Microbial fuel cell compared to a chemostat.

Microbial Fuel Cells (MFCs) represent a green and sustainable energy conversion system that integrate bacterial biofilms within an electrochemical two-electrode set-up to produce electricity from organic waste. In this review, we focus on a novel exploratory model, regarding "thin" biofilms forming on highly perfusable (non-diffusible) anodes in small-scale, continuous flow MFCs due to the unique properties of the electroactive biofilm. We discuss how this type of MFC can behave as a chemostat in fulfilling common properties including steady state growth and multiple steady states within the limit of biological physicochemical conditions imposed by the external environment. With continuous steady state growth, there is also continuous metabolic rate and continuous electrical power production, which like the chemostat can be controlled. The model suggests that in addition to controlling growth rate and power output by changing the external resistive load, it will be possible instead to change the flow rate/dilution rate.

Microbial fuel cell scale-up options: Performance evaluation of membrane (c-MFC) and membrane-less (s-MFC) systems under different feeding regimes.

In recent years, bioelectrochemical systems have advanced towards upscaling applications and tested during field trials, primarily for wastewater treatment. Amongst reported trials, two designs of urine-fed microbial fuel cells (MFCs) were tested successfully on a pilot scale as autonomous sanitation systems for decentralised area. These designs, known as ceramic MFCs ( c -MFCs) and self-stratifying MFCs ( s -MFC), have never been calibrated under similar conditions. Here, the most advanced versions of both designs were assembled and tested under similar feeding conditions. The performance and efficiency were evaluated under different hydraulic retention times (HRT), through chemical oxygen demand measures and polarisation experiments. Results show that c -MFCs displayed constant performance independently from the HRT (32.2 ± 3.9 W m-3) whilst displaying high energy conversion efficiency at longer HRT (NER COD  = 2.092 ± 0.119 KWh.Kg COD -1, at 24h HRT). The s -MFC showed a correlation between performance and HRT. The highest performance was reached under short HRT (69.7 ± 0.4 W m-3 at 3h HRT), but the energy conversion efficiency was constant independently from the HRT (0.338 ± 0.029 KWh.Kg COD -1). The c -MFCs and s -MFCs similarly showed the highest volumetric efficiency under long HRT (65h) with NER V of 0.747 ± 0.010 KWh.m-3 and 0.825 ± 0.086 KWh.m-3, respectively. Overall, c -MFCs seems more appropriate for longer HRT and s -MFCs for shorter HRT.

Microbial Fuel Cell Based Thermosensor for Robotic Applications.

On the roadmap to building completely autonomous artificial bio-robots, all major aspects of robotic functions, namely, energy generation, processing, sensing, and actuation, need to be self-sustainable and function in the biological realm. Microbial Fuel Cells (MFCs) provide a platform technology for achieving this goal. In a series of experiments, we demonstrate that MFCs can be used as living, autonomous sensors in robotics. In this work, we focus on thermal sensing that is akin to thermoreceptors in mammalian entities. We therefore designed and tested an MFC-based thermosensor system for utilization within artificial bio-robots such as EcoBots. In open-loop sensor characterization, with a controlled load resistance and feed rate, the MFC thermoreceptor was able to detect stimuli of 1 min directed from a distance of 10 cm causing a temperature rise of ∼1°C at the thermoreceptor. The thermoreceptor responded to continuous stimuli with a minimum interval of 384 s. In a practical demonstration, a mobile robot was fitted with two artificial thermosensors, as environmental thermal detectors for thermotactic application, mimicking thermotaxis in biology. In closed-loop applications, continuous thermal stimuli were detected at a minimum time interval of 160 s, without the need for complete thermoreceptor recovery. This enabled the robot to detect thermal stimuli and steer away from a warmer thermal source within the rise of 1°C. We envision that the thermosensor can be used for future applications in robotics, including as a potential sensor mechanism for maintaining thermal homeostasis.

Microbial fuel cells and their electrified biofilms.

Bioelectrochemical systems (BES) represent a wide range of different biofilm-based bioreactors that includes microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial desalination cells (MDCs). The first described bioelectrical bioreactor is the Microbial Fuel Cell and with the exception of MDCs, it is the only type of BES that actually produces harvestable amounts of electricity, rather than requiring an electrical input to function. For these reasons, this review article, with previously unpublished supporting data, focusses primarily on MFCs. Of relevance is the architecture of these bioreactors, the type of membrane they employ (if any) for separating the chambers along with the size, as well as the geometry and material composition of the electrodes which support biofilms. Finally, the structure, properties and growth rate of the microbial biofilms colonising anodic electrodes, are of critical importance for rendering these devices, functional living 'engines' for a wide range of applications.

