Effects of microgravity on the photoelectrodeposition of Rh electrocatalyst nanoparticles and on the diffusion coefficient of the Rh3+ ions in the precursor electrolyte.

Abstract

Electrodeposition and photoelectrodeposition are well-known techniques for the synthesis of catalyst materials on the surface of photoelectrodes, a prominent example being rhodium photoelectrodeposited on p-indium phosphide. Rhodium increases the efficiency of the hydrogen evolution reaction from acidic aqueous solutions by lowering the activation overpotentials of the device. The life support systems of space stations and of future Moon and Mars outposts require a variety of energy conversion technologies, several of which rely on catalyst materials. Monolithic photoelectrochemical systems that directly convert solar energy into chemical energy are a promising complementary system to conventional electrolysis. Further, the on-site synthesis of electrocatalysts on site prevents expensive, if not impossible, payload supply: photoelectrodeposition through solar light is a low-energy synthesis technique. Hence, it is an advantageous technique in a scarce environment, and is also applicable to the synthesis of nanomaterials for fuel cells, batteries, and functional materials for energetic and biomedical applications. At a fundamental level, microgravity affects the electrodeposition of metals and the nucleation of the nanoparticles: due to the absence of natural convection and sedimentation processes, the reaction becomes, if not stirred, diffusion-controlled. In the present work, the photoelectrodeposition of rhodium nanoparticles on p-InP photocathodes is examined as an example of heterogeneous nucleation in a diffusion-limited process. The effects of microgravity are shown by deriving the diffusion constant for Rh3+ ions using the chronoamperometric data recorded during the photoelectrodeposition process in microgravity and in terrestrial gravitation, and by comparing the morphology of the synthesised Rh nanoparticles on the surface of p-InP. The microgravity environment of choice was the Drop Tower of Bremen, at the Center for Applied Space Research and Microgravity (ZARM).

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