Proton Permeable, Oxygen Blocking, Ultrathin Al2O3 Layers for Electrocatalytic Applications

Abstract

Operating zero-gap electrolyzers at current densities above 1 Acm-2 is very challenging because ohmic losses associated with ion transport across the ion exchange membrane become predominant.[1] Technically, we can reach current densities up to 10 Acm-2. However, even a 100 micrometer thin nafion membrane would lead to cell voltages over 3 V [2] due to ohmic losses. Therefore, research and experimental development efforts are still needed to enable and increase the cell efficiency at elevated current densities.Implementing an ultrathin membrane of only a few nanometers in electrolyzers will minimize ion transport (i.e. protons) losses and will enable high-throughput electrolysis at low temperatures. However, to date, there is no concept available to perform electrochemical conversions using membranes of only of a few nanometers thickness that can block molecules but permeate protons.Multiple reports have demonstrated that dense ultrathin oxide layers, i.e. SiO2, can be used as electrocatalyst coatings to increase selectivities by blocking undesired ions/molecules from reaching the electrode.[3,4] The properties of such dense ultrathin oxide layers in principle fulfil the requirements of a proton exchange membrane, being only a couple of nanometer thin.To implement a nanometer thin proton exchange membrane into an electrolysis setup and enable mechanical integrity we envision a 3D-nanotube array design as shown in Figure 1a. In the ‘inside’ of the tube we perform the Hydrogen evolution reaction, and on the ‘outside’ the water oxidation reaction. Cathode and anode are separated only by a dense ultrathin oxide layer that permeates protons but blocks oxygen.Herein, we discuss the performance of ultrathin dense and amorphous Al2O3 layers made via pulsed laser deposition (2.5 nm, 3 nm, and 5 nm) with regard to proton and oxygen permeability. Electrochemical impedance spectroscopy allows us to determine diffusion coefficients and charge transfer resistances (Figure 1b). We use state-of-the-art operando FT-IR reflection-absorption spectroelectrochemistry that enables us to probe the dynamic structural and morphological changes of the dense alumina layer over time vs. applied potential (Figure 1c). In fact, we observe that proton permeation improves over time while we can exclude dissolution of the Al2O3 layer.