Applied Energy 262 (2020) 114475 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Microbial Fuel Cell stack performance enhancement through carbon veil T anode modification with activated carbon powder ⁎ ⁎ Iwona Gajdaa, , John Greenmana,b, Ioannis Ieropoulosa, a Bristol BioEnergy Centre, Bristol Robotics Laboratory, University of the West of England, Bristol BS16 1QY, UK b Department of Applied Sciences, University of the West of England, Bristol BS16 1QY, UK HIGHLIGHTS GRAPHICAL ABSTRACT Novel modification of carbon veil • anode with activated carbon in urine fed MFC stack. • Modified microstructure enhanced surface area and power performance up to 21.1 W m−3. • The modification exhibited 77% in- crease in power output of the MFC stack. • It improved COD reduction by 13.5% in comparison to control stack. • It exhibited a stable performance after 500 days and resilience to prolonged starvation. ARTICLE INFO ABSTRACT Keywords: The chemical energy contained in urine can be efficiently extracted into direct electricity by Microbial FuelCell Activated carbon stacks to reach usable power levels for practical implementation and a decentralised power source in remote Ceramic locations. Herein, a novel type of the anode electrode was developed using powdered activated carbon (PAC) Long-term operation applied onto the carbon fibre scaffold in the ceramic MFC stack to achieve superior electrochemical performance Microbial Fuel Cell during 500 days of operation. The stack equipped with modified anodes (MF-CV) produced up to 37.9 mW Urine (21.1 W m−3) in comparison to the control (CV) that reached 21.4 mW (11.9 W m−3) showing 77% increase in Stacking power production. The novel combination of highly porous activated carbon particles applied onto the con- ductive network of carbon fibres promoted simultaneously electrocatalytic activity and increased surface area, resulting in excellent power output from the MFC stack as well as higher treatment rate. Considering the low cost and simplicity of the material preparation, as well as the outstanding electrochemical activity during long term operation, the resulting modification provides a promising anode electrocatalyst for high-performance MFC stacks to enhance urine and waste treatment for the purpose of future scale-up and technology implementation as an applied off-grid energy source. ⁎ Corresponding authors. E-mail addresses: [email protected] (I. Gajda), [email protected] (I. Ieropoulos). https://doi.org/10.1016/j.apenergy.2019.114475 Received 2 September 2019; Received in revised form 26 December 2019; Accepted 28 December 2019 0306-2619/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). I. Gajda, et al. Applied Energy 262 (2020) 114475 1. Introduction activated carbon derived from the chestnut shell was proven suitable in the MFC anode [34] or as an addition to the constructed wetland MFC Microbial Fuel Cell is a type of bioelectrochemical device that em- [35]. Further development of new, efficient and cost effective materials ploys electroactive microorganisms to transform chemical energy into is needed for construction of sustainable bioelectrochemical technolo- direct electricity by means of a combination of microbial metabolic and gies, which can be deployed in wastewater treatment plants and in electrochemical reactions from a variety of substrates [1] including remote locations as the decentralised power sources. This work aims to wastewater [2] and urine [3]. In the anode of the MFCs, microbial explore for the first time, simple and inexpensive modification ofthe consortia break down organic “fuel” into electrons and protons. Elec- anodic carbon veil scaffold with the addition of powdered activated trons derived from the anaerobic oxidation by the microbial consortia carbon onto the fibres to produce a three-dimensional (3D) structure in are transported to the anodic electrode and travel through the external order to improve the output of a MFC stack. It aims to test the MFC circuit to the cathode electrode, while protons and other cations pass performance, as a result of this modification, over a long term period through a membrane, which separates both chambers. Urine is a va- (500 days) under feeding and starvation regimes and discusses scale-up luable “waste” that could potentially be used to off-set the need to mine challenges for wider implementation. This is in order to aid the de- and produce synthetic fertilisers [4]. It is also an energy-rich substrate velopment of an affordable system that is designed for the ease ofmass possessing good electrical conductivity due to high ionic content, which production of future urine-powered systems deployed as decentralised makes it an ideal substrate for the production of direct electric current energy sources in a wide range of real world environments. In order for as well as nutrient recovery [5] both in MFCs [6] and Microbial Elec- MFCs to be fully realised as off-the-grid power sources, research must trolysis Cells (MECs) [7]. Urine operated MFCs have the ability to focus on producing low cost and easy to manufacture electrode and generate electricity in remote locations as the off- grid energy source membrane materials, which have been tested in realistic environments. and can thus be used in areas of poor energy infrastructure. There are however major limiting factors that hinder the practical implementa- 2. Materials and methods tion of MFCs at larger scale, which are low power output, material cost and difficulties in the scale-up process as well as system longevity. 