Operando Infrared Spectroscopy Reveals the Dynamic Nature of Semiconductor-Electrolyte Interface in Multinary Metal Oxide Photoelectrodes.

Detailed knowledge about the semiconductor/electrolyte interface in photoelectrochemical (PEC) systems has been lacking because of the inherent difficulty of studying such interfaces, especially during operation. Current understandings of these interfaces are mostly from the extrapolation of ex situ data or from modeling approaches. Hence, there is a need for operando techniques to study such interfaces to develop a better understanding of PEC systems. Here, we use operando photoelectrochemical attenuated total reflection Fourier transform infrared (PEC-ATR-FTIR) spectroscopy to study the metal oxide/electrolyte interface, choosing BiVO4 as a model photoanode. We demonstrate that preferential dissolution of vanadium occurs from the BiVO4/water interface, upon illumination in open-circuit conditions, while both bismuth and vanadium dissolution occurs when an anodic potential is applied under illumination. This dynamic dissolution alters the surface Bi:V ratio over time, which subsequently alters the band bending in the space charge region. This further impacts the overall PEC performance of the photoelectrode, at a time scale very relevant for most lab-scale studies, and therefore has serious implications on the performance analysis and fundamental studies performed on this and other similar photoelectrodes.

In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode.

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

Tailored energy level alignment at MoOX/GaP interface for solar-driven redox flow battery application.

MoOX is commonly considered to be a high work-function semiconductor. From x-ray photoelectron spectroscopy and photo-electrochemical analysis, it is shown that MoOX can be considered as an effective hole transfer layer for the GaP-based device. Specifically, in the absence of carbon contamination using an ion beam cleaning step, the oxygen vacancy derived defect band located inside the bandgap becomes the main charge transfer mechanism. We demonstrate, for the first time, a device with a MoOX/GaP junction that functions as an unbiased photo-charging cell for the redox flow battery system with AQS/AQSH2∥I-/I3 - redox couples. This work has important implications toward enabling MoOX applications beyond the conventional solar cells, including electrochemical energy storage and chemical conversion systems.

In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction.

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure-activity relationships has been complicated, not only due to the ill-defined and intricate morphological and mesoscopic structure of electrocatalysts, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO2 electroreduction on copper electrodes at relatively low current densities, <10 mA/cm2. The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (-1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochemical CO2 reduction to hydrocarbons in CO2 saturated aqueous solutions were evaluated under mass transport limitations on copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure-activity relationships must rule out mass transport effects.

Hydrocarbon Synthesis via Photoenzymatic Decarboxylation of Carboxylic Acids.

A recently discovered photodecarboxylase from Chlorella variabilis NC64A ( CvFAP) bears the promise for the efficient and selective synthesis of hydrocarbons from carboxylic acids. CvFAP, however, exhibits a clear preference for long-chain fatty acids thereby limiting its broad applicability. In this contribution, we demonstrate that the decoy molecule approach enables conversion of a broad range of carboxylic acids by filling up the vacant substrate access channel of the photodecarboxylase. These results not only demonstrate a practical application of a unique, photoactivated enzyme but also pave the way to selective production of short-chain alkanes from waste carboxylic acids under mild reaction conditions.

In-Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2 : Theory, Practice and Challenges.

The field of electrochemical CO2 conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO2 have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO2 are reviewed, including challenges associated with the detection of reaction intermediates.

Light induced formation of a surface heterojunction in photocharged CuWO4 photoanodes.

Photocharging has recently shown the ability to significantly improve the performance of several metal oxide photoanodes, similar to the enhancements achieved with co-catalysts and passivation overlayers. Herein, we demonstrate the effect of photocharging on CuWO4 photoanodes for the first time, with prolonged AM 1.5 illumination under open-circuit conditions. The photocharging treatment on CuWO4 samples doubled the photocurrent obtained at 1.23 VRHE. This enhancement is attributed to the light induced formation of a surface bound copper complex with the solution anion species in the electrolyte. This thin semiconducting copper borate layer forms a heterojunction with the CuWO4, improving the charge separation near the surface and thus suppressing the recombination of charge carriers in the space charge region. The striking similarities in photocharging of different metal oxide semiconductors highlights that the metal oxide semiconductor-electrolyte interface is more complex than previously understood. The formation of this time-dependent light induced surface layer should therefore be considered in all experimental studies on photo-electrochemistry with metal oxide semiconductor photoanodes.

Lateral Adsorbate Interactions Inhibit HCOO- while Promoting CO Selectivity for CO2 Electrocatalysis on Silver.

