Self-stratified and self-powered micro-supercapacitor integrated into a microbial fuel cell operating in human urine

Bioelectricity generation from human urine and simultaneous nutrient recovery: Role of Microbial Fuel Cells

Urine is a 'valuable waste' that can be exploited to generate bioelectricity and recover key nutrients for producing NPK-rich biofertilizers. In recent times, improved and innovative waste management technologies have emerged to manage the rapidly increasing environmental pollution and to accomplish the goal of sustainable development. Microbial fuel cells (MFCs) have attracted the attention of environmentalists worldwide to treat human urine and produce power through bioelectrochemical reactions in presence of electroactive bacteria growing on the anode. The bacteria break down the complex organic matter present in urine into simpler compounds and release the electrons which flow through an external circuit generating current at the cathode. Many other useful products are harvested at the end of the process. So, in this review, an attempt has been made to synthesize the information on MFCs fuelled with urine to generate bioelectricity and recover value-added resources (nutrients), and their modifications to enhance productivity. Moreover, configuration and mode of system operation, and factors enhancing the performance of MFCs have been also presented.

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.

Bio-inspired graphene-based nano-systems for biomedical applications

The increasing demands of environmentally sustainable, affordable, and scalable materials have inspired researchers to explore greener nanosystems of unique properties which can enhance the performance of existing systems. Such nanosystems, extracted from nature, are state-of-art high-performance nanostructures due to intrinsic hierarchical micro/nanoscale architecture and generous interfacial interactions in natural resources. Among several, bio-inspired nanosystems graphene nanosystems have emerged as an essential nano-platform wherein a highly electroactive, scalable, functional, flexible, and adaptable to a living being is a key factor. Preliminary investigation project bio-inspired graphene nanosystems as a multi-functional nano-platform suitable for electronic devices, energy storage, sensors, and medical sciences application. However, a broad understanding of bio-inspired graphene nanosystems and their projection towards applied application is not well-explored yet. Considering this as a motivation, this mini-review highlights the following; the emergence of bio-inspired graphene nanosystems, over time development to make them more efficient, state-of-art technology, and potential applications, mainly biomedical including biosensors, drug delivery, imaging, and biomedical systems. The outcomes of this review will certainly serve as a guideline to motivate scholars to design and develop novel bio-inspired graphene nanosystems to develop greener, affordable, and scalable next-generation biomedical systems.

Supercapacitive biofuel cells

Supercapacitive biofuel cells' (SBFCs) most recent advancements are herein disclosed. In conventional SBFCs the biocomponent is employed as the pseudocapacitive component, while in self-charging biodevices it also works as the biocatalyst. The performance of different types of SBFCs are summarized according to the categorization based on the biocatalyst employed: supercapacitive microbial fuel cells (s-MFCs), supercapacitive biophotovoltaics (SBPV) and supercapacitive enzymatic fuel cells (s-EFCs). SBFCs could be considered as promising 'alternative' energy devices (low-cost, environmentally friendly, and technically undemanding electric power sources etc.) being suitable for powering a new generation of miniaturized electronic applications.

Biosupercapacitor with an enzymatic cascade at the anode working in a sucrose solution

In this report, we demonstrate the advantages of the dual-mode operation of an enzymatic biosupercapacitor with nanostructured polypyrrole/nanocellulose, gold nanoparticle-based paper electrodes, sucrose as the anode fuel and molecular oxygen as the oxidant. The device allowed conversion of the sucrose biofuel, and offered storage of the generated power in the same, small-scale device. The external and internal biosupercapacitor re-charging modes were compared. The specific capacitance of the device was 1.8 F cm-2 at a discharge current density of 1 mA cm-2. The cell used in the charge/discharge mode of operation allowed retention of 49% of the initial capacitance after eight days of exhaustive discharging under external load. The discontinuous capacitive mode, preserved the biocatalysts activity for much longer time. The use of such enzyme-based electrical energy sources in the capacitive mode i.e. under discontinuous charging was demonstrated as a solution for preserving high specific capacitance and long-term operational stability.

The Use of Electroactive Halophilic Bacteria for Improvements and Advancements in Environmental High Saline Biosensing

Halophilic bacteria are remarkable organisms that have evolved strategies to survive in high saline concentrations. These bacteria offer many advances for microbial-based biotechnologies and are commonly used for industrial processes such as compatible solute synthesis, biofuel production, and other microbial processes that occur in high saline environments. Using halophilic bacteria in electrochemical systems offers enhanced stability and applications in extreme environments where common electroactive microorganisms would not survive. Incorporating halophilic bacteria into microbial fuel cells has become of particular interest for renewable energy generation and self-powered biosensing since many wastewaters can contain fluctuating and high saline concentrations. In this perspective, we highlight the evolutionary mechanisms of halophilic microorganisms, review their application in microbial electrochemical sensing, and offer future perspectives and directions in using halophilic electroactive microorganisms for high saline biosensing.

