Performance Modeling of Simultaneous CO2 and Water Electrolysis By Practical Photo-Electrochemical Devices

The increasing demand of transportation fuels and challenges associated with the combustion of fossil fuels (emission, pollution) has led to the search for alternative sustainable fuels and fuel processing routes. The solar-driven production of synthetic fuels like methanol or kerosene can provide a solution to these problems if a carbon-neutral approach is chosen, i.e. the carbon-based source is recycled in the production-consumption cycle by capturing the CO₂ directly from the air or concentrated sources such as exhaust of large power plants. These fuel synthesis routes are based on photo-electrochemical (PEC) water and CO₂ electrolysis and subsequent reaction of the synthesis gas (a mixture of H₂ and CO) in a Fischer-Tropsch (FT) process for the production of liquid hydrocarbons. FT reactions can produce hydrocarbon fuels only when an appropriate mixture of H₂ and CO is provided (H₂/CO ratios of 1.7 and 2.15 when FT reactors with iron and cobalt-based catalyst are used, respectively). The production of this exact mixture requires a well-designed PEC device incorporating water and CO₂ splitting catalysts, or a combination of individual water-splitting PEC devices and CO₂-splitting PEC devices. We expect that the simultaneous conversion of CO₂ and water in a single device –if precisely designed– will operate at a reduced cost and a higher efficiency. We aim to provide quantifiable device design guidelines.

A numerical multi-physics model of a characteristic PEC device for simultaneous CO₂ and water splitting was developed. The catalysts were modeled by the Butler-Volmer correlations, for which exchange current densities, charge transfer coefficients, and limiting current densities were extracted from published experiments. Since most experimental studies focus on only one half-reaction, challenges were posed by finding operational conditions (electrolyte, temperature) simultaneously sustained by all components necessary for a working device. Current-potential performance characteristics of realistic PV cells were obtained by experimental fitting, and of idealized PV cells were obtained by the Shockley-Queisser limit and incorporating series and shunt resistances to account for transport losses. Steady and transient solar irradiation measured in Barstow-California were used for the calculations. The electrolyte and product-separating ion-conducting membrane were modeled as ohmic resistances. Electrolytes considered were 1M NaOH and 1M KOH for alkaline solutions, and 0.1M KHCO₃ for pH-neutral solutions.

Existing photoabsorbers and electrocatalyst materials were used for the investigation in order to assess the possibility of producing syngas at high efficiency and in the desired H₂/CO ratio using readily available components.

PEC devices using silver-cobalt catalysts in alkaline solution showed low and almost constant H₂/CO ratios of around 0.13, regardless of the photoabsorber combination used. Silver-platinum catalysts showed H₂/CO ratios below 0.13. Copper-platinum catalyst in pH-neutral solution showed H₂/CO ratios between 5 and 11 but a lower solar-to-product efficiencies compared to devices operating in alkaline solutions due to larger reaction overpotentials and ohmic losses in pH-neutral environments.

The influence of the photoabsorber choice on the performance of the PEC device using silver-cobalt catalysts was analyzed in-depth by considering hypothetical components made of dual-junction solar cells composed of materials with different band gaps, and different series and shunt resistances. The performance was improved as the band gap of the bottom cell was smaller. This, performance increase was limited when further reducing the bottom cell band gap due to similar generation rates of H₂ and CO at high operating currents, and due to mass transfer limitations in the catalyst. The desired catalytic characteristics (charge transfer coefficient and exchange current density) required to produce H₂/CO ratios of about 2 were quantified in order to guide the research of suitable-catalysts.

As an engineering solution to increase the H₂/CO ratio, the addition of a hydrogen-selective catalyst (i.e. nickel) was investigated. The positioning and the amount of this second catalyst was analyzed, allowing to provide design guidelines for reaching H₂/CO ratios of around 2 at minimized cost.

Finally, a silver-nickel-cobalt PEC device was isothermally modeled under realistic, transient irradiation conditions. The results showed a large variation of the product ratio during the day and year, which provided evidence that adequate on-site H₂ and CO storage options are required to smooth these variations. During cold seasons an average daily H₂/CO ratio of 1.66 was achieved (see figure 1-left side), whilst in warm seasons an average daily H₂/CO ratio of 2.24 was obtained (see figure 1-rigth side).

The developed model proved to be useful for the analysis and the optimization of PEC devices for simultaneous CO₂ and water splitting under different operating conditions and material choices, and allowed for the formulation of design guidelines for practical device development.