Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

Bismuth doped TiO2 as an excellent photocathode catalyst to enhance the performance of microbial fuel cell

Bismuth impregnation on pure TiO₂ (BiTiO₂) was carried out and tested in microbial fuel cell (MFC) as photocathode catalyst. UV–Visible spectral observation confirmed higher catalytic activity of BiTiO₂ under visible light irradiation with reduced band gap of 2.80 eV as compared to pure TiO₂ (3.26 eV). Electrochemical impedance spectroscopy also showed two times higher exchange current density with lower charge transfer resistance for BiTiO₂ (1.90 Ω) than pure TiO₂ (3.95 Ω), thus confirming it as superior oxygen reduction reaction catalyst. MFC operated with BiTiO₂ could generate a maximum power density of 224 mW m^?2, which was higher than MFC with Pt as cathode catalyst (194 mW m^?2) and much higher than MFCs with TiO₂ catalyzed cathode (68 mW m^?2) and without any cathode catalyst (60 mW m^?2). The results thus promote Bi doped TiO₂ as a superior low-cost alternative to the costly Pt catalyst to take this MFC technology forward for field application.     

Poly (acrylamide-co-acrylonitrile) hydrogel-derived iron-doped carbon foam electrocatalyst for enhancing oxygen reduction reaction in microbial fuel cell

Developing advanced non-precious metal catalysts for oxygen reduction reaction (ORR) is critical for microbial fuel cells (MFCs). Fe–N–C catalysts are considered the best successor to platinum-based catalysts for ORR. Herein, we have synthesized environmental friendly, cost-effective Fe–N-doped carbon foam catalyst [Fe-embedded poly (acrylamide-co-acrylonitrile) hydrogel-based carbon foam(Fe@Am-co-An/CF)] by using Fe-embedded poly (Am-co-An) hydrogel for MFCs. Poly(Am-co-An) hydrogel is used as a carbon and nitrogen precursor. The synthesized catalysts are characterized by FTIR, SEM, TEM, XRD and XPS. Furthermore, four different catalysts based on different ratios of the metal such as Fe@Am-co-An/CF (1:22), Fe@Am-co-An/CF (2:22), Fe@Am-co-An/CF (3:22), and Am-co-An/CF have been prepared. Results indicate that the Fe@Am-co-An/CF (2:22) catalyst exhibits the highest power density (736.06 mWm^?2 at the current density of 1132.04 mAm^?2) compared to the other catalysts. The results of CV, LSV, EIS, and chronoamperometry indicate that Fe@Am-co-An/CF (2:22) is the most promising catalyst for ORR activity in MFCs.     

Enhanced bioelectrochemical performance by NiCoAl-LDH/MXene hybrid as cathode catalyst for microbial fuel cell

In this work, NiCoAl-layered double hydroxide (LDH)/MXene was successfully prepared through straightforward hydrothermal method. NiCoAl-LDH was tightly and uniformly coated on MXene, forming a kind of porous structure. NiCoAl-LDH/MXene exhibited the (002) (012) (105) (100) crystal planes of hydrotalcite reflection. NiCoAl-LDH/MXene also showed superior catalytic oxygen reduction reaction (ORR) in response current according to electrochemical test (cyclic voltammetry (CV) etc.). The maximum power density and output voltage of NiCoAl-LDH/MXene as cathode in microbial fuel cell (MFC) was 362.404 mW/m² and 450 mV, respectively, which was 1.54 times of MXene-MFC (234.256 mW/m²) and 1.71 times of NiCoAl-LDH-MFC (211.56 mW/m²). The results indicated that NiCoAl-LDH/MXene was a kind of potential cathode catalyst for MFC and was full of future application.     