Electrosynthesis, modulation, and self-driven electroseparation in microbial fuel cells.

Microbial electrosynthesis (MES) represents a sustainable platform that converts waste into resources, using microorganisms within an electrochemical cell. Traditionally, MES refers to the oxidation/reduction of a reactant at the electrode surface with externally applied potential bias. However, microbial fuel cells (MFCs) generate electrons that can drive electrochemical reactions at otherwise unbiased electrodes. Electrosynthesis in MFCs is driven by microbial oxidation of organic matter releasing electrons that force the migration of cationic species to the cathode. Here, we explore how electrosynthesis can coexist within electricity-producing MFCs thanks to electro-separation of cations, electroosmotic drag, and oxygen reduction within appropriately designed systems. More importantly, we report on a novel method of in situ modulation for electrosynthesis, through additional "pin" electrodes. Several MFC electrosynthesis modulating methods that adjust the electrode potential of each half-cell through the pin electrodes are presented. The innovative concept of electrosynthesis within the electricity producing MFCs provides a multidisciplinary platform converting waste-to-resources in a self-sustainable way.

A new method for urine electrofiltration and long term power enhancement using surface modified anodes with activated carbon in ceramic microbial fuel cells.

This work is presenting for the first time the use of inexpensive and efficient anode material for boosting power production, as well as improving electrofiltration of human urine in tubular microbial fuel cells (MFCs). The MFCs were constructed using unglazed ceramic clay functioning as the membrane and chassis. The study is looking into effective anodic surface modification by applying activated carbon micro-nanostructure onto carbon fibres that allows electrode packing without excessive enlargement of the electrode. The surface treatment of the carbon veil matrix resulted in 3.7 mW (52.9 W m-3 and 1626 mW m-2) of power generated and almost a 10-fold increase in the anodic current due to the doping as well as long-term stability in one year of continuous operation. The higher power output resulted in the synthesis of clear catholyte, thereby i) avoiding cathode fouling and contributing to the active splitting of both pH and ions and ii) transforming urine into a purified catholyte - 30% salt reduction - by electroosmotic drag, whilst generating - rather than consuming - electricity, and in a way demonstrating electrofiltration. For the purpose of future technology implementation , the importance of simultaneous increase in power generation, long-term stability over 1 year and efficient urine cleaning by using low-cost materials, is very promising and helps the technology enter the wider market.

Urine in Bioelectrochemical Systems: An Overall Review.

In recent years, human urine has been successfully used as an electrolyte and organic substrate in bioelectrochemical systems (BESs) mainly due of its unique properties. Urine contains organic compounds that can be utilised as a fuel for energy recovery in microbial fuel cells (MFCs) and it has high nutrient concentrations including nitrogen and phosphorous that can be concentrated and recovered in microbial electrosynthesis cells and microbial concentration cells. Moreover, human urine has high solution conductivity, which reduces the ohmic losses of these systems, improving BES output. This review describes the most recent advances in BESs utilising urine. Properties of neat human urine used in state-of-the-art MFCs are described from basic to pilot-scale and real implementation. Utilisation of urine in other bioelectrochemical systems for nutrient recovery is also discussed including proofs of concept to scale up systems.

Microbial Fuel Cell stack performance enhancement through carbon veil anode modification with activated carbon powder.

The chemical energy contained in urine can be efficiently extracted into direct electricity by Microbial Fuel Cell stacks to reach usable power levels for practical implementation and a decentralised power source in remote locations. Herein, a novel type of the anode electrode was developed using powdered activated carbon (PAC) applied onto the carbon fibre scaffold in the ceramic MFC stack to achieve superior electrochemical performance during 500 days of operation. The stack equipped with modified anodes (MF-CV) produced up to 37.9 mW (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 power production. The novel combination of highly porous activated carbon particles applied onto the conductive 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.

Long-term bio-power of ceramic microbial fuel cells in individual and stacked configurations.

