Selective Catalyst for Oxygen Evolution in Neutral Brine Electrolysis: An Oxygen-Deficient Manganese Oxide Film

A Study on Measures to Preserve Chlorine and Ammonia Oxygen Removal

Ammonia zinc refining has the benefits of low energy consumption, high zinc recovery, and good environmental protection compared with traditional acid and alkaline zinc refining. However, in the production process of refining zinc with ammonia, the anode undergoes chlorine precipitation, and then the oxidation of the ammonia precipitation of some nitrogen occurs. Ammonia replenishment is a cumbersome process that results in large amounts of ammonia volatilization and environmental pollution. In ammonia zinc refining, it is important to ensure the concentration of ammonia and chlorine, as the graphite anodes used in conventional ammonia zinc refining do not retain chlorine and ammonia and dissolve slowly due to oxidation. Therefore, this paper proposes a new measure to conserve chlorine and ammonia to reduce anode chlorine generation by adding an anionic barrier layer and selecting manganese anode materials with selective oxygen precipitation. Under the conditions of 50 × 100 mm sized electrodes, a current density of 350 A/m2, and a temperature of 60 °C, a graphite anode and manganese anode were used for electrowinning and for the collection of anode gas under different additive conditions. For the first time, we present a comparative analysis of gas composition, using gas chromatography to demonstrate the feasibility of the different measures used to preserve chlorine, ammonia, and oxygen for industrial applications, as well as the advantages of using these methods in reducing costs. And the experiments show that, by adding the anionic barrier layer, adding urea, and using manganese anode materials with selective oxygen precipitation, the nitrogen precipitation in the anode gas can be reduced to 40-50%, and oxygen precipitation reaches 48.76%.

Electrodeposited Manganese Dioxides and Their Composites as Electrocatalysts for Energy Conversion Reactions

Enhancing the efficiencies of electrochemical reactions for converting renewable energy into clean chemical fuels as well as generating clean energy is critical to achieving carbon neutrality. However, this enhancement can be achieved using materials that are not constrained by resource limitations and those that can be converted into devices in a scalable manner, preferably for industrial applications. This review explores the applications of electrochemically deposited manganese dioxides (MnO2) and their composites as electrochemical catalysts for oxygen evolution (OER) and hydrogen evolution reactions for converting renewable energy into chemical fuels. It also explores their applications as electrochemical catalysts for oxygen reduction reaction (ORR) and bifunctional OER/ORR for the efficient operation of fuel cells and metal-air batteries, respectively. Manganese is the second most abundant transition metal in the Earth's crust, and electrodeposition represents a binder-free and scalable technique for fabricating devices (electrodes). To propose an improved catalyst design, the studies on the electrodeposition mechanism of MnO2 as well as the fabrication techniques for MnO2-based nanocomposites accumulated in the development of electrodes for supercapacitors are also included in this review.

Unilamellar MnO2 nanosheets confined Ru-clusters combined with pulse electrocatalysis for biomass electrooxidation in neutral electrolytes

The electrochemical oxidation of 5-hydroxymethylfurfural (HMFOR) in alkaline electrolyte is a promising strategy for producing high-value chemicals from biomass derivatives. However, the disproportionation of aldehyde groups under strong alkaline conditions and the polymerization of HMF to form humic substances can impact the purity of 2,5-furandicarboxylic acid (FDCA) products. The use of neutral electrolytes offers an alternative environment for electrolysis, but the lack of OH- ions in the electrolyte often leads to low current density and low yields of FDCA. In this study, a sandwich-structured catalyst, consisting of Ru clusters confined between unilamellar MnO2 nanosheets (S-Ru/MnO2), was used in conjunction with an electrochemical pulse method to realize the electrochemical conversion of 5-hydroxymethylfurfural into FDCA in neutral electrolytes. Pulse electrolysis and the strong electron transfer between Ru clusters and MnO2 nanosheets help maintain Ru in a low oxidation state, ensuring high activity. The increased *OH generation led to a groundbreaking current density of 47 mA/cm2 at 1.55 V vs. reversible hydrogen electrode (RHE) and an outstanding yield rate of 98.7% for FDCA in a neutral electrolyte. This work provides a strategy that combines electrocatalyst design with an electrolysis technique to achieve remarkable performance in neutral HMFOR.

