Silica: ubiquitous poison of metal oxide interfaces

Abstract

In this review, we consider electrochemical applications, broadly conceived, in which both ions and electrons play key roles in device operation and where exchange of oxygen between the gas and solid phase is likewise essential for operation. Examples include metal oxide gas sensors (semiconducting and electrolytic), solid oxide fuel and electrolysis cells, oxygen permeation membranes, oxygen storage materials and metal oxide-based catalysts. As might be expected, surfaces and interfaces play key or essential roles in the operation of all of these types of devices. Semiconducting metal oxide (SMOX) sensors depend on the adsorption/diffusion of gas species on the surfaces or exposed grain boundaries and subsequent charge transfer between the semiconductor and the adsorbed gas atoms or molecules for their operation. Fuel cells require reduction of oxygen molecules at the cathode and subsequent incorporation into the electrolyte, diffusion across the electrolyte, (including across grain boundaries within the electrolyte) and subsequent oxidation of the oxygen ions at the anode and their exit back into the gas phase by reaction with reactive gas phase hydrocarbons or hydrogen to form CO, CO₂ and or H₂O. Understandably, any process that interferes with these surface or interface reactions is undesirable. This may take the form of inert layers of materials that literally block access of oxygen, electrons or other reactive species to respective surfaces or interfaces or “poison” the catalytic activity of surface or interface sites needed for the efficient functioning of the devices. One omnipresent chemical species that appears to serve to “poison” the operation of many such electrochemical devices is some form of silica or SiO_x, or in combination of other cations to form silicates. Interestingly, while silica and other poisons, e.g. chromia for solid oxide fuel cell cathodes, are known to be the sources of degradation of many functional electrochemical devices, the detailed mechanisms behind their effectiveness in depressing functionality remain controversial or poorly understood. Since a major focus of Professor John Kilner's research and publications have been concerned with chemical species that interfere with transport across grain boundaries and electrochemical reactions at metal oxide surfaces, including that of silica, we dedicate this review in his honour. We also take the opportunity in this review to more broadly introduce our recent findings regarding the critical role that the relative acidity or basicity of surface additives may have on hindering or accelerating the reaction kinetics on metal oxide surfaces, thereby providing a more detailed understanding of the reactivity of common poisons like silica in degrading device performance and means for combating their impact.