In order to improve the potential of Microbial Fuel Cells (MFCs) as an applicable technology, the main challenge is to engineer practical systems for bioenergy production at larger scales and to test how the prototypes withstand the challenges occurring during the prolonged operation under constant feeding regime with real waste stream. This work presents the performance assessment of low-cost ceramic MFCs in the individual, stacked (modular) and modular cascade (3 modules) configurations during long term operation up to 19 months, utilising neat human urine as feedstock. During 1 year, the performance of the individual MFC units reached up to 1.56 mW (22.3 W/m3), exhibiting only 20% power loss on day 350 which was significantly smaller in comparison to conventional proton or cation exchange membranes. The stack module comprising 22 MFCs reached up to 21.4 mW (11.9 W/m3) showing power recovery to the initial output levels after 580 days, whereas the 3-module cascade reached up to 75 mW (13.9 W/m3) of power, showing 20% power loss on day 446. In terms of chemical oxygen demand (COD) removal, the 3-module cascade configuration achieved a cumulative reduction of >92%, which is higher than that observed in the single module (56%).

Electroosmotically generated disinfectant from urine as a by-product of electricity in microbial fuel cell for the inactivation of pathogenic species.

This work presents a small scale and low cost ceramic based microbial fuel cell, utilising human urine into electricity, while producing clean catholyte into an initially empty cathode chamber through the process of electro-osmostic drag. It is the first time that the catholyte obtained as a by-product of electricity generation from urine was transparent in colour and reached pH>13 with high ionic conductivity values. The catholyte was collected and used ex situ as a killing agent for the inactivation of a pathogenic species such as Salmonella typhimurium, using a luminometer assay. Results showed that the catholyte solutions were efficacious in the inactivation of the pathogen organism even when diluted up to 1:10, resulting in more than 5 log-fold reduction in 4 min. Long-term impact of the catholyte on the pathogen killing was evaluated by plating Salmonella typhimurium on agar plates and showed that the catholyte possesses a long-term killing efficacy and continued to inhibit pathogen growth for 10 days.

Multi-functional microbial fuel cells for power, treatment and electro-osmotic purification of urine.

BACKGROUND: In this work, a small-scale ceramic microbial fuel cell (MFC) with a novel type of metal-carbon-derived electrocatalyst containing iron and nicarbazin (Fe-NCB) was developed, to enhance electricity generation from neat human urine. Substrate oxidation at the anode provides energy for the separation of ions and recovery from urine without any chemical or external power additions. RESULTS: The catalyst was shown to be effective in clear electrolyte synthesis of high pH, compared with a range of carbon-based metal-free materials. Polarisation curves of tested MFCs showed up to 53% improvement (44.8 W m-3) in performance with the use of Fe-NCB catalyst.Catholyte production rate and pH directly increased with power performance while the conductivity decreased showing visually clear extracted liquid in the best-performing MFCs. CONCLUSIONS: Iron based catalyst Fe-NCB was shown to be a suitable electrocatalyst for the air-breathing cathode, improving power production from urine-fed MFCs. The results suggest electrochemical treatment through electro-osmotic drag while the electricity is produced and not consumed. Electro-osmotic production of clear catholyte is shown to extract water from urine against osmotic pressure. Recovering valuable resources from urine would help to transform energy intensive treatments to resource production, and will create opportunities for new technology development. © 2018 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Increased power generation in supercapacitive microbial fuel cell stack using Fe-N-C cathode catalyst.

The anode and cathode electrodes of a microbial fuel cell (MFC) stack, composed of 28 single MFCs, were used as the negative and positive electrodes, respectively of an internal self-charged supercapacitor. Particularly, carbon veil was used as the negative electrode and activated carbon with a Fe-based catalyst as the positive electrode. The red-ox reactions on the anode and cathode, self-charged these electrodes creating an internal electrochemical double layer capacitor. Galvanostatic discharges were performed at different current and time pulses. Supercapacitive-MFC (SC-MFC) was also tested at four different solution conductivities. SC-MFC had an equivalent series resistance (ESR) decreasing from 6.00â€¯Ω to 3.42â€¯Ω in four solutions with conductivity between 2.5 mScm-1 and 40 mScm-1. The ohmic resistance of the positive electrode corresponded to 75-80% of the overall ESR. The highest performance was achieved with a solution conductivity of 40 mS cm-1 and this was due to the positive electrode potential enhancement for the utilization of Fe-based catalysts. Maximum power was 36.9 mW (36.9 W m-3) that decreased with increasing pulse time. SC-MFC was subjected to 4520 cycles (8 days) with a pulse time of 5 s (ipulse 55 mA) and a self-recharging time of 150 s showing robust reproducibility.

Ceramic Microbial Fuel Cells Stack: power generation in standard and supercapacitive mode.

