Matthew T. Mayer

Silver (Ag) electrodes are among the best catalysts for producing CO in electrochemical CO2 reduction (CO2R). Beyond the catalyst material, it is increasingly recognized that electrolyte effects and dynamic processes that shape the interfacial microenvironment play critical roles in determining product selectivity. Herein, we reported unique observations of a variety of other CO2R products from Ag foil electrodes, highly dependent on electrolyte concentration and impurities. Notably, we found that trace levels of Cu (<5 ppb) could induce significant selectivity toward methane (CH4). This suggested that (sub)monolayer amounts of impurity metal deposited on Ag electrodes can drastically alter product selectivity. Density functional theory attributed this surprising CH4 selectivity to a d-band center upshift induced by the lattice mismatch with the Ag substrate. We furthermore observed that product selectivity shifts significantly over time during extended testing, attributable to dynamic electrolyte changes enabled by cation-exchange membranes. This study offers new insights into the behavior of well-studied Ag catalysts, highlighting the importance of trace electrolyte constituents and suggesting a possible new strategy for selectivity tuning.

Ambient electrochemical CO ₂ capture powered by renewable energy offers a promising carbon removal route, exemplified by the emerging electrochemically mediated amine regeneration (EMAR) process demonstrated in lab-scale single cells and stacks. However, molecular-level insight into EMAR interfacial kinetics is still missing, particularly at the anode, where CO ₂ release involves a mechanistically non-trivial re-complexation process at the electrode–electrolyte interface, coupling heterogeneous metal-ion release with bulk complexation. Here, we report the time-resolved characterization of the interfacial molecular processes of the EMAR CO ₂ release process. Using in situ Fourier-transform infrared (FTIR) spectroscopy and ultraviolet-visible (UV-vis) spectroscopy, cyclic voltammetry, and real-time differential electrochemical mass spectrometry (DEMS), we examine how the nature of the electrolyte anion affects the CO ₂ release onset potentials. The time-resolved analyses reveal that Cl⁻ ions are more effective in releasing Cu ions and hence CO ₂ than nitrate or perchlorate. Molecular dynamics simulations show that strong surface Cu–Cl interactions likely facilitate favorable CO ₂ and carbamate adsorption kinetics. We expect that this study paves the way for broader use of interfacial in-situ analytics in electrified CO ₂ capture and release.

Electrifying hydrogen peroxide production via the 2-electron oxygen reduction reaction (2e ^- ORR) has emerged as a promising alternative to the hazardous centralized anthraquinone process that is the current standard for industrial production. Our group recently identified the van der Waals layered material PdSe ₂ as a selective, active, and stable 2e ^- ORR electrocatalyst in neutral and acidic solutions [1]. DFT calculations suggest that the selectivity of the ORR reaction depends on the facets of this layered material, however, hydrothermally-grown PdSe ₂ samples exhibit small aspect-ratios and large variations in the surface facets. Through chemical vapor deposition synthesis of PdSe ₂ , we were able to select towards preferential exposure of the basal plane facets and directly grow the PdSe ₂ catalyst on electrode surfaces. Electrochemical and structural characterization of the different PdSe ₂ morphologies are being carried out through a multimodal approach, including rotating ring disk electrode (RRDE), operando soft and hard x-ray absorption studies, and spatially-selective electrochemical measurements, towards the goal of revealing the facet-dependent selectivity and performance of PdSe ₂ . We hope our results will further elucidate the catalytic mechanism(s) of this new 2e ^- ORR electrocatalyst. Our results deepen understanding of interfacial effects in nanostructured electrocatalysts and inform general design principles for selective electrochemical processes. [1] Ross et al, J. Am. Chem. Soc. 2024, 146, 23, 15718–15729

