S doped Cu2O-CuO nanoneedles array: Free standing oxygen evolution electrode with high efficiency and corrosion resistance for seawater splitting

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

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.

Direct Seawater Electrolysis: From Catalyst Design to Device Applications

Direct seawater electrolysis (DSE) for hydrogen production, using earth-abundant seawater as the feedstock and renewable electricity as the driving source, paves a new opportunity for flexible energy conversion/storage and smooths the volatility of renewable energy. Unfortunately, the complex environments of seawater impose significant challenges on the design of DSE catalysts, and the practical performance of many current DSE catalysts remains unsatisfactory on the device level. However, many studies predominantly concentrate on the development of electrocatalysts for DSE without giving due consideration to the specific devices. To mitigate this gap, the most recent progress (mainly published within the year 2020-2023) of DSE electrocatalysts and devices are systematically evaluated. By discussing key bottlenecks, corresponding mitigation strategies, and various device designs and applications, the tremendous challenges in addressing the trade-off among activity, stability, and selectivity for DSE electrocatalysts by a single shot are emphasized. In addition, the rational design of the DSE electrocatalysts needs to align with the specific device configuration, which is more effective than attempting to comprehensively enhance all catalytic parameters. This work, featuring the first review of this kind to consider rational catalyst design in the framework of DSE devices, will facilitate practical DSE development.

Bubble Management for Electrolytic Water Splitting by Surface Engineering: A Review

During electrocatalytic water splitting, the management of bubbles possesses great importance to reduce the overpotential and improve the stability of the electrode. Bubble evolution is accomplished by nucleation, growth, and detachment. The expanding nucleation sites, decreasing bubble size, and timely detachment of bubbles from the electrode surface are key factors in bubble management. Recently, the surface engineering of electrodes has emerged as a promising strategy for bubble management in practical water splitting due to its reliability and efficiency. In this review, we start with a discussion of the bubble behavior on the electrodes during water splitting. Then we summarize recent progress in the management of bubbles from the perspective of surface physical (electrocatalytic surface morphology) and surface chemical (surface composition) considerations, focusing on the surface texture design, three-dimensional construction, wettability coating technology, and functional group modification. Finally, we present the principles of bubble management, followed by an insightful perspective and critical challenges for further development.

Design Strategy of Corrosion-Resistant Electrodes for Seawater Electrolysis

Electrocatalytic water splitting for hydrogen (H₂) production has attracted more and more attention in the context of energy shortages. The use of scarce pure water resources, such as electrolyte, not only increases the cost but also makes application difficult on a large scale. Compared to pure water electrolysis, seawater electrolysis is more competitive in terms of both resource acquisition and economic benefits; however, the complex ionic environment in seawater also brings great challenges to seawater electrolysis technology. Specifically, chloride oxidation-related corrosion and the deposition of insoluble solids on the surface of electrodes during seawater electrolysis make a significant difference to electrocatalytic performance. In response to this issue, design strategies have been proposed to improve the stability of electrodes. Herein, basic principles of seawater electrolysis are first discussed. Then, the design strategy for corrosion-resistant electrodes for seawater electrolysis is recommended. Finally, a development direction for seawater electrolysis in the industrialization process is proposed.

Electrolyte Engineering for Oxygen Evolution Reaction Over Non-Noble Metal Electrodes Achieving High Current Density in the Presence of Chloride Ion

Direct seawater electrolysis potentially simplifies the electrolysis process and leads to a decrease in the cost of green hydrogen production. However, impurities present in the seawater, especially chloride ions (Cl^- ), cause corrosion of the electrode material, and its oxidation competes with the anodic oxygen evolution reaction (OER). By carefully tuning electrode substrate and electrolyte solutions, the CoFeO_x H_y /Ti electrode with high double-layer capacitance actively and stably electro-catalyzed the OER in potassium borate solutions at pH 9.2 in the presence of 0.5 mol kg^-1 Cl^- . The electrode possesses an active site motif composed of either a Co- or Fe-domain and benefits from an enlarged surface area. Selective OER was demonstrated in Cl^- -containing electrolyte solutions at an elevated reaction temperature, stably achieving 500 mA cm^-2 at a mere potential of 1.67 V vs. reversible hydrogen electrode (RHE) at 353 K for multiple on-off and long-term testing processes with a faradaic efficiency of unity toward the OER.

Electronic and Structural Modification of Mn3O4 Nanosheets for Selective and Sustained Seawater Oxidation

The accomplishment of seawater electrolysis to produce green hydrogen energy needs an efficient and durable electrocatalyst with high selectivity and corrosion resistance. Here we report a free-standing amorphous nanostructured oxygen evolution reaction (OER) electrocatalyst with microvoids developed by embedding Gd-doped Mn₃O₄ nanosheets in a CuO-Cu(OH)₂ nanostructure array (Gd-Mn₃O₄@ CuO-Cu(OH)₂. The surface oxygen vacancies modulated the electronic structure of the catalyst and offered active sites with optimal chemisorption energy to OER intermediates. The hierarchical surface structure provides a large specific surface area, high electrical conductivity, ionic mobility, intrinsic activity for each active site, and efficient charge transfer, leading to an outstanding catalytic performance. The enhanced structural, chemical, and corrosion resistance ensures effectiveness as an anode in direct seawater electrolysis. Specifically, it needs an input voltage of 1.63 V to deliver a current density of 500 mA cm^-2 in alkaline seawater, with the stability of more than 75 h of continuous electrolysis without hypochlorite formation. The high Faradaic efficiency demonstrates its potential for hydrogen fuel production from seawater.