Catalytic behavior of Co3O4 in electroreduction of H2O2

Spinel structure Co₃O₄ nanoparticles with an average diameter of around 17 nm were prepared and evaluated as electrocatalysts for H₂O₂ reduction. Results revealed that Co₃O₄ exhibits considerable activity and good stability for electrocatalytic reduction of H₂O₂ in 3 M NaOH solution. The reduction occurs mainly via the direct pathway when H₂O₂ concentration is lower than 0.5 M. An Al-H₂O₂ semi fuel cell using Co₃O₄ as cathode catalyst was constructed and tested at room temperature. The fuel cell displayed an open circuit voltage of 1.45 V and a peak power density of 190 mW cm⁻² at a current density of 255 mA cm⁻² operating with a catholyte containing 1.5 M H₂O₂. This study demonstrated that Co₃O₄ nanoparticles are promising cathode catalysts, in place of precious metals, for fuel cells using H₂O₂ as oxidant.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

LaNi0.8Co0.2O3 as a cathode catalyst for a direct borohydride fuel cell

A perovskite-type oxide LaNi_0.8Co_0.2O₃ is prepared as a direct borohydride fuel cell (DBFC) cathode catalyst. Its electrochemical properties are studied by cyclic voltammetry. The results demonstrate that LaNi_0.8Co_0.2O₃ exhibits excellent electrochemical activity with respect to the oxygen reduction reaction (ORR) and good tolerance of BH₄⁻ ions. Maximum power densities of 114.5 mW cm⁻² at 30 °C and 151.3 mW cm⁻² at 62 °C are obtained, and good stability (300-h stable performance at 20 mA cm⁻²) is also exhibited, which shows that such perovskite-type oxides as LaNi_0.8Co_0.2O₃ can be excellent catalysts for DBFCs.     

Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared from sodium ferric ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD) supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes (CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well as the pore volumes of catalysts were examined by nitrogen physisorption characterization. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using domestic wastewater as fuel. The maximum current density and power density recorded are 110 (mA m⁻²) and 127 ± 0.9 (mW m⁻²), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m⁻² and 226 ± 0.2 mW m⁻²), demonstrating that these catalysts can be used as substitutes for commercial Pt/C.     

High-performance N,P-CNL nanocomposites as catalyst for oxygen reduction reaction in fuel cell

Transition metal and heteroatom codoped carbon materials have become the most promising materials to replace commercial platinum carbon (Pt / C) catalysts due to their low cost, high stability, and methanol resistance. In this work, iron-nitrogen and phosphorus codoped carbon nanorod-layer composites (N, P-CNL) derived from phosphorus-doped polyaniline (P-PANI) by phytic acid (PA) and iron salt were successfully obtained after high-temperature pyrolysis. As a result, the N, P-CNL materials exhibited good electrocatalytic performance due to abundant active sites. The N, P-CNL with 50% mass filling ratio of iron salt (named as N, P-CNL-1:1) displayed an enhanced limiting current density of −5.97 mA cm⁻² at 1600 rpm and outstanding onset potential (−0.004 V) and oxygen reduction peak potential (−0.144 V). In general, this work can give insights into understanding the mechanism of codoped catalysts and synthesis the catalyst with excellent long-term stability and resistance to methanol crossover and poisoning better than commercial Pt/C.     

Application of synthesized porous graphitic carbon nitride and it's composite as excellent electrocatalysts in microbial fuel cell

The performance of microbial fuel cells (MFCs) consisting of exfoliated porous graphitic carbon nitride (ep-GCN) and its composite with acetylene black (AB) as cathode catalyst is evaluated. The cyclic voltammetry and electrochemical impedance spectroscopy of composite ep-GCN-AB indicated excellent oxygen reduction reaction activity and comparable charge transfer resistance with respect to Pt–C. The absence of X-ray diffraction peak at 2θ = 13° (corresponding to stacked structure of bulk GCN) indicated reduction in thickness. Four MFCs were operated with simulated wastewater with chemical oxygen demand (COD) of 3000 mg L⁻¹. The maximum power densities of MFC-GAB (14.74 ± 0.17 W m⁻³), MFC-PAB (15.68 ± 0.58 W m⁻³) and MFC-G (12.47 ± 0.30 W m⁻³) using ep-GCN-AB, Pt–C and ep-GCN electrocatalyst, respectively, were 2.6, 2.7 and 2.2 times higher than MFC-AB operated with only acetylene black coated cathode. The investigation demonstrates that ep-GCN and its composites can be utilized as excellent cathode catalysts in MFCs at 20 folds lesser cost than Pt–C.     

