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The integration of supercapacitive features in microbial fuel cells (MFC) can solve one of the major limitations of this technology, i.e., the low power output. In this chapter, three different strategies to exploit the supercapacitive features of MFCs are reported and discussed: (i) the integration of high specific surface area elements in the electrode to maximize the capacitive response, (ii) the exploitation of a pulsed discharge, and (iii) the decoration of microbial fuel cell electrodes with inorganic pseudocapacitive components.

Making a consistency with the objectives of circular economy, herein, waste pistachios shells were utilized for the development of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) electrocatalysts which are the key bottleneck in the technological evolution of electrolyzers and fuel cells, respectively. As an alternative to scarce and expensive platinum-group-metal (PGM) electrocatalysts, metal nitrogen carbons (MNCs) are emerging as a promising candidate for both aforementioned electrocatalysis where iron and nickel are the metal of choice for ORR and HER, respectively. Therefore, FeNCs and NiNCs were fabricated utilizing inedible pistachio shells as a low-cost biosource of carbon. The steps involved in the fabrication of electrocatalyst were correlated with electrochemical performance in alkaline media. Encouraging onset potential of ~ 0.88 V vs RHE with a possibility of a 2 + 2 reaction pathway was observed in pyrolyzed and ball-milled FeNC. However, HF etching for template removal slightly affected the kinetics and eventually resulted in a relatively higher yield of peroxide. In parallel, the pyrolyzed NiNC demonstrated a lower HER overpotential of ~ 0.4 V vs RHE at − 10 mA cm−2. Nevertheless, acid washing adversely affected the HER performance and consequently, very high overpotential was witnessed.

Metal-nitrogen-carbons (M-N-Cs) as a reliable substitution for platinum-group-metals (PGMs) for oxygen reduction reaction (ORR) are emerging candidates to rationalize the technology of fuel cells. The development of M-N-Cs can further be economized by consuming waste biomass as an inexpensive carbon source for the electrocatalyst support. Herein, we report the simple fabrication and in-depth characterization of electrocatalysts using lignin-derived activated char. The activated char (LAC) was functionalized with metal phthalocyanine (FePc and MnPc) via atmosphere-controlled pyrolysis to produce monometallic M-N-Cs (L_Mn and L_Fe) and bimetallic M1-M2-N-Cs (L_FeMn) electrocatalysts. Raman spectroscopy and transmission electron microscopy (TEM) revealed a defect-rich architecture. XPS confirmed the coexistence of various nitrogen-containing active moieties. L_Fe and L_FeMn demonstrated appreciable ORR in both acidic and alkaline conditions whereas L_FeMn helped in restricting the peroxide yield, particularly in alkaline media. L_Fe and L_FeMn demonstrated remarkable onset potential (Eonset) of ∼0.942 V (vs RHE) with an E1/2 of 0.874 V (vs RHE) in 0.1 M KOH. In acid, L_FeMn had an Eonset of 0.817 V (vs RHE) and an E1/2 of ∼0.76 V (vs RHE). Finally, the L_FeMn as a cathode electrocatalyst was integrated and tested in PEMFC and AEMFC. AEMFC demonstrated optimistic performance with a peak power density of 261 mW cm−2 at the current density of ∼577 mA cm−2.

Following the core theme of a circular economy, a novel strategy to upcycle cigarette butt waste into platinum group metal (PGM)-free metal nitrogen carbon (M-N-C) electrocatalysts for oxygen reduction reaction (ORR) is presented. The experimental route was composed of (i) the transformation of the powdered cigarette butts into carbonaceous char via pyrolysis at 450 °C, 600 °C, 750 °C and 900 °C, (ii) the porosity activation with KOH and (iii) the functionalization of the activated chars with iron (II) phthalocyanine (FePc). The electrochemical outcomes obtained by the rotating disk electrode (RRDE) technique revealed that the sample pyrolyzed at 450 °C (i.e., cig_450) outperformed the other counterparts with its highest onset (Eon) and half-wave potentials (E1/2) and demonstrated nearly tetra-electronic ORR in acidic, neutral and alkaline electrolytes, all resulting from the optimal surface chemistry and textural properties.

