Small pores and thin layers for big gains in energy conversion

Significance 

The current global focus on green energy has motivated immense research on how to best exploit the most readily available form of energy: solar energy. Consequently, conversion of solar-derived electrical energy to chemical energy through the electrochemical splitting of water has of late become a topic of much concern amongst scientists. The success of this process depends on the oxidative half-reaction which entails four-electron evolution of dioxygen. The dioxygen evolution presents a kinetic barrier that incapacitates the entire reaction. In industrial setups optimized for hydrogen production, this drawback has been overcome by application of large overpotentials that yield current densities in the range of 1-10 A cm-2. However, the overpotential represents the energy loss. Thus, water splitting as a viable energy storage strategy but must be operated at lower overpotentials. To achieve this, research has shown that one can either vary the material or optimize the electrode geometry. At present, little has been done with regard to the former despite the fact that it would present an economical solution in addition to optimizing the electrocatalytic turnover of the expensive noble metals utilized.

Recently, Friedrich-Alexander-Universität Erlangen-Nürnberg scientists Stefanie Schlicht, Sandra Haschke and Julien Bachmann in collaboration with Vladimir Mikhailovskii and Alina Manshina at Saint-Petersburg State University used atomic layer deposition technique for the preparation of iridium electrodes with accurately defined and tunable pore geometry. They purposed to utilize atomic layer deposition owing to its self-limiting surface reaction mechanism, which imparts to it the potential to uniquely generate thin coatings not only on planar substrates but also nanostructured ones. Their work is currently published in the research journal ChemElectroChem.

In brief, the research method used entailed the preparation of nano-porous iridium electrodes and their investigation towards the water oxidation reaction. Specifically, the preparation was based on ‘anodic’ aluminum oxide templates, which provided straight, cylindrical nanopores that were then coated using atomic layer deposition with the newly developed reaction which resulted in a metallic iridium layer. The atomic layer deposition film growth was then quantified by spectroscopic ellipsometry and X-ray reflectometry. Lastly, their catalytic activity was quantified for various pore geometries by cyclic voltammetry, steady-state electrolysis, and electrochemical impedance spectroscopy.

The authors observed that when the optimal pore length was set to L ≈ 17-20 mm, current densities of J = 0.28 mA cm-2 at pH 5 and J = 2.4 mA cm-2 at pH 1 were achieved at a moderate overpotential (η ≈ 0.25 V). This proved that the atomic layer deposition treatment enabled the limiting kinetic reaction to be performed at very low overpotentials with moderate current densities. Such near-reversibility is a step in the right direction for the development of more efficient energy storage systems.

In a nutshell, the study introduced a novel iridium atomic layer deposition reaction and its exploitation in investigating the electrochemical performance of nano-porous water oxidation electrodes as it depends on the nano- and microscale geometry systematically. Based on a systematic geometric optimization, their system allowed one to perform the storage of electrical energy in chemical form under conditions of high reversibility. Altogether, their system presents a platform on which to study geometric effects on electrocatalytic transformations in a systematic manner.

Small pores and thin layers for big gains in energy conversion - Advances in Engineering

About the author

Julien Bachmann studied chemistry at the University of Lausanne, Switzerland, and  performed his doctoral research work at the Massachusetts Institute of Technology in the USA After obtaining his Ph. D. in inorganic chemistry from MIT in 2006, he moved with a Humboldt Fellowship to the Max Planck Institute of Microstructure Physics in Germany, then obtained an Assistant Professor position in physics and chemistry at the University of Hamburg. He was appointed as an Associate Professor of Inorganic Chemistry at the Friedrich Alexander University of Erlangen-Nürnberg (FAU) in 2012 and promoted to Full Professor in 2017.

He now leads the Chair for ‘Chemistry of Thin Film Materials’ (CTFM) at FAU, and holds a secondary appointment at Saint Petersburg State University in the Russian Federation. Research at CTFM encompasses the development of preparative methods towards ordered nanoporous surfaces and interfaces with highly controlled and systematically tunable geometry, as well as the functional performance investigation as it depends on the geometric parameters. The functions under investigation include energy conversion in electrolysis, fuel cell reactions, photovoltaics and batteries. Stefanie Schlicht studied chemistry at FAU and is a doctoral student at CTFM.

Her research focuses on noble metal coatings by atomic layer deposition and their application in electrochemical reactions. She has developed reactions to deposit platinum and iridium catalysts, optimized their activity, and minimized their loading in order to optimize both the energy efficiency and economics of electrochemical water splitting.

Reference

Stefanie Schlicht, Sandra Haschke, Vladimir Mikhailovskii, Alina Manshina, Julien Bachmann. Highly Reversible Water Oxidation at Ordered Nanoporous Iridium Electrodes Based on an Original Atomic Layer Deposition. ChemElectroChem 2018, volume 5, page 1259- 1264.

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