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Article

Design and Optimization of Critical-Raw-Material-Free Electrodes towards the Performance Enhancement of Microbial Fuel Cells

by
Khair Un Nisa
,
Williane da Silva Freitas
*,
Alessandra D’Epifanio
and
Barbara Mecheri
*
Department of Chemical Science and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 385; https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060385
Submission received: 18 May 2024 / Revised: 6 June 2024 / Accepted: 7 June 2024 / Published: 15 June 2024
(This article belongs to the Section Electrocatalysis)
Browse Figures
Versions Notes

Abstract

:
Microbial fuel cells (MFCs) are sustainable energy recovery systems because they use organic waste as biofuel. Using critical raw materials (CRMs), like platinum-group metals, at the cathode side threatens MFC technology’s sustainability and raises costs. By developing an efficient electrode design for MFC performance enhancement, CRM-based cathodic catalysts should be replaced with CRM-free materials. This work proposes developing and optimizing iron-based air cathodes for enhancing oxygen reduction in MFCs. By subjecting iron phthalocyanine and carbon black pearls to controlled thermal treatments, we obtained Fe-based electrocatalysts combining high surface area (628 m2 g−1) and catalytic activity for O2 reduction at near-neutral pH. The electrocatalysts were integrated on carbon cloth and carbon paper to obtain gas diffusion electrodes whose architecture was optimized to maximize MFC performance. Excellent cell performance was achieved with the carbon-paper-based cathode modified with the Fe-based electrocatalysts (maximum power density-PDmax = 1028 mWm−2) compared to a traditional electrode design based on carbon cloth (619 mWm−2), indicating the optimized cathodes as promising electrodes for energy recovery in an MFC application.

Graphical Abstract

1. Introduction

Global development heavily depends on conventional energy sources derived from fossil fuels. However, the growing use of these sources raises concerns about a potential energy and environmental crisis. Therefore, there is an increasing interest in developing sustainable technologies for energy conversion and storage [1,2,3,4,5,6]. As the research in bioelectrochemical technologies progresses, integrating microbial fuel cells (MFCs) into an energy transition framework could significantly contribute to a more sustainable future [7,8,9,10]. An MFC converts biochemical energy into electrical energy through the metabolism of exoelectrogen microorganisms [11,12,13,14,15]. They operate in near-neutral pH as galvanic cells fed with wastewater, allowing the harvesting of electrical energy from removing organics in waste [16,17,18,19,20]. Despite recent practical implementations of MFCs, the low power density output remains challenging for making their application more widespread [8,19,20,21,22,23].
MFC comprises an anode, cathode, and electrolyte compartment [24]. Organic waste in wastewater undergoes oxidation by exoelectrogen microorganisms colonizing the anode [25,26,27], among which Shewanella oneidensis, Pseudomonas aeruginosa, and Geobacter sulfurreducens are the exoelectrogens explored for the anodic reaction [28,29,30]. At the same time, the electron acceptor is reduced at the cathode; the most used electron acceptor is oxygen due to its accessible availability and redox features [31]. The oxygen reduction reaction (ORR) is a pH-dependent reaction, and the low availability of H+/OH− in neutral pH electrolytes significantly affects ORR kinetics and, consequently, the MFC performance. A common strategy to accelerate ORR kinetics involves using electrocatalysts at the cathode [32].
According to the literature, three leading families of MFC electrocatalysts can be identified: (i) platinum-group metals (PGMs) [33,34,35], (ii) metal-free carbon-based materials [36], and (iii) PGM-free catalysts [33,37,38,39]. While PGM-based catalysts have been conventionally used in MFCs due to their high activity, they are not applicable in MFCs because of their low abundance, high cost, and low-performance durability due to deactivation of the catalytic sites in the MFC environment [40]. Electrocatalysts based on the heteroatom doping (usually B, N, O, S, and P) of carbonaceous materials have been reported as a cost-effective alternative [41]. However, metal-free active sites cannot efficiently convert O2 through a four-electron pathway, requiring high overpotential and yielding high peroxide intermediate concentrations known to degrade the active sites [42]. Extensive research has focused on developing transition-metal-based (TM-based) ORR electrocatalysts for a multiplicity of fuel cell applications, and catalysts based on metal, nitrogen, and carbon (M-N-C) exhibit remarkable activity toward ORR in a wide pH range [43,44].
Several approaches involving different families of precursors have been reported to develop M-N-C catalysts, including the use of pyrroles, corroles, and porphyrins [45]; N-containing organic molecules [46]; metal-organic frameworks [47,48]; and polymers [49,50]. The strategies for obtaining the M-Nx-C active sites from such precursors can be divided into two categories: molecular catalysts involving room- or low-temperature synthesis steps (wet impregnation, mechanical mixing, ball milling, etc.) [51] and TM single-atom catalysts obtained via high-temperature pyrolysis of the TM-based and nitrogen-rich precursors [52,53]. Molecular catalysts, having pre-formed M-N4-C active sites, have shown high ORR activity at neutral pH [54,55,56]; however, they lack stability under varying pH conditions. High-temperature pyrolysis was demonstrated to be crucial for obtaining stable M-Nx-C sites [57,58,59]. Fe-based catalysts show superior performance compared to catalysts based on Mn, Ni, and Co when integrated into the catalytic layer of MFC cathodes. [60,61]. Despite progress in developing M-N-C catalysts with high activity, their cost and stability remain challenging for their application in MFCs [62,63,64].
Besides the required features for suitable PGM-free catalysts in an MFC application, the air cathode design is another concern that plays a pivotal role in MFC performance [65]. Air cathodes were found to adapt to the MFC’s scale and application, with variations for lab-scale prototypes, wastewater treatment systems, or portable energy generators [4,66]. Their architecture’s optimization is central to enhancing MFC performance and efficiency, since it significantly affects the diffusion of reactive species and their related products, activity, stability of the catalyst layer, electron transfer, and water management at the electrode–electrolyte interface [67]. Moreover, long-term stability is essential, necessitating materials that resist microbial fouling, the presence of contaminants from the organic waste, and corrosion [68,69,70,71].
The use of Fe-phthalocyanines (FePc) as a precursor for iron and nitrogen has been reported as an efficient approach for obtaining highly active Fe-Nx-C sites after high-temperature pyrolysis steps, highlighting the FePc-derived Fe-N-C materials as a standard catalyst for ORR at a wide pH range [54,58,72,73]. Since the use of cost-effective and stable catalysts at the cathode side of MFC is required for their implementation in the energy scenario, we have adopted a facile synthesis approach based on the pyrolysis of FePc and carbon black pearls for obtaining Fe-N-C O.R.R. catalysts. In this work, we have focused on the design of the air electrode, which can boost the performance of a single-chamber MFC by using critical-raw-material-free catalysts. Combining a stainless-steel mesh-based current collector with a porous carbon-based electrode substrate and an optimized catalyst layer (CL), gas diffusion layer (GDL), and preparation effectively improved MFC performance in terms of power output and durability under operating conditions compared to a traditional electrode design taken as the control. This study highlights the optimized electrode design as a promising CRM-free air cathode for membraneless MFCs.