This study aims at demonstrating that wastes from anaerobic biodigester plants can be effectively valorized as functional materials to be implemented in technologies that enable efficient energy management and water treatment, therefore simultaneously addressing the Water-Energy-Waste Nexus challenges. Lignin, the main solid residue of the biodigester plant, has been valorized into activated biochar with a mild activation agent, like KHCO3, to produce electrode of supercapacitors and microbial fuel cells. In addition, the same sludge that is the liquid effluent of the biodigester plant has been exploited as inoculum and electrolyte for the MFC. The lignin-derived carbons obtained at lignin/KHCO3 mass ratios of 1:0.5 (LAC-0.5) and 1:2 (LAC-2) comprised of mesopores and micropores displaying BETs of 1558 m2g−1 and 1879 m2g−1, respectively. LAC-2 carbon exhibited a superior specific capacitance of 114 F g−1 in 2.5 M KNO3 with respect to LAC-0.5. A supercapacitor with LAC-2 electrodes was built displaying specific energy specific power up to 10 Wh kg−1 and 6.9 kW kg−1, respectively. Durability tests showed that the device was able to maintain a capacitance retention of 84.5% after 15,000 charge-discharge cycles. The lignin-derived carbons were also studied as electrocatalysts for ORR in a neutral medium. The LAC-2 showed higher ORR electrocatalytic activity than LAC-0.5. The interconnected porous network and the high surface area made the lignin-derived porous carbons suitable electrode materials for dual applications. The feasibility of the use of LAC 2 carbon incorporated in an air breathing cathode for MFC applications is also reported.

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.

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 ipulse 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.

Microbial fuel cells (MFCs), despite representing a promising technology, suffer from low power generation that hinders, in most of the cases, their application as power sources. In fact, MFCs are usually coupled with supercapacitors or batteries and these storage units accumulate the energy harvested by MFCs and deliver it on demand. In this work, the electrodes of a MFC are used as electrodes of an internal supercapacitor and discharges and self-recharges are performed and investigated. Discharges between 1.5 mA and 4 mA were presented producing a maximum power of 1.59 mW. Discharges between 1 mA and 100 mA and recharges are systematically studied for three commercial supercapacitors (SCs) with different capacitances of 1 F, 3 F, and 6 F. The MFC was also connected in parallel with external SCs and discharged galvanostatically. The SC was self-recharged by the MFC without any additional external power sources. At lower current pulses, the MFC contributed to the overall capacitance, probably owing to its faradaic component. At higher current pulses, the use of SCs enables the energy to be harvested by MFCs at power levels that could not be achieved with the MFC alone. This study demonstrates that, through the proper connection and operation mode of the MFC and SC, it is possible to improve and maximize the performance of every single unit. Understanding the MFC−SC combination is important for identifying the right practical application for which the combination is suitable.

Microbial fuel cells (MFCs), despite representing a promising technology, suffer from low power generation that hinders, in most of the cases, their application as power sources. In fact, MFCs are usually coupled with supercapacitors or batteries and these storage units accumulate the energy harvested by MFCs and deliver it on demand. In this work, the electrodes of a MFC are used as electrodes of an internal supercapacitor and discharges and self-recharges are performed and investigated. Discharges between 1.5 mA and 4 mA were presented produc- ing a maximum power of 1.59 mW. Discharges between 1 mA and 100 mA and recharges are systematically studied for three commercial supercapacitors (SCs) with different capacitances of 1 F, 3 F, and 6 F. The MFC was also connected in parallel with external SCs and discharged galvanostatically. The SC was self- recharged by the MFC without any additional external power sources. At lower current pulses, the MFC contributed to the overall capacitance, probably owing to its faradaic component. At higher current pulses, the use of SCs enables the energy to be harvested by MFCs at power levels that could not be achieved with the MFC alone. This study demonstrates that, through the proper connection and operation mode of the MFC and SC, it is possible to improve and maximize the performance of every single unit. Understanding the MFCSC combination is important for identifying the right practical application for which the combination is suitable.

Supercapacitive microbial fuel cells (SC-MFCs) are an emerging and promising field that has captured the attention of scientists in the past few years. This hybridization consists in the integration of supercapacitive features in the MFC electrodes to boost the performance output. The MFC anaerobic and aerobic enviroments induce self-polarization of the electrodes. The electrodes can be discharged galvanostatically and then self-recharged by the biotic/abiotic environments. During the discharge, two main phenomena named electrostatic and faradaic take place but the separation and quantification of the two contributes seems to be challenging. Galvanostatic discharges of SC-MFC produce at least one order of magnitude higher current/power compared with continuous operations, making it promising for pulsed type applications.