With the rising anthropogenic CO₂ concentrations in the atmosphere and the growing interest in converting solar energy into value-added products, interest in (photo)electrochemical reduction of CO₂ is increasing. A key component for this process is an optically transparent and electrically conductive substrate, usually in the form of a transparent conducting oxide (TCO) film deposited on glass. However, most traditional TCO materials, such as fluorine-doped SnO₂ (FTO) or tin-doped In₂O₃ (ITO), are not stable under cathodic conditions. TiO₂ has been widely reported as an excellent protection layer material in photo-electrochemical (PEC) research, and niobium is one prospective dopant to improve its conductivity. In this work, we demonstrate that commercial FTO glass can be protected against (photo)corrosion under cathodic reaction conditions in pH-neutral electrolytes by direct-current sputtering of thin Nb-doped TiO₂ (Nb:TiO₂) overlayers. Combined with a pre-deposition FTO etching procedure, the Nb:TiO₂-covered FTO electrode offers excellent electrochemical stability while maintaining good transparency and conductivity.

Glycerol, often considered a waste byproduct of biodiesel production, holds the potential for conversion into chemicals of varying economic value, such as dihydroxyacetone (DHA) and formic acid (FA). Hence, accurate identification and quantification of glycerol oxidation reaction (GOR) products are crucial for glycerol valorization research and practical deployment. High-performance liquid chromatography (HPLC) is the preferred analytical method for these purposes due to its proficiency in separating and quantifying components in liquid mixtures, even in the presence of diluted solutes. On the other hand, peak overlap in chromatograms, especially among glycerol, DHA, and FA, poses a notable challenge in the analysis of GOR products. This study introduces a quantification method aimed at resolving peak overlaps in HPLC analysis of GOR products. Initially, we examine the optical properties of glycerol and GOR products to identify optimal wavelengths for spectrophotometric HPLC analysis and detection. Subsequently, we propose an algebraic approach to resolve the peak overlap of glycerol, DHA, and FA using various detectors, including the refractive index detector (RID) and the variable wavelength detector (VWD). This method is applied to analyze the GOR products of undoped, nonco-catalyzed nanoporous BiVO₄ photoanodes, which have shown an intrinsic catalytic activity toward GOR products in previous studies.

Electrochemical CO₂ conversion can result in a variety of products, often as a mixture, and controlling the product selectivity remains a key challenge. It has been shown that pulsing the electrochemical potential can lead to altered product distributions, influenced by effects on, e.g., transport, double-layer rearrangement, adsorption/desorption, and changes to electrode structure and composition (1). Herein we report our observations using metal electrodes normally selective for the 2-electron formation of CO as major product under steady-state (potentiostatic) conditions, finding that they can produce significant amounts of higher-order products (including methane and ethylene) under the application of pulse potential waveforms. We confirm this is not due to metal impurities in the system, but is significantly affected by phenomena such as surface restructuring and accumulation of liquid products. Furthermore, time-resolved differential electrochemical mass spectrometry (DEMS) measurements reveal distinctly different transient behaviors between the different gaseous products, providing key new mechanistic insight for clarifying the roles of pulsing. (1) Casebolt, R.; Levine, K.; Suntivich, J.; Hanrath, T. Pulse Check: Potential Opportunities in Pulsed Electrochemical CO₂ Reduction. Joule 2021, 5 (8), 1987–2026. https://doi.org/10.1016/j.joule.2021.05.014. Figure: Pulsed CO₂ reduction on Ag (manuscript pending) -- a) Pulsing waveform with parameter definitions. b) Gas product evolution rates as measured by GC during continuous electrochemical pulsing of a silver foil electrode. c) DEMS study of porous Ag film electrodes, revealing product formation transient behaviors. d) Selectivity dependence on varying the cathodic step potential (E_c) while E_a is fixed. Figure 1

