Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies.

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Renewable electricity storage using electrolysis.

Electrolysis converts electrical energy into chemical energy by storing electrons in the form of stable chemical bonds. The chemical energy can be used as a fuel or converted back to electricity when needed. Water electrolysis to hydrogen and oxygen is a well-established technology, whereas fundamental advances in CO2 electrolysis are still needed to enable short-term and seasonal energy storage in the form of liquid fuels. This paper discusses the electrolytic reactions that can potentially enable renewable energy storage, including water, CO2 and N2 electrolysis. Recent progress and major obstacles associated with electrocatalysis and mass transfer management at a system level are reviewed. We conclude that knowledge and strategies are transferable between these different electrochemical technologies, although there are also unique complications that arise from the specifics of the reactions involved.

2-Aminobenzenethiol-Functionalized Silver-Decorated Nanoporous Silicon Photoelectrodes for Selective CO2 Reduction.

A molecularly thin layer of 2-aminobenzenethiol (2-ABT) was adsorbed onto nanoporous p-type silicon (b-Si) photocathodes decorated with Ag nanoparticles (Ag NPs). The addition of 2-ABT alters the balance of the CO2 reduction and hydrogen evolution reactions, resulting in more selective and efficient reduction of CO2 to CO. The 2-ABT adsorbate layer was characterized by Fourier transform infrared (FTIR) spectroscopy and modeled by density functional theory calculations. Ex situ X-ray photoelectron spectroscopy (XPS) of the 2-ABT modified electrodes suggests that surface Ag atoms are in the +1 oxidation state and coordinated to 2-ABT via Ag-S bonds. Under visible light illumination, the onset potential for CO2 reduction was -50 mV vs. RHE, an anodic shift of about 150 mV relative to a sample without 2-ABT. The adsorption of 2-ABT lowers the overpotentials for both CO2 reduction and hydrogen evolution. A comparison of electrodes functionalized with different aromatic thiols and amines suggests that the primary role of the thiol group in 2-ABT is to anchor the NH2 group near the Ag surface, where it serves to bind CO2 and also to assist in proton transfer.

Effects of Mesoporosity and Conductivity of Hierarchically Porous Carbon Supports on the Deposition of Pt Nanoparticles and Their Performance as Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media.

The structural characteristics of supports, such as surface area and type of porosity, affect the deposition of electrocatalysts and greatly influence their electrochemical performance in fuel cells. In this work, we use a series of high surface area hierarchical porous carbons (HPCs) with defined mesoporosity as model supports to study the deposition mechanism of Pt nanoparticles. The resulting electrocatalysts are characterized by several analytical techniques, and their electrochemical performance is compared to a state-of-the-art, commercial Pt/C system. Despite the similar chemical composition and surface area of the supports, as well as similar amounts of Pt precursor used, the size of the deposited Pt nanoparticles varies, and it is inversely proportional to the mesopore size of the system. In addition, we show that an increase in the size of the catalyst particles can increase the specific activity of the oxygen reduction reaction. We also report on our efforts to improve the overall performance of the above electrocatalyst systems and show that increasing the electronic conductivity of the carbon support by the addition of highly conductive graphene sheets improves the overall performance of an alkaline fuel cell.

Improving the efficiency of CO2 electrolysis by using a bipolar membrane with a weak-acid cation exchange layer.

The efficient conversion of electricity to chemicals is needed to mitigate the intermittency of renewable energy sources. Driving these electrochemical conversions at useful rates requires not only fast electrode kinetics, but also rapid mass and ion transport. However, little is known about the effect of local environments on ionic flows in solid polymer electrolytes. Here, we show that it is possible to measure and manipulate the local pH in membrane electrolysers with a resolution of tens of nanometres. In bipolar-membrane-based gas-fed CO2 electrolysers, the acidic environment of the cation exchange layer results in low CO2 reduction efficiency. By using ratiometric indicators and layer-by-layer polyelectrolyte assembly, the local pH was measured and controlled within an ~50-nm-thick weak-acid layer. The weak-acid layer suppressed the competing hydrogen evolution reaction without affecting CO2 reduction. This method of probing and controlling the local membrane environment may be useful in devices such as electrolysers, fuel cells and flow batteries, as well as in operando studies of ion distributions within polymer electrolytes.

A high throughput optical method for studying compositional effects in electrocatalysts for CO2 reduction.

In the problem of electrochemical CO2 reduction, the discovery of earth-abundant, efficient, and selective catalysts is essential to enabling technology that can contribute to a carbon-neutral energy cycle. In this study, we adapt an optical high throughput screening method to study multi-metallic catalysts for CO2 electroreduction. We demonstrate the utility of the method by constructing catalytic activity maps of different alloyed elements and use X-ray scattering analysis by the atomic pair distribution function (PDF) method to gain insight into the structures of the most active compositions. Among combinations of four elements (Au, Ag, Cu, Zn), Au6Ag2Cu2 and Au4Zn3Cu3 were identified as the most active compositions in their respective ternaries. These ternary electrocatalysts were more active than any binary combination, and a ca. 5-fold increase in current density at potentials of -0.4 to -0.8 V vs. RHE was obtained for the best ternary catalysts relative to Au prepared by the same method. Tafel plots of electrochemical data for CO2 reduction and hydrogen evolution indicate that the ternary catalysts, despite their higher surface area, are poorer catalysts for the hydrogen evolution reaction than pure Au. This results in high Faradaic efficiency for CO2 reduction to CO.