2. Results and Discussion

The electrocatalysts were synthesized by impregnating carbon black pearls (BPs) with Fe-phthalocyanine followed by pyrolysis in Ar or NH3 flows at 900 °C for 1 h, as described in the Supplementary Materials. The prepared catalysts were labeled Fe-N-C_Ar and Fe-N-C_NH3.
Thermogravimetric analysis of the catalyst precursors (Figure S1) indicated the fragmentation and complete carbonization of the iron phthalocyanine macrocycle [74], which is supported by the absence of the diffraction peaks (2θ = 10–40°) of pristine FePc (Figure S2), in the XRD patterns of the pyrolyzed Fe-N-C_Ar and Fe-N-C_NH3 catalysts (Figure S3). As shown in Figure S3, the structure of the catalysts is a combination of carbonaceous material, as indicated by the (002) and (101) planes at 2θ = 25° and 2θ = 43°, respectively [75], with iron-based phases. Pyrolysis under an argon atmosphere promoted the formation of iron carbide (Fe3C, JCPDS reference pattern: 00-003-0989), with a slight contribution of iron oxide (magnetite phase—FeOFe2O3, JCPDS reference pattern: 96-900-5815), and pyrolysis under ammonia led to the formation of FeOFe2O3. Oxides can be related to the saturation of in situ formed Nx-C moieties due to an iron content higher than 0.5 wt.% [62,76].
The N2 adsorption–desorption curves for both catalysts exhibited an IV-type isotherm with a notable H4 hysteresis loop [77] (Figure 1a). The specific surface area (SSA), determined using the BET theory, and the maximum pore volume were 628.42 m2 g−1 and 0.56 cm3 g−1 for Fe-N-C_NH3, 588.27 m2 g−1 and 0.44 cm3 g−1 for Fe-N-C_Ar. The beneficial effect of NH3 as a pyrolysis atmosphere in forming pores and increasing the surface area is also evident in the BJH pore size distribution (Figure 1b). The pore volume distribution is relatively higher for both samples, which have a 1.8 to 19.5 nm pore size. Interestingly, a higher mesopore volume distribution was observed for Fe-N-C_NH3 (Figure 1b), indicating that pyrolysis under NH3 is also beneficial for promoting the formation of mesopores, which facilitate reactant diffusion into the electrodes [78].

