Catalytic, Conductive Bipolar Membrane Interfaces through Layer‐by‐Layer Deposition for the Design of Membrane‐Integrated Artificial Photosynthesis Systems

Abstract

In the presence of an electric field, bipolar membranes (BPMs) are capable of initiating water disassociation (WD) within the interfacial region, which can make water splitting for renewable energy in the presence of a pH gradient possible. In addition to WD catalytic efficiency, there is also the need for electronic conductivity in this region for membrane‐integrated artificial photosynthesis (AP) systems. Graphene oxide (GO) was shown to catalyze WD and to be controllably reduced, which resulted in electronic conductivity. Layer‐by‐layer (LbL) film deposition was employed to improve GO film uniformity in the interfacial region to enhance WD catalysis and, through the addition of a conducting polymer in the process, add electronic conductivity in a hybrid film. Three different deposition methods were tested to optimize conducting polymer synthesis with the oxidant in a metastable solution and to yield the best film properties. It was found that an approach that included substrate dipping in a solution containing the expected final monomer/oxidant ratio provided the most predictable film growth and smoothest films (by UV/Vis spectroscopy and atomic force microscopy/scanning electron microscopy, respectively), whereas dipping in excess oxidant or co‐spraying the oxidant and monomer produced heterogeneous films. Optimized films were found to be electronically conductive and produced a membrane ohmic drop that was acceptable for AP applications. Films were integrated into the interfacial region of BPMs and revealed superior WD efficiency (≥1.4 V at 10 mA cm−2) for thinner films (<10 bilayers≈100 nm) than for either the pure GO catalyst or conducting polymer individually, which indicated that there was a synergistic effect between these materials in the structure configured by the LbL method.

Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes

“…36,38 Recently, various metal oxides and graphene oxide have been explored as water-dissociation catalysts. [38][39][40][41][42] Figure 1: Schematic of a bipolar membrane with a cation-exchange layer, an anion-exchange layer and a catalyst layer in (a) forward and (b) reverse bias. As depicted in the insets, the CEL contains sulfonate negative fixed-charged groups, the AEL contains quaternary ammonium positive fixedcharge groups, and the water dissociation catalyst is Al(OH)3.…”

On the Nature of Field-Enhanced Water Dissociation in Bipolar Membranes

“…Bipolar membranes (BPMs) will be critical to the future of sustainable electrochemical synthesis due to their ability to enable stable optimized pH microenvironments at individual electrodes. This property is especially intriguing for reverse-bias applications, such as CO 2 reduction and water splitting, where optimal pH conditions for each electrode can exist that also minimize CO 2 crossover. − However, BPMs have long been limited in their application due to the considerable applied potentials required to drive the water dissociation (WD) occurring at the interface of the anion-exchange layer (AEL) and cation-exchange layer (CEL) of the BPM. ,− Many prior BPMs require approximately 1 V of membrane potential to achieve 10 mA cm –2 of WD current density. − Further research is required to understand the nature of the issue and ameliorate it to develop BPMs that can enhance the WD current densities achievable.…”

Recent Advances in Bipolar Membrane Design and Applications

“…The number of catalyst sites can be approximated by the IEC of GO, where GO nanosheets with a 16:1 carbon:oxygen ratio can have an IEC around 5 mequiv g –1 , 2–5 times that of the IEC of AEL layers. , This high density of functional groups (catalyst sites) can facilitate O–H bond reorganization in the presence of an electric field . Optimized integration of GO into the IL of BPMs using either spin-coating or layer-by-layer approaches has led to consistent improvements on the membrane voltage under WD operation with minimal increase in the co-ion leakage currents (Figure ), in some cases surpassing the capabilities of the commercially available equivalent. ,,− …”

“…For instance, chemically reducing a GO layer at the BPM interface decreases the electrical resistance of the membrane but simultaneously decreases the WD capabilities due to the reduction in the number of WD catalytic sites . Alternatively, a composite of a WD catalyst and an electrically conductive material can be incorporated into the BPM, where the film thickness influences the electrical conductivity, WD capability, and optical transparency. , Analysis of these additional criteria will become vital for BPM integration into PEC systems, as the vast majority of BPM analyses have only investigated the electrochemical properties to date.…”