(Invited) Design Strategies for Efficient Solar Seawater Splitting Devices
Authors: Vayssieres Lionel 
Published: May 2020

This decade is experiencing the widest consequences of man-made activities on our planet with environmental risks for public health now at a record high. Indeed, staggering air and water pollution worldwide, chronic in major cities worldwide, due to toxic gases, chemicals, and ultrafine particles from industry, agriculture, and transportation sectors has become one of the most, if not The most, important problem that humanity is facing. It is now crucial to transition to new societies where environmental, energy, and economic policies are no longer based on endless-growth financial models and fossil fuel technologies to substantially decrease our ecological footprint and environmental and health impacts. The origin of this strong imbalance between human activities and the environment could be found in the endless-growth economic system in place in major countries worldwide as it inherently requires the use of endless cheap energy to be sustained, hence the massive use of coal and fossil fuels as energy sources for a more profitable energy return on energy invested, which might be good for the economy but clearly is not for our environment, health, and sustainable future. Technological innovation has always helped boost the economy and has done it successfully numerous times throughout civilizations, even recently with solid-state lighting technology for instance. These technological advancements must involve large-scale, clean, and cost-effective fabrication techniques at moderate to low temperatures and be based on highly efficient materials that mostly contain earth-abundant elements, easily extractable and fully recyclable rather than expensive, scarce, and toxic metals and rare earths. In addition, given that conventional technologies that attempt to improve the efficiency and performance of existing materials and devices by further development along the same incremental approaches are reaching their limits, it is crucial to develop novel (multi)functional materials where bulk limitations are overcome by changing the fundamental underlying physics and chemistry by nanoscale design and quantum confinement effects.

The transition to an hydrogen-based energy and economy would be ideal as it produces zero carbon emission (only water), and consumer hydrogen fuel cell powered cars and public transportation are available. Yet, most of the hydrogen produced nowadays still comes from nonrenewable sources, made by steam reforming of methane which produces large amount of carbon dioxide. The most natural and cleanest way to sustainably produce hydrogen at large scale is by photocatalytically splitting (sea)water[1].

Our strategy is to fabricate devices consisting of oriented arrays of quantum rods and dots of high purity synthesized by aqueous chemical growth at low temperature without surfactant and with controlled dimensionalities and surface chemistry[2] with intermediate bands for high visible-light conversion, bandgap and band edges optimized for stability against photocorrosion and operation conditions at neutral pH and low bias without sacrificial agent[1]. Such characteristics, combined with the in-depth investigation of their size-dependent and interfacial electronic structure[3], and electrical conductivity effects[4] do provide better fundamental understanding and structure-efficiency relationships for a cost-effective and sustainable generation of hydrogen from the two most abundant and geographically-balanced free resources available, i.e. the sun and seawater.

An overview along with the latest advances in low-cost controlled fabrication of efficient devices for solar seawater splitting will be presented including oxides, nitrides, sulfides as well as highly ordered hybrids consisting of visible-light-active semiconductors and molecular co-catalysts.

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