2.1. Half-Cell Tests: ORR Activity and Mechanism

Figure 2 shows cyclic voltammograms and RRDE experiments to assess the catalyst’s ORR activity and mechanisms when pyrolyzed under Ar or NH3, Fe-N-C_Ar, and Fe-N-C-NH3, respectively. The Fe-N-C-NH3 showed superior catalytic activity performance compared with Fe-N-C_Ar, as evidenced by the more positive potential peak for Fe-N-C_NH3 in the CV (Figure 2a,b) than Fe-N-C-Ar. The linear sweep voltammetry (Figure 2c) confirms the higher performance of Fe-N-C-NH3, as indicated by the higher onset potential, half-wave potential, and limiting current for Fe-N-C_NH3 (0.82 V, 0.66 V and 4.70 mA cm−2) compared with Fe-N-C_Ar (0.80 V, 0.58 V and 4.34 mA cm−2). Additionally, the ring current detected for Fe-N-C-NH3 was lower than Fe-N-C_Ar (Figure 2d), from which we calculated the electrons transferred n (Figure 2e) and H2O2 yield % (Figure 2f). For Fe-N-C_NH3, the peroxide production was lower than 5% and n approaches 4, indicating a direct oxygen reduction to water as a reduction pathway during ORR. According to previous studies, the higher ORR activity of the Fe-N-C_NH3 catalyst can be correlated to the use of NH3 as a nitrogen secondary source, promoting the content increase in Nx-C active sites and the change in the electronic properties and surface characteristics, combined with the effect of NH3 on increasing the porosity of the carbon matrix [63,79,80]. The half-cell tests confirm the effectivity of pyrolysis under the NH3 atmosphere, obtaining a highly active ORR catalyst. Therefore, Fe-N-C_NH3 was selected for integration in cathodes.