Efficient and Ultrastable Seawater Electrolysis at Industrial Current Density with Strong Metal-Support Interaction and Dual Cl--Repelling Layers

Direct seawater electrolysis is emerging as a promising renewable energy technology for large-scale hydrogen generation. The development of Os-Ni4Mo/MoO2 micropillar arrays with strong metal-support interaction (MSI) as a bifunctional electrocatalyst for seawater electrolysis is reported. The micropillar structure enhances electron and mass transfer, extending catalytic reaction steps and improving seawater electrolysis efficiency. Theoretical and experimental studies demonstrate that the strong MSI between Os and Ni4Mo/MoO2 optimizes the surface electronic structure of the catalyst, reducing the reaction barrier and thereby improving catalytic activity. Importantly, for the first time, a dual Cl- repelling layer is constructed by electrostatic force to safeguard active sites against Cl- attack during seawater oxidation. This includes a strong Os─Cl adsorption and an in situ-formed MoO4 2- layer. As a result, the Os-Ni4Mo/MoO2 catalyst exhibits an ultralow overpotential of 113 and 336 mV to reach 500 mA cm-2 for HER and OER in natural seawater from the South China Sea (without purification, with 1 m KOH added). Notably, it demonstrates superior stability, degrading only 0.37 µV h-1 after 2500 h of seawater oxidation, significantly surpassing the technical target of 1.0 µV h-1 set by the United States Department of Energy.

Progress in Anode Stability Improvement for Seawater Electrolysis to Produce Hydrogen

Seawater electrolysis for hydrogen production is a sustainable and economical approach that can mitigate the energy crisis and global warming issues. Although various catalysts/electrodes with excellent activities have been developed for high-efficiency seawater electrolysis, their unsatisfactory durability, especially for anodes, severely impedes their industrial applications. In this review, attention is paid to the factors that affect the stability of anodes and the corresponding strategies for designing catalytic materials to prolong the anode's lifetime. In addition, two important aspects-electrolyte optimization and electrolyzer design-with respect to anode stability improvement are summarized. Furthermore, several methods for rapid stability assessment are proposed for the fast screening of both highly active and stable catalysts/electrodes. Finally, perspectives on future investigations aimed at improving the stability of seawater electrolysis systems are outlined.

Recycled Cathodes in Rechargeable Aqueous Batteries as Ready-Made Electrodes for Oxygen Evolution Catalysis

The development of a low-cost and efficient oxygen evolution reaction (OER) electrode is of critical importance for water electrolysis technologies. The general approach to achieving a high-efficiency OER electrode is to regulate catalytic material structures by synthetic control. Here we reported an orthogonal approach to obtaining the OER electrode without intentional design and synthesis, namely, recycling MnO2 cathodes from failed rechargeable aqueous batteries and investigating them as ready-made catalytic electrodes. The recycled MnO2 cathode showed very little Zn2+ storage capacity but surprisingly high OER activity with a low overpotential of 307 mV at 10 mA cm-2 and a small Tafel slope of 77.9 mV dec-1, comparable to the state-of-the-art RuO2 catalyst (310 mV, 86.9 mV dec-1). In situ electrochemical and theoretical studies jointly revealed that the accelerated OER kinetics of the recycled MnO2 electrode was attributed to the enlarged active surface area of MnO2 and optimized electronic structure of Mn sites. This work suggests failed battery cathodes as successful catalysis electrodes for sustainable energy development.

SOx Functionalized NiOOH Nanosheets Embedded in Ni(OH)2 Microarray for High-Efficiency Seawater Oxidation

A nano-micro heterostructure has been established to address the challenges of selectivity, stress, pitting corrosion, and long-term durability of anodes in unpurified seawater. The heterostructure comprised NiOOH nanosheets embedded within a high surface area Ni(OH)2 microarray, and the surface structure is further functionalized with sulfate (SOx). This cation-selective protective layer impedes chloride (Cl-) diffusion and abstracts H from reaction intermediates, leading to enhanced selectivity and corrosion resistance of the anode. The multilevel porosity within the randomly oriented nanosheets and the underlying support provide short diffusion channels for ions and mass migration, ensuring efficient ion transport and long-term structural and mechanical durability of the active sites, even at high current density. Remarkably, the catalyst requires a small input voltage of 400 mV to deliver a current density of 1 A cm-2 and maintains it for over 168 h without noticeable degradation or hypochlorite formation. Spectroscopic analysis and density functional theory (DFT) calculations reveal that the Ni electronic structure in the +3 valence state, its strong structural interaction with the underlying microarray, and the functionality of SOx significantly reduce the required potential for O-O coupling.

