Nanostructured polypyrrole cathode based dual rotating disk photo fuel cell for textile wastewater purification and electricity generation
Graphical abstract
Introduction
Photo fuel cell (PFC), as a separate class of photoelectrochemical cell with a single cathode connecting with photoanode, has been paid more and more attention in recent years [[1], [2], [3], [4]]. With light irradiation, the potential difference resulting from photoanode and cathode can help the transport of photo generated electrons from anode to cathode through external circuit, which can not only generate electrical energy, but is also beneficial to the separation of electrons and holes, and thus the improvement in photocatalytic (PC) activity. To apply PFC in PC reactor with high light utilization efficiency, such as thin solution film reactor [[5], [6], [7]], can not only improve organic pollutants degradation efficiency, but can also utilize the chemical energy existing in wastewater more efficiently to generate electricity [[8], [9], [10]].
Cathode always plays more important role in electricity generation in PFC because cathode material may affect the rate of electron consumption through oxygen reduction, and further affect the voltage, current and output power of the PFC [11,12]. Moreover, the electricity generation performance may affect the separation of photogenerated electrons and holes, and further affect the PC degradation performance that occurs on photoanode. Though the most common used Pt based catalysts exhibit excellent catalytic activity on cathode, the expensive price restricts its real application in PC reactor [13,14]. Nickel, cobalt and other metal based catalysts are also employed as cathode catalysts by researchers in recent years and exhibit good electricity generation performances [15,16], but their stabilities still need to be improved for complex industrial wastewater purification, especially for the water with low pH value. Low cost cathode material with high electricity generation performance, large surface area and strong stability still needs to be developed to be applied in PFC reactor to treat real industrial wastewater.
Polypyrrole (PPy), as one of the conductive polymers, has been widely used in sensors, anticorrosive coatings and supercapacitors in recent years due to its easy synthesis, high environmental and thermal stabilities, distinct oxidation/reduction activities and tunable electrical conductivity [[17], [18], [19]]. PPy films based on FTO, silicon, nickel foam, stainless steel and titanium are always excellent candidates as electrode catalysts in batteries and supercapacitors in the past decades [[20], [21], [22]]. In our former work, we also found that PPy films grown on nickel foam and titanium substrates through electrochemical polymerization are promising cathode catalysts to replace Pt in PC hydrogen production and electricity generation, respectively [23,24]. However, the PPy film grown on metal substrate usually shows poor mechanical malleable property [24]. The film may easily pack together and fall off from the substrate with the film growing thicker (Fig. S1). Moreover, the film grown on metal substrate is usually built by large sized particles [23,24]. Nanostructures are difficult to be formed, resulting in small surface area and relative low activity. Carbon material always owns large surface area and exhibits certain mechanical, thermal and electronic properties [[25], [26], [27], [28]]. To combine PPy with carbon material may improve its mechanical and electronic properties to a certain extent. More important, nanostructured PPy film can grow on the surface of carbon substrate more easily than on titanium surface [29,30]. Nanostructured film is high desirable in fuel cells due to its high surface area and short ions diffusion path lengths, and thus strong catalytic activity.
In this work, three nanostructured PPy films, including nanosilk, nanowire array and nanorod array, are synthesized on carbon cloth substrate through a facile electrochemical polymerization method with only adjusting the concentration of p-toluenesulfonate acid (TsOH). The effects of polymerization conditions such as TsOH concentration, polymerization time and current density on the morphologies of the films and their activities being as cathodes in PFC are comprehensively investigated. The films are further applied in a dual rotating disk PFC reactor for advanced purification of real textile wastewater. The electricity generations and PC degradation performances obtained by different cathodes are compared and evaluated.
Section snippets
Materials and reagents
Titanium plate (99.6% purity, thickness 0.5 mm) purchased from Shanghai Hongtai Metal Production Co. Ltd. (Shanghai, China) was employed as substrate for photoanode. WOS1002 carbon cloth (CC) purchased from CeTech Co. Ltd. (Taiwan) was employed as substrate for cathode. All chemical reagents purchased from Shanghai Chemical Reagent Co. (Shanghai, China) were in analytical grade without further purifications. All solutions were prepared in doubly distilled water. The real textile wastewater
Characterizations of PPy films
As shown in Fig. 2a and b, a thin and flat PPy film (Fig. 2b) is covered on the surface of carbon fibers (Fig. 2a) when there is no TsOH in electrolyte. PPy films with different nanostructures are formed in the presence of different concentrations of TsOH in electrolyte. Nanosilk film (Fig. 2c and d) is observed when TsOH is as low as 0.005 mol L−1. With TsOH increasing to 0.025 and 0.050 mol L−1, nanowire arrays (Fig. 2e and f) and nanorod arrays (Fig. 2g and h) are observed, respectively.
Conclusions
PPy films with different nanostructures are synthesized on carbon cloth substrate through electrochemical polymerization in this work by adjusting the concentration of TsOH in electrolyte. PPy nanosilk, nanowire array and nanorod array films are formed with TsOH increasing from 0.005 to 0.025 and 0.050 mol L−1. The nanostructures grow longer with polymerization time, and become denser and shorter when the current density is at relative high level. The nanostructured films exhibit much better
Acknowledgements
Financial supports from the National Natural Science Foundation of China (Project No. 21607103, 21507082 and 21737002), the Science and Technology Opening Cooperation Project of Henan Province (No. 182106000010) are gratefully acknowledged.
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