2.2. Catalyst Integration into the Air Electrodes and Cell Testing

After optimizing the procedure for obtaining carbon-cloth-based cathodes (CC-cathodes) and carbon-paper-based cathodes (CP-cathodes), Fe-N-C_NH3 was integrated into CC-cathodes and CP-cathodes as described in Section 3 (Materials and Methods).
The cathode durability was evaluated by monitoring voltage generation cycles over time (90 days), as shown in Figure 3a. The CP-cathode generated more stable cell voltage cycles than the CC-cathode. During the initial 10–20 days, MFCs equipped with CC-cathodes demonstrated a relatively stable voltage response compared to days 30–40, where a drop in cell voltage was observed. The subsequent recovery in cell voltage for the CC-equipped M.F.C. during days 70 to 80 can be ascribed to biofilm stabilization and cathode activation and stabilization processes. Variations in oxygen diffusion rates to the cathode can also contribute to fluctuations, especially in the early stages of operation when inconsistent oxygen availability may occur, potentially influenced by non-optimized cathode architecture.
At the beginning of the experiments, the cell voltage obtained for the MFC equipped with the CP-cathode was 0.61 V, reaching 0.52 and 0.58 V after 60 and 90 days, respectively. In contrast, the cell voltage for the CC-cathode was 0.51 (day 0), 0.35 V (day 60), and 0.34 V (day 90), indicating that the CP-cathode’s architecture improved the MFC energy output and performance stability.
The cathode and anode polarization curves were separately acquired in a three-electrode configuration using an Ag/AgCl reference electrode (Figure 3b). The anode polarization was similar for all the MFCs investigated, indicating that the main difference in the overall MFC performance was caused by the cathode behavior as enhanced by the cathode polarization. The cathode polarization curves exhibited a linear trend, indicating that the MFCs were predominantly influenced by ohmic behavior, corroborating previous studies reported in the literature [81,82,83]. The MFC equipped with the CP-cathode consistently outperformed the CC-cathodes in terms of open-circuit voltage, activation overpotential, ohmic overpotential, and mass transport limitation.
EIS spectra were recorded for the freshly assembled (day 0) and aged (after 90 days) CC-cathode and the CP-cathode to investigate their electrochemical behavior under operating conditions. The Nyquist plots (Figure 4) were recorded under open-circuit voltage conditions, showing a high-frequency process associated with charge transfer at the cathode and a low-frequency process linked to mass transfer/diffusion [84]. The Nyquist plots were fitted with the equivalent electrical circuit depicted in the inset of Figure 4a–d. The equivalent circuit comprised ohmic resistance (Rs) corresponding to the solution resistance ranging between 5 and 10 Ω, subtracted from each spectrum. The first CPE denotes a constant phase element associated with the double-layer capacitance. Rct corresponds to the charge transfer resistance, and the second CPE can be ascribed to the biofilm growth onto the cathode. The Rd element indicates the mass transfer/diffusion resistance. For the aged CP cathode, CC-cathode (day 0), and aged CC-cathode (Figure 4b–d), the equivalent circuit comprises an additional Warburg diffusion (W) element in series with Rd, which is included to describe the rising diffusion limitations of reactants and products within the cathode structure due to electrode aging. The Warburg element is absent when modeling the EIS spectra of the fresh CP-cathode due to facilitated oxygen diffusion through the electrode. Nyquist plot fittings for the CP-cathode and CC-cathode are presented in Table 1. The Rct and Rd values for the CP-cathode and CC-cathode at day 0 are 0.88 vs. 2.46 Ω and 14.22 vs. 41.80 Ω, respectively, assuring the advantage of the optimized cathode for reducing activation overpotential. As the cathode ages, it may undergo structural changes or degradation, leading to changes in the pathways for mass transport since MFCs often involve the growth of microbial biofilms on the electrode surfaces. Both types of cathodes underwent aging and exhibited an increase in Rct and Rd values at the end of day 90. However, upon comparison, the Rct and Rd values for the aged CP-cathode and CC-cathode are reported as 1.58 vs. 3.8 Ω and 200 vs. 342 Ω, indicating that the CP-cathode outperforms the CC-cathode.
Visual, optical, and scanning electron microscopy (SEM) analyses were performed on CC- and CP-cathodes to investigate possible changes in the electrode surface morphology after MFC operation (Figure 5). The cathodes exhibited the formation of a thick biofilm (Figure 5d–h and Figure S4), typically observed for MFC cathodes after long-term operation [70]. Optical microscopy and SEM pictures were taken at the interface between the biofilm and the cathode unexposed area, as illustrated in the red highlighted area in Figure 5d,h.
Optical micrographs revealed a smoother surface for both CP-cathodes as compared to the CC-cathodes. Comparing the biofilm-covered surfaces illustrates that the CP-cathode maintains a more preserved morphology (Figure S5d,e). In contrast, the CC-cathode exhibits more prominent holes (Figure S5a,b). Further comparison of Figure S5c–f underscores the presence of holes in the CC-cathode, suggesting that the integration of the stainless-steel (SS) mesh to the gas diffusion layer (GDL) in the CP-cathode results in more uniform adhesion of the homemade GDL to the electrode, enhancing mechanical stability.
The morphology of the fresh and aged catalyst layers in both electrodes was further investigated by SEM analysis (Figure 5b,c,f,g). SEM micrographs of the fresh CC-cathode (Figure 5b) and CP-cathode (Figure 5f) show that the hot-pressing does not alter the catalyst layer (CL) morphology compared to that of the CC-cathode, showing uniformly distributed carbon-based particles with a porous structure. However, after aging, the catalyst particles on the CC-cathode surface (Figure 5c) appear more aggregated than those on the CP-cathode (Figure 5g).
These findings are consistent with the optical microscopy analysis, showing a more preserved CP-cathode CL morphology. This suggests that the combination of carbon paper as the electrode substrate, SS mesh as the current collector, and the hot-pressing steps effectively maintain the integrity of the electrode CL even after long-term aging.
The curves of potential vs. current and power density recorded for the assembled cells are illustrated in Figure 6 for 0-, 20-, 40-, 60-, and 90-day intervals, and Table 2 shows a comparison between the open-circuit voltage and PDmax of the two cathodes and power loss over 90 days.
The MFCs assembled with the fresh (day 0) CP-cathode achieved the highest PDmax (1028 ± 3 mWm−2), while PDmax was 619 ± 2 mWm−2 at day 0 for the MFCs equipped with the CC-cathodes. A slight performance fluctuation can be observed for the CC-cathode during the first 40 days, which can be ascribed to cathode activation, also reflected in changes in oxygen diffusion through the electrode [85]. Between day 60 and day 90, both cathodes experienced a drop in power density. The CP-cathode showed a 30.5% PDmax drop, and the CC-cathode exhibited a slightly larger PDmax drop of 31.3%. At the end of the 90 days, the PDmax for the CP-cathode was 616 mWm−2, surpassing the CC-cathode, which had a 412 mWm−2 PDmax. These data highlight the CP-cathode as the best-performing electrode in the single-chamber MFC system. The high performance observed for the CP-cathode arises from the lower thickness and higher porosity of CP compared to that of CC, combined with the optimization of the GDL and catalyst layer preparation, promoting oxygen transport and electron transfer. Additionally, incorporating a stainless-steel-based current collector with an appropriate pore size improved the GDL performance, increasing the power density.
The MFC performance achieved in this work, when compared with that obtained in previously published works for similar cathodes (Table 3), highlights CP-cathodes as potential electrodes for an MFC application since a versatile approach is adopted to obtain catalysts with high ORR activity and air electrodes with competitive performance in the MFC conditions.

