The absolute band-edge potentials of semiconductors and their positions relative to solution redox potentials are often invoked as design principles for photoelectrochemical devices and particulate photocatalysts. Here we show that these criteria are not necessary and limit the exploration of materials that may advance the fields of photoelectrochemistry, photochemistry and photocatalysis. We discuss how band-edge energies are not singular parameters and instead shift with pH, electrolyte type and surface chemistry. The free energies of electrons and holes, rather than those of solution redox couples, dictate overall reaction spontaneity and thus reactivity. Favourable charge-transfer kinetics can occur even when the relevant electrolyte redox potential(s) appear outside the bandgap, enabled by the inversion or accumulation of electronic charge at the semiconductor surface. This discussion informs design principles for photocatalytic systems engineering for both one-electron and multi-electron redox reactions (for example, H2 evolution, H2O oxidation and CO2 reduction). (Figure presented.)

Integrated solar fuels devices for CO2 reduction (CO2R) are a promising technology class towards achieving net-negative carbon emissions. Designing integrated CO2R solar fuels devices requires careful co-design of electrochemical and photovoltaic components as well as consideration of the diurnal and seasonal effects of solar irradiance, temperature, and other meteorological factors expected for ‘on-sun’ deployment. Using a photovoltaic-electrochemical (PV-EC) platform, we developed a temperature and potential-dependent diurnal and annual model using experimental CO2R performance of Cu-based electrocatalysts, local meteorological data from the National Solar Radiation Database (NSRD), and modeled performance of commercial c-Si PVs. We simulated diurnal product outputs with and without the effects of ambient temperature to determine gaseous product temperature sensitivity. From these outputs, we observed seasonal variation in gaseous product generation, with up to two-fold increases in ethylene productivity between the Winter and Summer, analyzed the consequences of dynamic cloud coverage, and identified periods where device cooling/heating mechanisms could be implemented to maximize ethylene generation. Finally, we modeled the annual ethylene generation for a scaled 1 MW solar farm at three different locations (Beijing, CN; Sydney, AUS; Barstow, CA) to determine the consequences of local meteorological climates on PV-EC CO2R product output, recording a maximum ethylene output of 18.5 tonne/yr at Barstow. Overall, this model presents a critical tool for streamlining the translation of experimental solar-driven electrochemical research to real-world implementation.

Generation of H2 and O2 from untreated water sources represents a promising alternative to ultrapure water required in contemporary proton exchange membrane-based electrolysis. Bipolar membrane-based devices, often used in electrodialysis and CO2 electrolysis, facilitate impure water electrolysis via the simultaneous mediation of ion transport and enforcement of advantageous microenvironments. Herein, we report their application in direct seawater electrolysis; we show that upon introduction of ionic species such as Na+ and Cl− from seawater, bipolar membrane electrolyzers limit the oxidation of Cl− to corrosive OCl− at the anode to a Faradaic efficiency (FE) of 0.005%, while proton exchange membrane electrolyzers under comparable operating conditions exhibit up to 10% FE to Cl− oxidation. The effective mitigation of Cl− oxidation by bipolar membrane electrolyzers underpins their ability to enable longer-term seawater electrolysis than proton exchange membrane assemblies by a factor of 140, suggesting a path to durable seawater electrolysis.

Integrated solar fuels devices for CO2 reduction (CO2R) are a promising technology class towards reducing carbon emissions. Designing integrated CO2R solar fuels devices requires careful co-design of electrochemical and photovoltaic components as well as consideration of the diurnal and seasonal effects of solar irradiance, temperature, and other meteorological factors expected for ‘on-sun’ deployment. Using a photovoltaic-electrochemical (PV-EC) platform, we developed a temperature and potential-dependent diurnal and annual model using experimentally-determined CO2R performance of Cu-based electrocatalysts, local meteorological data from the National Solar Radiation Database (NSRD), and modeled performance of commercial c-Si PVs. We simulated gaseous diurnal product outputs with and without the effects of ambient temperature. From these outputs, we observed seasonal variation in gaseous product generation, with up to two-fold increases in ethylene productivity between the Winter and Summer, analyzed the consequences of dynamic cloud coverage, and identified periods where device cooling/heating mechanisms could be implemented to maximize ethylene generation. Finally, we modeled the annual ethylene generation for a scaled 1 MW solar farm at three different locations (Beijing, CN; Sydney, AUS; Barstow, CA) to determine the consequences of local meteorological climates on PV-EC CO2R product output, recording a maximum ethylene output of 18.5 tonne per year at Barstow. Overall, this model presents a critical tool for streamlining the translation of experimental solar-driven electrochemical research to real-world implementation.