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Hole utilization in solar hydrogen production

Subjects

Abstract

In photochemical production of hydrogen from water, the hole-mediated oxidation reaction is the rate-determining step. A poor solar-to-hydrogen efficiency is usually related to a mismatch between the internal quantum efficiency of photon-induced hole generation and the apparent quantum yield of hydrogen. This waste of photogenerated holes is unwanted yet unavoidable. Although great progress has been made, we are still far away from the required level of dexterity to deal with the associated challenges of wasted holes and its consequential chemical effects that have placed one of the greatest bottlenecks in attaining high solar-to-hydrogen efficiency. A critical assessment of the hole and its related phenomena in solar hydrogen production would, therefore, pave the way moving forward. In this regard, we focus on the contextual and conceptual understanding of the dynamics and kinetics of photogenerated holes and its critical role in driving redox reactions, with the objective of guiding future research. The main reasons behind and consequences of unused holes are examined and different approaches to improve overall efficiency are outlined. We also highlight yet unsolved research questions related to holes in solar fuel production.

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Fig. 1: Thermodynamics and kinetics of photocatalytic hydrogen production from water.
Fig. 2: Hole is a catalyst in a redox reaction.
Fig. 3: Chemistry perspective of hole generation and separation.
Fig. 4: Relaxation, recombination, separation and transport of dissociated charge carriers.
Fig. 5: Hole transport kinetics at the interface of a photocatalyst and molecular adsorbate.
Fig. 6: Innovation in catalyst design for utilization of wasted holes.

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Acknowledgements

Funding from King Abdullah University of Science and Technology through the CRG programme is gratefully acknowledged. T.E. acknowledges generous supports through funding from the Swedish Research Council (VR-2015-03814), the Swedish Research Council for Sustainable Development (grant no. 2016-00908) and the Swedish Energy Agency (grant no. 44648-1).

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Authors and Affiliations

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Contributions

M.Z.R. researched data, conceived the structure and wrote the first draft of the manuscript. T.E. and J.G. edited and revised the manuscript. All authors agreed on the manuscript before submission.

Corresponding authors

Correspondence to Mohammad Z. Rahman, Tomas Edvinsson or Jorge Gascon.

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The authors declare no competing interests.

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Related links

Charge carrier: https://en.wikipedia.org/wiki/Charge_carrier

Gibbs free energy: https://www.sciencedirect.com/topics/chemistry/gibbs-free-energy

Photon: https://en.wikipedia.org/wiki/Photon

Valence band: https://www.sciencedirect.com/topics/chemistry/valence-band

Supplementary information

Glossary

Water photolysis

Photon-driven chemical reaction that breaks a water molecule into hydrogen and oxygen.

Co-catalyst

A metal or semiconductor that cooperatively improves the catalytic activity when it embeds into a catalyst without being consumed in the chemical reaction.

Sacrificial reagents

Chemical substance that can either donate electrons or remove holes.

Fermi levels

Measure of the highest energy level that an electron can occupy at absolute zero.

Quasi-Fermi level

A displaced Fermi level from equilibrium due to occupancy of electrons/holes generated by external stimuli.

Redox Fermi level

Fermi level of reductant and oxidant species in solution.

Lattice

A crystal structure composed of a series of atoms in a distinct pattern.

Internal photovoltage

The chemical potential of the separated electrons and holes.

Dielectric constant

Coulomb electrostatic attraction between point charges is inversely proportional to the dielectric constant.

Conduction band

The band of electron orbitals above the Fermi level where electrons can move freely or jump up into from the conduction band when excited.

Emissive state

An excited state of electrons in the conduction band or in energy states below the conduction band.

Valence band

The outermost electron orbital of an atom of any specific material that electrons actually occupy.

Ground state

The lowest energy state where electrons are in equilibrium.

Excitons

Excited electron–hole pairs that are bound by Coulomb electrostatic force.

Impact ionization

Process by which one energetic charge carrier can lose energy by the creation of other charge carriers.

Multiple electron generation

Phenomenon wherein the absorption of a single photon leads to the excitation of multiple electrons from the valence band to the conduction band.

Shockley–Queisser limit

Maximum theoretical efficiency of a single p–n junction solar cell.

Geminate recombination

A process where an electron recombines with the same hole that is created in the photogeneration process.

Auger recombination

When the electron–hole recombine by transferring excess energy to a third electron (or hole).

Shockley–Read–Hall recombination

Trap-assisted recombination of electron–hole pairs.

Hole scavengers

A sacrificial chemical that consumes holes in the photocatalytic process.

Nanostructuring

Scaling down of bulk materials into the nanoscale.

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Rahman, M.Z., Edvinsson, T. & Gascon, J. Hole utilization in solar hydrogen production. Nat Rev Chem 6, 243–258 (2022). https://doi.org/10.1038/s41570-022-00366-w

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