3. Materials and Methods

3.1. Electrocatalysts’ Synthesis and Air Cathode Assembly

The reagents and materials used in this work are listed in the Supplementary Materials, together with the procedure for preparing activated carbon black pearls (activated BPs).
The Fe-N-C catalysts were prepared by N- and Fe-functionalization of activated BPs, which consisted of dispersing 0.12 g of iron phthalocyanine (FePc) in ethanol (50 mL). Separately, 0.48 g of activated BPs was dispersed in the same volume (50 mL) of solvent. Both suspensions were then sonicated for 15 min. Subsequently, the two suspensions were combined and stirred for 60 min at around 25 °C. The final dispersion was dried (T = 70 °C) for 3 to 4 h. The resulting powder was then ground with the assistance of a mortar and pestle. The samples were then subjected to pyrolysis under either an Ar or NH3 atmosphere, increasing the temperature at a rate of 20 °C per minute until it reached 900 °C, which was kept for 1 h, obtaining two samples labeled as Fe-N-C_Ar and Fe-N-C_NH3.
Figure 7 shows the methodology for preparing gas diffusion layer (GDL) cathodes.
CC-cathodes were prepared by applying 4 PTFE-based diffusion layers on the opposite side of the microporous layer (MPL) of ELAT LT1400W MPL carbon cloth (Figure 6a) following established methods [95,96,97]. A 7 cm2 electrode was modified by brush painting an ink of Fe-N-C_NH3 onto the opposite side of the PTFE layers. The catalyst layer comprised 14 mg of the prepared Fe-N-C materials, 47 μL of 2-propanol, 12 μL of deionized water, and 94 μL of 5 wt.% Nafion solution to a total catalyst loading of 2 mgcm−2. As previously reported, carbon-paper-based catalysts were prepared by modifying stainless-steel (SS) mesh with a carbon Vulcan/PTFE GDL [98]. Carbon paper was assembled onto the GDL-modified stainless-steel mesh by pressing the surface at 80 °C with a pressure of 4.96 MPa for 1 min (Figure 6b). A catalyst ink based on a suspension of Fe-N-C_NH3 catalyst into 160 μL of Nafion 5 wt.% and 200 μL of isopropanol solution was then deposited on the CP, maintaining a catalyst loading of 2 mgcm−2.

