Modeling of Concurrent CO₂and Water Splitting by Practical Photoelectrochemical Devices

Abstract

Concurrent solar generation of hydrogen and CO through photoelectrochemical (PEC) water and CO 2 electrolysis, and the subsequent use of the product gas mixture in conventional Fischer-Tropsch processes, has the potential to provide a flexible pathway for direct solar generation of a variety of liquid fuels. In order for this approach to be practical, PEC devices must be designed to continuously and selectively provide a well-defined ratio of hydrogen to CO, independent of operating conditions. We develop a computational PEC device model providing insight into the dynamics and design requirements of such a device. We investigate a variety of combinations of catalysts (Ag, Cu, Ni, Pt, Co) and photoabsorbers (Si and Ga-based) under steady and transient solar irradiation conditions. Typical H 2 /CO ratios of 0.1 were observed for Ag-based electrodes, and ratios of 5 when using Cu-based electrodes. Variation in catalyst and photoabsorber properties provided guidance for the development of catalysts allowing for a H 2 /CO product ratio close to 2. Device design variations and the addition of Ni as a second cathode-side catalyst improved the generation of hydrogen, allowing H 2 /CO ratios to reach between 1.7 and 2.15. Transient simulations showed that product ratios vary significantly over the day and year, implying the use of storage or controlling measures or the addition of a water gas shift reactor. Our model provides insights and practical considerations for the design and implementation of a PEC device for the concurrent production of hydrogen and CO. Solar liquid fuel production can potentially be achieved by the reduction of water and CO 2 through two non-biological pathways: [1][2][3][4] i) solar thermochemical (STC) approaches, and ii) photoelectrochemical (PEC) approaches. In both cases, water and CO 2 is split into hydrogen and CO (a mixture called synthesis gas) and, for PEC approaches only, other lower order hydrocarbon products. Synthesis gas can subsequently be used in a Fischer-Tropsch process to produce liquid fuels such as gasoline and diesel. In STC approaches, redox couples are used and alternatingly reduced then oxidized at temperatures above 1000 K while splitting water and CO 2 . Synthesis gas is very selectively produced at a tailorable hydrogen to CO ratio. 5In PEC approaches, water electrolysis has been achieved, 6 and the reduction of CO 2 has been demonstrated, 7-10 but with challenges in respect to product selectivity and efficiency. While the PEC production of CO requires the involvement of two electrons only, methanol or higher order hydrocarbon products require the involvement of six, eight, or even twelve electrons (Eqs. 5 and 6), presenting significant challenges for the development of efficient and selective catalysts. Assuming that STC and PEC approaches can be complementary solar fuel processing pathways, synthesis gas appears to be the obvious fuel choice, as it can be produced by both pathways (comparatively easy in the case of PEC compared to other products) and would subseque...

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

“…These parameters are typically the most sensitive when fitting and facilitating agreement between the polarization curves or product distributions collected experimentally and those simulated through continuum modeling. Figure displays a distribution of Butler–Volmer kinetic parameters for electrochemical CO 2 reduction on various catalysts (Cu, Ag, Au, and Sn) employed in various continuum models of electrochemical CO 2 reduction; ,,,,,,,,− the specific values used for these figures can be found in Table . When examining the fit parameters in these distributions, the spread in the fit values, even across a single catalyst, is quite large, even though some variation in these parameters is expected due to the drastically different experimental architectures and conditions (e.g., the choice of the electrolyte cation, which greatly impacts selectivity). ,− For instance, in the case of CO 2 reduction to ethanol on Cu, there is a variance in the fit exchange current densities of 25 orders of magnitude, and the transfer coefficients vary from 0.02 to 1.1.…”

Silver Nanowire/Carbon Sheet Composites for Electrochemical Syngas Generation with Tunable H2/CO Ratios

“…For instance, the optimal H 2 /CO ratio for syngas to generate hydrocarbons is 1.7 when using an iron-based catalyst and 2.15 when using a cobalt-based catalyst. 7,8…”

“…In the F–T process, the ratio of syngas is crucial to maximize the product yield. For instance, the optimal H 2 /CO ratio for syngas to generate hydrocarbons is 1.7 when using an iron-based catalyst and 2.15 when using a cobalt-based catalyst. , …”

“…For instance, the optimal H 2 /CO ratio for syngas to generate hydrocarbons is 1.7 when using an iron-based catalyst and 2.15 when using a cobalt-based catalyst. 7,8 Metal catalysts such as Au, Ag, and Zn can be used as electrochemical catalysts to reduce CO 2 to CO in aqueous solutions. 5,9 With these metal catalysts, syngas can be directly produced by electrochemical reduction of CO 2 to CO in aqueous solutions because electrochemical CO 2 RR in an aqueous solution always accompanies the H 2 evolution reaction (HER) as a competitive reaction.…”

Performance Limits of Photoelectrochemical CO2 Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

“…However, it is worth noting that even in this case, hydrogen is still the main product. The complicated details of polarization losses and other cell dependent nonidealities have been modeled in detail for PEC-based CO 2 reducing cells. , …”