ni sites in the AMO nanolayer act as active sites leading to a strong charge transfer for effective H2O adsorption and an optimization of rate-determining step of OER process. Such in situ AMO-MG hierarchical structure therefore provides a novel structure-integration strategy and win–win situation to design and commercialize high-performance alloy catalysts for OER.

Novel BDD-loaded bimetallic phosphide electrodes enable interesting bifunctional seawater electrolysis

The direct electrolysis of seawater for the production of green hydrogen energy is an economically attractive approach. However, this technique confronts technical problems because to the oxygen evolution reaction (OER) weak stability and inadequate selectivity due to competition with the chlorine evolution process. This work focuses on improving the stability of the electrode and electrolysis efficiency. The phosphating NiCoP active layer with 3D porous morphology was created by electrodeposition on the B-doped diamond (BDD) as an electrode inspired by the bifunctional catalysis strategy. The successfully constructed unique cube-on-nanosheets structure in the NiCoP active layer enables the electrode to exhibit excellent electrocatalytic performance. The porous NiCoP/BDD (Por-NiCoP/BDD) electrodes showed good catalytic activity in alkaline-simulated seawater with the OER and hydrogen evolution reaction (HER) overpotentials of 400 and 261 mV, respectively, at a current density of 100 mA cm−2. More importantly, the electrode exhibits interesting stability of the catalytic performance and structural in alkaline-simulated seawater electrolysis, with the current density decaying by only 1.52 % and 4.06 % after OER and HER processes for 50 h, respectively. This work expands the valuable application scenarios of BDD electrode and provides novel ideas for the development of seawater electrolysis.

Microwave-edge-thermal effect induced growth of coral-like self-supporting CoFe2O4/NF electrocatalysts for efficient water splitting at high-current–density

High-efficiency and long-term water-splitting to produce hydrogen under the high-current–density (HCD) environment is a severe challenge because the swift-consumption of electrolyte and large-production of bubbles require better charge/mass-transfer capability and durability of electrocatalysts. Rationally design and effectively construct of three-dimensional (3D) hierarchical electrocatalysts is the promising solutions to accelerate the charge/mass transfer rates during water splitting process. Herein, a 3D self-supporting coral-like oxygen evolution reaction (OER) electrode was synthesized using a microwave-edge-thermal (MET) effect induced heterogeneous-nucleation/oriented-attachment growth process by in-situ growing CoFe2O4 nanocrystals on nickel-foams (CoFe2O4/NF). SEM, TEM, electrolyte/catalyst contact angles, in-situ observation of O2 bubbles generation-evolution process, and three-electrode configuration (TEC) and anion-exchange-membrane (AEM) electrocatalytic OER tests at the HCD were used to investigated the structure–activity relationship of the CoFe2O4/NF electrode. The CoFe2O4/NF electrode offered super hydrophilic/aerophobic surfaces and firmly bonded interfaces, resulting in quicker charge/mass-transfer rates and long-term durability under the HCD conditions. The typical electrode exhibited a low OER overpotential of 0.24 V at 10 mA cm−2 and a Tafel slope of 35.2 mV dec−1; its alkaline AEM electrolyzer (Pt || CoFe2O4/NF) yielded a HCD of 1200 mA cm−2 at only 2.36 V and 60˚C and kept stable over 120 h. This work provides a new insight into the design and fabrication of active and stable non-noble metal-based OER electrocatalysts for cost-effective water-electrolysis applications.

Paired array electrocatalyst coupling NiFe-MOF and NiFe-LDH for boosting oxygen evolution reaction

Developing affordable and highly efficient electrocatalytic materials is of utmost importance and indispensable for the process of water splitting, specifically for the oxygen evolution reaction (OER). To improve the OER process, a remarkable pair sites OER catalyst was successfully synthesized by coupling array NiFe-LDH and NiFe-MOF, which grew close together and support each other, some evidences prove that NiFe-LDH and NiFe-MOF heterostructure could enhance the electrocatalytic activities. The array NiFe-LDH/NiFe-MOF/NF-20 exhibits exceptional performance in the OER as evidenced by remarkably low overpotentials of 244 mV and 261 mV at the current densities of 50 and 100 mA/cm2, respectively. These significantly reduced overpotentials demonstrate the ability of the catalyst to facilitate the OER with minimal energy input, highlighting its remarkable efficiency. Furthermore, the OER overpotential was merely increased 23 mV after a rigorous 100 hours durability test. The results reveal that the heterostructures of NiFe-MOF and NiFe-LDH of the catalyst are the key factors for maintaining its excellent OER performance of over extended periods of operation. All the evidences show an excellent OER catalyst is fabricated resoundingly. This work could offer a feasibility reference of a simple way to construct a heterostructure electrocatalyst.

