High Rate Electrochemical Reduction of Carbon Monoxide to Ethylene using Cu-Nanoparticle-Based Gas Diffusion Electrodes

Gas diffusion electrodes (GDEs) with high electrochemically active surface areas (ECSAs) and triple-phase boundaries for efficient gas, electron, and ion transport offer a unique opportunity for high-rate electrochemical CO reduction (COR) in relative to traditional aqueous configurations. Cu-nanoparticle-based GDEs were fabricated by applying a mixture of carbon powders, copper acetate aqueous solution, and Teflon onto a Cu gauze substrate. The catalyst-coated substrate was air-dried, mechanically pressed, and subsequently annealed under forming gas to produce GDEs. Two distinctive types of GDE configurations, a flow-through configuration and a flow-by configuration, were constructed, characterized, and tested to quantitatively evaluate the effects of reactant gas transport on the activity and the selectivity of the GDE materials for COR. In the flow-through configuration, a high partial current density of 50.8 mA cm^(–2) for COR to C_2H_4 was achieved at −0.85 V vs RHE in 10 M KOH at −15 °C, while in the flow-by configuration with the same catalyst materials the partial current density for C_2H_4 generation was limited to <1 mA cm^(–2)

Modeling an integrated photoelectrolysis system sustained by water vapor

Two designs for an integrated photoelectrolysis system sustained by water vapor have been investigated using a multi-physics numerical model that accounts for charge and species conservation, electron and ion transport, and electrochemical processes. Both designs leverage the use of a proton-exchange membrane that provides conductive pathways for reactant/product transport and prevents product crossover. The resistive losses, product gas transport, and gas crossovers as a function of the geometric parameters of the two designs have been evaluated systematically. In these designs, minimization of pathways in the membrane that can support the diffusive transport of product gases from the catalyst to the gas-collecting chamber was required to prevent supersaturation of hydrogen or oxygen gases at the Nafion/catalyst interface. Due to the small, thin membrane layer that was required, a small electrode width (<300 μm) was also required to produce low resistive losses in the system. Alternatively, incorporation of a structured membrane that balances the gas transport and ionic transport allows the maximum electrode width to be increased to dimensions as large as a few millimeters. Diffusive gas transport between the cathode and anode was the dominant source for crossover of the product gases under such circumstances. The critical dimension of the electrode required to produce acceptably low rates of product crossover was also investigated through the numerical modeling and device simulations

Modeling the Performance of A Flow-Through Gas Diffusion Electrode for Electrochemical Reduction of CO or CO₂

A flow-through gas diffusion electrode (GDE) consisting of agglomerate catalysts for CO or CO₂ reduction, gas channels for reactants, aqueous electrolytes for ionic transport, and metallic current collectors was simulated and evaluated using a numerical model. The geometric partial current densities and Faradaic Efficiencies (FE) for CH₄, C₂H₄ and H₂ generation in GDEs were calculated and compared to the behavior of analogous aqueous-based planar electrodes. The pH-dependent kinetics for CH₄ and C₂H₄ generation were used to represent the intrinsic catalytic characteristics for the agglomerate catalyst. The modeling indicated that relative to planar electrodes for either CO reduction (COR) or CO₂ reduction (CO₂R), substantial increases in electrochemical reduction rates and Faradaic efficiencies are expected when flow-through GDEs are used. The spatially resolved pH and reaction rates within the flow-through GDEs were also simulated for two different operating pHs, and the resulting transport losses were analyzed quantitatively. For CO₂ reduction, substantial loss of CO₂ via chemical reaction with the locally alkaline electrolyte was observed due to the increased pH in operating GDEs

Modeling the Performance of A Flow-Through Gas Diffusion Electrode for Electrochemical Reduction of CO or CO₂

