Section S1: Materials and Electrolysis Cell Design Materials. Materials were purchased in the grade indicated and used as received. Ammonium carbonate ((NH4)2CO3, 99.999%), ammonium chloride (NH4Cl. BioUltra), ammonium hydroxide (NH4OH, BioUltra), ammonium perchlorate (NH4ClO4, 99.999%), ammonium sulfate ((NH4)2SO4, 99.999%), ammonium tetrathiomolybdate ((NH4)[MoS4], 99.97%), boric acid (H3BO3, BioUltra), chromium(III) sulfate monohydrate (Cr2(SO4)3•H2O, 99.999%), cobalt(II) acetate tetrahydrate (Co(OAc)2•4H2O, 99.999%), cobalt(II) chloride hexahydrate (CoCl2•6H2O, ACS 98%), cobalt(II) nitrate hexahydrate (Co(NO3)2•6H2O, 99.999%), copper(II) sulfate heptahydrate (CuSO4•5H2O, 99.995%), glycine (NH2CH2COOH, BioUltra), iridium(III) acetylacetonate (Ir(acac)3, 97%), iron(II) chloride tetrahydrate (FeCl2•4H2O, 99.99%), iron(III) chloride heptahydrate (FeCl3•6H2O, ACS 97%), iron(III) nitrate nonahydrate (Fe(NO3)3•9H2O, 99.99%), iron(II) sulfate heptahydrate (FeSO4•7H2O, ACS 99%), manganese(II) sulfate hydrate (MnSO4•H2O, 99.99%), nickel(II) nitrate hexahydrate (Ni(NO3)2•6 H2O, 99.999%), nickel(II) sulfate hexahydrate (NiSO4•6H2O, 99.99%),nickel(II) chloride hexahydrate (

The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar-energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (electrical plus chemical) divided by the total input power (electrical plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures-of-merit, are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures-of-merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit analysis formalism to obtain "optimal" system efficiencies that can be compared between various approaches, when the component of concern is used in a reference fuel-producing energy-conversion system. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight.As the fields of photoelectrochemical (PEC) energy conversion and solar fuels have grown, a number of metrics have been adopted for evaluating the performance of electrodes and systems. These metrics are often contradictory, irreproducible, or not properly standardized, which prevents researchers from accurately comparing the performance of materials, even within the PEC community itself. We explore herein these different metrics to evaluate their strengths and applicability, as well as to demonstrate the knowledge derived from each approach. We also present a framework for reporting these metrics in an unambiguous and reproducible manner. Additionally, we outline a method to estimate two-electrode system efficiencies from three-electrode potentiostatic measurements, to accelerate the identification of promising system components without requiring the actual construction of a full system. Clarifying these issues will benefit the PEC community by facilitating the consistent reporting of electrode performance metrics, and will allow photoelectrodes and solar fuels systems to be appropriately compared in performance to other solar energy-conversion technologies. Table of contents graphic textWe outline the significance and advantages of different metrics used to characterize photoelectrodes ...