Fe-Co co-doped 1D@2D carbon-based composite as an efficient catalyst for Zn-air batteries

A Fe-Co dual-metal co-doped nitrogen-containing carbon composite (FeCo-HNC) was prepared by adjusting the ratio of iron to cobalt as well as the pyrolysis temperature with the assistance of functionalized silica template. Fe1Co-HNC, which was made from 1D carbon nanotubes and 2D carbon nanosheets with a rich mesoporous structure, presented exceptional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity. The ORR half-wave potential is 0.86 V (vs. reversible hydrogen electrode, RHE), and the OER overpotential is 0.76 V at 10 mA cm−2 with the Fe1Co-HNC catalyst, which can be applied to zinc-air batteries with superior performance. It is worth noting that the zinc-air batteries with Fe1Co-HNC-1000 as the cathodic catalyst achieved the highest power density of 94.9 mW cm−2 under a current density of 159.8 mA cm−2. This method provides a promising strategy for fabricating efficient transition metal-based carbon catalysts.

A High-Entropy Oxyhydroxide with a Graded Metal Network Structure for Efficient and Robust Alkaline Overall Water Splitting.

Designing high-entropy oxyhydroxides (HEOs) electrocatalysts with controlled nanostructures is vital for efficient and stable water-splitting electrocatalysts. Herein, a novel HEOs material (FeCoNiWCuOOH@Cu) containing five non-noble metal elements derived by electrodeposition on a 3D double-continuous porous Cu support is created. This support, prepared via the liquid metal dealloying method, offers a high specific surface area and rapid mass/charge transfer channels. The resulting high-entropy FeCoNiWCuOOH nanosheets provide a dense distribution of active sites. The heterostructure between Cu skeletons and FeCoNiWCuOOH nanosheets enhances mass transfer, electronic structure coupling, and overall structural stability, leading to excellent activities in the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and water splitting reaction. At 10mAcm-2, the overpotentials for OER, HER, and water splitting in 1.0m KOH solution are 200, 18, and 1.40V, respectively, outperforming most current electrocatalysts. The catalytic performance remains stable even after operating at 300mAcm-2 for 100, 100, and over 1000h, correspondingly. This material has potential applications in integrated hydrogen energy systems. More importantly, density functional theory (DFT) calculations demonstrate the synergy of the five elements in enhancing water-splitting activity. This work offers valuable insights for designing industrial water electrolysis systems.

Catalytic Activities of Ir-Based Pyrochlore-Type Oxides Doped with Divalent Cations for Oxygen Evolution Reaction

Alternative energy sources to fossil fuels are essential for a sustainable society, and hydrogen energy is expected to be a solution to environmental and energy problems. Currently, proton exchange membrane water electrolyzers are becoming widely used for the electrochemical production of hydrogen due to their advantages such as high efficiency. However, the highly acidic conditions and the large overpotentials of oxygen evolution reaction (OER) at the anode are problems, and the development of catalysts that are stable and highly active in acidic aqueous solutions is important for further popularization.Ir-based oxides with pyrochlore structure (A2B2O7) are known to exhibit high catalytic activities in acidic aqueous solutions. Recently, it has been reported that the improved catalytic activities of Pr2Ir2O7 by Zn doping, which controlled the composition of A-site elements in the pyrochlore structure [1]. In this study, we synthesized Pr2-x MgxIr2O7 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5), in which a part of trivalent cation Pr3+ was replaced by divalent cation Mg2+ and improved the OER catalytic activity in acidic aqueous solutions.Pr2-x MgxIr2O7 was synthesized by the hydrothermal method. The catalyst was mixed with a conductivity aid carbon (Vulcan XC-72) and a cationic conductive ionomer (Nafion) at a weight ratio of 5:1:1 and dispersed in 2-propanol to form a catalyst ink. The catalytic activity was evaluated by cyclic voltammetry (CV) using a glassy carbon rotating disk electrode (RDE) as a working electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and a platinum wire as a counter electrode. The electrolyte was a 0.1 mol dm-3 HClO4 solution saturated with oxygen, and the CV scanning range was 1.1 to 1.7 V with respect to the RHE, scanning speed was 20 mV s-1, and the RDE rotation speed was 1200 rpm. The stability of the catalyst in acidic aqueous solutions was evaluated by chronopotentiometry (CP) using the same cell.X-ray diffraction and energy-dispersive X-ray analysis measurements of the synthesized materials confirmed that the desired compounds were obtained, and CV measurements showed that the OER activity of Pr2-x MgxIr2O7 increased in the first 20 cycles for all compositions, and then stabilized over several tens of cycles. The cycle change was similar to that of Pr2Ir2O7, suggesting that the surface rearrangement proceeds in acidic aqueous solutions [2]. Linear sweep voltammogram (LSV) and OER activity normalized by Brunauer-Emmett-Teller (BET) specific surface area were measured at the 20th cycle, and the catalytic activity stabilized at a high value. The OER activity was enhanced by Mg substitution in the catalysts of Pr2-x Mg x Ir2O7.References L. Zhu, C. Ma, Z. Wang, X. Gong, L. Cao, and J. Yang, Appl. Surf. Sci., 31, 151840 (2022).C. Shang, C. Cao, D. Yu, Y. Yan, Y. Lin, H. Li, T. Zheng, X. Yan, W. Yu, S. Zhou, and J. Zeng, Adv. Mater., 31, 1805104 (2018).

