Decoupling Oxygen and Chlorine Evolution Reactions in Seawater using Iridium‐based Electrocatalysts

Ultrafast Synthesis of IrB1.15 Nanocrystals for Efficient Chlorine and Hydrogen Evolution Reactions in Saline Water

The production of storable hydrogen fuel through water electrolysis powered by renewable energy sources such as solar, marine, geothermal, and wind energy presents a promising pathway toward achieving energy sustainability. Nevertheless, state-of-the-art electrolysis requires support from ancillary processes which often incur financial and energy costs. Developing electrolysers capable of directly operating with water that contains impurities can circumvent these processes. Herein, we demonstrate the efficient and durable electrolysis of saline water to produce chlorine gas (Cl2) and hydrogen using structurally ordered IrB1.15, synthesized through ultrafast joule heating. IrB1.15 exhibits remarkable performance, achieving overpotentials of 75 mV for the chlorine evolution reaction (CER) and 12 mV for hydrogen evolution reactions (HER) at current densities of 10 mA cm-2. Moreover, IrB1.15 displays a durability of over 90 h towards both CER and HER. Density functional theory reveals that IrB1.15 has adsorption energies significantly closer to 0 eV for Cl and H, compared to IrO2 and Pt/C. Furthermore, in situ Raman investigations reveal that Ir in IrB1.15 serves as the active center for CER, while the introduction of B atoms to Ir lattices mitigates the formation of absorbed hydrogen species on the Ir surface, thereby enhancing the performance of IrB1.15 in HER.

High Catalytic Selectivity of Electron/Proton Dual-Conductive Sulfonated Polyaniline Micropore Encased IrO2 Electrocatalyst by Screening Effect for Oxygen Evolution of Seawater Electrolysis

Acidic seawater electrolysis offers significant advantages in high efficiency and sustainable hydrogen production. However, in situ electrolysis of acidic seawater remains a challenge. Herein, a stable and efficient catalyst (SPTTPAB/IrO2) is developed by coating iridium oxide (IrO2) with a microporous conjugated organic framework functionalized with sulfonate groups (-SO3H) to tackle these challenges. The SPTTPAB/IrO2 presents a -SO3H concentration of 5.62 × 10-4 mol g-1 and micropore below 2 nm numbering 1.026 × 1016 g-1. Molecular dynamics simulations demonstrate that the conjugated organic framework blocked 98.62% of Cl- in seawater from reaching the catalyst. This structure combines electron conductivity from the organic framework and proton conductivity from -SO3H, weakens the Cl- adsorption, and suppresses metal-chlorine coupling, thus enhancing the catalytic activity and selectivity. As a result, the overpotential for the oxygen evolution reaction (OER) is only 283 mV@10 mA cm-2, with a Tafel slope of 16.33 mV dec-1, which reduces 13.8% and 37.8% compared to commercial IrO2, respectively. Impressively, SPTTPAB/IrO2 exhibits outstanding seawater electrolysis performance, with a 35.3% improvement over IrO2 to 69 mA cm-2@1.9 V, while the degradation rate (0.018 mA h-1) is only 24.6% of IrO2. This study offers an innovative solution for designing high-performance seawater electrolysis electrocatalysts.

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.

Corrosion of Titanium Electrode Used for Solar Saline Electroflotation

The solar electroflotation (EF) processes using saline electrolytes are today one of the great challenges for the development of electrochemical devices, due to the corrosion problems that are generated during the operation by being in permanent contact with Cl^- ions. This manuscript discloses the corrosion behavior of titanium electrodes using a superposition model based on mixed potential theory and the evaluation of the superficial performance of the Ti electrodes operated to 4 V/SHE solar electroflotation in contact with a solution of 0.5 M NaCl. Additionally provided is an electrochemical analysis of Ti electrodes regarding HER, ORR, OER, and CER that occur during the solar saline EF process. The non-linear superposition model by mixed potential theory gives electrochemical and corrosion parameters that complement the information published in scientific journals, the corrosion current density and corrosion potential in these conditions is 0.069 A/m² and -7.27 mV, respectively. The formation of TiO₂ and TiOCl on the anode electrode was visualized, resulting in a reduction of its weight loss of the anode electrode.

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.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

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.

Large-Scale Synthesis of Spinel Nix Mn3-x O4 Solid Solution Immobilized with Iridium Single Atoms for Efficient Alkaline Seawater Electrolysis

Seawater electrolysis not only affords a promising approach to produce clean hydrogen fuel but also alleviates the bottleneck of freshwater feeds. Here, a novel strategy for large-scale preparing spinel Ni_x Mn_3-x O₄ solid solution immobilized with iridium single-atoms (Ir-SAs) is developed by the sol-gel method. Benefitting from the surface-exposed Ir-SAs, Ir₁ /Ni_1.6 Mn_1.4 O₄ reveals boosted oxygen evolution reaction (OER) performance, achieving overpotentials of 330 and 350 mV at current densities of 100 and 200 mA cm^-2 in alkaline seawater. Moreover, only a cell voltage of 1.50 V is required to reach 500 mA cm^-2 with assembled Ir₁ /Ni_1.6 Mn_1.4 O₄ ‖Pt/C electrode pair under the industrial operating condition. The experimental characterizations and theoretical calculations highlight the effect of Ir-SAs on improving the intrinsic OER activity and facilitating surface charge transfer kinetics, and evidence the energetically stabilized *OOH and the destabilized chloride ion adsorption in Ir₁ /Ni_1.6 Mn_1.4 O₄ . This work demonstrates an effective method to produce efficient alkaline seawater electrocatalyst massively.

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.

Nitrate Removal in an Electrically Charged Granular-Activated Carbon Column

Nitrate removal from groundwater remains a challenge. Here, we report on the development of a flow-through, electrically charged, granular-activated carbon (GAC)-filled column, which effectively removes nitrate. In this system, the GAC functioned as an anode, while a titanium sheet acted as a cathode. The high removal rate of nitrate was achieved through a combination of electrosorption and electrochemical transformation to N₂. The column could be readily regenerated in situ by reversing the polarity of the applied potential. We demonstrate that in the presence of chloride, the mechanism responsible for the observed nitrate removal involves a combination of electroadsorption of nitrate to the anodically charged GAC, electroreduction of nitrate to ammonium, and the oxidation of ammonium to N₂ gas by reactive chlorine and other oxidative radicals (with nearly 100% N₂ selectivity). Given the ubiquitous presence of chloride in groundwater, this method represents a ready, green, and sustainable treatment process with significant potential for the remediation of contaminated groundwater.

Recent Progress in the Development of Advanced Functionalized Electrodes for Oxygen Evolution Reaction: An Overview

Presently, the global energy demand for increasing clean and green energy consumption lies in the development of low-cost, sustainable, economically viable and eco-friendly natured electrochemical conversion process, which is a significant advancement in different morphological types of advanced electrocatalysts to promote their electrocatalytic properties. Herein, we overviewed the recent advancements in oxygen evolution reactions (OERs), including easy electrode fabrication and significant action in water-splitting devices. To date, various synthetic approaches and modern characterization techniques have effectively been anticipated for upgraded OER activity. Moreover, the discussed electrode catalysts have emerged as the most hopeful constituents and received massive appreciation in OER with low overpotential and long-term cyclic stability. This review article broadly confers the recent progress research in OER, the general mechanistic approaches, challenges to enhance the catalytic performances and future directions for the scientific community.