En Route to a Unified Model for Photoelectrochemical Reactor Optimization. II–Geometric Optimization of Perforated Photoelectrodes

Abstract

Results have been reported previously of a model describing the performance of photoelectrochemical reactors, which utilize semiconductor | liquid junctions. This model was developed and verified using SnIV-doped α-Fe2O3 as photoanodes. Hematite films were fully characterized to obtain parameter inputs to a model predicting photocurrent densities. Thus, measured photocurrents were described and validated by the model in terms of measurable quantities. The complete reactor model, developed in COMSOL Multiphysics, accounted for gas evolution and desorption in the system. Hydrogen fluxes, charge yields and gas collection efficiencies in a photoelectrochemical reactor were estimated, revealing a critical need for geometric optimization to minimize H2-O2 product recombination as well as undesirable spatial distributions of current densities and “overpotentials” across the electrodes. Herein, the model was implemented in a 3D geometry and validated using solid and perforated 0.1 × 0.1 m2 planar photoanodes in an up-scaled photoelectrochemical reactor of 2 dm3. The same model was then applied to a set of simulated electrode geometries and electrode configurations to identify the electrode design that would maximize current densities and H2 fluxes. The electrode geometry was modified by introducing circular perforations of different sizes, relative separations and arrangements into an otherwise solid planar sheet for the purpose of providing ionic shortcuts. We report the simulated effects of electrode thickness and the presence or absence of a membrane to separate oxygen and hydrogen gases. In a reactor incorporating a membrane and a photoanode at 1.51 V vs RHE and pH 13.6, an optimized hydrogen flux was predicted for a perforation geometry with a separation-to-diameter ratio of 4.5 ± 0.5; the optimal perforation diameter was 50 µm. For reactors without a membrane, this ratio was 6.5 and 8.5 for a photoanode in a “wired” (monopolar) and “wireless” (photo-bipolar) design, respectively. The results and methodologies presented here will serve as a framework to optimize composite photoelectrodes (semiconductor | membrane | electrolyte), and photoelectrochemical reactors in general, for the production of hydrogen (and oxygen) from water using solar energy.

Reply to the ‘Comment on “Flat band potential determination: avoiding the pitfalls”’ by M. I. Díez-García, D. Monllor-Satoca and R. Gómez, J. Mater. Chem. A, 2022, 10, DOI: 10.1039/D1TA06474F

“…2, did not include a limitation by the photon flux. Although we imposed such a constraint in several of our other publications, 5–7 where we modelled the photocurrent, we did not do so in ref. 2.…”

Earth-Abundant Metal Oxides for Monolithic Tandem Photoelectrochemical Water Splitting Devices: Current Trends and Perspectives

“…Previous studies have also explored modifications to the tandem device structure to facilitate ion transport. For example, a tandem nanowire embedded in a gas separation membrane or perforated tandem PEC device , can make both electron and ion transport paths short. In addition, a PEC cell with a louvered structure was also suggested to enhance the ion transport …”

“…As previously discussed, one of the key advantages of PEC lies in its ability to minimize electrical and ion transport paths by employing appropriate structures. While suggestions such as tandem nanowires embedded in gas separation membranes or perforated tandem cells have been proposed to achieve this advantage, research in this area remains limited. − We anticipate that these perspectives will offer valuable guidance to the field of PEC, encouraging further exploration and innovation in addressing the existing challenges and advancing the development of efficient tandem PEC devices.…”

Simulation of photoelectrochemical CO2 conversion to useful chemicals

“…To improve the performance of the PEC CO 2 process, the effective factors like light intensity, operating mode (batch reaction system and continuous flow reaction system), operating conditions (temperature, pressure, flow rate), bandgap (type of semiconductors as photocatalyst), pH (type of electrolyte), product separator (type of ion-exchange membrane), and cell configurations were reviewed in many pieces of research for laboratory scale. The arrangement and PEC cell geometric configuration of the main components include photoelectrode, counter-electrode, electrolyte, and ionic conducting membrane which are still challenging for investigation in the laboratory and pilot plant, therefore, the numerical study for CO 2 reduction should be applied to model and simulate the process [40].…”

“…𝜙0 is equilibrium potential (V) Therefore, the rate of electrochemical reactions at the interface od electrodes expressed as 𝑅 𝑖 is shown in Eq. (40).…”