A comprehensive review on emerging constructed wetland coupled microbial fuel cell technology: Potential applications and challenges

Constructed wetlands (CWs) integrated with bioelectrochemical systems (BESs) are being intensively researched with the names like constructed wetland-microbial fuel cell (CW-MFC), electro-wetlands, electroactive wetlands, and microbial electrochemical technologies-based constructed wetland since the last decade. The implantation of BES in CW facilitates the tuning of redox activities and electron flow balance in aerobic and anaerobic zones in the CW bed matrix, thereby alleviating the limitation associated with electron acceptor availability and increasing its operational controllability. The benefits of CW-MFC include high treatment efficiency, electricity generation, and recalcitrant pollutant abatement. This article presents CW-MFC technology's journey since its emergence to date, encompassing the research done so far, including the basic principle and functioning, bio-electrocatalysts as its machinery, influential factors for microbial interactions, and operational parameters controlling different processes. A few key challenges and potential applications are also discussed for the CW-MFC systems.

Highly sensitive and stable fructose self-powered biosensor based on a self-charging biosupercapacitor

Herein, we present an alternative approach to obtain a highly sensitive and stable self-powered biosensor that was used to detect D-fructose as proof of concept.In this platform, we perform a two-step process, viz. self-charging the biosupercapacitor for a constant time by using D-fructose as fuel and using the stored charge to realize the detection of D-fructose by performing several polarization curves at different D-fructose concentrations. The proposed BSC shows an instantaneous power density release of 17.6 mW cm^-2 and 3.8 mW cm^-2 in pulse mode and at constant load, respectively. Moreover, the power density achieved for the self-charging BSC in pulse mode or under constant load allows for an enhancement of the sensitivity of the device up to 10 times (3.82 ± 0.01 mW cm^-2 mM^-1, charging time = 70 min) compared to the BSC in continuous operation mode and 100 times compared to the normal enzymatic fuel cell. The platform can potentially be employed as a self-powered biosensor in food or biomedical applications.

Technologies for the recovery of nutrients, water and energy from human urine: A review

The global demand for a constant supply of fertilizer is increasing with the booming of the population. Nowadays more focus is given to the recovery and reuse of the nutrients rather than synthesis of the fertilizer from chemicals. Human urine is the best available resource for the primary macronutrients (Nitrogen, Phosphorus and Potassium) for the fertilizer as it contains 10-12 g/L nitrogen, 0.1-0.5 g/L phosphorous and 1.0-2.0 g/L potassium. For the recovery of these nutrients from human urine, various technologies are available which requires source separation and treatment. . In this review, a wide range of the technologies for the treatment of source-separated human urine are covered and discussed in detail. This review has categorized the technologies based on the recovery of nutrients, energy, and water from human urine. Among the various technologies available, Bio-electrochemical technologies are environmental friendly and recovers energy along with the nutrients. Forward Osmosis is the best available technology for the water recovery and for concentrating the nutrients in urine, without or minimal consumption of energy. However, experimental work in this technology is at its prior stage. A single technology is still not sufficient to recover nutrients, water and energy. Therefore, integration of two or more technologies seems essential.

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P _max  = 13 mW; large: ESR = 4.2 Ω and P _max  = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm^-2) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm^-2 and 633 μW cm^-2, respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL^-1, 265 μW mL^-1 and 249 μW cm^-1, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g^-1-C- and 3270 μW g^-1-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.

Air-breathing cathode self-powered supercapacitive microbial fuel cell with human urine as electrolyte

In this work, a membraneless microbial fuel cell (MFC) with an empty volume of 1.5 mL, fed continuously with hydrolysed urine, was tested in supercapacitive mode (SC-MFC). In order to enhance the power output, a double strategy was used: i) a double cathode was added leading to a decrease in the equivalent series resistance (ESR); ii) the apparent capacitance was boosted up by adding capacitive features on the anode electrode. Galvanostatic (GLV) discharges were performed at different discharge currents. The results showed that both strategies were successful obtaining a maximum power output of 1.59 ± 0.01 mW (1.06 ± 0.01 mW mL⁻¹) at pulse time of 0.01 s and 0.57 ± 0.01 mW (0.38 ± 0.01 mW mL⁻¹) at pulse time of 2 s. The highest energy delivered at i_pulse equal to 2 mA was 3.3 ± 0.1 mJ. The best performing SC-MFCs were then connected in series and parallel and tested through GLV discharges. As the power output was similar, the connection in parallel allowed to roughly doubling the current produced. Durability tests over ≈5.6 days showed certain stability despite a light overall decrease.

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Air-breathing microbial fuel cell was tested in supercapacitive mode.

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A double cathode addition lead to a decrease in ohmic resistance.

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Apparent capacitance was boosted up by adding capacitive features.

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Maximum power output of 1.59 mW (1.06 mW mL⁻¹) was reached at t_pulse 0.01s.