Ag is a promising catalyst for the production of carbon monoxide (CO) via the electrochemical reduction of carbon dioxide (CO2 ER). Herein, we study the role of the formate (HCOO- ) intermediate *OCHO, aiming to resolve the discrepancy between the theoretical understanding and experimental performance of Ag. We show that the first coupled proton-electron transfer (CPET) step in the CO pathway competes with the Volmer step for formation of *H, whereas this Volmer step is a prerequisite for the formation of *OCHO. We show that *OCHO should form readily on the Ag surface owing to solvation and favorable binding strength. In situ surface-enhanced Raman spectroscopy (SERS) experiments give preliminary evidence of the presence of O-bound bidentate species on polycrystalline Ag during CO2 ER which we attribute to *OCHO. Lateral adsorbate interactions in the presence of *OCHO have a significant influence on the surface coverage of *H, resulting in the inhibition of HCOO- and H2 production and a higher selectivity towards CO.

Selective and Efficient Reduction of Carbon Dioxide to Carbon Monoxide on Oxide-Derived Nanostructured Silver Electrocatalysts.

In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide-derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide-derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO2 to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH(.) intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO2 with suppressed H2 evolution.

Controllable Hydrocarbon Formation from the Electrochemical Reduction of CO2 over Cu Nanowire Arrays.

In this work, the effect of Cu nanowire morphology on the selective electrocatalytic reduction of CO2 is presented. Cu nanowire arrays were prepared through a two-step synthesis of Cu(OH)2 and CuO nanowire arrays on Cu foil substrates and a subsequent electrochemical reduction of the CuO nanowire arrays to Cu nanowire arrays. By this simple synthesis method, Cu nanowire array electrodes with different length and density were able to be controllably synthesized. We show that the selectivity for hydrocarbons (ethylene, n-propanol, ethane, and ethanol) on Cu nanowire array electrodes at a fixed potential can be tuned by systematically altering the Cu nanowire length and density. The nanowire morphology effect is linked to the increased local pH in the Cu nanowire arrays and a reaction scheme detailing the local pH-induced formation of C2  products is also presented by a preferred CO dimerization pathway.

In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity.

Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni(Fe)-B(i)) and nickel oxyhydroxide (Ni(Fe)OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO(-)), which can be described as adsorbed "active oxygen". Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negatively charged surface sites that act as OER precursors. This work emphasizes the relevance of the electrolyte effect to obtain catalytically active phases in Ni-based OECs, in addition to the key role of the Fe impurities. This effect should be carefully considered in the development of Ni-based compounds meant to catalyze the OER at moderate pHs. Complementarily, UV-vis spectroscopy measurements show strong darkening of those catalysts in the catalytically active state. This coloration effect is directly related to the oxidation of nickel and can be an important factor limiting the efficiency of solar-driven devices utilizing Ni-based OECs.

Improved charge separation via Fe-doping of copper tungstate photoanodes.

Photoelectrochemical (PEC) water splitting offers a clean pathway to renewable and sustainable energy in the near future. The key to improving the efficiency of PEC devices is the ability to find materials with suitable optoelectronic properties, and identifying, then overcoming their limitations. In this paper, we explore the photoelectrochemical performance of CuWO4 photoanodes for solar water splitting, and find that charge separation is the dominant limitation for this material. As a result, we attempt to dope the material with Fe, the first such example of doping this semiconductor for PEC water splitting. An improvement in performance is achieved for doped films which show 1.5 times the photocurrent density and a 50% higher charge separation efficiency at 1.23 V vs. RHE.

Selective electrochemical reduction of CO2 to CO on CuO-derived Cu nanowires.

In this work, we report a new synthesis method to prepare a Cu nanowire electrocatalyst for selective CO2 reduction at room temperature and atmospheric pressure. Cu nanowire array electrodes were prepared through a two-step synthesis of Cu(OH)2 and CuO nanowire arrays on Cu foil substrates and a subsequent electrochemical reduction of the CuO nanowire arrays. The Cu nanowire arrays are able to electrochemically reduce CO2 to CO with a faradaic efficiency of ∼50% at a moderate overpotential of 490 mV, which is significantly higher than that of polycrystalline Cu foil catalysts at identical conditions. The improved faradaic efficiency for the reduction of CO2 to CO is ascribed to the enhanced stabilization for the CO2˙(-) intermediate on the high surface area Cu nanowire arrays.

Layered Sn-Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement.

