Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

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Abstract

Sunlight is by far the most plentiful renewable energy resource, providing Earth with enough power to meet all of humanity's needs several hundred times over. However, it is both diffuse and intermittent, which presents problems regarding how best to harvest this energy and store it for times when the sun is not shining. Devices that use sunlight to split water into hydrogen and oxygen could be one solution to these problems, because hydrogen is an excellent fuel. However, if such devices are to become widely adopted, they must be cheap to produce and operate. Therefore, the development of electrocatalysts for water splitting that comprise only inexpensive, earth-abundant elements is critical. In this Review, we investigate progress towards such electrocatalysts, with special emphasis on how they might be incorporated into photoelectrocatalytic water-splitting systems and the challenges that remain in developing these devices.

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Figure 1: Artificial photosynthesis systems for solar-to-hydrogen applications.
Figure 2: Trasatti's HER volcano plot.
Figure 3: OER at neutral pH mediated by a cobalt-oxide catalyst.

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Acknowledgements

M.D.S. thanks the University of Glasgow for a Lord Kelvin Adam Smith Research Fellowship.

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Correspondence to Mark D. Symes.

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The authors declare no competing interests.

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Glossary

Tafel slopes

Tafel slopes are the gradient of the linear portion of a graph of overpotential versus the logarithm of the current density for a given electrocatalyst performing a particular reaction. Hence, the value of each slope gives an idea as to how the electrocatalytic performance of a material changes over a given potential range. These slopes are often quoted in millivolts per decade, where a decade in this sense relates to a decade of current density on the log scale. Catalysts with lower slopes require smaller increments of overpotential to give increased current densities, making them more effective.

Faradaic yield

In electrochemistry, this is the ratio between the amount of product actually detected and quantified, and the amount of that product that could theoretically have been formed based on the charge passed in the experiment.

Exchange current densities

Exchange current densities reflect the intrinsic rate of electron transfer between an analyte in solution and the electrode, and can thus be viewed as a measure of the effectiveness of a catalyst for a particular electrochemical reaction under a particular set of conditions (the greater the magnitude of the exchange current density, the greater the activity of the catalyst).

Photosystem II

Photosystem II is a key protein complex involved in photosynthesis. It uses sunlight to oxidize water, producing high energy electrons for subsequent chemical reactions.

Reversible hydrogen electrode

(RHE). The RHE is based on the following electrochemical half-reaction: 2e + 2H+ → H2. Therefore, it is a subtype of the standard hydrogen electrode. However, the measured potential of the RHE does not change with pH, which means it can be used to make a ready comparison between the theoretical position of onset of the HER (always 0 V versus RHE) and the position of a redox event of interest, regardless of the pH of the solution.

Solar-to-hydrogen conversion efficiency

The energy that would be released upon complete oxidation of the hydrogen produced by, for example, an artificial photosynthesis system in a given time, divided by the energy required by the artificial photosynthesis system to produce that amount of hydrogen.

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Roger, I., Shipman, M. & Symes, M. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat Rev Chem 1, 0003 (2017) doi:10.1038/s41570-016-0003

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