The conversion of CO₂ into formate (HCOO⁻), a techno‐economically feasible product, can be achieved using earth‐abundant Cu_xS electrocatalysts, but questions remain regarding how catalyst structure, composition, and reaction environment influence product selectivity. A novel synthesis method based on electrodeposition of Cu foam and its subsequent sulfidation via immersion in sulfur saturated toluene solution resulted in Cu_xS foams. Catalytic activity studies found that HCOO⁻ selectivity is dependent on electrochemical activation at higher overpotentials. To understand the effects of activation, determine the active forms of the catalysts, and identify the role of sulfur, the electrodes are carefully characterized as well as gaseous and sulfur dissolved in electrolyte. This included study of the effects of intentional addition of solution sulfur species, identification of the sulfur loss, determination of the electrode composition and relating sulfur speciation to observed product selectivity. It is found that residual sulfur stabilizes Cu⁺ during electrolysis at potentials favoring HCOO⁻ production, in contrast to pristine Cu that undergoes complete reduction and shows poor HCOO⁻ selectivity. Sulfur in both the catalyst and dissolved in electrolyte are of dynamic nature, and surface residues of SO₄²⁻ species are identified in all activated catalysts which correspond with enhanced HCOO⁻ production.

Glycerol, a primary by-product of biodiesel production, can be oxidized into various value-added chemicals, significantly enhancing the techno-economic value of photoelectrochemical (PEC) devices. Several studies have explored various photoelectrode materials and cocatalysts, but the influence of electrolytes on PEC glycerol oxidation has remained relatively unexplored despite its significance. Here, we explore the impact of various acidic (pH = 2) electrolytes, namely NaNO₃, NaClO₄, Na₂SO₄, K₂SO₄, and KP_i, on PEC glycerol oxidation using nanoporous thin film BiVO₄ as a model photoanode. Our experimental findings reveal that the choice of electrolyte anion and cation significantly affects the PEC performance (i.e., photocurrent, onset potential, stability, and selectivity towards value-added products) of BiVO₄ for glycerol oxidation. To explain this interesting phenomenon, we correlate the observed performance trend with the ion specificity in the Hofmeister series as well as the buffering capacity of the electrolytes. Notably, NaNO₃ is identified as the optimal electrolyte for PEC glycerol oxidation with BiVO₄, when considering various factors such as stability and production rates for glycerol oxidation reaction (GOR) products, surpassing the previously favored Na₂SO₄. Glycolaldehyde emerges as the most dominant product with ~50% selectivity in NaNO₃. The general applicability of our findings is confirmed by similar observation in electrochemical (EC) GOR with a polycrystalline platinum anode. Overall, these results emphasize the critical role of electrolyte selection in enhancing the efficiency of EC / PEC glycerol oxidation.

Membrane electrode assemblies in gas diffusion cells enable CO₂ reduction at industrially relevant rates, yet their long-term operational stability is often limited by the formation of solid precipitates (e.g. K₂CO₃) in the cathode pores. This is a consequence of a combination of 1) local alkalization due to the electrochemical reaction, 2) generation of (bi)carbonate by chemical reaction of CO₂ with the alkaline electrolyte, and 3) the presence of alkali metal cations. In catholyte-free, zero-gap cells using anion exchange membranes, the presence of electrolyte cations at the cathode is the result of unintended crossover from the anolyte, and a detailed understanding of the factors enabling this crossover is lacking. Here we show that the anolyte concentration governs the flux of cation migration through the membrane, and this substantially influences the behaviors of copper catalysts in catholyte-free CO₂ electrolysers. Systematic variation of the anolyte ionic strength (using aqueous KOH or KHCO₃) correlated with drastic changes in the observed product selectivity – most notably, at low ionic strength, Cu catalysts produced predominantly CO, in contrast to the mixture of C₂₊ products typically observed on Cu. In this talk, we examine the factors influencing ion crossover and the resulting effects on catalyst structure and activity, under conditions of both CO₂ and CO reduction. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy were used to study how the catalyst is affected by operation conditions. Our results show that even in catholyte-free cells, cation effects (including unintended ones) can significantly influence reaction pathways, and this must be considered in future development of catalysts and devices. El-Nagar, G. A., Haun, F., Gupta, S., Stojkovikj, S. & Mayer, M. T. Unintended cation crossover influences CO2 reduction activity in Cu-based zero-gap electrolysers. Nature Communications14, 2062 (2023) doi: doi.org/10.1038/s41467-023-37520-x Figure 1