Operando electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (EC ATR-SEIRAS) is a valuable method for a fundamental understanding of electrochemical interfaces under real operating conditions. The applicability of this method depends on the ability to tune the optical and catalytic properties of an electrode film, and it thus requires unique optimization for any given material. Motivated by the growing interest in Sn-based electrocatalysts for selective reduction of CO2 to formate species, we investigate several Sn thin-film synthesis routes for the resulting SEIRA signal response. We compare the SEIRA performance of thermally evaporated metallic Sn to a series of Sn-based films on top of a SEIRA-active Au substrate (metallic Sn, oxide-derived metallic Sn, and metal oxide SnOx). Using alkanethiol self-assembled monolayers as a probe, we find that electrodepositing metallic catalyst films on top of SEIRA-active Au substrates yield higher signal relative to thermal evaporation as well as higher signal than the independent SEIRA-active Au underlayer. These observations come despite the fact that thermally evaporated Sn has a significantly higher surface roughness (and thus higher adsorbate population), suggesting specific SEIRA-magnifying effects for the stacked films. Finally, we applied these films to observe the electrochemical conversion of CO2. Differences are observed in spectral features based on the composition of the electrode being either metallic or oxide-derived metallic Sn, implying differences in their respective reaction pathways.

Insights into Electrochemical CO2 Reduction on Metallic and Oxidized Tin Using Grand-Canonical DFT and In Situ ATR-SEIRA Spectroscopy.

Electrochemical CO2 reduction (CO2R) to formate is an attractive carbon emissions mitigation strategy due to the existing market and attractive price for formic acid. Tin is an effective electrocatalyst for CO2R to formate, but the underlying reaction mechanism and whether the active phase of tin is metallic or oxidized during reduction is openly debated. In this report, we used grand-canonical density functional theory and attenuated total reflection surface-enhanced infrared absorption spectroscopy to identify differences in the vibrational signatures of surface species during CO2R on fully metallic and oxidized tin surfaces. Our results show that CO2R is feasible on both metallic and oxidized tin. We propose that the key difference between each surface termination is that CO2R catalyzed by metallic tin surfaces is limited by the electrochemical activation of CO2, whereas CO2R catalyzed by oxidized tin surfaces is limited by the slow reductive desorption of formate. While the exact degree of oxidation of tin surfaces during CO2R is unlikely to be either fully metallic or fully oxidized, this study highlights the limiting behavior of these two surfaces and lays out the key features of each that our results predict will promote rapid CO2R catalysis. Additionally, we highlight the power of integrating high-fidelity quantum mechanical modeling and spectroscopic measurements to elucidate intricate electrocatalytic reaction pathways.

Closing the Loop: Unexamined Performance Trade-Offs of Integrating Direct Air Capture with (Bi)carbonate Electrolysis.

CO2 from carbonate-based capture solutions requires a substantial energy input. Replacing this step with (bi)carbonate electrolysis has been commonly proposed as an efficient alternative that coproduces CO/syngas. Here, we assess the feasibility of directly integrating air contactors with (bi)carbonate electrolyzers by leveraging process, multiphysics, microkinetic, and technoeconomic models. We show that the copresence of CO32- with HCO3- in the contactor effluent greatly diminishes the electrolyzer performance and eventually results in a reduced CO2 capture fraction to ≤1%. Additionally, we estimate suitable effluents for (bi)carbonate electrolysis to require 5-14 times larger contactors than conventionally needed contactors, leading to unfavorable process economics. Notably, we show that the regeneration of the capture solvent inside (bi)carbonate electrolyzers is insufficient for CO2 recapture. Thus, we suggest process modifications that would allow this route to be operationally feasible. Overall, this work sheds light on the practical operation of integrated direct air capture with (bi)carbonate electrolysis.

Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene.

This work utilizes EIS to elucidate the impact of catalyst-ionomer interactions and cathode hydroxide ion transport resistance (RCL,OH-) on cell voltage and product selectivity for the electrochemical conversion of CO to ethylene. When using the same Cu catalyst and a Nafion ionomer, varying ink dispersion and electrode deposition methods results in a change of 2 orders of magnitude for RCL,OH- and ca. a 25% change in electrode porosity. Decreasing RCL,OH- results in improved ethylene Faradaic efficiency (FE), up to ∼57%, decrease in hydrogen FE, by ∼36%, and reduction in cell voltage by up to 1 V at 700 mA/cm2. Through the optimization of electrode fabrication conditions, we achieve a maximum of 48% ethylene with >90% FE for non-hydrogen products in a 25 cm2 membrane electrode assembly at 700 mA/cm2 and <3 V. Additionally, the implications of optimizing RCL,OH- is translated to other material requirements, such as anode porosity. We find that the best performing electrodes use ink dispersion and deposition techniques that project well into roll-to-roll processes, demonstrating the scalability of the optimized process.