The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, like those we recently invented. Systems using low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently operates at low voltages through use of an ensemble of optically thin photoabsorbers each operating at a low rate. Collectively the ensemble exhibits larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 and BiVO4 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively, our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.
Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers�such as pinholes, cracks, or particle inclusions�may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiO x ) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions�the hydrogen evolution and Fe(III) reduction reactions�can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60−90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiO x overlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Finally, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.
Similar to natural photosynthesis, Z-scheme photocatalytic water splitting relies on two different light absorbing components that are coupled by a redox active mediator that shuttles charge between them. Such a two-absorber system possesses several advantages over single absorber photocatalytic system, including higher theoretical solar-to-hydrogen conversion efficiency, relaxed band alignment requirements, and the potential for inherently safe operation whereby H2 and O2 evolution occur in separated compartments. However, a major disadvantage and challenge for Z-scheme photocatalysis is that the presence of a redox mediator introduces two undesirable back-reactions on top of parasitic H2 oxidation and O2 reduction reactions that can occur in a single absorber photocatalytic system. Previous research efforts have identified the use of semi-permeable oxide coatings as an attractive approach to suppress these thermodynamically favored redox reactions while still permitting the desired water splitting and mediator redox reactions to occur. Here, we present a combined experimental and computational approach based on model thin films that is used to (i) probe the performance limits of oxide-encapsulated photocatalysts, (ii) quantify the effects of coating defects on performance, and (iii) guide the rational design of coatings aimed at maximizing the solar-to-hydrogen conversion efficiency of a target photocatalytic system. This work specifically focusses on the development of silicon and titanium oxide coatings for Z-scheme water splitting based on a Fe(II)/Fe(III) mediator, showing that the best coatings can achieve selectivities > 90 % towards the H2 and O2 evolution reactions over undesired Fe(II)/Fe(III) back reactions. Another key finding from this work is that coating defects can have a significant influence on the performance of encapsulated electrodes, as revealed by scanning electrochemical microscopy (SECM) measurements that were used to locally quantify the parasitic back reaction rates around individual defects to determine their impact on the global selectivity of an encapsulated electrode.
The U.S. Department of Energy recently announced its first Energy Earthshot on Clean Hydrogen, with a cost target of $1/kg-H2 by 2031. Assuming future utility-scale grid electricity prices from photovoltaics ($0.02/kWh), 80% of the cost of H2 would come from performing low-temperature water electrolysis at its thermoneutral voltage, with zero additional overpotential. This fact motivates alternative, less-expensive means of using light to generate mobile charge carriers than photovoltaics, and reactor designs with exceedingly low capital costs, which we recently invented. Systems that use low capital cost reactors benefit from low-voltage operation, which represents a paradigm shift from current state-of-the-art electrolyzers that aim to operate at high current densities. Analytical models predict that solar photocatalytic water splitting inherently exhibits such low-voltage operation through use of an ensemble of optically thin photoabsorbers, which results in larger overall solar-to-hydrogen conversion efficiencies in comparison to optically thick designs. In efforts to attain these predicted higher efficiencies, we are performing detailed studies on the properties of state-of-the-art doped SrTiO3 photocatalyst particles. During my talk, I will share our recent efforts in atomic-layer deposited ultrathin oxide coatings to impart redox selectivity and materials stability, single-photocatalyst-particle current–potential behavior and mobile charge carrier properties, and atomic-level information on dopant distributions and materials interfaces obtained from electron microscopies and X-ray spectroscopies. Collectively our discoveries provide new design guidelines and additional research pathways for the development of effective composite materials to serve as active components in techno-economically viable artificial photosynthetic devices.
Application of ultrathin oxide encapsulation layers on co-catalysts employed in z-scheme photocatalysis has the potential to increase overall efficiency through the prevention of undesirable back reactions and increased charge separation. [1,2] However, defects within the semipermeable oxide coatings – such as pinholes, cracks or particle protrusions – have been postulated to facilitate locally high rates of undesirable reactions by creating pathways for facile transport of undesired reactants to exposed active sites.[3] Local probe measurements, such as scanning electrochemical microscopy (SECM), can estimate the relative rates of production of product species generated from competing electrochemical reactions with high spatial resolution over the electrode. This can be leveraged to determine the influence of local defects on global performance metrics, such as the apparent permeability of the overlayer and selectivity of the electrode. In this presentation, we report the use of SECM to determine the influence of overlayer defects on the performance of a model silicon oxide (SiOx) -encapsulated Pt thin film electrocatalyst when operated under conditions where two competing reactions can occur. Motivated by Z-scheme photocatalysis, the hydrogen evolution reaction (HER) and Fe(III)/Fe(II) redox reaction were studied. After introducing new methodology to determine local selectivity towards HER using SECM tip current, the resulting selectivity maps are compared against defects seen in optical images, scanning electron micrographs, and atomic force microscopy images. This analysis reveals that that certain types of defects in the oxide overlayer can be responsible for a large percentage of the partial current density towards the undesired Fe(III) reduction reaction. Correcting for the defect contributions to the undesired reaction, it is determined that the true Fe(III) permeability values for the SiOx overlayers are almost two orders of magnitude lower than permeabilities determined from conventional analysis that ignored the presence of defects. Finally, different types of defects were studied revealing that the defect morphology can have varying influence on both the selectivity and calculated permeability. This work highlights the need for local measurements in addition to macroscopic measurements of oxide encapsulated catalysts as they are applied to more complex geometries such as particle photocatalysts. References [1] Qi, Y. et al. Applied Catalysis B: Environmental 224, 579–585 (2018). [2] T. Zhao, et al., PNAS, 2021, 118 (7), [3] N. Y. Labrador, et al. , ACS Catalysis, 2018, vol. 8, 1767–1778.
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