In this work, a microbial fuel cell (MFC) stack containing 28 ceramic MFCs was tested in both standard and supercapacitive modes. The MFCs consisted of carbon veil anodes wrapped around the ceramic separator and air-breathing cathodes based on activated carbon catalyst pressed on a stainless steel mesh. The anodes and cathodes were connected in parallel. The electrolytes utilized had different solution conductivities ranging from 2.0 mScm-1 to 40.1 mScm-1, simulating diverse wastewaters. Polarization curves of MFCs showed a general enhancement in performance with the increase of the electrolyte solution conductivity. The maximum stationary power density was 3.2 mW (3.2 Wm-3) at 2.0 mScm-1 that increased to 10.6 mW (10.6 Wm-3) at the highest solution conductivity (40.1 mScm-1). For the first time, MFCs stack with 1 L operating volume was also tested in supercapacitive mode, where full galvanostatic discharges are presented. Also in the latter case, performance once again improved with the increase in solution conductivity. Particularly, the increase in solution conductivity decreased dramatically the ohmic resistance and therefore the time for complete discharge was elongated, with a resultant increase in power. Maximum power achieved varied between 7.6 mW (7.6 Wm-3) at 2.0 mScm-1 and 27.4 mW (27.4 Wm-3) at 40.1 mScm-1.

Improved power and long term performance of microbial fuel cell with Fe-N-C catalyst in air-breathing cathode.

Power output limitation is one of the main challenges that needs to be addressed for full-scale applications of the Microbial Fuel Cell (MFC) technology. Previous studies have examined electrochemical performance of different cathode electrodes including the development of novel iron based electrocatalysts, however the long-term investigation into continuously operating systems is rare. This work aims to study the application of platinum group metals-free (PGM-free) catalysts integrated into an air-breathing cathode of the microbial fuel cell operating on activated sewage sludge and supplemented with acetate as the carbon energy source. The maximum power density up to 1.3 Wm-2 (54 Wm-3) obtained with iron aminoantipyrine (Fe-AAPyr) catalyst is the highest reported in this type of MFC and shows stability and improvement in long term operation when continuously operated on wastewater. It also investigates the ability of this catalyst to facilitate water extraction from the anode and electroosmotic production of clean catholyte. The electrochemical kinetic extraction of catholyte in the cathode chamber shows correlation with power performance and produces a newly synthesised solution with a high pH > 13, suggesting caustic content. This shows an active electrolytic treatment of wastewater by active ionic and pH splitting in an electricity producing MFC.

Miniaturized Ceramic-Based Microbial Fuel Cell for Efficient Power Generation From Urine and Stack Development.

One of the challenges in Microbial Fuel Cell (MFC) technology is the improvement of the power output and the lowering of the cost required to scale up the system to reach usable energy levels for real life applications. This can be achieved by stacking multiple MFC units in modules and using cost effective ceramic as a membrane/chassis for the reactor architecture. The main aim of this work is to increase the power output efficiency of the ceramic based MFCs by compacting the design and exploring the ceramic support as the building block for small scale modular multi-unit systems. The comparison of the power output showed that the small reactors outperform the large MFCs by improving the power density reaching up to 20.4 W/m3 (mean value) and 25.7 W/m3 (maximum). This can be related to the increased surface-area-to-volume ratio of the ceramic membrane and a decreased electrode distance. The power performance was also influenced by the type and thickness of the ceramic separator as well as the total surface area of the anode electrode. The study showed that the larger anode electrode area gives an increased power output. The miniaturized design implemented in 560-units MFC stack showed an output up to 245 mW of power and increased power density. Such strategy would allow to utilize the energy locked in urine more efficiently, making MFCs more applicable in industrial and municipal wastewater treatment facilities, and scale-up-ready for real world implementation.

Recent advancements in real-world microbial fuel cell applications.

This short review focuses on the recent developments of the Microbial Fuel Cell (MFC) technology, its scale-up and implementation in real world applications. Microbial Fuel Cells produce (bio)energy from waste streams, which can reduce environmental pollution, but also decrease the cost of the treatment. Although the technology is still considered "new", it has a long history since its discovery, but it is only now that recent developments have allowed its implementation in real world settings, as a precursor to commercialisation.

Scaling-up of a novel, simplified MFC stack based on a self-stratifying urine column.