3.2. Methods

Thermogravimetric analysis (TGA) was conducted in a TGA/DSC1 Star System analyzer (Mettler Toledo, Milan, Italy). Mass loss curves were recorded under an N2 flow, investigated in a temperature range of 25 to 800 °C, at a rate of 5 °C min−1. A Philips PW1730 diffractometer (Cu Kα radiation, λ = 1.5406 A) was used to investigate the phase composition of the precursors and pyrolyzed catalysts. BET SA analysis was carried out by using a Micromeritics® TriStar II Plus (Norcross, GA, USA) surface area and porosity analyzer. The specific surface area (p/p0 = 0.05–0.3) of the samples was estimated by applying the BET theory, while the pore size distribution was estimated from the Barrett–Joyner–Halenda (BJH) model, applied to the N2-desorption curve. Before measurements, samples underwent a pretreatment consisting of drying (T = 270 °C) over 12 h in a vacuum system (BUCHI Glass Oven B-585, Milan, Italy) and 1 h under N2 flow at 300 °C.
The surface morphology of the catalyst layer and GDL of the air cathodes were investigated by optical microscopy using a Nikon SMZ-U, Norcross, GA, USA (zoom 1:10) optical microscope equipped with a Visicam 10.0 photo-camera after an extensive aging period of 11 months in operating microbial fuel cells (MFCs). Before the micrograph acquisition, the electrodes were dried at 50 °C for 1 h. The catalyst layer morphology of the dried electrodes was investigated by scanning electron microscopy using an FE-SEM, Leo Supra 35, Carl Zeiss, Oberkochen, Germany scanning electron microscope.
The electrochemical tests in a standard cell were carried out using a rotating ring disk working electrode (RRDE-AFE6R2GCPT, Pine Research Instrumentation, Durham, NC, USA), a graphite rod counter electrode, and a saturated silver/silver chloride electrode (Ag/AgCl 3.3 M) as the reference electrode. Data recording was carried out using a potentiostat model VMP3 (Bio-Logic Science Instruments, Seyssinet-Pariset, France) under the control of EC-LabV10.18 software. The potential scale vs. the adopted reference electrode was subsequently converted to the reversible hydrogen electrode (RHE), which normalizes the potential values to the pH effect. A 100 mM phosphate-buffered solution (PBS, pH = 7.4) was used as the electrolyte. Before the catalyst ink deposition, the working electrode was first polished with alumina suspension (0.3 μm) and finally polished with a 0.05 μm suspension in a micro-cloth pad (Pine polishing kit, Durham, NC, USA). Then, the glassy carbon disk/platinum ring surface was rinsed and ultrasonicated for 5 min with distilled water. Catalyst inks were prepared by dispersing a proper amount of catalyst in a solution containing isopropanol (425 μL) and Nafion (0.5 wt.% in H2O, 75 μL) to obtain 0.20 mgcm−2 catalyst loading after drop-casting 5 μL of the ink onto the glassy carbon disk (geometric area = 0.196 cm2). The dispersion was prepared by ultrasonication for 2 h at around 15 °C. The ink layer on the glassy carbon was dried in an oven (T = 40 °C, time = 5 min). Before electrochemical testing, the PBS electrolyte was purged with N2 for at least 20 min and the surface of the modified WE was activated through cyclic voltammetry (CV) between 0.3 and 1.2 V vs. RHE at 500 mV s−1 potential scan rate over 200 CV cycles. Both CV and linear sweep voltammetry (LSV) were adopted to record capacitive and ORR currents after purging the electrolyte with N2 or O2. LSV at the disk and the chronoamperometry at the ring (LSV-CA) performed with the RRDE setup during ORR were conducted at a 5 mV s−1 scan rate and hydrodynamic conditions (ω = 1600 rpm), scanning the potential between 1.0 and 0.0 V vs. RHE. The Pt-ring was polarized at 1.2 V vs. RHE to record ring current during ORR. The CVs under static conditions were acquired at the same scan rate and potential window. Capacitive current contributions on both disk and ring currents during the ORR were corrected by subtracting the current measured after saturation of the electrolyte with N2. The reported potentials for the ohmic drop were also corrected by measuring the cell resistance.
The number of electrons transferred (n) and the yielding percentage of hydrogen peroxide (H2O2%) were calculated as described in the Supplementary Materials and reported in previous papers [47].
MFC tests were conducted in a membraneless reactor (28 mL volume, 4 cm length). The cathode is a 7 cm2 disk of either the CC-cathode or CP-cathode and the anode consists of a graphite fiber brush, moved from running MFCs, precolonized with the inoculum of wastewaters from Ortona (Zona Industriale Tamarete-Chieti, Italy). The cell was fed with a phosphate-buffered solution (100 mM, pH = 7.4) composed of NH4Cl (0.62 g L−1), KCl (0.26 g L−1), and CH3COONa (0.75 g L−1). The MFC tests were conducted in batch mode [99] at room temperature and included two independent replicates for each measurement. The solution was removed from the reactors and replaced by a fresh one whenever the measured voltage (R = 1 kΩ) fell below 40 mV, marking the completion of one operational cycle. The cells were considered acclimated when the voltage output was similar for three consecutive cycles (0.4 V ± 0.05).
Polarization curves were recorded using either a multichannel VSP potentiostat/galvanostat (BioLogic, France) controlled by EC-Lab software in a three-electrode configuration (with Ag/AgCl reference electrode) or a multimeter (2700, Keithley, Cleveland, OH, USA) connected to a personal computer in a two-electrode configuration. In the three-electrode configuration, the cathode and anode polarization curves were separately recorded at a scan rate of 5 mV s−1 and the current density was normalized to the cathode geometric area. Electrochemical impedance spectra of cathodes were carried out between 10 mHz and 10 kHz, with an alternating current amplitude of 10 mV. In the two-electrode configuration, polarization curves were obtained by changing a series of external resistances (from 10 to 10 KΩ) on the circuit connected to each cell and measuring the cell voltage every 30 min for each different resistance.