Investigating the Effect of KOH Feed Concentration on a Stainless-Steel Anode Using a 3-Electrode Anion Exchange Water Electrolyser Cell Set-up

Anion-exchange membrane water electrolysers (AEM-WE) have the potential to lower the capital cost of hydrogen production compared to proton-exchange membranes water electrolysers (PEM-WE). The alkaline conditions of AEM-WE result in: (i) a kinetically more favourable oxygen evolution reaction (OER), (ii) the ability to use platinum group metal (PGM)-free metal catalysts, and (iii) reduced cell component costs because of the less corrosive operating conditions. However, AEM-WE is held back from commercialisation by limited current density and durability.1 Compared to conventional alkaline water electrolysis, AEM-WEs have the advantage of lower ohmic resistance thanks to a solid polymer electrolyte which provides intrinsic ionic conductivity instead of relying on 6 M KOH and a porous separator. Most studies show, however, that a dilute KOH feed (< 1 M), rather than pure water, is critical to reaching a high current density in AEM-WE. Varying the concentration of KOH in the feed solution impacts catalyst activity and membrane resistance thereby making performance measurements in AEM-WE more complex. Moreover, the overpotential for the alkaline hydrogen evolution reaction (HER) is significant due to its sluggish kinetics. This means full-cell measurements are insufficient for understanding the behaviour of OER and HER catalysts in AEM-WE. It is therefore crucial to make half-cell measurements in AEM-WEs to decouple the effect of the anode and cathode electrocatalysts from the overall cell performance.In this context, our group recently demonstrated a 3-electrode, 5 cm2 membrane electrode assembly electrolyser cell2 that uses a reference electrode to decouple the anode and cathode contributions to the overall water splitting reaction. The technique was demonstrated for AEM-WEs and is soon to be published by Malone et al. (Figure 1A)3. In this study, we aimed to investigate the effect of KOH concentration in the feed solution on OER by using this 3-electrode cell setup. We used a stainless steel (SS)-felt at the anode as both catalyst and porous transport layer (PTL) in different concentrations of KOH feed while maintaining Pt/C as the cathode catalyst. We also repeated the tests with various anode catalyst layers (IrOx, NiFeOx) in combination with the SS PTL. For each test, full and half-cell polarisation curves and impedance spectroscopy were recorded before and after a sixteen-hour durability test at 1 A cm-2 at 40 °C. The results revealed the catalytic significance of the SS-felt in 1 M KOH as recently shown by Chen et al.4 Additionally, we demonstrated how the SS masks the performance of an IrOx catalyst layer in 1 M KOH. Reducing the KOH concentration to 0.18 M resulted in ≈ 7 % increase in the overall cell voltage at 1 A cm-2 (from 1.88 V to 2.00 V at 40 °C in Figure 1B). Aside from the ≈ 35 % increase in membrane resistance (e.g. 180 to 240 mΩ cm2 at 0.9 A cm-2) in 0.18 M KOH, the higher overpotential arose from the anode (Figure 1C), which could be attributed to the higher OER overpotential of SS-felt in a lower pH solution. Meanwhile, there was no appreciable change in cathode overpotential when the KOH concentration was reduced. These findings demonstrate the importance of 3-electrode cell measurements for understanding the impact of each component in an AEM-WE cell. Furthermore, the results reveal that both the choice of PTL material as well as the concentration of feed solution should be considered when testing different catalysts in AEM-WE.

Unveiling the Synergistic Effect of Two-Dimensional Heterostructure NiFeP@FeOOH as Stable Electrocatalyst for Oxygen Evolution Reaction

Introducing multiple active sites and constructing a heterostructure are efficient strategies to develop high-performance electrocatalysts. Herein, two-dimensional heterostructure NiFeP@FeOOH nanosheets supported by nickel foam (NF) are prepared by a hydrothermal–phosphorization–electrodeposition process. The synthesis of self-supporting heterostructure NiFeP@FeOOH nanosheets on NF increases the specific surface region, while bimetallic phosphide realizes rapid charge transfer, improving the electron transfer rate. The introduction of FeOOH and the construction of a heterostructure result in a synergistic effect among the components, and the surface-active sites are abundant. In situ Raman spectroscopy showed that the excellent oxygen evolution reaction (OER) performance was due to reconstruction-induced hydroxyl oxide, which achieved a multi-active site reaction. The NiFeP@FeOOH/NF electrocatalytic activity was then significantly improved. The findings indicate that in a 1.0 M KOH alkaline solution, NiFeP@FeOOH/NF showed an OER overpotential of 235 mV at 100 mA cm−2, a Tafel slope of 46.46 mV dec−1, and it worked stably at 50 mA cm−2 for 80 h. This research proves that constructing heterostructure and introducing FeOOH are of great significance to the study of the properties of OER electrocatalysts.