BACKGROUND: The microbial fuel cell (MFC) is a technology in which microorganisms employ an electrode (anode) as a solid electron acceptor for anaerobic respiration. This results in direct transformation of chemical energy into electrical energy, which in essence, renders organic wastewater into fuel. Amongst the various types of organic waste, urine is particularly interesting since it is the source of 75 % of the nitrogen present in domestic wastewater despite only accounting for 1 % of the total volume. However, there is a persistent problem for efficient MFC scale-up, since the higher the surface area of electrode to volume ratio, the higher the volumetric power density. Hence, to reach usable power levels for practical applications, a plurality of MFC units could be connected together to produce higher voltage and current outputs; this can be done by combinations of series/parallel connections implemented both horizontally and vertically as a stack. This plurality implies that the units have a simple design for the whole system to be cost-effective. The goal of this work was to address the built configuration of these multiple MFCs into stacks used for treating human urine. RESULTS: We report a novel, membraneless stack design using ceramic plates, with fully submerged anodes and partially submerged cathodes in the same urine solution. The cathodes covered the top of each ceramic plate whilst the anodes, were on the lower half of each plate, and this would constitute a module. The MFC elements within each module (anode, ceramic, and cathode) were connected in parallel, and the different modules connected in series. This allowed for the self-stratification of the collective environment (urine column) under the natural activity of the microbial consortia thriving in the system. Two different module sizes were investigated, where one module (or box) had a footprint of 900 mL and a larger module (or box) had a footprint of 5000 mL. This scaling-up increased power but did not negatively affect power density (≈12 W/m(3)), a factor that has proven to be an obstacle in previous studies. CONCLUSION: The scaling-up approach, with limited power-density losses, was achieved by maintaining a plurality of microenvironments within the module, and resulted in a simple and robust system fuelled by urine. This scaling-up approach, within the tested range, was successful in converting chemical energy in urine into electricity.

Electricity and disinfectant production from wastewater: Microbial Fuel Cell as a self-powered electrolyser.

This study presents a simple and sustainable Microbial Fuel Cell as a standalone, self-powered reactor for in situ wastewater electrolysis, recovering nitrogen from wastewater. A process is proposed whereby the MFC electrical performance drives the electrolysis of wastewater towards the self-generation of catholyte within the same reactor. The MFCs were designed to harvest the generated catholyte in the internal chamber, which showed that liquid production rates are largely proportional to electrical current generation. The catholyte demonstrated bactericidal properties, compared to the control (open-circuit) diffusate, and reduced observable biofilm formation on the cathode electrode. Killing effects were confirmed using bacterial kill curves constructed by exposing a bioluminescent Escherichia coli target, as a surrogate coliform, to catholyte where a rapid kill rate was observed. Therefore, MFCs could serve as a water recovery system, a disinfectant/cleaner generator that limits undesired biofilm formation and as a washing agent in waterless urinals to improve sanitation. This simple and ready to implement MFC system can convert organic waste directly into electricity and self-driven nitrogen along with water recovery. This could lead to the development of energy positive bioprocesses for sustainable wastewater treatment.

Microbial Fuel Cell-driven caustic potash production from wastewater for carbon sequestration.

This work reports on the novel formation of caustic potash (KOH) directly on the MFC cathode locking carbon dioxide into potassium bicarbonate salt (kalicinite) while producing, instead of consuming electrical power. Using potassium-rich wastewater as a fuel for microorganisms to generate electricity in the anode chamber, has resulted in the formation of caustic catholyte directly on the surface of the cathode electrode. Analysis of this liquid has shown to be highly alkaline (pH>13) and act as a CO2 sorbent. It has been later mineralised to kalicinite thus locking carbon dioxide into potassium bicarbonate salt. This work demonstrates an electricity generation method as a simple, cost-effective and environmentally friendly route towards CO2 sequestration that perhaps leads to a carbon negative economy. Moreover, it shows a potential application for both electricity production and nutrient recovery in the form of minerals from nutrient-rich wastewater streams such as urine for use as fertiliser in the future.

A review into the use of ceramics in microbial fuel cells.

Microbial fuel cells (MFCs) offer great promise as a technology that can produce electricity whilst at the same time treat wastewater. Although significant progress has been made in recent years, the requirement for cheaper materials has prevented the technology from wider, out-of-the-lab, implementation. Recently, researchers have started using ceramics with encouraging results, suggesting that this inexpensive material might be the solution for propelling MFC technology towards real world applications. Studies have demonstrated that ceramics can provide stability, improve power and treatment efficiencies, create a better environment for the electro-active bacteria and contribute towards resource recovery. This review discusses progress to date using ceramics as (i) the structural material, (ii) the medium for ion exchange and (iii) the electrode for MFCs.