Water-Dissociation Catalysis Near the Reversible Limit in Bipolar Membrane Electrolyzers

Abstract

The voltage penalty encountered when driving water dissociation (WD) at a high current density represents a major obstacle in the commercialization of existing bipolar-membrane (BPM) technology for energy devices. Here we show that three materials descriptors, including the electrical conductivity, microscopic surface area, and (nominal) surface-hydroxyl coverage, effectively control the kinetics of WD in BPMs. Using these descriptors and optimal mass loading, we design new earth-abundant WD catalysts based on nanoparticle SnO2 synthesized at low temperature that exhibit exceptional performance by driving the WD reaction in a BPM electrolyzer at the remarkably low WD overvoltage (ηwd) of 100 ± 20 mV at 1.0 A cm−2. We demonstrate this new catalyst works equivalently well with hydrocarbon proton-exchange layers as it does with fluorocarbon-based Nafion, thus providing new pathways to commercializing advanced bipolar membranes for a broad array of electrolysis, fuel-cell, and electrodialysis applications.

Multi-scale physics of bipolar membranes in electrochemical processes

“…Instead they are better described as having a proton-absorption isotherm with an average proton binding strength and interaction term 53 . BPMs with optimal loadings of metal-oxide catalysts yield record voltaic efficiency 17 , and have nearly linear current-voltage response instead of the more-exponential shape observed for uncatalyzed and GrOx-based BPMs 13,51 . Electronically conductive, or high dielectric, metal-oxide catalysts have much faster WD kinetics than electronically insulating catalysts, suggesting a key role in electricfield screening, where catalyst mobile electronic charge redistributes and cancels the electric field in the solid, thereby resulting in a stronger field directly outside of the particle.…”

“…Typically, more hydrophobic ionomers like PFSAs possess phase-segregated domains, a hydrophobic backbone phase, and a development of advanced interfacial catalysts has significantly reduced BPM energy requirements [13][14][15] that have long been an important contributor to cost precluding their industrial application 16 , with recent demonstrations exhibiting performance close to the thermodynamic minimum even at high current densities approaching 1 A cm -2 (refs. 13,14,17). However, an untapped opportunity exists to leverage engineering approaches to optimize BPM systems.…”

“…Although Nafion is the prototypical research CEL due to its availability, BPMs with hydrocarbon CELs have demonstrated performance on par with Nafion-based BPMs 17,20 .…”

Divergent Synthesis of Bipolar Membranes Combining Strong Interfacial Adhesion and High-Rate Capability

“…One of the main hurdles in the development of electrochemical devices based on BPMs is the high resistance of commercial materials at practical current densities, which mainly originates from the slow water dissociation (WD) reaction at the interface between the anion and cation exchange layers . This has triggered tremendous research efforts spanning from mapping and probing of phenomena that govern water dissociation and ionic separation − to the design of new BPMs with enhanced water dissociation activity − and mass transport characteristics. , The evaluation of different water dissociation catalysts currently points to graphene oxide, − TiO 2 , and SnO 2 as good candidates that combine high activity, earth abundancy, and good stability. Remarkably high-rate capability has recently been reported for TiO 2 - and SnO 2 -catalyzed BPMs with cation exchange layers (CELs) and anion exchange layers (AELs) based on perfluorosulfonic acid (Nafion) and poly(arylene piperidinium) (PiperION), respectively. ,, …”