A bipolar membrane (BPM), consisting of a cation and an anion exchange layer (CEL and AEL), can be used in an electrochemical cell in two orientations: reverse bias and forward bias. A reverse bias is traditionally used to facilitate water dissociation and control the pH at either side. A forward bias has been proposed for several applications, but insight into the ion transport mechanism is lacking. At the same time, when implementing a BPM in a membrane electrode assembly (MEA) for CO2 reduction, the BPM orientation determines the environment of the CO2 reduction catalyst, the anolyte interaction and the direction of the electric field at the interface layer. In order to understand the transport mechanisms of ions and carbonic species within a bipolar membrane electrode assembly (BPMEA), these two orientations were compared by performing CO2 reduction. Here, we present a novel BPMEA using a Ag catalyst layer directly deposited on the membrane layer at the vapour-liquid interface. In the case of reverse bias, the main ion transport mechanism is water dissociation. CO2 can easily crossover through the CEL as neutral carbonic acid due to the low pH in the reverse bias. Once it enters the AEL, it will be transported to the anolyte as (bi)carbonate because of the presence of hydroxide ions. When the BPM is in the forward bias mode, with the AEL facing the cathode, no net water dissociation occurs. This not only leads to a 3 V lower cathodic potential but also reduces the flux of carbonic species through the BPM. As the pH in the AEL is higher, (bi)carbonate is transported towards the CEL, which then blocks the majority of those species. However, this forward bias mode showed a lower selectivity towards CO production and a higher salt concentration was observed at the cathode surface. The high overpotential and CO2 crossover in reverse bias can be mitigated via engineering BPMs, providing higher potential for future application than that of a BPM in forward bias owing to the intrinsic disadvantages of salt recombination and poor faradaic efficiency for CO2 reduction. This journal is

Electrochemical atomic force microscopy (EC-AFM) enables measurement of electrode topography and mechanical properties during electrochemical reactions. However, for aqueous-based reactions that make gas products, such as CO2 reduction and water splitting into CO/H2, current densities below 1 mA cm-2 have been necessary to prevent formation of bubbles at the electrode; such bubbles can stick to the AFM probe and prevent further AFM imaging. Here, we demonstrate a novel cell design with a gas-diffusion electrode (GDE) to exhaust the gas products, thereby enabling high current density EC-AFM measurements at 1, 10, and 100 mA cm-2 that are not disturbed by bubble formation at the electrode surface. These experiments revealed a stable morphological structure of Cu catalysts deposited on GDEs during high current density operation. Systematic spatially resolved maps of deformation and adhesion showed no signs of a gas-liquid interface between catalyst particles of the GDE.

Carbon dioxide can be electrochemically converted into feedstocks for many industrial processes, such as the manufacturing of synthetic fuels and chemicals. This work focuses on the structure-functionality relationship between Au, Sn, and bimetallic AuSn catalysts and their CO2 reduction performance in an H-Cell at varying current densities. X-Ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) were used to determine the crystal structure, surface morphology, and composition of compositionally variant bimetallic thin films of Au-Sn before and after electrolysis. The electrochemical activity for each bimetallic film was measured in terms of electrode current and product selectivity as a function of applied current density and catalyst composition. The results of this work show that not all combinations of metals for CO2 reduction can improve catalyst activity toward a desired product and that a detailed material characterization can help in drawing structure-functionality relationships between a catalyst and its activity.

X-ray absorption spectroscopy (XAS) offers the unique possibility to study metal electrocatalysts such as silver and copper while they are performing electrochemical carbon dioxide (CO2) reduction. In this work, we present an approach to perform operando XAS experiments on an electrochemical cell performing CO2 reduction with a gas diffusion electrode (GDE) as cathode. The experimental set-up, advantages and drawbacks, XAS data analysis, and XAS theory are discussed. Results on copper and silver GDEs obtained through the presented procedures are then presented and discussed. Structural and compositional catalyst data acquired under operando conditions can help further density functional theory calculations, and catalytic, and systems studies on CO2 reduction. Structural and compositional data including crystallite size were obtained while performing high current density (up to 200 mA cm-2) CO2 reduction. On the silver catalysts at higher than 100 mA cm-2 applied current density, a Ag-X contribution was found and is ascribed to Ag-O. For both silver and copper, the XAS experiments revealed that the crystallite size of the ex situ samples is smaller than the samples during CO2 reduction. Furthermore, metal particle size polydispersity was found in the silver catalysts by comparing the obtained coordination numbers with theoretical values. The operando EXAFS data was of such high quality (k: 3-14 Å-1) that four shells could be fitted. The value of combining ex situ material characterisation and electrocatalyst performance data with operando XAS experiments is discussed and found to be of great importance to further CO2 reduction research.

Electrochemical CO₂ reduction can convert CO₂ into fuels and valuable chemicals using renewable electricity, which provides a prospective path toward large-scale energy storage. Au nanostructured electrodes have demonstrated the best catalytic performance for CO₂ conversion: high catalytic selectivity for CO formation at low overpotentials, high current density, and long-term durability. Here, we report selective electrocatalytic CO₂ reduction to CO on nanostructured Au with various morphologies, prepared via electrocrystallization with a megahertz potential oscillation. X-ray diffraction showed that the proportion of {100} and {110} to {111} surfaces increased at more negative deposition potentials. Cyclic voltammetry showed the potential of zero charge on an Au film was approximately 0.35 V vs standard hydrogen electrode (SHE) and that the surface energy decreased by ∼1 eV/nm² at -0.5 V vs SHE, tending to 0 within several volts in either direction. Scanning electron micrograms showed that the Au crystals grow primarily in the 〈110〉 directions. From these data, a model for crystallization from melts was adapted to calculate the roughening temperature of the {111}, {100}, and {110} Miller indices as 7000, 4000, and 1000 K, decreasing for more negative deposition potentials. This offers a framework for exposed facet control in electrocrystallization. In CO₂ electrocatalysis, -0.35 V vs reversible hydrogen electrode was observed to be a turn-on potential for improved CO₂ reduction activity; dendrites showed 50% Faradaic efficiency for CO production at more cathodic potentials. The Tafel slope was measured to be 40 mV/decade for {100} and {110}-rich Au dendrites and 110 mV/decade for {111}-dominated Au plates, suggesting the higher surface energy crystal facets may stabilize all of the CO₂ reduction reaction intermediates.