CFD simulation and development of an improved photoelectrochemical reactor for H₂ production

Highlights

• An improved photoelectrochemical (PEC) reactor for water splitting is developed.

• Electrolyte flow profile in the proposed PEC reactor is improved significantly.

• Potential drop problem across the transparent conductive Films (TCFs) is studied.

• Using a golden grid on the TCFs is proposed to address the potential drop problem.

• The potential drop is decreased after applying the golden grid on TCFs.

Abstract

In this paper an improved dual chamber photoelectrochemical (PEC) reactor for water splitting is developed. COMSOL Multiphysics (4.2b) software is utilized to simulate two and three-dimensional PEC reactor. Different geometries are studied to have a uniform flow with minimum recirculation zones inside the photoreactor chamber. Furthermore, the gas evolution rate in the photoreactor at different current densities is studied. As expected, the simulation results showed that by increasing the current density, the gas production rate increases. Integration of a thin layer of golden grid on the transparent conductive films (TCFs) is proposed to reduce potential drop across the sheet resistance of TCFs in photoelectrods which is one of the major problems in construction of larger photoelectrods. The simulation results showed that the potential drop is decreased significantly (from about 25% drop to roughly 6% drop) after applying the golden grid on the fluorine doped tin oxide (F-TO) film. Finally, some practical considerations and data are provided for fabrication of the PEC reactor.

Introduction

Nowadays most of the world energy demand is being supplied by the fossil fuels which is easy to use and access. However, apart from depleting sources of the fossil fuels, burning them to produce energy causes devastating effects on our environment. Therefore, development of technologies for more sustainable and cleaner energy sources is inevitable. Even though hydrogen is not an ideal fuel, it is one of the promising options. Hydrogen is an energy carrier that produces water and energy when used in fuel cells. Water can be split into hydrogen and oxygen using a suitable semiconductor when subjected to sunlight [1]. Therefore, splitting water into hydrogen and oxygen provides a feasible solution to store sun's energy. Fujishima and Honda observed that the interaction of light with TiO₂ produces electron–hole-pairs [2]. The holes oxidize water at the TiO₂ surface to form oxygen while the electrons reduce water to form hydrogen at the counter electrode [3] (see equations (1), (2), (3)).

$$2\text{hν}\stackrel{\text{Semiconductor}}{\to }2{\text{h}}^{+}+2{\text{e}}^{-}$$

$$2{\text{h}}^{+}+{\text{H}}_2{\text{O}}_{\left(\text{l}\right)}\to \frac{1}{2}{\text{O}}_{2\left(\text{g}\right)}+2{\text{H}}^{+}$$

$$2{\text{H}}^{+}+2{\text{e}}^{-}\to {\text{H}}_{2\left(\text{g}\right)}$$

In recent years many photoelectrochemical researches has been done to develop new materials with increased stability and energy conversion efficiency [4], [5], [6], [7], [8], [9], [10], and little attention have been paid to the development of PEC reactors for hydrogen generation [8], [11], [12]. Even though, there are different types of photocells developed for testing the photoactive materials [8], there is dearth of research to develop more practical and optimized PEC reactors that targets scaled-up hydrogen production photoreactors.

Baniasadi et al. have proposed a hybrid water splitting reactor for H₂ production [13]. They improved the energy conversion efficiency by applying an external electric potential. However, they did not consider important design criteria such as: electrolyte flow profile distribution, potential drop problem across the photoelectrods, and light absorbtion. In recent times, some researchers investigated and developed new continuous-type PEC systems for hydrogen production [14], [15], [16]. Carver et al. recently have developed a relatively new PEC reactor with a 0.01 m² photoanode [11]. The main design criteria that they used to build the PEC reactor were light collection/absorption and improvement of the electrolyte flow distribution. In this work, we improved the photoreactor design considering the main design criteria mentioned in Section simulation results and discussion. Furthermore, the practical problems for constructing larger photoelectrodes is studied and addressed in the proposed PEC reactor. The author's main objectives in this paper was, eradication of limits imposed by potential drop across the sheet resistance of TCFs in photoelectrods, and improvement of PEC reactors for possible industrial uses.

The reset of this paper is organized as follows: 3D modeling of the reactor is explained in Section mathematical modeling, simulation results and discussions are provided in Section simulation results and discussion, and extra information for construction of the designed PEC reactor are given in Section fabrication of PEC reactor. The conclusions are stated in Section conclusions.