Solar photocatalytic fuel cell using CdS–TiO₂ photoanode and air-breathing cathode for wastewater treatment and simultaneous electricity production

Highlights

• Photocatalytic fuel cell was designed for wastewater treatment and electricity production.

• CdS–TiO₂ nanorods were used as effective photoanode for solar photocatalytic fuel cell.

• Air-breathing cathode was first reported in solar photocatalytic fuel cell.

• Our experimental study facilitates quantitative understanding of PFC wastewater treatment.

Abstract

Solar photocatalytic fuel cell (PFC) is a promising technology for environmental-friendly wastewater treatment and simultaneous production of electricity. In this study, PFC was enhanced by using CdS quantum-dot-sensitized TiO₂ nanorod array deposited onto FTO glass as effective photoanode. Moreover, gas diffusion electrode was employed to improve oxygen reduction reaction at the cathode. The material characterization shows that an array of 1.2-μm TiO₂ nanorods is decorated with 10-nm CdS quantum dots, which significantly improve solar light harvesting ability. The results of the PFC performance study indicate that light irradiation, acetic acid concentration, electrolyte pH and conductivity have significant influence on the short-circuit current and maximum power density. When the PFC operates at the optimum pH of 4.6, the short-circuit current and maximum power density are 1.79 mA/cm² and 1134 mW/cm², respectively. It is found that increasing the electrolyte conductivity is an effective approach to improve the PFC performance. The highest short-circuit current of 5.1 mA/cm² and maximum power density of 3980 mW/cm² are obtained with electrolyte having a conductivity of 63.1-mS/cm. In addition, the test results of various pure and practical organic substances in PFC further suggest that it is feasible to use sunlight as a driving force to clean up wastewater with simultaneous electricity production.

Introduction

Environmental pollution and energy crisis have significant impacts to our environment. Recovery of energy from wastewater is a promising approach to mitigate these two problems simultaneously. The organic compounds in wastewater, such as carbohydrates, fatty acids and amino acids, store substantial chemical energy [1]. Proper energy conversion technology can be applied to turn the potential energy into a useful form, such as electricity and/or hydrogen fuel. Not only can the energy recovery contribute to carbon–neutral energy supply, but also it can significantly reduce the burden of wastewater treatment. Recently, microbial fuel cell (MFC) is proposed as a candidate technology for achieving wastewater treatment and simultaneous production of electricity [2], [3], [4]. However, the present MFC technologies reveal slow bio-electrochemical activity because of the complex electron transfer mechanisms. Other limitations of MFC include complicated bacterial cultivation, long start-up time and stringent working conditions [5].

Alternatively, photocatalytic fuel cell (PFC), which is an integration of photocatalysis and fuel cell technologies, is a more promising approach. The photoanode of PFC is deposited with a thin layer of active photocatalyst (e.g. TiO₂). When the photoanode is exposed to light irradiation, photo-induced electrons (e⁻) and holes (h⁺) are produced at the photocatalytic sites. The holes can oxidize OH⁻ to produce hydroxyl free radicals (OH) or oxidize photodegradable organic substances in the solution to form CO₂, electrons and protons. Electrons generated from photocatalysis (TiO₂ + hν → h⁺ + e⁻) and oxidation of organics (C_xH_yO_z + OH → CO₂ + H⁺ + e⁻) at the anode side can move through the external circuit to the cathode side, where reduction reaction occurs. Taking Pt/C catalyst as an example, the reaction varies depending on the availability of oxygen. In case oxygen is absent, the reduction reaction involved is hydrogen evolution reaction. On the other hand, if oxygen is present in the solution, the reaction is oxygen reduction to produce water. Therefore, it is feasible to convert organic substances including pollutants into electricity and/or hydrogen fuel.

Photocatalytic fuel cell is recently regarded as a re-emerging research field [6]. The architecture can be categorized into single-compartment or two-compartment cell [7], [8], [9]. Most of previous studies were based on single-compartment cell because the simple design is of high potential for commercialization. The two-compartment cell configuration requires additional separator, such as proton exchange membrane (Nafion®) [10], that will increase the cost of the PFC. The key components of PFC include photoanode, cathode and photodegradable organic substances in the supporting electrolytes. The commonly used photocatayst for the PFC photoanode is the commercially available Degussa P25 TiO₂ powder, which can be activated by UV [11], [12], [13]. Thus, the performance of solar PFC is hindered by the material itself, due to the limited activating UV spectrum (4–5% in sunlight). Various deposition methods were employed in the fabrication of photoanode using powdered TiO₂, such as spin coating [14], screen printing [15] and doctor blade method [8]. Direct growth of TiO₂ onto electric conducting substrates (e.g. Ti sheet and FTO glass) is a promising strategy to enhance the photoelectrochemical performance. TiO₂ nanotube array as photocatalyst could be fabricated by direct anodization of Ti sheets in fluoride containing electrolyte [16], [17]. However, the high cost of Ti sheets and strong toxicity of fluorides pose barriers for large-scale applications.

For the cathode, previous work employed Pt/C electrocatalysts for oxygen reduction reaction [18]. However, the cathode performance was greatly limited by the low oxygen solubility in the electrolyte. Additional energy-intensive process, such as air-sparging, was required to enhance oxygen transfer to the cathode [19]. Therefore, it is desirable to produce effective solar-light-responsive photoanode and cathode to enhance the PFC performance.

Recently, aligned TiO₂ nanorod array on FTO glass has shown good photocatalytic performance in energy harvesting from sunlight [20], [21], [22]. Many techniques were proposed to modify TiO₂ nanorods to enhance the visible-light absorption ability [23], [24], [25], [26]. CdS quantum-dot-sensitized TiO₂ is a promising photocatalyst in harvesting solar energy [27], [28], [29], [30]. Quantum dot (QD) is a kind of semiconductor with crystal in dimension of nanoscale which can exhibit quantum mechanical properties. The application of QDs attracts much attention in photocatalytic field especially in solar cells as photosensitizer [31]. Up to the present time, the literature has not reported any study on CdS modified TiO₂ nanorod arrays for PFC application. Besides, with reference to previous works, air-breathing electrode could improve the fuel cell applications. Previous studies on air-breathing microbial fuel cell and microfluidic fuel cell for enhancing the power output are useful references for the present investigation [32], [33], [34].

This study reports the use of solar-light-responsive photocatalyst CdS–TiO₂ deposited onto FTO transparent conducting glass as effective photoanode in a single-compartment photocatalytic fuel cell with air-breathing cathode. In the experiments, acetic acid was used to represent the primary organic compound of wastewater. The parametric effects of acetic acid concentration, electrolyte pH and conductivity on the PFC performance were systematically investigated. In addition, various organic substances were tested in the PFC to demonstrate the feasibility of recovering energy from wastewater.