Material requirements for membrane separators in a water-splitting photoelectrochemical cell

Abstract

A fully integrated model of a photoelectrochemical cell for water electrolysis is applied to the case of light-absorbing particles embedded in a membrane separator. Composition of the product gases is shown to be one critical measure of device performance. Not only must the composition be kept outside the explosive window for mixtures of H₂ and O₂, but also product purity is a concern. For the absorber-in-membrane geometry and the model assumptions used here, results show purely water-saturated H₂ on the cathode side and water-saturated O₂ on the anode side. Since it is possible to design devices that violate these assumptions, it should not be assumed that a polymer separator or an absorber-in-membrane geometry will be effective in preventing explosive mixtures in all cases. Net H₂ collected, i_H2,net, is the second essential performance metric, and it is shown to differ significantly from the more commonly reported total H₂ produced and operating current density. Schemes which co-evolve H₂ and O₂ violate the first metric and do not provide the second. A composite of triple-junction silicon absorbers in a Nafion membrane is shown to have an optimum thickness of 30 μm, dependent on the properties of the light absorber. Varying membrane properties reveals a tradeoff between conductivity, κ_m, and gas permeabilities, ψ_H2 and ψ_O2, that can potentially be exploited differently than in a fuel cell. Modulating the relative humidity (RH) is insufficient. The maximum i_H2,net is calculated to be 6.97 mA cm⁻² at RH = 30% relative to a value of 6.92 mA cm⁻² at RH = 100%. The model identifies target material properties for new polymers. If ψ is dropped one order of magnitude below that of Nafion (ψ/ψ_Nafion = 0.1), the optimum value for i_H2,net increases by 63.5%. For ψ/ψ_Nafion = 0.01, the optimum i_H2,net increases by 73.5%, which compares favorably to the 74.5% improvement that would result if Nafion were made impermeable (ψ/ψ_Nafion = 0). Meanwhile, κ_m can drop to a value of 1.2 × 10⁻³ S cm⁻¹ (two orders of magnitude below liquid-equilibrated Nafion) with less than a 5% decline in i_H2,net.