ABSTRACT The storage of solar energy as formic acid generated electrochemically from carbon dioxide has been identified as a viable solar fuel pathway. We report that this transformation can be accomplished by separating light absorption...
moreABSTRACT The storage of solar energy as formic acid generated electrochemically from carbon dioxide has been identified as a viable solar fuel pathway. We report that this transformation can be accomplished by separating light absorption and CO2 reduction through the use of a commercial solar panel illuminated with natural AM1.5 sunlight to power a custom closed-loop electrochemical flow cell stack. Faradaic yields for formate of up to 67% have been demonstrated in this system, yielding a solar energy to fuel thermionic conversion efficiency above 1.8%. ----------------------------------------------------------------------------------------------------- Artificial photosynthesis is often associated with schemes that utilize light to split water, generating hydrogen as a fuel. However, over the past decade, interest in the generation of liquid solar fuels has grown due to their enhanced energy density and improved storage properties, both of which are important attributes for dispatchable power [1]. In addition to liquid fuels, the generation of energy-dense carbon-based products provides a new source of chemical feedstocks derived from precursors other than fossil fuels [2]. However, a practical system must exceed the efficiency of natural photosynthesis, which converts up to 1% of incident sunlight to biomass [3]. Since CO2 can be recovered in a relatively pure form from a variety of industrial processes [4], it is a prime candidate for a feedstock for solar fuel production. A second advantage to this approach is the mitigation of anthropogenically produced carbon dioxide, a greenhouse gas whose atmospheric concentrations have drastically increased in the past 200 years. Several approaches have been employed in the light-assisted reduction of CO2 using illuminated semiconductor electrodes, both with photoanodes to supply electrons to a dark cathode and with photocathodes to reduce the CO2 directly [5–7]. Recently, Panasonic claimed ‘‘a world’s top efficiency’’ of solar energy to formic acid conversion of 0.2% using an n-GaN photoanode to drive an indium cathode [8,9]. Although the long term stability of this system has not been reported, similar III–V n-type semiconductors tend to photodegrade in aqueous solution, limiting their usability [10]. An alternative to a photoelectrochemical system is to use an external photovoltaic (PV) array to power an electrolyzer utilizing metallic electrodes [11,12]. Only limited research has been reported using this approach [13–15]. A variety of metal electrodes have been previously examined for the generation of formate from CO2 [16–20]. However, in general, these systems have required relatively large overpotentials, and have exhibited limited operational lifetime. There are benefits and drawbacks to both photoelectrochemical (PEC) and coupled photovoltaic-electrochemical (PV-EC) systems, and the relative merits of both have been discussed for years, without resolution, since the cost–benefit analysis depends subtly on project specific details that may not directly be controlled by the chemistry and physics of the solar converting elements [21]. PEC cells are self-contained and do not require a separate set of electrodes to perform the electrochemical transformation. As a result, PEC systems have lower balance-of-plant costs and a higher theoretical system efficiency. However, PEC cells have severe materials constraints. High efficiency PEC cells are often achieved in the laboratory by employing single-crystal semiconductors as electrodes. Scale-up to a field-implementable system requires developing either polycrystalline, nanostructured or thin-film semiconductor electrodes. Such electrodes typically yield reduced activity compared to their monocrystalline counterparts. Further, it has been empirically and theoretically found [22,23] that photocorrosion is a major problem for semiconductor electrodes having band gaps providing an optical response well matched to the solar spectrum. Although a variety of chemistries have been developed to circumvent this problem, long-term cell stability can still be an issue. PV-EC systems, such as the one described herein, do have the disadvantages of greater balance-of-plant costs and increased complexity due to the need to impedance match the various components between the photovoltaic and electrochemical cells. Proper mating of PV power output with the number and sizes of the EC cells is also difficult and may require an additional conversion step between the two. The coupling of the components, regardless of power-matching, leads to losses in the photogenerated electricity that do not occur with the simpler PEC cells [24]. However, the separation of light collection and carbon dioxide reduction into two distinct systems makes the coupling of the two into a single efficient system attractive since each subsystem can be independently optimized. The use of metal-based electrodes, in place of the semiconductor- based electrodes…
