Water-Splitting Electrocatalysts Synthesized Using Ionic Liquids

Abstract

Over the last 2–3 years, the number of papers reporting the use of ionic liquids for the synthesis of electrocatalysts for the oxygen and hydrogen evolution reactions has increased by a factor of ten. Electrocatalysts derived from ionic liquids often display very different morphologies and structures relative to analogous materials prepared using aqueous media. Ionic liquids provide a number of simple and straightforward routes for electrocatalyst doping, allowing material compositions that are hard to obtain using other solvents. Examples of ionic liquid-derived water-splitting electrocatalysts for both of the half-reactions of water splitting are discussed, including materials prepared from deep eutectic electrolytes. Electrocatalytic water splitting is a promising sustainable route for generation of clean-burning hydrogen. To realize this goal, more effective catalysts for the oxygen and hydrogen evolution reactions are required. In this review, we investigate the recent use of ionic liquids (ILs) in the synthesis of electrocatalysts for the water-splitting reactions. ILs have an attractive range of properties (e.g., high boiling points, wide electrochemical windows, and high vapor pressures) that facilitate the synthesis of materials that would be hard (or impossible) to obtain using other solvents. The number of reports using ILs for electrocatalyst synthesis has increased significantly in recent years. Therefore, the time is ripe for a critical review of current progress and outstanding challenges in this dynamic field. Electrocatalytic water splitting is a promising sustainable route for generation of clean-burning hydrogen. To realize this goal, more effective catalysts for the oxygen and hydrogen evolution reactions are required. In this review, we investigate the recent use of ionic liquids (ILs) in the synthesis of electrocatalysts for the water-splitting reactions. ILs have an attractive range of properties (e.g., high boiling points, wide electrochemical windows, and high vapor pressures) that facilitate the synthesis of materials that would be hard (or impossible) to obtain using other solvents. The number of reports using ILs for electrocatalyst synthesis has increased significantly in recent years. Therefore, the time is ripe for a critical review of current progress and outstanding challenges in this dynamic field. absorbs on the surface of a growing particle and thus helps to control the size of the resulting material. electric current flowing per unit area of electrode (normally based on the simple geometric surface area of the electrode). It provides an area-normalized measure of the rate of the electrochemical reaction(s) occurring at the electrode. added to a reaction as a means of introducing a small amount of an element (a dopant) to a material. a solvent’s electrochemical window defines the potential region in which it will neither be oxidized at the anode nor reduced at the cathode. It therefore corresponds to that solvent’s zone of electrochemical stability. the process by which an applied potential is used to induce the formation of insoluble oxidized species on the anode and/or insoluble reduced species on the cathode of an electrochemical cell. in a galvanic replacement reaction, one metal is oxidized by ions of a second metal, where the second metal has a more positive reduction potential than the first. The ions of the second metal are thus reduced. This approach can be used to replace the surface of the first metal with a coating of the (now reduced) second metal, whilst oxidized ions of the first metal are released into solution. the lattice enthalpy is the energy that is released when ions in the gaseous state combine to make an ionic solid. A high lattice enthalpy therefore implies that the resulting salt is very stable with respect to the gaseous ions. voltage (potential) difference between the thermodynamically determined reduction potential of an electrochemical half-reaction, and the potential at which that redox event is observed experimentally. Typically, the overpotential required to achieve a rate of reaction (expressed as a current or current density) is quoted. coordinates to the surface of a particle and preferentially stabilizes faces or crystal facets of the particle’s surface. It thus plays a role in controlling particle shape. directs the formation of a material, typically using weak noncovalent interactions to arrange the reactant material into specific orientations.