(Invited) Advanced Semiconductor-Electrocatalyst Systems for Photoelectrochemical Hydrogen and Oxygen Production in Microgravity Environment

Abstract

Efficient artificial photosynthesis systems are currently realized as catalyst- and surface-functionalized photovoltaic tandem and triple junction devices [1,2] enabling photoelectrochemical water oxidation while simultaneously recycling CO2 and generating hydrogen as a solar fuel for storable renewable energy. The successful implementation of an efficient photoelectrochemical (PEC) water splitting cell is not only a highly desirable approach to solving the energy challenge on earth: an effective air revitalization system generating a constant flux of O2 while simultaneously recycling CO2 and providing a sustainable fuel supply is also essential for the International Space Station and long-term space missions, where a regular resupply from earth is not possible.We recently demonstrated in a series of drop tower experiments that efficient direct hydrogen production can be realized photoelectrochemically in microgravity environment, providing an alternative route to existing life support technologies for space travel [3]. Current limiting factors such as the absence of macroconvection processes were overcome by controlling the micro- and nanotopography of the electrocatalyst using shadow nanosphere lithography (SNL), generating so-called catalytic ‘hot-spots’ on the electrode surface which prevent gas bubble coalescence [3,4]. We found that the J-V characteristics of the half-cell and the overall device efficiency in microgravity environment are significantly affected by alterations in the electrocatalyst nanotopography [5]. By varying the shape and distance of catalytic ‘hot-spots’ on the electrode surface, we could control the gas bubble radius upon detachment from the electrode surface and the light absorption properties of the semiconductor in free fall. Shadow nanosphere lithography can therefore be used as a prosperous tool to develop custom-tailored electrocatalyst nanostructures of high fidelity on a light-absorbing semiconductor surface for an optimized device performance in microgravity and terrestrial applications.