Monolithic cells for solar fuels

Abstract

Hybrid energy generation models based on a variety of alternative energy supply technologies are considered the best way to cope with the depletion of fossil energy resources and to limit global warming. One of the currently missing technologies is the mimic of natural photosynthesis to convert carbon dioxide and water into chemical fuel using sunlight. This idea has been around for decades, but artificial photosynthesis of organic molecules is still far away from providing real-world solutions. The scientific challenge is to perform in an efficient way the multi-electron transfer reactions of water oxidation and carbon dioxide reduction using holes and single electrons generated in an illuminated semiconductor. In this tutorial review the design of photoelectrochemical (PEC) cells that combine solar water oxidation and CO2 reduction is discussed. In such PEC cells simultaneous transport and efficient use of light, electrons, protons and molecules has to be managed. It is explained how efficiency can be gained by compartmentalisation of the water oxidation and CO2 reduction processes by proton exchange membranes, and monolithic concepts of artificial leaves and solar membranes are presented. Besides transferring protons from the anode to the cathode compartment the membrane serves as a molecular barrier material to prevent cross-over of oxygen and fuel molecules. Innovative nano-organized multimaterials will be needed to realise practical artificial photosynthesis devices. This review provides an overview of synthesis techniques which could be used to realise monolithic multifunctional membrane-electrode assemblies, such as Layer-by-Layer (LbL) deposition, Atomic Layer Deposition (ALD), and porous silicon (porSi) engineering. Advances in modelling approaches, electrochemical techniques and in situ spectroscopies to characterise overall PEC cell performance are discussed.

Nanoscale Effects in Water Splitting Photocatalysis

“…PECs can reach efficiencies of up to 12.4% [10], about half of the theoretical efficiency limit for these devices (24.4% for a tandem [11,12] and 30% for a multijunction device [4]). However, issues with stability continue to plague these devices [13,14]. Of all known solar hydrogen technologies, solar water electrolysis with suspended photocatalysts (PCs) has the greatest potential to induce a revolution in fuel production on this planet.…”

Performance Limits of Photoelectrochemical CO2 Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

“…[18][19][20] This is in spite of the fact that a substantial research effort currently devoted to finding electrocatalysts capable of directly reducing CO 2 into energetic compounds such as methane, ethane, and ethanol and, in turn, to integrate such CO 2 reduction catalysts into PEC solar harvesting systems. [21][22][23][24] Only in late 2015 did the first analysis of PEC-driven CO 2 reduction appear. 25 …”

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

“…This concept underlies biofuel production, [2][3][4] as well as a number of solar-to-fuel or ''artificial photosynthesis'' approaches. [5][6][7] This review concentrates on approaches that use sunlight to split water into hydrogen and oxygen, 8 noting the recent review by Rongé et al, 9 which also covers solar-driven carbon dioxide reduction. Hydrogen is a storable fuel that can be used as a feedstock for fuel cells that generate power for transportation and, potentially, grid-scale energy storage.…”

“…6 Solar irradiation can be used for thermal and/or electrochemical water splitting. [14][15][16][17] Electrochemical water splitting requires the following overall cathodic and anodic half-reactions (in acid): 8,9 2H + + 2e À -H 2 (1)…”