Bipolar Membrane and Interface Materials for Electrochemical Energy Systems

Abstract

Bipolar membranes (BPMs) are recently emerging as a promising material for application in advanced electrochemical energy systems such as (photo)­electrochemical CO2 reduction and water splitting. BPMs exhibit a unique property to accelerate water dissociation and ionic separation that allows for maintaining a steady-state pH gradient in electrochemical devices without a significant loss in process efficiency, thereby allowing a broader catalyst material selection for the respective oxidation and reduction reactions. However, the formation of high-performance BPMs with significantly reduced overpotentials for driving water dissociation and ionic separation at conditions and rates that are relevant to energy technologies is a key challenge. Herein, we perform a detailed assessment of the requirements in base materials and optimal design routes for the BPM layer and interface formation. In particular, the interface in the BPM presents a critical component with its structure and morphology influencing the kinetics of water dissociation reaction governed by both electric field and catalyst driven mechanisms. For this purpose, we present, among others, the advantages and drawbacks in the utilization of a bulk heterojunction formed in 3D structures that recently have been reported to demonstrate a possibility of designing stable and high-performance BPMs. Also, the outer layers of a BPM play a crucial role in kinetics and mass transport, particularly related to water and ion transport at electrolyte–membrane and membrane–catalyst interfaces. This work aims at identifying the gaps in the structure–property of the current monopolar materials to provide prospective facile design routes for BPMs with excellent water dissociation and ionic separation efficiency. It extends to a discussion about material selection and design strategies of advanced BPMs for application in emerging electrochemical energy systems.

When Flooding Is Not Catastrophic─Woven Gas Diffusion Electrodes Enable Stable CO2 Electrolysis

“…Compared to MEA-based CO 2 electrolyzers with anion exchange membranes, , the use of a catholyte layer in our proposed design would act as a buffer between the membrane and the catalyst. This would allow the utilization of, for example, a bipolar membrane, which could reduce CO 2 crossover ,, and allow the deployment of a non-precious anode made from nickel. ,, Although the catholyte channel introduces additional ohmic resistance, it allows better control of the local reaction environment at high current densities , in comparison to MEA electrolyzers, in which the water management at the membrane and the cathode or salt formation in the gas channel , can hinder performance. From a practical perspective, the reactor (Figure d) has to be fed with a sufficiently high electrolyte flow rate to ensure that it does not run dry.…”

Catalytic Proton–Hydroxide Recombination for Forward-Bias Bipolar Membranes

“…When sufficient reverse bias is applied, water dissociation (WD; H 2 O → H + + OH – ) is driven at the bipolar junction to form OH – and H + , which are moved out of the BPM in opposite directions by the electrochemical potential gradient . In forward-bias mode (Figure b), however, the anode is acidic and the cathode basic; ions are transported through their respective layer toward the bipolar junction, where they accumulate and react . In the absence of additional co-ions, H + and OH – accumulate at the BPM junction and recombine (H + /OH – recombination; H + + OH – → H 2 O) .…”

“…13 In forward-bias mode (Figure 1b), however, the anode is acidic and the cathode basic; ions are transported through their respective layer toward the bipolar junction, where they accumulate and react. 14 In the absence of additional co-ions, H + and OH − accumulate at the BPM junction and recombine (H + /OH − recombination; H + + OH − → H 2 O). 12 Forward-bias BPMEAs are of interest for fuel cells, 15−17 redox-flow batteries, 18 and CO 2 reduction.…”

Vanadium Oxide Nanosheet-Infused Functionalized Polysulfone Bipolar Membrane for an Efficient Water Dissociation Reaction

“…Thus, under extreme pH difference (ΔpH = 14) across the BPM, the membrane potential reaches ∼0.83 V, representing the minimum potential for the initiation of the water dissociation (WD) reaction. , Under the reverse bias condition, the BPM in an electrochemical cell, containing electrolytic solutions, restricts the transverse of coions (ions with the same charge as that of the fixed on the membrane polymer) through the membrane and carries a limited current depending on the layer permselectivity. ,,, A large electric field is generated in the IL at a higher potential across the BPM, resulting in the dissociation of the residing water molecules due to polarization. ,− The generated H + and OH – upon WD are the major ionic current carriers above migrating toward the cathode and anode through the CEL and AEL, respectively, creating a pH gradient over the BPM.…”