In the 1950s the first all-solid state fuel cell was developed based on a polymer membrane. It was the first electrochemical energy conversion device of its sort – safe, low temperature, low weight and high performance. Solid electrochemical interfaces allow for gaseous reactants, improved mass transport in the gas phase and extremely small separation distance between both electrodes. Polymer electrolyte membranes (PEMs) were also found useful in other electrochemical energy applications, such as PEM electrolysis and photoelectrolysis. The latest innovation in these fields has been the use of air as the inlet feed.[1-3] Air contains enough water to sustain solar-driven water splitting in the vapor phase. The use of air closes the water cycle, reduces device cost and resolves a number of engineering challenges (electrolyte circulation, frost, corrosion, bubble formation, …). Air-based solar water splitting provides a global solution to a global challenge. Small-scale, autonomous devices could be deployed anywhere, producing hydrogen fuel from the air and sunlight that are available all over the planet. However, the scientific basis of vapor phase water splitting is yet underdeveloped. 

Fundamental investigation of solid electrochemical interfaces is complicated by the need for a conducting solid phase connecting the working, counter and reference electrodes. This solid electrolyte is influenced by experimental conditions such as humidity and pressure, which convolute with their effect on the electrode surface. In this work, electrochemical chips with ultramicroelectrodes were used to isolate the kinetic effects of vapor composition on catalyst performance. Proper design of the electrochemical chips on the µm scale was needed to minimize the influence of the solid electrolyte. Electrochemical impedance spectroscopy and cyclic voltammetry were applied to identify electrolyte resistance, mass transport limitations and the kinetic reaction order. By elucidating the behavior of water splitting catalysts in the vapor phase, solar fuel research is brought one step closer towards achieving a grand challenge: making fuel out of thin air.

[1] Rongé, J. et al. RSC Adv. 4 29286–29290 (2014).

[2] Modestino, M. A. et al. Vapor-fed microfluidic hydrogen generator. Lab Chip 15, 2287–2296 (2015).

[3] Kumari, S. et al. Solar Hydrogen Production from Seawater Vapor Electrolysis. Energy Environ. Sci. 9, 1725–1733 (2016).