4. Conclusions

This study adopted a facile synthesis approach to develop iron–nitrogen–carbon catalysts derived from iron phthalocyanine, a single source of Fe and N, which were incorporated into a modified carbon-based support. The precursors were subjected to a controlled thermal treatment to promote Fe-Nx-C catalytic site formation as oxygen-reducing centers. The optimization of the pyrolysis conditions under an ammonia flow allows obtaining an electrocatalyst with a high specific surface area (628.42 m2 g−1) and a high oxygen reduction activity in terms of half-wave potential (0.66 V vs. RHE) and limiting current density (4.70 mA cm−2), with the transferred electrons close to 4, indicating a direct 4e- pathway of oxygen reduction to water.
The Fe-N-C_NH3 catalyst was integrated at the cathode side of a single-chamber MFC, using either carbon paper (CP-cathode) or carbon cloth (CC-cathode) as catalyst layer substrates. As indicated by the open-circuit voltage, peak power density, and voltage generation cycles, CP-cathodes outperformed CC-cathodes. Electrochemical impedance spectroscopy analysis of the MFC cathode during operation evidenced lower charge transfer and diffusion resistance values for CP-cathodes than CC-cathodes, indicating that optimized electrode design promoted reactive species transport and electron transfer into the cathode. This higher performance is correlated with the thickness and porosity differences between CP and CC, combined with using the stainless-steel mesh current collector and the hot-pressing of the catalyst layer. The use of such components and the optimization of the procedure for their assembly contributed to the integration of the catalyst and the improvement of the energy output and performance durability of the MFCs. The optimized electrode also showed an outstanding performance compared to other PGM-free air cathodes assembled in MFCs. The results highlighted the adopted approach for synthesizing the catalysts and the design of the air cathodes as effective in improving the MFCs energy output, indicating the prepared CRM-free electrodes as promising oxygen-reducing cathode candidates for application in bioelectrochemical systems for energy recovery.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/catal14060385/s1, Figure S1. TGA curves of the catalyst’s precursors [74,100,101,102]; Figure S2. XRPD patterns for bare Fe phthalocyanine and activated carbon black pearls precursors [103,104]; Figure S3. XRD patterns for the pyrolyzed samples. Figure S4. Pictures of the CC-cathode and CP-cathode after 90 days of MFC operation. Figure S5. Optical micrographs of the catalyst layer and GDL for the CC-cathode and CP-cathode at different magnifications after 90 days of MFC operation.

Author Contributions

Conceptualization, W.d.S.F., A.D. and B.M.; Data curation, K.U.N. and W.d.S.F.; Funding acquisition, B.M.; Investigation, K.U.N. and W.d.S.F.; Methodology, W.d.S.F., B.M. and A.D.; Project administration, B.M. and A.D.; Resources, A.D. and B.M.; Supervision, W.d.S.F., A.D. and B.M.; Writing—original draft, K.U.N. and W.d.S.F.; Writing—review and editing, W.d.S.F., B.M. and A.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work received funding from the European Union—Next Generation EU in response to the MUR (Ministry of University and Research) call “PRIN (Project of National Interest) 2022”: Project code: 20224WLXRK.