Characterizing Biogenic MnOx Produced by Pseudomonas putida MnB1 and Its Catalytic Activity towards Water Oxidation

Mn-oxidizing microorganisms oxidize environmental Mn(II), producing Mn(IV) oxides. Pseudomonas putida MnB1 is a widely studied organism for the oxidation of manganese(II) to manganese(IV) by a multi-copper oxidase. The biogenic manganese oxides (BMOs) produced by MnB1 and similar organisms have unique properties compared to non-biological manganese oxides. Along with an amorphous, poorly crystalline structure, previous studies have indicated that BMOs have high surface areas and high reactivities. It is also known that abiotic Mn oxides promote oxidation of organics and have been studied for their water oxidation catalytic function. MnB1 was grown and maintained and subsequently transferred to culturing media containing manganese(II) salts to observe the oxidation of manganese(II) to manganese(IV). The structures and compositions of these manganese(IV) oxides were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction, and their properties were assessed regarding catalytic functionality towards water oxidation in comparison to abiotic acid birnessite. Water oxidation was accomplished through the whole-cell catalysis of MnB1, the results for which compare favorably to the water-oxidizing ability of abiotic Mn(IV) oxides.

Anode Reaction Control for a Single-Compartment Electrochemical CO2 Reduction Reactor with a Surface-Activated Diamond Cathode

The electrochemical reaction of carbon dioxide (CO2 ) in aqueous electrolyte solutions is attracting increasing attention for sustainable chemical production. Boron-doped diamond (BDD) electrodes have been previously shown to be very effective for the stable electrochemical production of formic acid from CO2 . Typically, the electrochemical production of formic acid by CO2 reduction (CO2 R) reaction is performed with a dual-compartment flow reactor equipped with a membrane separator. The problems caused by the membrane separator, such as scaling-up, complicated operational control and materials costs can be solved using a membrane free single-compartment reactor. Here we demonstrate anode reaction control for a single-compartment CO2 R flow reactor using a surface-activated BDD cathode and achieve a Faradaic efficiency for formic acid production of over 70 %.

Selectively Enhanced Electrocatalytic Oxygen Evolution within Nanoscopic Channels Fitting a Specific Reaction Intermediate for Seawater Splitting

Abundant availability of seawater grants economic and resource-rich benefits to water electrolysis technology requiring high-purity water if undesired reactions such as chlorine evolution reaction (CER) competitive to oxygen evolution reaction (OER) are suppressed. Inspired by a conceptual computational work suggesting that OER is kinetically improved via a double activation within 7 Å-gap nanochannels, RuO₂ catalysts are realized to have nanoscopic channels at 7, 11, and 14 Å gap in average (d_gap ), and preferential activity improvement of OER over CER in seawater by using nanochanneled RuO₂ is demonstrated. When the channels are developed to have 7 Å gap, the OER current is maximized with the overpotential required for triggering OER minimized. The gap value guaranteeing the highest OER activity is identical to the value expected from the computational work. The improved OER activity significantly increases the selectivity of OER over CER in seawater since the double activation by the 7 Å-nanoconfined environments to allow an OER intermediate (*OOH) to be doubly anchored to Ru and O active sites does not work on the CER intermediate (*Cl). Successful operation of direct seawater electrolysis with improved hydrogen production is demonstrated by employing the 7 Å-nanochanneled RuO₂ as the OER electrocatalyst.

Modulating Co-catalyst/Facet Junction for Enhanced Photoelectrochemical Water Splitting

Rational construction of electric field via assembling appropriate co-catalysts on anisotropic facets is of great significance for improving the photogenerated charge separation efficiency. However, this strategy usually gives rise to Fermi-level pinning which is not contributive to the charge separation but deleterious to the photoelectrochemical performance through consuming the measurable photovoltage. Herein, we demonstrate that manganese dioxide electrodeposited on the (111) facet of titanium dioxide nanorods could tremendously boost the catalytic activity of pristine photoanode via a stronger interface electric field and less photovoltage decay compared with the counterpart grown on the (110) facet. A photocurrent density of 1.65 mA·cm^-2 at 1.23 V (vs reversible hydrogen electrode), nearly the theoretical maximum of titanium dioxide, is achieved by the optimum photoanode with an extremely high separation efficiency of 95.15%. This study offers more in-depth insights into the design of carrier separation strategy through loading co-catalysts on different substrate surfaces for more efficient solar energy conversion.