Mass generation of scrap tires presents a major challenge for environmental safety, however, their upcycling into carbon-based nanomaterials by the virtue of pyrolysis treatments can open up new windows for energy conversion and storage technologies in the context of the circular economy. Herein, we report the synthesis of Fe-N-C oxygen reduction reaction (ORR) electrocatalyst for fuel cell (FC) applications using carbonaceous char derived from scrap tires through microwave-assisted pyrolysis (MAP). The char obtained from MAP was activated with potassium hydroxide and then pyrolyzed at a high temperature to fabricate Fe-N-C after mixing with iron and nitrogen precursors. Finally, the developed Fe-N-C was ball-milled and acid-etched for homogenization and leaching of iron oxide nanoparticles. In this study, structural evaluation during each synthesis step was elucidated and correlated with the ORR activity in all three pHs i.e. acidic, neutral, and alkaline. Moreover, the effect of electrocatalyst loading on ORR kinetics was also analyzed using two different loadings (0.2 and 0.6 mg cm−2) on the rotating ring disk electrode (RRDE). The developed Fe-N-C demonstrated encouraging onset potentials of 0.881, 0.822, and 0.936 V vs RHE in acidic, neutral, and alkaline conditions, respectively. Whereas the ORR activity was slightly reduced after the milling-etching step. Lower peroxide yield together with a tetra-electronic reduction of oxygen was witnessed in acidic and neutral conditions, however, peroxide production was increased in the alkaline medium.

A self-stratified microbial fuel cell fed with human urine with a total internal volume of 0.55 ml was investigated as an internal supercapacitor, for the first time. The internal self-stratification allowed the development of two zones within the cell volume. The oxidation reaction occurred on the bottom electrode (anode) and the reduction reaction on the top electrode (cathode). The electrodes were discharged galvanostatically at different currents and the two electrodes were able to recover their initial voltage value due to their red-ox reactions. Anode and cathode apparent capacitance was increased after introducing high surface area activated carbon embedded within the electrodes. Peak power produced was 1.20 ± 0.04 mW (2.19 ± 0.06 mW ml−1) for a pulse time of 0.01 s that decreased to 0.65 ± 0.02 mW (1.18 ± 0.04 mW ml−1) for longer pulse periods (5 s). Durability tests were conducted over 44 h with ≈2600 discharge/recharge cycles. In this relatively long-term test, the equivalent series resistance increased only by 10% and the apparent capacitance decreased by 18%.

Super-concentrated electrolyte solutions are of increasing interest for safer and more stable lithium and post-lithium batteries. The combination of 7Li and 17O (at natural abundance) nuclear magnetic resonance (NMR) and electrochemical characterization is proposed here as an effective approach to investigate the Li+ solvation structures and properties of electrolytes featuring tetraethylene glycol dimethyl ether (TEGDME) and lithium-bis(trifluoromethane sulfonyl) imide (LiTFSI). Five different formulations from salt-in-solvent to solvent-in-salt with LiTFSI at different concentrations (0.1 m, 0.5 m, 2 m, 4 m, 5 m) are investigated. The NMR results, also supported by physico-chemical characterizations such as thermal gravimetric analyses, differential scanning calorimetry, specific conductivity and viscosity, give information about the association of Li+ ions with anion and solvent molecules, allowing a deeper knowledge on the relationships among structure and functional properties of super-concentrated solutions.

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.

Three-dimensional graphene nanosheets (3D-GNS) were used as cathode catalysts for microbial fuel cells (MFCs) operating in neutral operating conditions. 3D-GNS catalysts showed high performance towards oxygen electroreduction in neutral media with high current densities and low hydrogen peroxide generation compared to activated carbon (AC). 3D-GNS was incorporated into air-breathing cathodes based on AC with three different loadings (2, 6 and 10 mgcm-2). Performances in MFCs showed that 3D-GNS had the highest performances with power densities of 2.059±0.003 Wm-2, 1.855±0.007 Wm-2 and 1.503±0.005 Wm-2 for loading of 10, 6 and 2 mgcm-2 respectively. Plain AC had the lowest performances (1.017±0.009 Wm-2). The different cathodes electrodes were also investigated in supercapacitive MFCs (SC-MFCs). The addition of 3D-GNS decreased the ohmic losses by 14-25%. The decrease in ohmic losses allowed the SC-MFC with 3D-GNS (loading 10 mgcm-2) to have the maximum power (Pmax) of 5.746±0.186 Wm-2. At 5 mA, the SC-MFC featured a capacitive response that increased from 0.027±0.007 F with AC to 0.213±0.026 F with 3D-GNS (loading 2 mgcm-2) and further to 1.817±0.040 F with 3D-GNS (loading 10 mgcm-2). This is the first time that a capacitance of the order of magnitude of Farads is shown related to microbial fuel cell system.

