Catalyzing water dissociation (WD) into protons and hydroxide ions is important both for fabricating bipolar membranes (BPMs) that can couple different pH environments into a single electrochemical device and for accelerating electrocatalytic reactions that consume protons in neutral to alkaline media. We designed a BPM electrolyzer to quantitatively measure WD kinetics and show that, for metal nanoparticles, WD activity correlates with alkaline hydrogen evolution reaction activity. By combining metal-oxide WD catalysts that are efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer, we demonstrate a BPM driving WD with overpotentials of <10 mV at 20 mA·cm−2 and pure water BPM electrolyzers that operate with an alkaline anode and acidic cathode at 500 mA·cm−2 with a total electrolysis voltage of ~2.2 V.

Anion exchange membrane (AEM) electrolysis is a promising technology to produce hydrogen through the splitting of pure water. In contrast to proton-exchange-membrane (PEM) technology, which requires precious-metal oxide anodes, AEM systems allow for the use of earth-abundant anode catalysts. Here we report a study of first-row transition-metal (oxy)­hydroxide/oxide catalyst powders for application in AEM devices and compare physical properties and performance to benchmark IrO x catalysts as well as typical catalysts for alkaline electrolyzers. We show that the catalysts’ oxygen-evolution activity measured in alkaline electrolyte using a typical three-electrode cell is a poor indicator of performance in the AEM system. The best oxygen-evolution-reaction (OER) catalysts in alkaline electrolyte, NiFeO x H y oxyhydroxides, are the worst in AEM electrolysis devices where a solid alkaline electrolyte is used along with a pure water feed. NiCoO x -based catalysts show the best performance in AEM electrolysis. The performance can be further improved by adding Fe species to the particle surface. We attribute the differences in performance in part to differences in the electrical conductivity of the catalyst phases, which are also measured and reported.

No abstract

In this study, we have taken advantage of a pulsed CO 2 electroreduction reaction (CO 2 RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO 2 RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of −0.7 V RHE ) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from E an = 0.6 to 1.5 V RHE , followed by 1 s pulses at −0.7 V RHE ) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for E an = 0.9 V RHE we obtained 10% enhanced C 2 product selectivity (FE C 2 H 4 = 43.6% and FE C 2 H 5 OH = 19.8%) in comparison to the potentiostatic CO 2 RR at −0.7 V RHE (FE C 2 H 4 = 40.9% and FE C 2 H 5 OH = 11%), (ii) while for E an = 1.2 V RHE , high CH 4 selectivity (FE CH 4 = 48.3% vs 0.1% at constant −0.7 V RHE ) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with E an = 0.9 V RHE , including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C 2 product formation. In turn, pulsed electrolysis with E an = 1.2 V RHE caused the consumption of OH – species near the catalyst surface, leading to an OH-poor environment favorable for CH 4 production.

Water electrolysis powered by renewable electricity produces green hydrogen and oxygen gas, which can be used for energy, fertilizer, and industrial applications and thus displace fossil fuels. Pure-water anion-exchange-membrane (AEM) electrolyzers in principle offer the advantages of commercialized proton-exchange-membrane systems (high current density, low cross over, output gas compression, etc.) while enabling the use of less-expensive steel components and nonprecious metal catalysts. AEM electrolyzer research and development, however, has been limited by the lack of broadly accessible materials that provide consistent cell performance, making it difficult to compare results across studies. Further, even when the same materials are used, different pretreatments and electrochemical analysis techniques can produce different results. Here, we report an AEM electrolyzer comprising commercially available catalysts, membrane, ionomer, and gas-diffusion layers operating near 1.9 V at 1 A cm–2 in pure water. After the initial break in, the performance degraded by 0.67 mV h–1 at 0.5 A cm–2 at 55 °C. We detail the key preparation, assembly, and operation techniques employed and show further performance improvements using advanced materials as a proof-of-concept for future AEM-electrolyzer development. The data thus provide an easily reproducible and comparatively high-performance baseline that can be used by other laboratories to calibrate the performance of improved cell components, nonprecious metal oxygen evolution, and hydrogen evolution catalysts and learn how to mitigate degradation pathways.

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.

No abstract

No abstract