Concurrent solar generation of hydrogen and CO through photoelectrochemical (PEC) water and CO2 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 awell-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 H2/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 H2/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 H2/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.