Mixed ion and electron conducting polymer composite membranes for artificial photosynthesis

Abstract

Inspired by the fact that OH- has a very high mobility in water, highly conductive OH⁻conducting membranes were developed for alkaline water electrolysis. The membranes were semi-interpenetrating networks of crosslinked poly(vinyl alcohol) (PVA) and a polycation miscible with PVA. It is analogous to aqueous strong base solution. The polycation is a OH- containing polymer; PVA solvates this polycation and facilitates the ion conduction via Grotthuss mechanism. The membrane with proper composition has an exceptionally high OH⁻ conductivity of 151 mS/cm, 6.51 times as high as the commercial membrane Neosepta AHA. At the same time, the hydrogen bonds and covalent crosslinks in the system give this membrane a high tensile strength of 41 MPa in the wet state, 46% higher than the Neosepta AHA membrane. Insight in the ion conduction mechanism was gained by spectroscopic studies and the measurement of OH- conduction activation energy.

This material system is also highly anion perm-selective, a feature critical to sustaining the pH gradient in bipolar membrane based artificial photosynthesis devices. A highly transparent mixed proton and electron conducting membrane was developed. The Nafion and reduced graphene oxide (rGO) were chosen as the proton conducting polymer matrix and the electrically conductive filler respectively. The filler has a high aspect ratio. As predicted by simulations, it will have low percolation threshold if homogeneously dispersed. To achieve this homogeneity, water-aided mixing was employed followed by fast freezing in liquid nitrogen. Though rGO is a light absorber, the extremely low percolation threshold (0.1%) allows us to achieve sufficient electrical conductivity with only a small volume fraction of rGO. Therefore, the membrane was highly transparent in addition to its ability to conduct both electrons and protons.

Detailed modeling of the energy loss from conduction, light absorption, and gas crossover was conducted, showing that this material system is promising for the artificial photosynthesis application.