Efficiency limits for photoelectrochemical water-splitting.

Katherine T Fountaine,Hans Joachim Lewerenz,Harry A Atwater

doi:10.1038/ncomms13706

Abstract

Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community's focus on detailed balance. However, due to their multicomponent nature, photoelectrochemical devices do not have an equivalent analogue to detailed balance, and reported theoretical efficiency limits vary depending on the assumptions made. Here we introduce a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood and contextualized. Ideal and experimentally realistic limiting efficiencies are presented, and then generalized using five representative parameters—semiconductor absorption fraction, external radiative efficiency, series resistance, shunt resistance and catalytic exchange current density—to account for imperfect light absorption, charge transport and catalysis. Finally, we discuss the origin of deviations between the limits discussed herein and reported water-splitting efficiencies. This analysis provides insight into the primary factors that determine device performance and a powerful handle to improve device efficiency.

Highlights

Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community’s focus on detailed balance
We aim to present a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood
We first present the analytic equations and solutions for the limiting efficiencies of photoelectrochemical water-splitting devices based on the ultimate limits of device physics; limiting efficiencies reported here are consistent with those presented in references[3,13,15]

Summary

Introduction

Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community’s focus on detailed balance. We examine the validity of these ideal limits and consider more realistic limits based on existing materials, similar to those presented in references[10,11,12,14]; the realistic limits are presented and discussed as a function of five parameters: semiconductor absorption fraction, semiconductor external radiative efficiency (ERE), series resistance, shunt resistance and catalytic exchange current density These five parameters directly correlate with the three governing physical phenomena of photoelectrochemical device operation—light absorption (absorption fraction), charge carrier transport (ERE, series resistance and shunt resistance), catalysis (catalytic exchange current density); the parameter variation study demonstrates the varying impact of each phenomenon on overall device efficiency and efficiency limits. This analysis is contextualized via a comparison of the discussed limits with reported water-splitting efficiencies

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Results

Conclusion