Extraordinary Hydrogen Evolution and Oxygen Evolution Reaction Activity From PPy@FeCo-LDH/NF Bifunctional Electrocatalyst in Alkaline Solution

Alkaline water electrolysis is a promising technique for the production of hydrogen and oxygen. Nevertheless, the development of low-cost, high-activity metal-based electrocatalysts that can effectively catalyze the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remains a significant challenge. Herein, we polymerized Polypyrrole (PPy) with FeCo layered double metal hydroxide grown in situ on nickel foam (NF) (FeCo-LDH/NF) by electrochemical polymerization to acquire composite material PPy@FeCo-LDH/NF. As a promising electrocatalyst with dual functionality for the HER and OER, the HER overpotential of PPy@FeCo-LDH/NF was 153 mV, and the OER overpotential was 245 mV at a current density of 10 mA·cm−2. It was because that PPy increased the number of active adsorption sites, which in turn regulated the ion transfer rate between the electrolyte and the prepared catalyst. At the same time, after 24 h of stability testing, the HER and OER capacitance retention rates were 96.7% and 97.1%, respectively.

Ti3C2 MXene nanosheets integrated cobalt-doped nickel hydroxide heterostructured composite: An efficient electrocatalyst for overall water-splitting

Developing an efficient electrocatalyst for superior electrochemical water splitting (EWS) is crucial for achieving comprehensive hydrogen production. A heterostructured electrocatalyst, free of noble metals, Ti3C2 MXene nanosheet-integrated cobalt-doped nickel hydroxide (NHCoMX) composite was synthesized via a hydrothermal method. The abundant pores in the Ti3C2 MXene nanosheet (MX)–integrated microarchitecture increased the number of active sites and facilitated charge transfer, thus enhancing electrocatalysis. Specifically, the MX-enhanced charge transfer considerably transformed the microelectronic structure of cobalt-doped Ni(OH)2 (NHCo), which promoted its hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Hence, as an EWS catalyst, NHCoMX exhibited an exceptional electrocatalytic activity, demonstrating OER and HER overpotentials of 310 mV and 73 mV, respectively, with low Tafel slopes of 65 mV dec−1 and 85 mV dec−1, respectively; it exhibited a current density of 10 mV cm−2 in 1.0 mol L−1 KOH, representing the closest efficiency to the noble state-of-the-art RuO2 and Pt/C catalyst. Furthermore, the developed electrocatalyst improved the activities of both HER and OER, leading to an overall EWS current density of 10 mA cm−2 at 1.72 V in an alkaline electrolyte with two electrodes. This study describes an efficient heterostructured NHCoMX composite electrocatalyst. It is significantly comparable to the noble state-of-the-art electrocatalysts and can be extended to fabricate resourceful catalysts for large-scale EWS applications.

Single-atom nickel on defective g-C4N3 as a promising bifunctional electrocatalyst for the OER and ORR: a first-principles analysis

Searching for inexpensive and highly active materials to replace noble metals for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is an important challenge. The two-dimensional semi-metallic carbon nitride g-C4N3 is an excellent candidate for electrochemical reactions owing to its eminent conductivity and stability. Here, using density functional theory (DFT) a series of low-budget non-noble transition metal single-atom (SAC) loaded on the g-C4N3 (named as TM@C4N3, TM = Cr, Mn, Fe, Co, Ni, Cu) were investigated as electrocatalysts for OER and ORR. Ni@C4N3 shows an enormous application potential as electrode material, with OER and ORR overpotentials of 0.59 V and 0.23 V, respectively. The second lowest binding energy between Ni and support g-C4N3 indicates that Ni@C4N3 possesses good overall stability. The d-band center as well as metal valence as the descriptor exhibit good agreement with the adsorption energy of the intermediate. A linear relationship exists among the adsorption energies of the three intermediates O, OH, and OOH, given the similarity that they are all bonded to the metal through one oxygen atom. By adjusting the adsorption energy of these intermediates, the catalytic activity of electrocatalysts can be tuned for OER and ORR. This study confirmed that Ni@C4N3 is a good bifunctional catalyst and provides important guidance for the design of bifunctional electrocatalysts for OER and ORR.