Design principles for water dissociation catalysts in high-performance bipolar membranes

Abstract

Water dissociation (WD, H2O → H+ + OH−) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO2 as an earth-abundant WD catalyst, and generate O2 and H2 at 500 mA cm−2 with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.

Membrane Strategies for Water Electrolysis

“…This work reported a water dissociation overpotential of 1.5 V when operating pure water-fed BPMWE at 3.4 A cm –2 which resulted in a total cell voltage of 4 V. In a 2022 report, a BPM featuring a low-cost, earth-abundant TiO 2 water dissociation catalyst showed 0.5 A cm –2 at 2 V in a BPMWE. All the BPMWE demonstrations by Boettcher et al deployed IrO x and Pt in the anode and cathode, respectively, at 2 mg cm –2 Figure presents the polarization data of said BPMWE demonstrations. , These papers almost showed BPMWE parity with an AEMWE under 1 A cm –2 .…”

“…All the BPMWE demonstrations by Boettcher et al deployed IrO x and Pt in the anode and cathode, respectively, at 2 mg cm –2 Figure presents the polarization data of said BPMWE demonstrations. , These papers almost showed BPMWE parity with an AEMWE under 1 A cm –2 . However, the BPMWE displayed 200 mV more polarization at the current density of values of 1 A cm –2 or higher when compared to the AEMWE.…”

“…Polarization curves of BPMWE cells using BPMs at 50 to 55 °C and ambient pressure. , Cathode: Pt black (2.0 mg cm –2 ) with 10 wt% Nafion binder and anode: IrO 2 (2.0 mg cm –2 ) with 10 wt% AEI binder. Black, red, blue, and orange traces used Sustanion for fabricating the BPM and AEI binder.…”

“…By judiciously selecting metal oxide-based water dissociation catalysts with acidic pH at the point of zero charge (PZC) adjacent to the PEM and an alkaline pH at the PZC adjacent to the AEM, the BPM displayed <10 mV overpotential for water dissociation at 20 mA cm –2 (for more details on PZC, see Supporting Information). Modeling efforts on BPMs dual-layer water dissociation catalysts also show that metal oxide particles with acidic and alkaline pH values at the PZC increase the surface charge. , Furthermore, the semiconducting nature of the particles also enhances the local electric field and focuses it at the PEM–interfacial junction and AEM–interfacial junction regions rather than distributing it broadly over the junction region with the water dissociation catalysts. , These effects are posited to increase water dissociation kinetics. The nature of the electric field and its strength over the BPM junction region on water dissociation is a subject of research and debate .…”

“…The nature of the electric field and its strength over the BPM junction region on water dissociation is a subject of research and debate . Onsager’s theory of the second Wien effect is often cited as the main mechanism for water dissociation in reverse bias BPMs under an applied electric field . The second Wien effect asserts that the extent of a weak electrolyte dissociation is exponentially dependent on the applied electric field.…”