Hydrogen Gas Bubble Evolution on Electrocatalytic Nanostructures in Microgravity Environment

Photoelectrochemical (PEC) monolithic devices for oxygen and chemical production are attractive for space applications due to predominant weight and volume constraints as well as a high system tunability. Here, advanced semiconductor-electrocatalyst systems integrate the processes of light absorption, charge separation and catalysis in one device - processes, which are usually carried out e.g., on the International Space Station (ISS) by an electrolyser coupled to external PV cells. Due to the near absence of buoyancy and buoyancy-induced convection in micro gravity, all (photo)electrolyser systems in space naturally have lower efficiencies, as gas bubble desorption and phase separation are severely hindered. We report on a new approach to design the electrocatalyst morphology on a nanoscale in order to modify the physicochemical properties of the photoelectrode and thus allow for continuous gas bubbles desorption and efficient operation of the PEC device in microgravity. Using a rhodium-coated p-InP model photoelectrochemical system, we discuss different Rh electrocatalyst nanotopographies fabricated by shadow nanosphere lithography and report on their characterisation as well as performance in photoelectrochemical measurements realised during 9.2 s of free fall at the Bremen Drop Tower, Center of Applied Space Technology and Microgravity (ZARM). In particular, we elucidate on how the electrocatalyst nanostructures affect the gas bubble size during growth and at detachment from the electrode and conclude on an efficient (photo-)electrode design for hydrogen evolution in microgravity.