Electrical conductivity, ionic conductivity, optical absorption, and gas separation properties of ionically conductive polymer membranes embedded with Si microwire arrays

Abstract

The optical absorption, ionic conductivity, electronic conductivity, and gas separation properties have been evaluated for flexible composite films of ionically conductive polymers that contain partially embedded arrays of ordered, crystalline, p-type Si microwires. The cation exchange ionomer Nafion, and a recently developed anion exchange ionomer, poly(arylene ether sulfone) that contains quaternary ammonium groups (QAPSF), produced composite microwire array/ionomer membrane films that were suitable for operation in acidic or alkaline media, respectively. The ionic conductivity of the Si wire array/Nafion composite films in 2.0 M H₂SO₄(aq) was 71 mS cm⁻¹, and the conductivity of the Si wire array/QAPSF composite films in 2.0 M KOH(aq) was 6.4 mS cm⁻¹. Both values were comparable to the conductivities observed for films of these ionomers that did not contain embedded Si wire arrays. Two Si wire array/Nafion membranes were electrically connected in series, using a conducting polymer, to produce a trilayer, multifunctional membrane that exhibited an ionic conductivity in 2.0 M H₂SO₄(aq) of 57 mS cm⁻¹ and an ohmic electrical contact, with an areal resistance of ∼0.30 Ω cm², between the two physically separate embedded Si wire arrays. All of the wire array/ionomer composite membranes showed low rates of hydrogen crossover. Optical measurements indicated very low absorption (<3%) in the ion-exchange polymers but high light absorption (up to 80%) by the wire arrays even at normal incidence, attesting to the suitability of such multifunctional membranes for application in solar fuels production.