A flow-through gas diffusion electrode (GDE) consisting of agglomerate catalysts for CO or CO₂ reduction, gas channels for reactants, aqueous electrolytes for ionic transport, and metallic current collectors was simulated and evaluated using a numerical model. The geometric partial current densities and Faradaic Efficiencies (FE) for CH₄, C₂H₄ and H₂ generation in GDEs were calculated and compared to the behavior of analogous aqueous-based planar electrodes. The pH-dependent kinetics for CH₄ and C₂H₄ generation were used to represent the intrinsic catalytic characteristics for the agglomerate catalyst. The modeling indicated that relative to planar electrodes for either CO reduction (COR) or CO₂ reduction (CO₂R), substantial increases in electrochemical reduction rates and Faradaic efficiencies are expected when flow-through GDEs are used. The spatially resolved pH and reaction rates within the flow-through GDEs were also simulated for two different operating pHs, and the resulting transport losses were analyzed quantitatively. For CO₂ reduction, substantial loss of CO₂ via chemical reaction with the locally alkaline electrolyte was observed due to the increased pH in operating GDEs

Evaluation and optimization of mass transport of redox species in silicon microwire-array photoelectrodes

Physical integration of a Ag electrical contact internally into a metal/substrate/microstructured Si wire array/oxide/Ag/electrolyte photoelectrochemical solar cell has produced structures that display relatively low ohmic resistance losses, as well as highly efficient mass transport of redox species in the absence of forced convection. Even with front-side illumination, such wire-array based photoelectrochemical solar cells do not require a transparent conducting oxide top contact. In contact with a test electrolyte that contained 50 mM/5.0 mM of the cobaltocenium^(+/0) redox species in CH_3CN–1.0 M LiClO_4, when the counterelectrode was placed in the solution and separated from the photoelectrode, mass transport restrictions of redox species in the internal volume of the Si wire array photoelectrode produced low fill factors and limited the obtainable current densities to 17.6 mA cm^(-2) even under high illumination. In contrast, when the physically integrated internal Ag film served as the counter electrode, the redox couple species were regenerated inside the internal volume of the photoelectrode, especially in regions where depletion of the redox species due to mass transport limitations would have otherwise occurred. This behavior allowed the integrated assembly to operate as a two-terminal, stand-alone, photoelectrochemical solar cell. The current density vs. voltage behavior of the integrated photoelectrochemical solar cell produced short-circuit current densities in excess of 80 mA cm^(-2) at high light intensities, and resulted in relatively low losses due to concentration overpotentials at 1 Sun illumination. The integrated wire array-based device architecture also provides design guidance for tandem photoelectrochemical cells for solar-driven water splitting

Principles and implementations of electrolysis systems for water splitting

Efforts to develop renewable sources of carbon-neutral fuels have brought a renewed focus to research and development of sunlight-driven water-splitting systems. Electrolysis of water to produce H_2 and O_2 gases is the foundation of such systems, is conceptually and practically simple, and has been practiced both in the laboratory and industrially for many decades. In this Focus article, we present the fundamentals of water splitting and describe practices which distinguish commercial water-electrolysis systems from simple laboratory-scale demonstrations

Evaluation of flow schemes for near-neutral pH electrolytes in solar-fuel generators

The electrochemical performance of three different types of membrane-containing electrolyte-flow schemes for solar-driven water splitting has been studied quantitatively using 1-dimensional and 2-dimensional multi-physics models. The three schemes include a recirculation scheme with a well-mixed bulk electrolyte, a recirculation scheme with laminar flow fields, and a fresh-feed scheme with laminar flow fields. The Nernstian potential loss associated with pH gradients at the electrode surfaces, the resistive loss between the cathode and anode, the product-gas crossovers, and the required pumping energy in all three schemes have been evaluated as a function of the operational current density, the flow rates for the electrolyte, and the physical dimensions of the devices. The trade-offs in the voltage loss, safety considerations, and energy inputs from the balance-of-systems required to produce a practical device have been evaluated and compared to membrane-free devices as well as to devices that operate at extreme pH values

A sensitivity analysis to assess the relative importance of improvements in electrocatalysts, light absorbers, and system geometry on the efficiency of solar-fuels generators

A sensitivity analysis has been performed for a variety of generic designs for solar-fuels generators. The analysis has revealed the relative importance of reductions in the overpotentials of electrocatalysts, of improvements in the materials properties of light absorbers, and of optimization in the system geometry for various different types of solar-fuels generators, while considering operation at a range of temperatures as well as under a variety of illumination intensities including up to 10-fold optical concentration