Modeling the Performance of an Integrated Photoelectrolysis System with 10 × Solar Concentrators

Abstract

Two designs for an integrated photoelectrolysis system that uses a 10× concentrating solar collector have been investigated in detail. The system performance was evaluated using a multi-physics model that accounted for the properties of the tandem photoabsorbers, mass transport, and the electrocatalytic performance of the oxygen-evolution and hydrogen-evolution reactions (OER and HER, respectively). The solar-to-hydrogen (STH) conversion efficiencies and the ohmic losses associated with proton transport in the solution electrolyte and through the membrane of the photoelectrolysis system were evaluated systematically as a function of the cell dimensions, the operating temperatures, the bandgap combinations of the tandem cell, and the performance of both the photoabsorbers and electrocatalysts. Relative to designs of optimized systems that would operate without a solar concentrator, the optimized 10× solar concentrator designs possessed larger ohmic losses and exhibited less uniformity in the distribution of the current density along the width of the photoelectrode. To minimize resistive losses while maximizing the solar-to-hydrogen conversion efficiency, η STH , both of the designs, a two-dimensional "trough" design and a three-dimensional "bubble wrap" design, required that the electrode width or diameter, respectively, was no larger than a few millimeters. As the size of the electrodes increased beyond this limiting dimension, the η STH became more sensitive to the performance of the photoabsorbers and catalysts. At a fixed electrode dimension, increases in the operating temperature reduced the efficiency of cells with smaller electrodes, due to degradation in the performance of the photoabsorber with increasing temperature. In contrast, cells with larger electrode dimensions showed increases in efficiency as the temperature increased, due to increases in the rates of electrocatalysis and due to enhanced mass transport. The simulations indicted that cells that contained 10% photoabsorber area, and minimal amounts of Nafion or other permselective membranes (i.e. areal coverages and volumetric fractions of only a few percent of the cell), with the remaining area comprised of a suitable, low-cost inert, non porous material (flexible polymers, inert inorganic materials, etc.) should be able to produce high values of η STH , with η STH = 29.8% for an optimized design with a bandgap combination of 1.6 eV/0.9 eV in a tandem photoabsorber system at 350 K. Artificial photosynthesis could provide a promising route to large-scale solar energy conversion and storage. 1-4 Recent techno-economic studies have evaluated various designs for integrated photoelectrolysis systems, including a very promising system that makes use of concentrated illumination. 5,6 A discrete III-V photovoltaic cell connected electrically in series with a discrete polymer-electrolyte membrane (PEM) electrolyzer has demonstrated a solar-to-hydrogen (STH) conversion efficiency of 18% under 500 Suns. 7 Although concentrated photovoltaics (CPV) typically incorporate multi-stage optical systems to achieve high optical concentration (∼ 400 Suns to 1200 Suns), 8-10 integrated photoelectrochemical systems for large scale, distributed solar-to-fuel applications are most likely operate efficiently and scal-ably at lower solar concentration factors (5-100) due to limitations associated with electrocatalytic overpotential losses, ohmic losses, and mass transport restrictions associated with high current densities in a system operating under very concentrated sunlight. Notably, systems that utilize a low-multiple concentrating solar collector, such as a 10× concentrator, requires little or no active solar tracking or temperature-regulation systems. 11-13 Conceptual designs of coupling low concen-trator solar collectors with a photoelectrochemical cell have been proposed. 14,15 A principal advantage of a sunlight-concentrating design for a solar-to-fuels generator is the potential reduction of the usage per unit area of photoabsorber materials, which could result in a significant reduction in the system cost. 5,6 Although extensive model-ing and simulation efforts have been completed for solar fuel generator system designs without solar concentrators, including various operating conditions and using different types of input feed-stocks including liquid electrolytes 16-18 and water vapor feeds 19 , the design criteria and constraints for an integrated system that exploits concentrating solar collectors have not yet been evaluated. The high-intensity illumination, and the expected elevated operating temperatures in a light-concentrating photoelectrolysis system, could have significant impacts on the performance of the individual * Electrochemical Society Active Member. z E-mail: nslewis@caltech.edu components of the system, and thus on the efficiency of the system as a whole. Increases in the illumination intensity would increase the photocurrent density and would concomitantly improve the open-circuit voltage and the fill factor of the current-voltage characteristic of the photoabsorber materials. 20 The increased current density would also, however, result in an increase in the ohmic losses of the cell, as well as produce an increase to the overpotentials required to drive the oxygen-evolution reaction (OER) and the hydrogen-evolution reaction (HER). 21 Increases in the operating temperatures would introduce similar trade-offs to the system design, because the increased temperature would degrade the performance of the semiconducting light absorbers while improving both the mass transport in the elec-trolyte and the performance of the HER and OER catalysts. As a result, the overall system efficiency as a function of the cell geometry , the illumination intensity, and the operating temperature depends upon the working principles for each component as well as upon the detailed mathematical relationships between the components. Previously reported results from the modeling of integrated pho-toelectrolysis system designs without solar concentrators have shown that the geometric parameters of the cell dominate the cell performance. Specifically, the width of the photoelectrode must be less than a few centimeters to minimize the ohmic losses from the ionic transport in the electrolyte and membrane. 17,22 At the higher operating current density produced by concentrated sunlight, the details of the cell geometry are likely to play an even more important role. Although a smaller cell will typically reduce the efficiency losses due to mass transport in the electrolyte, the assembly and integration of absorber materials and other system components could be easier with macroscopic, rather than microscopic, components. Thus, an optimal design would likely maximize the photoelectrode width while minimizing any efficiency losses due to mass transport limitations and ohmic losses. In this work, two types of integrated photoelectrolysis systems that use a concentrated light source have been investigated: a two-dimensional "trough" design and a three-dimensional "bubble wrap" design. The concentration of the illumination was chosen to be tenfold relative to natural sunlight. The solar-to-hydrogen (STH) conversion) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.215.70.231 Downloaded on 2014-09-25 to IP