Data Availability Statement

Data used to support the findings of this study are available within the article and its Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00385 g001
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distribution for Fe-N-C_NH3 and Fe-N-C_Ar catalysts.
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distribution for Fe-N-C_NH3 and Fe-N-C_Ar catalysts.
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Figure 2. (a,b) Cyclic voltammetry, (c) disk current density, (d) ring current, (e) number of electrons transferred, and (f) hydrogen peroxide percentage for Fe-N-C_Ar and Fe-N-C_NH3.
Figure 2. (a,b) Cyclic voltammetry, (c) disk current density, (d) ring current, (e) number of electrons transferred, and (f) hydrogen peroxide percentage for Fe-N-C_Ar and Fe-N-C_NH3.
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Catalysts 14 00385 g003
Figure 3. (a) Cell voltage generation cycles (under 1 kΩ resistance) over 90 days from the beginning of the experiment and (b) separate anode (dotted line) and cathode (solid line) polarization curve for freshly assembled MFCs with the CC-cathode or CP-cathode.
Figure 3. (a) Cell voltage generation cycles (under 1 kΩ resistance) over 90 days from the beginning of the experiment and (b) separate anode (dotted line) and cathode (solid line) polarization curve for freshly assembled MFCs with the CC-cathode or CP-cathode.
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Figure 4. Nyquist plots for freshly assembled (day 0) and aged (day 90) cathodes. (a,b) CP-cathode and (c,d) CC-cathodes.
Figure 4. Nyquist plots for freshly assembled (day 0) and aged (day 90) cathodes. (a,b) CP-cathode and (c,d) CC-cathodes.
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Figure 5. Optical micrographs (a,e), SEM images (c,g), and pictures (d,h) of the CC-cathode (upper images) and CP-cathode (bottom images) after 90 days of MFC operation. SEM images of fresh (day 0) CC-cathode (b) and CP-cathode (f).
Figure 5. Optical micrographs (a,e), SEM images (c,g), and pictures (d,h) of the CC-cathode (upper images) and CP-cathode (bottom images) after 90 days of MFC operation. SEM images of fresh (day 0) CC-cathode (b) and CP-cathode (f).
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Figure 6. Polarization and power density curves for the MFCs equipped with (a,b) CP-cathodes and the (c,d) CC-cathodes over 3 months (0-, 20-, 30-, 40, 60-, and 90-day intervals).
Figure 6. Polarization and power density curves for the MFCs equipped with (a,b) CP-cathodes and the (c,d) CC-cathodes over 3 months (0-, 20-, 30-, 40, 60-, and 90-day intervals).
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Figure 7. Air cathode preparation schematics. (a) Carbon-cloth-based cathodes (CC-cathode) and (b) carbon-paper-based cathodes (CP cathode).
Figure 7. Air cathode preparation schematics. (a) Carbon-cloth-based cathodes (CC-cathode) and (b) carbon-paper-based cathodes (CP cathode).
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Table 1. Charge and mass transfer resistances of newly assembled vs. aged cathodes.
Table 1. Charge and mass transfer resistances of newly assembled vs. aged cathodes.
Rct (Ohm)Rd (Ohm)
CP-cathode (day 0)Aged CP-cathode CP-cathode (day 0)Aged CP-cathode
0.88 ± 0.01 1.58 ± 0.0714.22 ± 1.2200 ± 3.0
CC-cathode (day 0)Aged CC-cathode CC-cathode (day 0)Aged CC-cathode
2.46 ± 0.23.80 ± 0.941.80 ± 2.0342 ± 4.20
Table 2. Performance comparison of the MFCs equipped with CP- and CC-cathodes.
Table 2. Performance comparison of the MFCs equipped with CP- and CC-cathodes.
Aging TimeOpen-Circuit Voltage
(V)		PDmax
(mWm−2)		Power Loss
(%)
CP-cathodeCC-cathodeCP-cathodeCC-cathodeCP-cathodeCC-cathode
Day 00.690.661028619--
Day 200.680.659676525.93-
Day 400.670.659406462.791.00
Day 600.630.648876005.637.12
Day 900.630.5961641230.531.3
Table 3. Performance comparison of MFCs equipped with different PGM-free catalysts based on the literature.
Table 3. Performance comparison of MFCs equipped with different PGM-free catalysts based on the literature.
CatalystMaximum Power Density (mWm−2)Reference
AC/PTFE584[86]
MgO350[87]
Fe-N-C643[88]
Fe-N-C1092[89]
Fe-N-C699[90]
TMO450[91]
Biochar528[92]
Fe-N-C1200[93]
Fe-N-C375[94]
Fe-N-C_NH31028This work
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Nisa, K.U.; da Silva Freitas, W.; D’Epifanio, A.; Mecheri, B. Design and Optimization of Critical-Raw-Material-Free Electrodes towards the Performance Enhancement of Microbial Fuel Cells. Catalysts 2024, 14, 385. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060385

AMA Style

Nisa KU, da Silva Freitas W, D’Epifanio A, Mecheri B. Design and Optimization of Critical-Raw-Material-Free Electrodes towards the Performance Enhancement of Microbial Fuel Cells. Catalysts. 2024; 14(6):385. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060385

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Nisa, Khair Un, Williane da Silva Freitas, Alessandra D’Epifanio, and Barbara Mecheri. 2024. "Design and Optimization of Critical-Raw-Material-Free Electrodes towards the Performance Enhancement of Microbial Fuel Cells" Catalysts 14, no. 6: 385. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060385

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