In this work, the electrodes of a microbial desalination cell (MDC) are investigated as the positive and negative electrodes of an internal supercapacitor. The resulting system has been named a supercapacitive microbial de- salination cell (SC-MDC). The electrodes are self-polarized by the red-ox reactions and therefore the anode acts as a negative electrode and the cathode as a positive electrode of the internal supercapacitor. In order to overcome cathodic losses, an additional capacitive electrode (AdE) was added and short-circuited with the SC- MDC cathode (SC-MDC-AdE). A total of 7600 discharge/self-recharge cycles (equivalent to 44 h of operation) of SC-MDC-AdE with a desalination chamber filled with an aqueous solution of 30 g L−1 NaCl are reported. The same reactor system was operated with real seawater collected from Pacific Ocean for 88 h (15,100 cycles). Maximum power generated was 1.63 ± 0.04 W m−2 for SC-MDC and 3.01 ± 0.01 W m−2 for SC-MDC-AdE. Solution conductivity in the desalination reactor decreased by ∼50% after 23 h and by more than 60% after 44 h. There was no observable change in the pH during cell operation. Power/current pulses were generated without an external power supply.

Supercapacitive microbial fuel cells with various anode and cathode dimensions were investigated in order to determine the effect on cell capacitance and delivered power quality. The cathode size was shown to be the limiting component of the system in contrast to anode size. By doubling the cathode area, the peak power output was improved by roughly 120% for a 10 ms pulse discharge and internal resistance of the cell was decreased by $47%. A model was constructed in order to predict the perfor- mance of a hypothetical cylindrical MFC design with larger relative cathode size. It was found that a small device based on conventional materials with a volume of approximately 21 cm3 would be capable of delivering a peak power output of approximately 25 mW at 70 mA, corresponding to 1300 W m"3.

Supercapacitors (SCs) are playing a key role for the development of self-powered and self-sustaining integrated systems for different fields ranging from remote sensing, robotics and medical devices. SC miniaturization and integration into more complex systems that include energy harvesters and func- tional devices are valuable strategies that address system autonomy. Here, we discuss about novel SC fabrication and integration approaches. Specifically, we report about the results of interdisciplinary ac- tivities on the development of thin, flexible SCs by an additive technology based on Supersonic Cluster Beam Deposition (SCBD) to be implemented into supercapacitive electrolyte gated transistors and supercapacitive microbial fuel cells. Such systems integrate at materials level the specific functions of devices, like electric switch or energy harvesting with the reversible energy storage capability. These studies might open new frontiers for the development and application of new multifunction-energy storage elements.

In this work, four different supercapacitive microbial fuel cells (SC-MFCs) with carbon brush as the anode and an air-breathing cathode with Fe-Aminoantipyrine (Fe-AAPyr) as the catalyst have been investigated using galvanostatic discharges. The maximum power (Pmax) obtained was in the range from 1.7 mW to 1.9 mW for each SC-MFC. This in-series connection of four SC-MFCs almost quadrupled Pmax to an operating voltage of 3025 mV and a Pmax of 8.1 mW, one of the highest power outputs reported in the literature. An additional electrode (AdHER) connected to the anode of the first SC-MFC and placed in the fourth SC-MFC evolved hydrogen. The hydrogen evolution reaction (HER) taking place at the electrode was studied on Pt and two novel platinum group metal-free (PGM-free) catalysts: Fe-Aminoantipyrine (Fe-AAPyr) and Fe-Mebendazole (Fe-MBZ). The amount of H2 produced was estimated using the Faraday law as 0.86 mMd-1cm-2 (0.132 LH2 day-1) for Pt, 0.83 mMd-1cm-2 (0.127 LH2 day-1) for Fe-AAPyr and 0.8 mMd-1cm-2 (0.123 LH2 day-1) for Fe-MBZ. Hydrogen evolution was also detected using gas chromatography. While HER was taking place, galvanostatic discharges were also performed showing simultaneous H2 production and pulsed power generation with no need of external power sources.

In this work, for the first time, we demonstrate a supercapacitive microbial fuel cell which integrates the energy harvesting function of a microbial fuel cell (MFC) with the high-power operation of an internal supercapacitor. The pursued strategies are: (i) the increase of the cell voltage by the use of high potential cathodes like bilirubin oxidase (BOx) or iron-aminoantipyrine (Fe-AAPyr); (ii) the use of an additional capacitive electrode (additional electrode, AdE) which is short-circuited with the MFC cathode and coupled with the MFC anode (MFC-AdE). The high working potential of BOx cathode and the low impedances of the additional capacitive electrode and the MFC anode permitted to achieve up to 19mW (84.4Wm-2, 152Wm-3), the highest power value ever reported for MFCs. Exploiting the supercapacitive properties of the MFC electrodes allows the system to be simpler, cheaper and more efficient without additional electronics management added with respect to an MFC/external supercapacitor coupling. The use of the AdE makes it possible to decouple energy and power and to achieve recharge times in the order of few seconds making the system appealing for practical applications.