Chapter I: Introduction
1.2 Solar-to-fuel conversion system
1.2.1 Solar-driven water-splitting cell
1.2.1.1 Sensitivity analysis
Figure 1.9: Schematic energy band diagrams of an integrated photoelectrochemical (PEC) system under 1 Sun illumination for (a) the photocathode + photoanode PEC system, and (b) the tandem light absorber + electrocatalyst PEC system. Types I and II illustrate the semiconductor–liquid junctions and buried junctions, respectively.
An integrated solar-driven water-splitting cell generally has two configurations, photocathode + photoanode PEC and tandem absorber + electrocatalysts PEC. Figure 1.9a shows the energy band diagram for the first configuration, where a photoanode and a photocathode are connected back-to-back with an Ohmic contact. The photogenerated minority-carrier electrons drift and diffuse to the photocathode-electrolyte interface and reduce H+ to H2, while the photogenerated minority-carrier holes drift and diffuse to the photoanode-electrolyte interface and oxidize water to O2. In the meantime, the majority- carriers (holes in photocathode and electrons in photoanode) recombine at the Ohmic contact.
Both the semiconductor-liquid junction, which is formed at the photoelectrode-electrolyte interface, as seen in Type I, and the ‘buried’ junction, which is formed inside the photoelectrode, as seen in Type II, can be served as the asymmetrical barrier to effectively
separate the photogenerated carriers in this configuration. The energy band diagram for the second configuration is indicated in Figure 1.9b. This model consists a tandem two- junction light absorber and HER/OER catalysts that are in electrical connection to the electron/hole collectors of the light absorber. For this configuration, both junctions can be
‘buried’, as seen in Type I, or one of the junctions can be at the photoelectrode-electrolyte interface, as seen in Type II.
Figure 1.10: Overlaid current density-potential behavior for a p-type photocathode and an n-type photoanode, with overall efficiency projected by the power generated PSTH = Jop (1.23 V) by the cell for splitting water.
The overall solar-to-hydrogen conversion efficiency (STH) of the water-splitting system is calculated through
𝜂𝜂𝑆𝑆𝑆𝑆𝑆𝑆 =1.23(𝑉𝑉)∙𝐽𝐽𝑜𝑜𝑜𝑜(𝑚𝑚𝑚𝑚 𝑐𝑐𝑚𝑚−2)
𝑃𝑃𝑖𝑖𝑖𝑖 (𝑚𝑚𝑚𝑚 𝑐𝑐𝑚𝑚−2) , (Eq. 1.1) where 𝐽𝐽𝑜𝑜𝑜𝑜 stands for the operating photocurrent density and Pin represents the total incident solar irradiance. By independently characterizing photoanodes and photocathodes, the expected performance of an integrated system can be directly calculated. 𝐽𝐽𝑜𝑜𝑜𝑜 can be obtained by overlapping the individually calculated J-V data for each photoanode/photocathode, as
shown in Figure 1.10, in which the red shaded area illustrates the maximal power generated for each component of the cell while the blue shaded area illustrates the power generated at the operating current density.
Solar to hydrogen conversion efficiency of such a system depends on the performance and materials properties of all the individual components as well as the design of the system.
Significant research efforts are being devoted to improving the performance of all of the system components, yet some improvements will result in larger gains in the overall system efficiency than others. In Chapter II, a sensitivity analysis of the solar-to-hydrogen conversion efficiency with respect to the materials properties of light absorbers, electrocatalysts, and the geometric design parameters, for a series of specific but generic designs for solar-fuels generators, has been described. The analysis has revealed the relative importance of reductions in the overpotentials of electrocatalysts, of improvements in the materials properties of light absorbers, and of optimization in the system geometry for various types of solar-fuels generators, while considering operation at a range of temperatures as well as under a variety of illumination intensities including up to 10-fold optical concentration. Such a sensitivity analysis provides a quantitative framework within which to assess the gains in system performance that can be attained as a result of improving, relative to the current state-of-the-art, the performance of different components of the system, and provides a useful framework for setting a forward R&D agenda for such systems.
Figure 1.11: Schematic illustration of the trade-offs between optical obscuration and concentrated operational current densities at the catalyst surface for photoabsorbers (a), coated with continuous electrocatalyst films; (b) coated with patterned electrocatalyst films with high filling fractions, and (c) coated with patterned electrocatalyst films with low filling fractions.
Furthermore, efficient photoelectrochemical water splitting requires the use of electrocatalysts that reduce the kinetic barriers to the reduction and oxidation half-reactions.
However, the electrocatalysts can absorb or reflect light, and thus can limit the overall solar- to-hydrogen conversion efficiency. One strategy for reducing the optical obscuration that results from the presence of the electrocatalyst layer is to produce a patterned catalyst film that results in a low geometric filling fraction of the metal catalyst on the surface of the light absorbers, as seen in Figure 1.11, but reducing the filling fraction also increases the kinetic overpotentials required for the desired reactions. The trade-off between the optical obscuration and kinetic overpotentials of electrocatalyst films patterned onto the surface of tandem light-absorber structures in model photoelectrosynthetic water-splitting systems was investigated using a 0-dimensional load-line analysis and experimental measurements. The electrocatalytic performance of the catalyst at high current densities, normalized to the electrocatalyst surface area, is an important factor in the dependence of the optimal solar-to- hydrogen (STH) conversion efficiency, η𝑆𝑆𝑆𝑆𝑆𝑆,𝑜𝑜𝑜𝑜𝑜𝑜, on the filling fraction (fc) of the patterned catalysts, because even under conditions that produce minority-carrier current densities of
∼10 mA cm−2 at the solid/liquid interface, the current density at catalyst-bearing sites can be
>1–2 A cm−2 in low filling-fraction films. The maximum STH conversion efficiency, η𝑆𝑆𝑆𝑆𝑆𝑆,𝑜𝑜𝑜𝑜𝑜𝑜, using a hypothetical electrocatalyst that was optically transparent but which nevertheless exhibited a current-density versus potential behavior that is characteristic of the most active Pt films measured experimentally regardless of their optical obscuration, was calculated as 26.7%. By comparison, the maximum η𝑆𝑆𝑆𝑆𝑆𝑆,𝑜𝑜𝑜𝑜𝑜𝑜 of 24.9% for real patterned Pt electrocatalyst films closely approached this ideal-case limit. Hence, patterned electrocatalysts with very low filling fractions can provide a potentially promising path to the realization of efficient large-scale photoelectrolysis systems while minimizing the use of scarce noble metals.