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 membraneseparated catalysts and porous charge collectors), allow circumvention of some of the challenges imposed by solid-liquid interfaces in traditional 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][4][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][7][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. ...

Integrated Photo-Electrochemical Solar Fuel Generators under Concentrated Irradiation

“…We utilized our coupled 2-dimensional, non-isothermal multiphysics model 17 to simulate the performance of an integrated PEC device utilizing concentrated irradiation. The model accounts for concentrated solar irradiation, electromagnetic wave propagation, semiconductor charge generation and transport, heat transfer, fluid flow, mass transport, electrolyte and electrode charge transport, and electrochemical reactions.…”

“…19 The detailed modeling methodology is presented in Tembhurne et al 17 The net heat source term coming from EM wave propagation is defined to be the sum of the individual source and sink terms:…”

“…To incorporate this parameter, we have developed a fully automated coupled 2D multi-physics non-isothermal model which uses finite element and finite volume methods to predict the performance of the concentrated integrated PEC device. 17 The model has been validated 17 and accounts for: charge generation and transport in the triple/dual junction solar cell and the components of the integrated electrochemical system (polymeric electrolyte and solid electrode), electrochemical reaction at the catalytic sites, fluid flow and species transport in the channels delivering the reactant (water) and removing the products (hydrogen and oxygen), and radiation absorption and heat transfer in all components. Here, we use this model to quantify the gain in the performance of a CIPEC device, incorporating heat transfer between the different components and thermal management to actively steer the heat transfer process.…”

“…The 2D simulation domain is the plane shown in (a). A 3D schematic is shown in Tembhurne et al 17 The water from the top water channel is fed to the anodic chamber shown in (a) with green arrows. The PV's top p-type contact is connected to the titanium flow plate of the anode side via an electronic conductor, providing flow of positive charge (holes), shown in (b) with dark gray.…”

Evaluating particle-suspension reactor designs for Z-scheme solar water splittingviatransport and kinetic modeling

“…Several solutions are being pursued for solar fuels production including photovoltaic-powered electrochemical devices for water and carbon dioxide splitting [4][5][6][7], solar thermochemical cycles [8][9][10], integrated photoelectrochemical (PEC) devices [11][12][13][14][15][16], and suspended photocatalyst particles [17][18][19][20]. Fixed-electrode, wafer-based PEC devices to split water, that consist of photoactive electrodes immersed in an aqueous electrolyte, have been experimentally demonstrated [12,[21][22][23][24][25][26] and modeled extensively [27][28][29][30][31][32][33][34]. These designs have exhibited solar-tohydrogen energy-conversion (STH) efficiencies as high as 18% [25] at 1 sun illumination and over 30% [26] with concentrated sunlight; however, these demonstrations suffered from stability issues where operational lifetimes were less than one day [12].…”