Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation

Abstract

We investigate the direct conversion of solar energy and water into a storable fuel via integrated photo-electrochemical (IPEC) devices. Here we focus particularly on a device design which uses concentrated solar irradiation to reduce the use of rare and expensive components, such as light absorbers and catalysts. We present a 2-dimensional coupled multi-physics model using finite element and finite volume methods to predict the performance of the IPEC device. Our model accounts for charge generation and transport in the photoabsorber, charge transport in the membrane-separated catalysts, electrochemical reaction at the catalytic sites, fluid flow and species transport in the porous charge collectors and channels, and radiation absorption and heat transfer for all components. We then develop performance optimization strategies utilizing device design, component and material choice, and adaptation of operational conditions. Our model predicts that operation under high irradiation is possible and that dedicated thermal management can ensure high performant operation. The model shows to be a valuable tool for the design of IPEC devices under concentrated irradiation at elevated temperatures. To our knowledge, it is the most detailed yet computationally low-cost model of an IPEC device reported. The solar energy received on earth's surface can meet mankind's current and future energy demand. 1,2 Direct conversion of solar energy and water into chemical energy via photo-electrochemical (PEC) processes is one viable route for renewable fuel processing and energy storage. Integrated photo-electrochemical (IPEC) devices, i.e. composed of an integrated buried photovoltaic (PV) component and an integrated electrochemical component (consisting of membrane-separated catalysts and porous charge collectors), allow circumvention of some of the challenges imposed by solid-liquid interfaces in tradi-tional PEC devices, while also exhibiting potential to operate at higher efficiencies and lower cost than externally wired (non-integrated) PV plus electrolyzer (EC) devices. 3–5 We refer to the design as " integrated " to signify that the PV and electrolyzer components are area-matched and in direct contact, allowing heat transfer from one component to the other and for thermal management strategies to be applied. In order to increase the economic competitiveness of IPEC devices compared to conventional hydrogen generation pathways, we consider concentration of irradiation. 6–8 This leads to large driving current densities (approximately proportional to the concentration factor), and thus introduces larger overpotentials and potential mass transport limitations. 9 Concentration also decreases the performance of the photoactive components due to increased temperature. On the other hand, the kinetics are enhanced with increased temperature. Ionic transport in the solid electrolyte is also enhanced with increased temperature, but this increase stops abruptly and sharply drops at temperatures above 120 • C due to membrane dry out. 3,10,11 This com-peting and coupled behavior of the components requires a detailed understanding of the heat transfer, charge transport, fluid flow, and reaction kinetics in order to formulate performance optimization strategies for concentrated integrated photo-electrochemical (CIPEC) devices via device design and adaptation of the operational conditions. Multi-physics computational models are a crucial support in device design and engineering. They allow in-depth analysis of conceptual designs, support feasibility investigations of devices and integrated systems, and permit the quantification of performance. Modeling ef-forts of PEC and specifically IPEC devices are limited. The earliest attempts used lumped-circuit models of a photocell in series with a current-dependent electrochemical load. 12 Berger et al. 13 presented a basic 1-dimensional model for light absorbers and electrolysis with