Realization of a photoelectrochemical cascade for the generation of methanol Grace A. Rome 4,5 , Thomas Chan 1,3 , Calton J. Kong 1,2 , Myles A. Steiner 5 , Emily L. Warren 5 , Joel W. Ager 1,2 , Ann L. Greenaway 5 1 Lawrence Berkeley National Laboratory, Berkeley CA 94720, United States, 2 University of California, Berkeley CA 94720, United States, 3 University of California, San Diego, La Jolla CA 92093, United States, 4 Colorado School of Mines, Golden CO 80401, United States, 5 National Renewable Energy Laboratory, Golden CO 80401, United States Solar photons are a free and ubiquitous energy source, capable of meeting the global energy demand if captured and utilized. Although photovoltaics are increasingly efficient at converting photons to electricity, there is still a need to store that same photon energy due to solar energy being a variable resource. Photoelectrochemistry is a method to utilize photogenerated charge carriers to reduce feedstocks such as CO 2 into energy-dense chemicals that can later be consumed to extract energy on demand or as building blocks for higher-value chemicals. However, selectivity of the CO 2 reduction reaction is a known issue, as a plethora of products can be generated, requiring energy-intensive separation processes. 1 The success of natural photosynthesis suggests that multi-step (cascade) reactions can be leveraged to lead to higher product selectivity; this concept has been demonstrated electrochemically, but not photo electrochemically. 2 In this work we investigate a unique photocathode design that can facilitate photoelectrochemical CO 2 reduction cascades. 3 Typical photoelectrodes can produce only one current density-voltage function, as determined by the material band gap; this makes it impossible to match the reaction potential and electron flux (current density) of more than one reaction, as would be required in a cascade. In this work, we created a 3-terminal tandem (3TT) photocathode for the first ever demonstration of photoelectrochemical cascade fuel production, where CO 2 is first reduced to CO and then CO is reduced to methanol. 3 The 3TT was comprised of a series connected GaInP top cell and GaAs bottom cell; however, unlike typical 2-terminal tandems, 3TTs have an extra contact between the two subcells, which allows for two photoelectrochemical reaction sites with different current density-voltage functions at each. Cobalt phthalocyanine immobilized on multiwalled carbon nanotubes was used to catalyze both cascade reactions. To determine cascade operating conditions, dark electrochemical experiments mimicking the constraints of 3TTs were performed. Using that information, 3TTs were tested and were able to create methanol with a Faradaic efficiency of up to 3.8 ± 0.4%. To ensure that a cascade mechanism was occurring, light filters were used to deactivate one of the reaction sites and the resulting Faradaic efficiencies and partial current densities were compared to the fully illuminated photocathode. This first demonstration has paved the way for cascade photoelectrochemical fuel production. Efforts are ongoing to further enhance the cascade photoelectrode system based on lessons learned in this work. One aspect to improve upon is the understanding of the voltages experienced at the two solution contacts. Originally, potentials were measured between the front contact and counter electrode, leaving ambiguity about the voltages experienced at the two solution contacts. To investigate further, non-aqueous experiments with probes on all three contacts will be performed and compared to easily obtained dry photovoltaic measurements to determine a predictive relationship. References 1. Li, D.; et.al. Adv. Energy Mater. 2022 . https://doi.org/10.1002/aenm.202201070. 2. Gurudayal; Perone, et.al. ACS Appl. Energy Mater. 2019 . https://doi.org/10.1021/acsaem.9b00791. 3. Rome, G.; et.al. Energy Fuels 2024 . https://doi.org/10.1021/acs.energyfuels.4c04779.
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