Recent Advances on Hydrogen Evolution and Oxygen Evolution Catalysts for Direct Seawater Splitting

Producing hydrogen via water electrolysis could be a favorable technique for energy conversion, but the freshwater shortage would inevitably limit the industrial application of the electrolyzers. Being an inexhaustible resource of water on our planet, seawater can be a promising alternative electrolyte for industrial hydrogen production. However, many challenges are hindering the actual application of seawater splitting, especially the competing reactions relating to chlorine at the anode that could severely corrode the catalysts. The execution of direct seawater electrolysis needs efficient and robust electrocatalysts that can prevent the interference of competing reactions and resist different impurities. In recent years, researchers have made great advances in developing high-efficiency electrocatalysts with improved activity and stability. This review will provide the macroscopic understanding of direct seawater splitting, the strategies for rational electrocatalyst design, and the development prospects of hydrogen production via seawater splitting. The nonprecious metal-based electrocatalysts for stable seawater splitting and their catalytic mechanisms are emphasized to offer guidance for designing the efficient and robust electrocatalyst, so as to promote the production of green hydrogen via seawater splitting.

Interface Engineering of NixSy@MnOxHy Nanorods to Efficiently Enhance Overall-Water-Splitting Activity and Stability

Three-dimensional (3D) core-shell heterostructured Ni_xS_y@MnO_xH_y nanorods grown on nickel foam (Ni_xS_y@MnO_xH_y/NF) were successfully fabricated via a simple hydrothermal reaction and a subsequent electrodeposition process. The fabricated Ni_xS_y@MnO_xH_y/NF shows outstanding bifunctional activity and stability for hydrogen evolution reaction and oxygen evolution reaction, as well as overall-water-splitting performance. The main origins are the interface engineering of Ni_xS_y@MnO_xH_y, the shell-protection characteristic of MnO_xH_y, and the 3D open nanorod structure, which remarkably endow the electrocatalyst with high activity and stability. Exploring highly active and stable transition metal-based bifunctional electrocatalysts has recently attracted extensive research interests for achieving high inherent activity, abundant exposed active sites, rapid mass transfer, and strong structure stability for overall water splitting. Herein, an interface engineering coupled with shell-protection strategy was applied to construct three-dimensional (3D) core-shell Ni_xS_y@MnO_xH_y heterostructure nanorods grown on nickel foam (Ni_xS_y@MnO_xH_y/NF) as a bifunctional electrocatalyst. Ni_xS_y@MnO_xH_y/NF was synthesized via a facile hydrothermal reaction followed by an electrodeposition process. The X-ray absorption fine structure spectra reveal that abundant Mn-S bonds connect the heterostructure interfaces of Ni_xS_y@MnO_xH_y, leading to a strong electronic interaction, which improves the intrinsic activities of hydrogen evolution reaction and oxygen evolution reaction (OER). Besides, as an efficient protective shell, the MnO_xH_y dramatically inhibits the electrochemical corrosion of the electrocatalyst at high current densities, which remarkably enhances the stability at high potentials. Furthermore, the 3D nanorod structure not only exposes enriched active sites, but also accelerates the electrolyte diffusion and bubble desorption. Therefore, Ni_xS_y@MnO_xH_y/NF exhibits exceptional bifunctional activity and stability for overall water splitting, with low overpotentials of 326 and 356 mV for OER at 100 and 500 mA cm^-2, respectively, along with high stability of 150 h at 100 mA cm^-2. Furthermore, for overall water splitting, it presents a low cell voltage of 1.529 V at 10 mA cm^-2, accompanied by excellent stability at 100 mA cm^-2 for 100 h. This work sheds a light on exploring highly active and stable bifunctional electrocatalysts by the interface engineering coupled with shell-protection strategy.

Electrodeless hydrogen production from seawater using femtosecond laser pulses

This study presents the first experimental evidence of direct H2 production from seawater without harmful gas emissions (e.g., CO2, Cl2), which uses multiphoton ionization water splitting with a femtosecond pulse laser. According to H2 analysis using a gas chromatograph, the H2 production rate in seawater was 70 μmol h-1, which was approximately 3.3 times more than the ultrapure water case reported in the literature. This positive effect derives from focusing through the cuvette wall and the more significant Kerr effect in seawater. Such ion enhancement was observed in the case of seawater and diluted seawater compared with the ultrapure water case, but excessive salt can lead to ion suppression and adverse effects. These differences in salinity suggest appearances of nonlinear optical effects near the focal point and ionization of metallic elements with low ionization potential and are discussed in relation to results of bubble visualization, gas composition analysis, and pressure measurement in gaseous products.

