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Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory -based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining for the production of the two main products of furfural electroreduction on Cu, furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu’s ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction.

The high overpotential required for the oxygen evolution reaction (OER) represents a significant barrier for the production of closed-cycle renewable fuels and chemicals. Ruthenium dioxide is among the most active catalysts for OER in acid, but the activity at low overpotentials can be difficult to measure due to high capacitance. In this work, we use electrochemistry – mass spectrometry to obtain accurate OER activity measurements spanning six orders of magnitude on a model series of ruthenium-based catalysts in acidic electrolyte, quantifying electrocatalytic O2 production at potential as low as 1.30 VRHE. We show that the potential-dependent O2 production rate, i.e., the Tafel slope, exhibits three regimes, revealing a previously unobserved Tafel slope of 25 mV decade−1 below 1.4 VRHE. We fit the expanded activity data to a microkinetic model based on potential-dependent coverage of the surface intermediates from which the rate-determining step takes place. Our results demonstrate how the familiar quantities “onset potential” and “exchange current density” are influenced by the sensitivity of the detection method.

The operating conditions of low pH and high potential at the anodes of polymer electrolyte membrane electrolysers restrict the choice of catalysts for the oxygen evolution reaction (OER) to oxides based on the rare metals iridium or ruthenium. In this work, we investigate the stability of both the metal atoms and, by quantitative and highly sensitive 18O isotope labelling experiments, the oxygen atoms in a series of RuOx and IrOx electrocatalysts during the OER in the mechanistically interesting low overpotential regime. We show that materials based on RuOx have a higher dissolution rate than the rate of incorporation of labelled oxygen from the catalyst into the O2 evolved (“labelled OER”), while for IrOx-based catalysts the two rates are comparable. On amorphous RuOx, metal dissolution and labelled OER are found to have distinct Tafel slopes. These observations together lead us to a full mechanistic picture in which dissolution and labelled OER are side processes to the main electrocatalytic cycle. We emphasize the importance of quantitative analysis and point out that since less than 0.2% of evolved oxygen contains an oxygen atom originating from the catalyst itself, lattice oxygen evolution is at most a negligible contribution to overall OER activity for RuOx and IrOx in acidic electrolyte.

The high overpotential required for the oxygen evolution reaction (OER) represents a significant barrier for the production of closed-cycle renewable fuels and chemicals. Ruthenium dioxide is among the most active catalysts for OER in acid, but the activity at low overpotentials can be difficult to measure due to high capacitance. In this work, we use electrochemistry - mass spectrometry to obtain accurate OER activity measurements spanning six orders of magnitude on a model series of ruthenium-based catalysts in acidic electrolyte, quantifying electrocatalytic O

Electrochemistry-mass spectrometry is a versatile and reliable tool to study the interfacial reaction rates of Faradaic processes with high temporal resolutions. However, the measured mass spectrometric signals typically do not directly correspond to the partial current density toward the analyte due to mass transport effects. Here, we introduce a mathematical framework, grounded on a mass transport model, to obtain a quantitative and truly dynamic partial current density from a measured mass spectrometer signal by means of deconvolution. Furthermore, it is shown that the time resolution of electrochemistry-mass spectrometry is limited by entropy-driven processes during mass transport to the mass spectrometer. The methodology is validated by comparing the measured impulse responses of hydrogen and oxygen evolution to the model predictions and subsequently applied to uncover dynamic phenomena during hydrogen and oxygen evolution in an acidic electrolyte.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO

© 2016 American Chemical Society. The roles of hydrogen impurity and oxygen vacancy defects on defining the conductivity, and hence photoelectrochemical (PEC) performance characteristics, of monoclinic scheelite bismuth vanadate (BiVO4) are investigated using a combination of experiment and theory. We find that elemental hydrogen is present as an impurity in as-synthesized BiVO4and that increasing its concentration by annealing in H2at temperatures up to 290 °C leads to near-complete elimination of majority carrier transport limitations, a beneficial shift in the photoanodic current onset potential, and improved fill factor. Magnetic resonance measurements reveal that hydrogen can be incorporated in at least two different chemical environments, which are assigned to interstitial and substitutional sites. Incorporation of hydrogen leads to a shift of the Fermi level toward the conduction band edge, indicating that n-type character is correlated with increased hydrogen content. This finding is in agreement with theory and reveals that hydrogen acts as a donor in BiVO4. Sub-bandgap photoluminescence is observed from as-synthesized material and is consistent with deep electronic states associated with oxygen vacancies. Hydrogen treatment leads to reduced emission from these states. These findings support the conclusion that hydrogen, rather than oxygen vacancies, is dominant in determining the n-type conductivity of BiVO4. These findings have important implications for controlling the electronic properties and functional characteristics of this promising photoanode material.

Improving understanding of the electrocatalytic mechanisms for the oxygen evolution reaction (OER) will help in designing electrode materials with lower catalytic overpotential and greater stability, increasing the efficiency and economic viability of electrolysis. Isotope-labeling can be a powerful tool for elucidating catalytic mechanisms, including in electrocatalysis. Herein, we apply 18-O labeling, utilizing a fully quantitative electrochemistry - mass spectrometry method with unprecedented sensitivity[1] and post-characterization by low-energy ion scattering spectrometry, to investigate the electrocatalytic mechanism of oxygen evolution. Nickel-iron based electrodes are used in industrial alkaline water electrolysis, but questions remain about the intrinsic activity and electrocatalytic mechanism. By performing a series of isotope-labeling experiments on a model system of mass-selected Ni-Fe nanoparticles, we show that oxygen evolution does not proceed via lattice oxygen exchange and that only the surface of the nanoparticles are active[2]. This allows us to estimate the turn-over frequency of the active sites, 6 O2 molecules per site per second, which is higher than previous estimates. We also quantify lattice oxygen exchange in IrO2 and RuO2 thin films during oxygen evolution in acidic electrolyte as a function of electrochemical roughening, and perform oxygen stripping experiments to probe OER intermediates. This talk will describe these findings and further explore the potential of isotope labeling studies in electrocatalysis. [1] D. B. Trimarco, S. B. Scott et al, Enabling real-time detection of electrochemical desorption phenomena with sub-monolayer sensitivity. Electrochimica Acta. 2018, 268, 520-530 [2] C. Roy, B. Sebok, S. B. Scott et al, Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeOxHy. Nature Catalysis. 2018, 1(11), 820–829 Figure 1