CO ₂ reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions

Electrocatalytic CO 2 reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO 2 . The field has advanced immensely in recent years, taking significant strides towards commercial realization. Catalyst innovations have played a pivotal role in these advances, with a steady stream of new catalysts providing gains in CO 2 conversion efficiencies and selectivities of both C1 and C2 products. Comparatively few of these catalysts have been tested at commercially-relevant current densities (∼200 mA cm -2 ) due to transport limitations in traditional testing configurations and a research focus on fundamental catalyst kinetics, which are measured at substantially lower current densities. A catalyst's selectivity and activity, however, have been shown to be highly sensitive to the local reaction environment, which changes drastically as a function of reaction rate. As a consequence of this, the surface properties of many CO 2 reduction catalysts risk being optimized for the wrong operating conditions. The goal of this perspective is to communicate the substantial impact of reaction rate on catalytic behaviour and the operation of gas-diffusion layers for the CO 2 reduction reaction. In brief, this work motivates high current density catalyst testing as a necessary step to properly evaluate materials for electrochemical CO 2 reduction, and to accelerate the technology toward its envisioned application of neutralizing CO 2 emissions on a global scale. ChemE/Materials for Energy Conversion & Storag

Photocharged BiVO4 photoanodes for improved solar water splitting

Bismuth vanadate (BiVO4) is a promising semiconductor material for the production of solar fuels via photoelectrochemical water splitting, however, it suffers from substantial recombination losses that limit its performance to well below its theoretical maximum. Here we demonstrate for the first time that the photoelectrochemical (PEC) performance of BiVO4 photoanodes can be dramatically improved by prolonged exposure to AM 1.5 illumination in the open circuit (OC) configuration. Photoanodes subjected to such light treatment achieve a record photocurrent for undoped and uncatalysed BiVO4 of 3.3 mA cm?2 at 1.23 VRHE. Moreover, photoelectrochemical tests with a sacrificial agent yield significantly enhanced catalytic efficiency over the whole operating potential range, suggesting elimination of major losses at the semiconductor–electrolyte interface. Finally, we demonstrate that this so-called ‘photocharging’ technique induces a considerable cathodic shift in the photocurrent onset potential and increases the photovoltage extracted from BiVO4 photoanodes.Chemical EngineeringApplied Science

Photo-assisted water splitting with bipolar membrane induced pH gradients for practical solar fuel devices

Different pH requirements for a cathode and an anode result in a non-optimal performance for practical solar fuel systems. We present for the first time a photo-assisted water splitting device using a bipolar membrane, which allows a cathode to operate in an acidic electrolyte while the photoanode is in alkaline conditions. The bipolar membrane dissociates water into H+ and OH?, which is consumed for hydrogen evolution at the cathode and oxygen evolution at the anode, respectively. The introduction of such a bipolar membrane for solar fuel systems provides ultimate freedom for combining different (photo)cathodes and -anodes. This paper shows that photo-assisted water splitting at both extreme pH gradients (0–14) as well as mild pH gradients (0–7) yields current densities of 2–2.5 mA cm?2 using a BiVO4 photoanode and a bipolar membrane. The membrane potentials are within 30 mV of the theoretical electrochemical potential for low current densities. The pH gradient is maintained for 4 days of continuous operation and electrolyte analysis shows that salt cross-over is minimal. The stable operation of the bipolar membrane in extreme and mild pH gradients, at negligible loss, contributes to a sustainable and practically feasible solar fuel device with existing photoactive electrodes operating at different pH.Chemical EngineeringApplied Science

Assessing Silver Palladium Alloys for Electrochemical CO₂ Reduction in Membrane Electrode Assemblies

The field of electrochemical CO2 reduction has been transitioning to industrially relevant scales by changing the architecture of the electrochemical cells and moving away from the traditional aqueous H-cells to membrane electrode assemblies (MEA). The reaction environments in MEAs vary drastically from that of aqueous H-cells, which could result in significantly different catalytic activity. In this paper, we test AgPd alloys, one of the most promising CO producing catalysts reported, at industrially relevant scales (50 to 200 mA/cm2) in a MEA configuration. We report that, with increasing Pd composition in the electrode, the CO selectivity reduces from 99 % for pure Ag to 73 % for pure Pd at 50 mA/cm2. The MEA configuration helps attain a high CO partial current density of 123 mA/cm2. We find that catalytic activity reported in aqueous H-Cells does not translate at higher current densities and that cell architecture must play an important role in benchmarking catalytic activity.ChemE/Materials for Energy Conversion & Storag

Introductory Guide to Assembling and Operating Gas Diffusion Electrodes for Electrochemical CO ₂ Reduction

ChemE/Materials for Energy Conversion & Storag

Accelerating ¹H NMR Detection of Aqueous Ammonia

Direct electrolytic N2 reduction to ammonia (NH3) is a renewable alternative to the Haber-Bosch process. The activity and selectivity of electrocatalysts are evaluated by measuring the amount of NH3 in the electrolyte. Quantitative 1H nuclear magnetic resonance (qNMR) detection reduces the bench time to analyze samples of NH3 (present in the assay as NH4+) compared to conventional spectrophotometric methods. However, many groups do not have access to an NMR spectrometer with sufficiently high sensitivity. We report that by adding 1 mM paramagnetic Gd3+ ions to the NMR sample, the required analysis time can be reduced by an order of magnitude such that fast NH4+ detection becomes accessible with a standard NMR spectrometer. Accurate, internally calibrated quantification is possible over a wide pH range. ChemE/Materials for Energy Conversion & Storag

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.ChemE/Materials for Energy Conversion & Storag

Hidden Figures of Photo-charging: a thermo-electrochemical approach for a solar-rechargeable redox flow cell system

Achieving high current densities without thermal performance degradation at high temperatures is one of the main challenges for enhancing the competitiveness of photo-electrochemical energy storage systems. We describe a system that overcomes this challenge by incorporating an integrated photoelectrode with a redox flow cell, which functions as a coolant for the excess heat from the photo-absorber. We perform quantitative analyses to theoretically validate and highlight the merit of the system. Practical operation parameters, including daily temperature and redox reaction kinetics, are modeled with respect to heat and charge transfer mechanisms. Our analyses show a profound impact on the resulting solar-to-chemical efficiencies and stored power, which are 21.8% higher than that of a conventional photovoltaic-assisted energy storage system. This paves the way for reassessing the merit of photovoltaic-integrated systems, which have hitherto been underrated as renewable energy storage systems.ChemE/Materials for Energy Conversion & Storag

Introducing special issue on photocatalysis and photoelectrochemistry

ChemE/Materials for Energy Conversion & Storag

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. ChemE/Materials for Energy Conversion & Storag