Inexpensive energy conversion devices necessitate novel strategies towards reducing the need for rare functional materials. One such strategy consists in accurately controlling the interface’s geometry.
In our work, the pore walls of ‘anodic’ nanoporous template are coated with atomic layer deposition (ALD) to obtain structured interfaces in electrodes, photoelectrodes and photovoltaic stacks. The inert ‘anodic’ template provides a hexagonally ordered array of cylindrical pores, the diameter and length of which can be defined between 20 and 300 nm and between 0.5 and 100 μm, approximately. ALD allows for coating the deep pores with one or several layers of functional materials, the thickness of which we set to values between 0.5 and 50 nm. The functional materials can be metals, oxides or sulfides and may behave as electrical insulators, conductors, semiconductors, and/or catalysts. This family of functional samples provide a general model system in which the specific interface area of an energy conversion device can be increased and its effect on the functional performance parameters characterized systematically.
In the photovoltaics realm, we have lifted the veil on effects of individual layer thicknesses on ‘extremely thin absorber’ solar cell efficiencies. In electrochemical transformations, we have observed the difference between diffusion-limited reactions, which remain unaffected by changes in the length of the electrode's pores, and slow multielectron transformations the steady-state galvanic current density of which increases linearly with the pore length. Our approach has enabled us to achieve an increase of the electrochemical water oxidation turnover at iron oxide surfaces by three orders of magnitude. These results highlight a strategy for optimizing energy transformation devices that could be generalized: the geometric tuning of catalytically mediocre but abundant and cost-effective material systems.