An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems

Abstract

The solar-to-hydrogen (STH) efficiency limits, along with the maximum efficiency values and the corresponding optimal band gap combinations, have been evaluated for various combinations of light absorbers arranged in a tandem configuration in realistic, operational water-splitting prototypes. To perform the evaluation, a current-voltage model was employed, with the light absorbers, electrocatalysts, solution electrolyte, and membranes coupled in series, and with the directions of optical absorption, carrier transport, electron transfer and ionic transport in parallel. The current density vs. voltage characteristics of the light absorbers were determined by detailed-balance calculations that accounted for the ShockleyQueisser limit on the photovoltage of each absorber. The maximum STH efficiency for an integrated photoelectrochemical system was found to be $31.1% at 1 Sun (¼1 kW m À2, air mass 1.5), fundamentally limited by a matching photocurrent density of 25.3 mA cm À2 produced by the light absorbers. Choices of electrocatalysts, as well as the fill factors of the light absorbers and the Ohmic resistance of the solution electrolyte also play key roles in determining the maximum STH efficiency and the corresponding optimal tandem band gap combination. Pairing 1.6-1.8 eV band gap semiconductors with Si in a tandem structure produces promising light absorbers for water splitting, with theoretical STH efficiency limits of >25%. Broader contextAn integrated system that allows for the direct production of fuels from sunlight would provide a scalable, sustainable source of clean fuels for grid storage as well as for use in the transportation sector. Because all of the components of such a complete system must operate under mutually compatible conditions, an assessment of the required materials properties necessitates a systems-level analysis of the operational conditions of an integrated solar-fuel generator. Accordingly, the optimal system efficiency has been calculated for various solar-fuel generator system geometries and component dimensions, as a function of the band gaps of the light absorber components that serve to capture and convert sunlight into chemical fuels. Dual band gap light absorber congurations have been evaluated, with the dual band gap, tandem structure, providing optimal efficiency and thus a preferred approach, to effect direct solar-fuel production. The assessment has incorporated a variety of systems-level parameters that are present in an actual operating solar-fuel generator system, including the thermodynamic constraints on the light absorbers as given by the Shockley-Queisser detailed-balance limit, the overpotentials of earth-abundant catalysts for water oxidation and reduction reactions, and the effects of solution resistance and light absorber quality on the overall conversion process of sunlight into chemical fuels.

Combinatorial Discovery of Lanthanum–Tantalum Oxynitride Solar Light Absorbers with Dilute Nitrogen for Solar Fuel Applications

“…In particular, we investigate the chemistry and phase behavior of a thin film composition spread library in the La-Ta-O-N system. This investigation is motivated by i) the presence of a LaTaON 2 phase with a 2.0 eV band gap 17 nearly optimal for a photoanode in a tandem photoelectrochemical water splitting device 18 and for the top absorber in a triple-junction light absorber for CO 2 reduction 19 , and ii) the reported photocatalytic activity of La 3 TaO 7 for water splitting reactions. 20 To explore new oxynitride materials, combinatorial oxyntride synthesis must be combined with materials characterization methods that can map the structural and chemical properties of the thin film composition library.…”

CuO-Functionalized Silicon Photoanodes for Photoelectrochemical Water Splitting Devices

“…Several kinds of photoanodes have been used in this PEC set-up, including metal oxide semiconductors (Fe 2 O 3 , WO 3 , BiVO 4 , …), group III-V semiconductors (GaAs, GaP, InP, …) and amorphous silicon, all being excellent light absorbers and in most of the cases with good carrier mobility. [3][4][5] Among them, silicon is a particularly interesting candidate due to its low cost and good ability to absorb light. Unfortunately, silicon is only able to produce modest photocurrent densities of some µA/cm 2 , which decay very fast (in the minutes time scale) due to premature corrosion.…”

Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting

“…[94][95][96][97][98][99][100] While the assumptions regarding catalyst overpotentials and device architectures vary, the consensus of these studies is that a STH conversion efficiency of 425% is possible with a 2J approach for integrated systems in which the catalyst and absorber areas are equivalent. Both 1J and 3J approaches have lower efficiency limits.…”