Crosslinked poly(vinyl alcohol) membrane as separator for domestic wastewater fed dual chambered microbial fuel cells

Highlights

• Catalyst-free PVA membrane synthesized by crosslinking with GA.

• Performance of Crosslinked PVA membrane and Nafion 117 were tested.

• Power performance enhanced by 3-fold using the complex substrate.

• Thermomechanical and antimicrobial property of the fabricated membrane studied.

Abstract

The use of Nafion as a proton exchange membrane in microbial fuel cells (MFCs) is expensive with operational issues like biofouling and fuel crossover limiting the practical application of the device to harvest energy from wastewaters. In this connection, a facile route is adapted to fabricate a Nafion-alternative membrane using poly(vinyl alcohol) (PVA) crosslinked with glutaraldehyde (GA) as a relatively low-cost, effective membrane for MFCs. The crosslinking of the PVA membrane resulted in a reduction in hydroxyl groups and the formation of the acetal ring and ether linkage demonstrated by controlled water uptake and swelling ratio with enhanced thermo-mechanical stability. The crosslinked membrane displayed higher power density than those typically reported for domestic wastewater fed MFCs, reaching a maximum of 158.28 mW/m² for the fabricated membrane. The PVA-GA membrane with antimicrobial activity, high power performance, and negligible fuel crossover shows its potential as a separator in future MFCs based on its performance and low cost of installation.

Introduction

Microbial fuel cells (MFCs) are bio-electrochemical devices which with the aid of bacteria convert organic matter to produce electricity. Over the past decade, MFCs have evolved into a simple yet robust technology with researchers across the globe constantly working on various aspects namely material and architectural aspect of the electrodes, microbial inoculum, chemical substrate as the feed material as well as low-cost proton exchange membrane, to enhance the performance of MFCs and also decrease its cost of operation. A more suitable route to cut down the cost drastically is to fabricate a low-cost membrane to replace the Nafion, which accounts for 38% of the total capital cost [1]. Although membrane-less MFCs can be promising at a small scale because of their simple design, low cost, and relatively high power density, however, the design is restricted to flat plate and tubular MFCs. Secondly, an increase in the diffusion of substrate and oxygen in absence of a membrane makes the MFCs operation less efficient [2]. Thirdly, a closely packed electrode owing to limited spacing in such membrane-less MFCs adds to the risk of the short circuit [3,4]. Fourthly, although the membrane-less configuration reduced the cost by eliminating the PEM membrane it increased the internal resistance. For Air-cathode MFC configuration, fouling has led to a decrease in power production and an increase in the operational cost [5,6]. Therefore, membranes as separators are a necessity to ensure efficient operation in dual-chambered MFCs, to allow the passage of protons through the membrane to maintain electro-neutrality, and to prevent unfavorable crossover across the membrane which will result in an enhancement in OCV value. An ideal separator has to be hydrophilic for low resistance and high ion transport, neutral to avoid pH change and ion-selectivity, mechanically stable to resist any deformation and possess anti-biofouling property to ensure stable power generation for a long duration [7]. However, to make the process of using membranes practical three aspects need to be considered namely, (a) the cost of the membrane, (b) its internal resistance, (c) biofouling during operation [8,9].

Recently PVA based polymer membranes have attracted a lot of attention owing to their broad applications in the medical field, in fuel cells as proton exchange membrane, and so on [[10], [11], [12], [13], [14]]. PVA is a non-toxic, biodegradable [15], and inexpensive polymer. It is inherently hydrophilic which makes it favorable for wastewater treatment [16]. It has good film-forming property due to its ability to crosslink within available hydroxyl groups in presence of a chemical crosslinker to retain the compact nature of the membrane under pressure [[17], [18], [19]]. Various crosslinking agents have been explored so far in combination with PVA film such as sulfosuccinate acid [20], poly (styrene sulfonic acid-co-maleic acid) with glutaraldehyde (GA) [21] to name a few. Although, the effect of crosslinking of PVA with GA under the acidic condition on characteristics of the PVA membrane has been examined in single-chambered MFCs [22]. It has been rarely focused upon how the PVA membrane crosslinked with GA would behave under the neutral condition when applied as a membrane in dual-chambered MFCs with domestic wastewater as substrate.

This is a novel attempt to prepare a PVA membrane using a more straightforward technique that is free from the catalyst, controller, or quencher. A major advantage of this technique is the use of water for membrane preparation instead of chemical solvent which saves the preparation cost and also rules out the leaching of impregnated acid in the long term that might restrict its application. In this study, a Nafion-alternative membrane using biodegradable PVA cross-linked with GA has been synthesized and its performance as a low-cost separator in a dual-chambered MFCs (DC-MFCs) fed with domestic wastewater has been evaluated for practical application. Microbial consortia in the wastewater act as the biocatalyst while wastewater is used as the substrate. Detailed analysis on the performance of the crosslinked membrane, when employed in the MFC based on its composition, thermo-mechanical stability, and antimicrobial properties, has been reported.