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Series and parallel connections improved the galvanostatic discharges.

Scalability and stacking of self-stratifying microbial fuel cells treating urine

The scalability of Microbial fuel cells (MFCs) is key to the development of stacks. A recent study has shown that self-stratifying membraneless MFCs (S-MFCs) could be scaled down to 2 cm without performance deterioration. However, the scaling-up limit of S-MFC is yet unknown. Here the study evaluates the scale-up height of S-MFCs treating urine, from 2 cm, 4 cm to 12 cm high electrodes. The electrochemical properties of the S-MFCs were investigated after steady-states were established, following a 70-days longevity study. The electrochemical properties of the 2 cm and 4 cm conditions were similar (5.45 ± 0.32 mW per cascade). Conversely, the 12 cm conditions had much lower power output (1.48 ± 0.15 mW). The biofilm on the 12 cm cathodes only developed on the upper 5-6 cm of the immersed part of the electrode suggesting that the cathodic reactions were the limiting factor. This hypothesis was confirmed by the cathode polarisations showing that the 12 cm S-MFC had low current density (1.64 ± 9.53 µA cm-2, at 0 mV) compared to the other two conditions taht had similar current densities (192.73 ± 20.35 µA cm-2, at 0 mV). These results indicate that S-MFC treating urine can only be scaled-up to an electrode height of around 5-6 cm before the performance is negatively affected.

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 cells directly powering a microcomputer

Many studies have demonstrated that microbial fuel cells (MFC) can be energy-positive systems and power various low power applications. However, to be employed as a low-level power source, MFC systems rely on energy management circuitry, used to increase voltage levels and act as energy buffers, thus delivering stable power outputs. But stability comes at a cost, one that needs to be kept minimal for the technology to be deployed into society. The present study reports, for the first time, the use of a MFC system that directly and continuously powered a small application without any electronic intermediary. A cascade comprising four membrane-less MFCs modules and producing an average of 62  mA at 2550 mV (158 mW) was used to directly power a microcomputer and its screen (Gameboy Color, Nintendo®). The polarisation experiment showed that the cascade produced 164 mA, at the minimum voltage required to run the microcomputer (ca. 1.850 V). As the microcomputer only needed ≈70 mA, the cascade ran at a higher voltage (2.550 V), thus, maintaining the individual modules at a high potential (>0.55 V). Running the system at these high potentials helped avoid cell reversal, thus delivering a stable level of energy without the support of any electronics.

Combination of bioelectrochemical systems and electrochemical capacitors: Principles, analysis and opportunities

Bioelectrochemical systems combine electrodes and reactions driven by microorganisms for many different applications. The conversion of organic material in wastewater into electricity occurs in microbial fuel cells (MFCs). The power densities produced by MFCs are still too low for application. One way of increasing their performance is to combine them with electrochemical capacitors, widely used for charge storage purposes. Capacitive MFCs, i.e. the combination of capacitors and MFCs, allow for energy harvesting and storage and have shown to result in improved power densities, which facilitates the up scaling and application of the technology. This manuscript summarizes the state-of-the-art of combining capacitors with MFCs, starting with the theory and working principle of electrochemical capacitors. We address how different electrochemical measurements can be used to determine (bio)electrochemical capacitance and show how the measurement data can be interpreted. In addition, we present examples of the combination of electrochemical capacitors, both internal and external, that have been used to enhance MFC performance. Finally, we discuss the most promising applications and the main existing challenges for capacitive MFCs.

Supercapacitive paper based microbial fuel cell: High current/power production within a low cost design

Microbial fuel cells (MFCs) with paper separators and liquid containing elements were investigated in supercapacitive mode. MFCs (15 mL) in a supercapacitive configuration, consisted of plain wrapped carbon veil anode (negative) and conductive latex cathode (positive). The internal supercapacitor is discharged galvanostatically and is self-recharged as red-ox reactions occur on both electrodes. MFCs were discharged at different current pulses varying from 1 mA to 7 mA. The MFCs had an equivalent series resistance of 41.2 ± 3.5 Ω caused mainly by the cathode. A maximum power of 1.380 ± 0.083 mW (0.092 ± 0.006 mW mL⁻¹) was measured. Durability tests were conducted over 24 h collecting 1000 discharge cycles (0.5 s) and self-recharges (85 s) at a current of 1 mA. Over time the anode potential dropped causing a decline in performance perhaps due to evaporation of liquid from the pyramidal structure. Resistance and apparent capacitance measured during the durability test remained approximately constant during the cycles.

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Supercapacitor paper-based MFCs are reported for the first time.

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Galvanostatic discharges at different current pulses (1 mA to 7 mA) were analysed.

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Equivalent series resistance (ESR) and apparent capacitance (C) were identified.

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Maximum power achieved was 1.380 ± 0.083 mW (0.092 ± 0.006 mW mL⁻¹).

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The system stability was investigated through 1000 discharge/self-recharge cycles.