2.1. Experimental set-up Multiple findings suggest that the viability of the urine-operated sys- tems is directly correlated with its use [8,9] and its suitability for Individual MFCs were assembled using terracotta cylinders (Jain generating enough power to directly light an LED [10] or charge a Scientific Suppliers, India) as previously described [36]. The cylinder mobile phone [11]. Technology scale-up is of enormous importance as dimensions were 50 mm (h), 22 mm inside and 30 mm outside dia- it brings the bioelectrochemical technology innovations into real world meter. The cathodes were made of activated carbon (G Baldwins and environments, where it is tested in practice for electricity production Co., UK) and 20% PTFE blend applied onto the hydrophobic carbon veil [8,9], treatment [12] and hydrogen generation [13]. One approach into as previously described [31]. The cathodes (22.5 cm2) were placed in scale-up is through stacking and multiplication of MFC units [14] where the inner chamber of the cylinder, with the activated carbon layer ex- the miniaturisation of stacked [15] reactors achieves higher energy posed to the ceramic wall using stainless steel crocodile clips as con- efficiency through lower ohmic losses [14]. To improve the MFC nectors to the external circuit and data acquisition hardware. Blocks of output, efforts have been made into the reactor design focusing ona activated carbon foam (Finest Filters, UK) were inserted into the cy- membrane-based [15] or membrane-less configuration [9,16], anodic linders to maintain good electrical contact between the cathode and the [17] and cathodic modifications [18,19] as well as achieving low-cost ceramic membrane. and low-tech production procedures for materials including ceramic For the test, two types of anode materials were prepared: carbon [20] and cardboard [21]. Numerous anode modifications have been veil fibre (CV) as the control and modified carbon veil (MF-CV) with pursued over the past decades looking into the metallic [22], carbon activated carbon powder. The preparation is described as follows: [23], composite [24] and chemical modifications [25] that aim to in- CV- Carbon veil fibre was purchased from PRF Composites, UKwith crease its specific surface area as well as to increase electron transfer the carbon loading of 20 g/m−2 and cut to the dimensions of rates, however in light of the practical application of the technology, 200 × 140 mm. It was folded and wrapped around the terracotta cy- selected materials and methodology of their synthesis/preparation linder and tightened with stainless steel wire to hold the electrode in should take cost and ease of assembly into account [26] in order to place and to maintain an electrical connection for the external circuit. allow for the electrode materials to be mass produced in a cost-effective MF-CV- identical piece of carbon veil used as the control was coated way. However, the vast majority of electrode modification strategies with activated carbon ink. For the preparation of the ink, food-grade are unsuitable for practical applications due to complex manufacturing powdered activated carbon originating from coconut shells, was pur- processes and high costs, and their long-term stability is rarely tested chased from a health store (G Baldwins and Co., UK) and had a typical [27]. One of the most promising, readily available and affordable ma- surface area of 1 g between 800–1000 m2 and size: approx 100 µm. It terials is activated carbon [28] typically used in granular (GAC) form in was blended with 5% PTFE (60% water dispersion in H2O, Sigma the anodic half-cell [29] and powdered form (PAC) for the fabrication Aldrich) and 300 mL of deionised water. The mixture was stirred for of the cathode when applied onto stainless steel mesh [9,30] or carbon 2 min -in order to obtain a slurry and applied onto both sides of carbon matrix [10,31]. PAC-based cathodes are widely applied in MFC studies veil using a paintbrush. The sheets of such prepared material were then including field trials and prototypes of the membrane-based [8] and heat treated at 250 °C for 30 min. The final loading of the activated membrane-less systems [9] and its properties suggest it would be an carbon was 5 mg/cm−2.