Analysis of the electrolyte diode. Electro-diffusion and chemical reactionwithin a hydrogel reactor

A reaction-diffusion system describing the electrolyte diode is investigate d. This consists of a chemically crosslinked polyvinylalcohol (PVA) hydroge l cylinder in which a pH gradient is provided by having an acid and a base maintained at constant concentrations in reservoirs at each end of the one- dimensional reactor A potential difference of a given strength is also appl ied across the gel cylinder. Previous experimental studies of the current-v oltage characteristics (CVC) have shown two distinct cases, depending on wh ether a positive or negative potential difference was applied. The slopes o f the linear current-voltage response curve are substantially different in the two cases, that in the 'forward' case being typically several orders of magnitude greater than that in the 'backward' case. Thus the system behave s like a semiconductor diode, The stationary concentration distribution for the different ions is described by a system of reaction-diffusion equation s involving migration caused by the electric field. An approximate solution of these equations, using a simplified model, is presented and compared wi th results obtained by solving the full system numerically. The concentrati on profiles obtained from the numerical solution confirm the validity of th e simplified model.