Photoelectrochemical Devices for Oxygen and Fuel Production on Moon and Mars

Human space exploration and the realisation of a green energy economy on Earth are confronted with the same challenge: both require efficient, stable and reliable renewable energy systems. 1 Currently existing solutions such as the water electrolyzer in operation on the International Space Station (ISS) for oxygen and hydrogen production are not feasible in future lunar and Martian habitats as they are too bulky and heavy to transport and they lack the required reliability and efficiency 2 . Photoelectrochemical (PEC) devices not only meet the weight and volume constraints imposed onto devices for space applications by utilising a monolithic design for light absorption, charge separation and catalysis, 3 they can also directly be coupled to solar concentrators in order to harvest the abundant solar energy available in space. Despite the advantages, PEC devices are nearly exclusively researched and developed for terrestrial applications and only a very few examples explore the possibility of utilizing the integrated semiconductor-electrocatalyst systems for space applications. 4 Here, we present an unprecedented theoretical analysis of the applicability of PEC devices for lunar solar water-splitting and Martian CO 2 reduction. These target reactions are in line with roadmaps of international space organisations aiming at a habitat establishment on the lunar South Pole, where water ice is assumed to be present in the Shackleton Crater. Furthermore, the atmosphere on Mars is composed of about 96% gaseous CO 2 , motivating our choice to investigate Martian CO 2 to CO and CH 4 conversion. 5 Both reaction products could be used as propulsion additives or as precursors for the synthesis of hydrocarbon-based molecules in analogy to terrestrial applications. Firstly, we calculated a refined Martian solar irradiance spectrum and established the theoretical framework to describe PEC devices on Moon and Mars before calculating device limiting efficiencies and expected product rates for realistic devices at different solar irradiance scenarios. Our findings emphasise that PEC devices are indeed applicable on Moon and Mars, particularly when coupled to solar concentrators. In addition, it is evident that gas diffusion electrodes (GDEs), which mark a significant part of CO 2 reduction devices investigated terrestrially, are surprisingly not necessarily viable in situations of limited solar irradiance such as present on Mars. References Ross B, Haussener S, Brinkert K. Photoelectrochemical devices for oxygen and fuel production on Moon and Mars. In review. Brinkert K, Mandin P. Fundamentals and future applications of electrochemical energy conversion in space. npj Microgravity 8 , (2022). Cheng W-H , et al. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Letters 3 , 1795-1800 (2018). Brinkert K , et al. Efficient solar hydrogen generation in microgravity environment. Nature Communications 9 , (2018). Yoshizaki T, McDonough WF. The composition of Mars. Geochimica et Cosmochimica Acta 273 , 137-162 (2020).