Methods for comparing the performance of energy-conversion systems for use in solar fuels and solar electricity generation

Abstract

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 ...

Functional integration of Ni–Mo electrocatalysts with Si microwire array photocathodes to simultaneously achieve high fill factors and light-limited photocurrent densities for solar-driven hydrogen evolution

“…The light-limited current density was obtained through twodimensional full-wave electromagnetic simulations of a Si microwire array architecture. 3,4 The modeling indicated that utilization of a stand-alone n + p-Si microwire array PV device having a solar energy-conversion efficiency of 12.9% could, in principle, yield a maximum ideal regenerative cell efficiency (Z IRC ) 23 of 11.2% based on the hydrogen-evolution half-reaction being performed at such a photocathode. This value is essentially the same as the value that would be obtained by wiring the respective photovoltaic device to a discrete, catalytic electrode Fig.…”

Solar-Powered Plasmon-Enhanced Heterogeneous Catalysis

“…PV-electrolysis technology is currently available, however, this approach is limited by the price of PV and electrolyzer units, resulting in a price of H 2 produced around US$ 10/Kg [70]. While encouraging scientific research on PV may reduce this price, general drawbacks come from: (i) the strong dependence of PV performances on the illumination intensity and (ii) the low durability of materials used in the electrolyzer due to harsh operational conditions (high voltage and corrosive solutions).…”

“…On the other hand, despite lower projected cost, the low efficiency of colloidal photocatalysts (below 7%) [69] and the intrinsic need to implement an apparatus to separate H 2 and O 2 gases have pushed recent research efforts towards PEC water splitting. This path eliminates the need for a separate electrolyzer and reduces the potential losses in the system [63,70,71]. The PEC approach in perspective can be competitive with other H 2 production technologies, setting a H 2 cost around US$ 2-3/Kg (American DOE cost goal) considering a PEC device lifetime of 10 years, a 20% solarto-hydrogen efficiency (measured as electrical output from the conversion of photons into chemicals) and US$ 100-200/m 2 PEC material cost [72].…”

Thermodynamic and achievable efficiencies for solar-driven electrochemical reduction of carbon dioxide to transportation fuels

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