Eniola Sokalu’s scientific contributions

One of the major challenges human space exploration faces is the absence of buoyancy forces in orbit. Consequently, phase separation is severely hindered which impacts a large variety of space technologies including propellant management devices, heat transfer and life support systems e.g., during the production of oxygen and the recycling of carbon dioxide. Of particular interest are hereby (photo-)electrochemical (PEC) devices as they can produce essential chemicals such as oxygen and hydrogen in two set-ups: either, by coupling the electrochemical cell to external photovoltaic cells as currently utilized on the International Space Station or by direct utilization of sunlight in a monolithic device, where integrated semiconductor-electrocatalyst systems carry out the processes of light absorption, charge separation and catalysis in analogy to natural photosynthesis in one system. The latter device is particularly interesting for space applications due to present mass and volume constraints. Here, we discuss two combined approaches to overcome phase separation challenges in (photo-)electrolyzer systems in reduced gravitational environments: using the hydrogen evolution reaction (HER) as a model reaction, we combine nanostructured, hydrophilic electrocatalyst surfaces for efficient gas bubble desorption with magnetically-induced buoyancy to direct the produced hydrogen gas bubbles on specific trajectories away from the (photo-)electrode surface. (Photo-)current-voltage ( J-V ) profiles obtained in microgravity environments generated for 9.2 s at the Bremen Drop Tower show that our systems can operate with our two-fold approach near terrestrial efficiencies. Simulations of gas bubble trajectories accompany our experimental observations, allowing us to attribute the achieved phase separation in the PEC cells to the increased electrode wettability as well as the systematic use of diamagnetic and Lorentz forces.

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.