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16 Chapter 3. Aqueous PEC System mixed IrOx in acidic conditions (McCrory et al., 2015), showed stable and active performance with a typical overpotential of 250 mV at the operating current densities that match the solar photon flux. A range of electrolyte media can be employed in such a device, including an aqueous bicarbonate buffer solution or ionic liquids. The separation between the cathode and anode chambers, as well as the ionic transport between the two chambers, is often realized by the use of polymer membranes. Depending on the selection of catalyst materials, proton conductive membranes, such as Nafion, or hydroxide conductive membranes, such as Selemion, or a mix of cation and anion exchange membranes, such as bipolar membranes, will be used. The power-generating component and the fuel-forming component will be fully integrated together and the geometric operating current density of 5 mA cm-2 would be the target generation rate for the system. The auxiliary systems in Device-A will include the transport and delivery of 1 atm CO2 and water to the catalytic sites, recirculation of the water generated at the anode to the cathode, and efficient collection and separation of O2 from CO. For Device-A, a total cell voltage, which includes the thermodynamic voltage for the CO2 reduction and oxygen evolution reaction, kinetic overpotentials for the two reactions, concentration overpotentials at the electrode surfaces, and transport losses in the electrolyte and membrane, is expected to be 2 V. Based on the performance of the state-of-the-art CO2 reduction catalyst, a Faradic efficiency of 90% is expected. Hence, the overall solar-to-fuel conversion efficiency for Device-A is expected to be 15–18% with a 30% efficient PV module. Device-B is an aqueous-based methane generator, in which methane will be generated at the cathode surface, oxygen will be generated at the anode surface, and water will be consumed in the process to provide protons resulting in fuel generation. The major components and system architectures in Device-B are very similar to Device-A. The only difference is the required electrocatalysts for CO2 reduction to methane at the cathode. Currently, there are few electrocat- alysts reported to perform CO2 reduction to methane at high activity and selectively. Moreover, the mechanism for this eight-electron, eight-proton reaction is still under debate. Copper or copper-derived catalysts are among of the most studied materials systems that exhibite moderate selectivity towards methane. For instance, a Faradic efficiency of 33.3% at >1.0 V of overpotential was reported by Hori et al. (1989) and similar performances in copper based systems have been observed in recent years as well. More recently, rhodium nanomaterials have been described with promising potential for this process (Zhang et al., 2017). Based on the performance of Cu as the electrocatalyst for methane, a cell voltage of 3.4–3.5 V is expected with a Faradic efficiency of 30%. Hence, Device-B with the state-of-the-art materials assemblies is expected to deliver an electric-to-fuel conversion efficiency of 10–15% and an overall solar-to-fuel conversion efficiency 3%–4.5%. Because of the different technology readiness levels for Device-A and Device-B, different emphases in research development could be taken for the two systems. For Device-A, since existing materials could already deliver >10% efficient systems, full cell prototyping under realistic operatingPDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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