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

Bifunctional electrocatalytic performance of MgAlCe-LDH/β-Ni(OH)2/Ni foam in total natural seawater splitting

Electrocatalytic seawater splitting is a potential solution to environmental problems, as seawater is a plentiful supply of hydrogen sources in nature. Hence, the development of bifunctional electrodes exhibiting outstanding performance in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is essential for the efficacy of total seawater splitting. Herein, we employed a straightforward method for in situ creation of β-Ni(OH)2 on Ni foam, followed by a hydrothermal approach to manufacture MgAlCe-LDH on β-Ni(OH)2/Ni foam. The linear sweep voltammetry (LSV) experiments demonstrate that the MgAlCe-LDH/β-Ni(OH)2/Ni foam exhibits superior performance for both the OER and the HER in a natural seawater electrolyte compared to the MgAlCe-LDH/Ni foam, β-Ni(OH)2/Ni foam, and Ni foam. Consequently, the MgAlCe-LDH/β-Ni(OH)2/Ni foam requires an 80 mV overpotential for OER and 337 mV for HER to attain a current density of 10 mA cm-2. The enhanced electrocatalytic activity of MgAlCe-LDH/β-Ni(OH)2/Ni foam can be attributed to boosting exposed active sites, improving electronic interaction, and increasing charge transfer capacity resulting from the synthesis of MgAlCe-LDH on β-Ni(OH)2/Ni foam. Implementing a two-electrode configuration for the MgAlCe-LDH/β-Ni(OH)2/Ni foam, the total seawater splitting investigation exhibits 1.42 V cell voltage at a current density of 10 mA cm-2 and 25 h long-term stability.

Investigating the Effects of Copper Impurity Deposition on the Structure and Electrochemical Behavior of Hydrogen Evolution Electrocatalyst Materials

Electrolysis of impure water (such as seawater) has recently garnered research interest as it may enable hydrogen production at reduced costs. However, the tendency of impurity ions and other species to degrade electrocatalysts and membranes within an electrolyzer is a serious challenge. Here, we investigate the effects of copper impurities of varying concentrations on the hydrogen evolution reaction (HER) using platinum electrocatalysts. A decrease of current density is observed with an increasing copper concentration. By comparing the effect of ionic impurities on current density at different concentrations, we gain insight into how impurities can interfere with the HER at different potentials. Surface characterization of the electrodes reveals differences in the morphology and extent of copper deposition on HER-active platinum vs inactive gold electrodes. This enables an improved understanding of how copper nucleates and grows on the two types of electrodes under different electrochemical conditions while also confirming deposition in low-concentration cases, as present in seawater. The results indicate that copper electrodeposition competes with the HER, and the nature of copper electrodeposition varies depending on the electrocatalytic activity of the electrode. This study provides insight toward catalyst design that can withstand the effects of impurity-induced degradation over extended use.

Challenges and progress in oxygen evolution reaction catalyst development for seawater electrolysis for hydrogen production

Production of green hydrogen on a large scale can negatively impact freshwater resources. Therefore, using seawater as an electrolyte in electrolysis is a desirable alternative to reduce costs and freshwater reliance. However, there are limitations to this approach, primarily due to the catalyst involved in the oxygen evolution reaction (OER). In seawater, the OER features sluggish kinetics and complicated chemical reactions that compete. This review first introduces the benefits and challenges of direct seawater electrolysis and then summarises recent research into cost-effective and durable OER electrocatalysts. Different modification methods for nickel-based electrocatalysts are thoroughly reviewed, and promising electrocatalysts that the authors believe deserve further exploration have been highlighted.

Interface engineering of heterogeneous NiMn layered double hydroxide/vertically aligned NiCo2S4 nanosheet as highly efficient hybrid electrocatalyst for overall seawater splitting

We report the fabrication of a heterogeneous catalyst through vertically aligned NiCo2S4/Ni3S2 nanosheet with encapsulation of ultrathin NiMn layered double hydroxide over self-standing nickel foam (NM/NCS/NS/NF) via two-step hydrothermal processes. Benefiting from more adequate catalytic active centres and copious interfacial charge transfer channels, NM/NCS/NS/NF electrode demonstrates superior bifunctional activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) processes under alkaline fresh/simulated seawater electrolyte conditions. As a result, NM/NCS/NS/NF electrode requires the smallest overpotentials of 282 & 312 mV (OER) and 171 & 204 mV (HER) to attain current densities of 30 & 50 mA cm-2 respectively under alkaline simulated seawater electrolyte conditions. Besides, the presence of amorphous NiMn LDH layers over crystalline NiCo2S4/Ni3S2 catalyst stimulates surface adsorption of oxygen intermediate species, water dissociate ability on catalytic active centres, and mass transport with electron transfer at the interface. Further, the two-electrode configuration assisted electrolyser system delivers an efficient overall water splitting activity with minimum cell voltages of 1.54 V (in 1 M KOH) and 1.56 V (in 1 M KOH+0.5 M NaCl) at a current density of 10 mA cm-2. Besides, a fabricated electrolyser cell provides a more sustained water electrolysis process and robust durability for 20 h which displays NM/NCS/NS/NF electrode is a vibrant and potential candidate for realistic seawater electrolysis. Therefore, our proposed heterogeneous electrocatalyst could open up a new platform for developing efficient large-scale efficient seawater electrolysis.

Materials Design and System Innovation for Direct and Indirect Seawater Electrolysis

Green hydrogen production from renewably powered water electrolysis is considered as an ideal approach to decarbonizing the energy and industry sectors. Given the high-cost supply of ultra-high-purity water, as well as the mismatched distribution of water sources and renewable energies, combining seawater electrolysis with coastal solar/offshore wind power is attracting increasing interest for large-scale green hydrogen production. However, various impurities in seawater lead to corrosive and toxic halides, hydroxide precipitation, and physical blocking, which will significantly degrade catalysts, electrodes, and membranes, thus shortening the stable service life of electrolyzers. To accelerate the development of seawater electrolysis, it is crucial to widen the working potential gap between oxygen evolution and chlorine evolution reactions and develop flexible and highly efficient seawater purification technologies. In this review, we comprehensively discuss present challenges, research efforts, and design principles for direct/indirect seawater electrolysis from the aspects of materials engineering and system innovation. Further opportunities in developing efficient and stable catalysts, advanced membranes, and integrated electrolyzers are highlighted for green hydrogen production from both seawater and low-grade water sources.

Recent advances in direct seawater splitting for producing hydrogen

Hydrogen production from electrocatalytic water splitting driven by renewable energy sources provides a promising path for energy sustainability. The current water electrolysis technologies mainly use fresh water as feedstock, which will further aggravate the shortage of water resources in the world. Seawater has an innate advantage in large-scale electrolysis hydrogen production because of its abundant reserves. However, direct seawater electrolysis without any pre-treatment faces serious challenges due to the electrode side reactions and corrosion issues caused by the complex compositions of seawater. In this review, we first discuss the basic principles of seawater electrolysis. Second, the recent progress in designing efficient direct seawater electrolysis systems is discussed in detail, including catalyst design, electrolyser assembly, membrane regulation, and electrolyte engineering. In addition, the challenges and future opportunities are highlighted for the development of seawater splitting technologies toward large-scale hydrogen production.