Researchers have developed new characterization methods to study the surface of electrodes used in a water splitting reaction.
Water splitting is widely regarded as a potentially interesting step towards realization of hydrogen storage, a renewable energy source that is carbon-free. During the process, water is split into hydrogen and oxygen, and hydrogen separates at the surface of the cathode electrode. The hydrogen can subsequently be separated from the electrode and stored in a hydrogen fuel cell for later use.
Hydrogen storage from water splitting is facing a number of technical and economic challenges. Among those challenges are anode corrosion under acidic condition and anodic polarization, that shorten material lifetimes, leading to increased costs of use. The oxidation becomes much less of a problem if the anode is made of certain special materials, such as iridium and its oxides or hydroxides. Because iridium is one of the scarcest elements on earth, efforts are underway to synthesize such materials and optimize their use in anodes. To develop such synthesis and optimization strategies it is vital to understand what drives stability and electrochemical activity in these materials. Now, researchers have developed new characterization methods to quantify the amount of the material, and location of active species participating in the reaction, on the surface of iridium oxide electrodes. Knowledge of these parameters is key to the characterization of existing electrodes and development of improved ones.
Image from Velasco-Velez et al, J. Am. Chem. Soc. 2021, 143, 32, 12524-12534, reproduced under a CC 4.0 license.
In a paper published recently in the Journal of the American Chemical Society, researchers from five different countries describe the combined use of operando X-ray absorption spectroscopy (XAS) with operando X-ray photoelectron spectroscopy (XPS) to provide complementary information on anode properties. They enhanced the amount of information collected from XAS by precise nanostructuring of the electrode, which increased its surface-to-bulk ratio. By combining these measurements with calculations, the nature of the active species and their distribution in the surface and near-surface region of the anode during the electrocatalytic oxidation of water was determined. In subsequent steps, the scientists performed XPS on the same electrodes in the presence of an electrolyte, thus in conditions under which the anode actually operated in a real water-splitting device. Aside from confirming the results obtained with XAS, they obtained further information that was not available from XAS alone, such as near-surface oxidation states. These two tools combined with each other and with ab-initio calculations provide a powerful new method that can be used to improve iridium-based electrodes for water splitting and hydrogen storage.
Graphene played an important enabling role in the research. Graphenea team members produced and transferred monolayer CVD graphene on top of silicon nitride membranes, and the graphene was used as an electrode owing to graphene’s good electrical conductivity and leak-tight sealing. The electrodes were decorated with iridium nanoparticles for the XAS and XPS studies. The ultrathin, yet structurally robust graphene served as an ideal support to study the active material with these methods.