High Selectivity Electrocatalysts for Oxygen Evolution Reaction and Anti-Chlorine Corrosion Strategies in Seawater Splitting

Seawater is one of the most abundant and clean hydrogen atom resources on our planet, so hydrogen production from seawater splitting has notable advantages. Direct electrolysis of seawater would not be in competition with growing demands for pure water. Using green electricity generated from renewable sources (e.g., solar, tidal, and wind energies), the direct electrolytic splitting of seawater into hydrogen and oxygen is a potentially attractive technology under the framework of carbon-neutral energy production. High selectivity and efficiency, as well as stable electrocatalysts, are prerequisites to facilitate the practical applications of seawater splitting. Even though the oxygen evolution reaction (OER) is thermodynamically favorable, the most desirable reaction process, the four-electron reaction, exhibits a high energy barrier. Furthermore, due to the presence of a high concentration of chloride ions (Cl−) in seawater, chlorine evolution reactions involving two electrons are more competitive. Therefore, intensive research efforts have been devoted to optimizing the design and construction of highly efficient and anticorrosive OER electrocatalysts. Based on this, in this review, we summarize the progress of recent research in advanced electrocatalysts for seawater splitting, with an emphasis on their remarkable OER selectivity and distinguished anti-chlorine corrosion performance, including the recent progress in seawater OER electrocatalysts with their corresponding optimized strategies. The future perspectives for the development of seawater-splitting electrocatalysts are also demonstrated.

Nickel Boride/Boron Carbide Particles Embedded in Boron-Doped Phenolic Resin-Derived Carbon Coating on Nickel Foam for Oxygen Evolution Catalysis in Water and Seawater Splitting

Electrolysis of seawater can be a promising technology, but chloride ions in seawater can lead to adverse side reactions and the corrosion of electrodes. A new transition metal boride-based self-supported electrocatalyst was prepared for efficient seawater electrolysis by directly soaking nickel foam (NF) in a mixture of phenolic resin (PR) and boron carbide (B₄ C), followed by an 800 °C annealing. During PR carbonization process, the reaction of B₄ C and NF generated nickel boride (Ni_x B) with high catalytic activity, while PR-derived carbon coating was doped with boron atoms from B₄ C (B-C_PR ). The B-C_PR coating fixed Ni_x B/B₄ C particles in the frames and holes to improve the space utilization of NF. Meanwhile, the B-C_PR coating effectively protected the catalyst from the corrosion by seawater and facilitates the transport of electrons. The optimal Ni_x B/B₄ C/B-C_PR /NF required 1.50 and 1.58 V to deliver 100 and 500 mA cm^-2 , respectively, in alkaline natural seawater for the oxygen evolution reaction.

Insights on the Electrocatalytic Seawater Splitting at Heterogeneous Nickel-Cobalt Based Electrocatalysts Engineered from Oxidative Aniline Polymerization and Calcination

Given the limited access to freshwater compared to seawater, a growing interest surrounds the direct seawater electrolysis to produce hydrogen. However, we currently lack efficient electrocatalysts to selectively perform the oxygen evolution reaction (OER) over the oxidation of the chloride ions that are the main components of seawater. In this contribution, we report an engineering strategy to synthesize heterogeneous electrocatalysts by the simultaneous formation of separate chalcogenides of nickel (NiS_x, x = 0, 2/3, 8/9, and 4/3) and cobalt (CoS_x, x = 0 and 8/9) onto a carbon-nitrogen-sulfur nanostructured network. Specifically, the oxidative aniline polymerization in the presence of metallic cations was combined with the calcination to regulate the separate formation of various self-supported phases in order to target the multifunctional applicability as both hydrogen evolution reaction (HER) and OER in a simulated alkaline seawater. The OER’s metric current densities of 10 and 100 mA cm⁻² were achieved at the bimetallic for only 1.60 and 1.63 V_RHE, respectively. This high-performance was maintained in the electrolysis with a starting voltage of 1.6 V and satisfactory stability at 100 mA over 17 h. Our findings validate a high selectivity for OER of ~100%, which outperforms the previously reported data of 87–95%.