Polyaniline/carbon black composite-supported iron phthalocyanine as an oxygen reduction catalyst for microbial fuel cells

Polyaniline/carbon black (PANI/C) composite-supported iron phthalocyanine (FePc) (PANI/C/FePc) has been investigated as a catalyst for the oxygen reduction reaction (ORR) in an air-cathode microbial fuel cell (MFC). The electrocatalytic activity of the PANI/C/FePc toward the ORR is evaluated using cyclic voltammogram and linear scan voltammogram methods. In comparison with that of carbon-supported FePc electrode, the peak potential of the ORR at the PANI/C/FePc electrode shifts toward positive potential, and the peak current is greatly increased, suggesting the enhanced activity of FePc absorbed onto PANI/C. Additionally, the results of the MFC experiments show that PANI/C/FePc is well suitable to be the cathode material for MFCs. The maximum power density of 630.5 mW m⁻² with the PANI/C/FePc cathode is higher than that of 336.6 mW m⁻² with the C/FePc cathode, and even higher that that of 575.6 mW m⁻² with a Pt cathode. Meanwhile, the power per cost of the PANI/C/FePc cathode is 7.5 times greater than that of the Pt cathode. Thus, the PANI/C/FePc can be a potential alternative to Pt in MFCs.     

Modification of Ni-based cathode material for molten carbonate fuel cells using Co3O4

The degradation of cathode materials due to the so-called ‘NiO dissolution’ problem, is one of the most critical issues restricting the long-term operation of molten carbonate fuel cells (MCFCs). To overcome this problem, a modified NiO powder is prepared by the annealing of a pre-mixed powder consisting of Ni powder and Co₃O₄ nano-particles (n-Co₃O₄) at 650 °C. Annealing above 300 °C plays an important role in converting the physically bound mixed oxide system to a chemically bound system and, therefore, the separation of n-Co₃O₄ from Ni is prevented. This modification leads to the formation of a Ni_0.9Co_0.1O solid solution, and the lithiated Ni_0.9Co_0.1O has a core-shell structure that consists of different Li contents. Whereas the core phase has a low concentration of lithium, the outer layer is a highly lithiated phase. Since the highly lithiated outer phase, acting as a barrier, minimizes the dissolution of NiO, the modified cathode demonstrates good electrochemical properties and chemical stability under actual operating conditions. This study can provide an effective way to mass produce MCFC cathode materials.     

Methanol tolerant core-shell RuFeSe@Pt/C catalyst for oxygen reduction reaction

Pt decorated RuFeSe/C catalyst is prepared by reduction of Pt precursor on pre-formed RuFeSe/C for oxygen reduction reaction (ORR). The catalyst is characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalyst particles are found to disperse on the carbon support with an average particle size of 2.8 nm. Physical characterizations and electrochemical tests confirm that Pt is deposited on the surfaces of RuFeSe particles and RuFeSe@Pt/C catalyst has a core-shell structure. The as-prepared catalyst has high durability and shows high ORR activity through a four-electron transfer process. RuFeSe@Pt/C exhibits 1.3-fold greater specific activity and 1.4-fold greater mass activity for ORR than Pt/C. More importantly, it has excellent tolerance to methanol. Consequently, RuFeSe@Pt/C may be used as fine cathode catalyst in direct methanol fuel cells (DMFCs).     

Carbon supported nickel-phthalocyanine/MnOx as novel cathode catalyst for microbial fuel cell application

Performance of microbial fuel cells (MFCs) with carbon supported nickel phthalocyanine (NiPc)MnO_x composite (MFC-1) and nickel phthalocyanine (MFC-2) incorporated cathode was compared with a control MFC with non-catalysed carbon felt as cathode (MFC-3) and MFC-4 having Pt on cathode (as benchmark reference control). MFC-1 exhibited power density of 8.02 Wm^?3, which was four folds higher than control MFC-3 (2.08 Wm^?3) and 1.14 times higher than MFC-2 (6.97 Wm^?3). Coulombic efficiency of 30.3% obtained in MFC-1 was almost double of that obtained for control MFC-3 and it was 5.4% lesser as compared to MFC-4 (35.7%). Linear sweep voltammetry study of cathodes revealed that NiPc-MnO_x could enhance the electrocatalytic activity of oxygen reduction reaction (ORR) in comparison to control cathode. However, the power recovery from MFC-1 was noted little lower than what obtained from MFC-4 (10.58 Wm^?3), however the cost normalized power was two times higher than Pt catalyst on cathode. Thus, NiPc-MnO_x based catalyst developed in this study has potential to enhance ORR in cathodes of MFCs in order to harvest more power.     

Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells

Cathode oxygen reduction reaction (ORR) performance is crucial for power generation of microbial fuel cells (MFCs). The current study provides a novel strategy to prepare Fe/N-doped carbon (Fe/N/C) catalyst for MFCs cathode through high temperature pyrolyzing of biomass capsuling melamine resin polymer. The obtained Fe/N/C can effectively enhance activity, selectivity and stability toward 4 e^– ORR in pH neutral solution. Single chamber MFC with Fe/N/C air cathode produces maximum power density of 1166 mW m⁻², which is 140% higher than AC cathode. The improved performance of Fe/N/C can be attributed to the involvement of nitrogen and iron species. The excellent stability can be attributed to the preferential structure of the catalyst. The moderate porosity of the catalyst facilitates mass transfer of oxygen and protons and prevents water flooding of triple-phase boundary where ORR occurs. The biomass particles encapsulated in the catalyst act as skeletons, which prevents catalyst collapse and agglomeration.     

Using rhodium as a cathode catalyst for enhancing performance of microbial fuel cell

Rhodium with activated carbon as carbon base layer (Rh/AC) was exploited as an oxygen reduction reaction (ORR) catalyst to explore its applicability in microbial fuel cell (MFC). Four MFCs were fabricated using the Rh/AC catalyst, adopting varying Rh loadings of 0.5, 1.0 and 2.0 mg cm⁻² and without Rh on carbon felt cathode in order to understand the optimum loading of this catalyst to enhance the performance of MFC. The participation of Rh/AC in ORR was confirmed by cyclic voltammetry and electron impedance spectroscopy analysis, which supported the enhanced charge transfer capacity of the cathode modified with the prepared catalysts. Volumetric power density of MFC was found to be improved by 2.6 times when Rh/AC was used as cathode catalyst (9.36 W m⁻³) at a loading of 2.0 mg cm⁻² in comparison to the control MFC (3.65 W m⁻³) without Rh on the cathode. It was thus inferred that the increase in the Rh loading up to 2 mg cm⁻² can improve the performance of MFC significantly.     

Templating synthesis of hierarchically meso/macroporous N-doped microalgae derived biocarbon as oxygen reduction reaction catalyst for microbial fuel cells

N-doped carbons have been hailed as cost effective catalysts for the large-scale commercialization of microbial fuel cells (MFCs). In this paper, we developed a hierarchically meso/macroporous N-doped biocarbon by templating approach using Chlorella pyrenoidosa as precursor. The results showed that graphitic-N was the dominating functional group contributing to oxygen reduction reaction (ORR) performance. In addition, the role of pore structure was identified and the results suggested that mesopores exhibited a nearly linear correlation with limiting current density and half-wave potential, while electrochemical surface area almost linearly varied with macropores in the carbon materials. These results implied that mesopores play a dominating role in facilitating ion and oxygen supply and creating accessible active sites for ORR, while macropores mainly served as an electrolyte buffering reservoir shortening the electrolyte diffusion distances in the prepared catalysts. The optimized meso/macro pore structure enhanced the accessibility of the active sites and facilitated the mass transport of ion and oxygen, and consequently improved ORR performance of catalyst. The as-prepared catalyst exhibited a remarkably higher power generation than that of the commercial Pt/C in MFCs. This paper offered an insight into the effect of pore structure on the ORR performance of catalysts, and also provided an alternative avenue for synthesizing meso/macroporous carbon catalysts for the applications of MFCs.     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC applications.     

Novel multi walled carbon nanotube based nitrogen impregnated Co and Fe cathode catalysts for improved microbial fuel cell performance

For application in a microbial fuel cell (MFC), transition metal and nitrogen co-doped nanocarbon catalysts were synthesised by pyrolysis of multi-walled carbon nanotubes (MWCNTs) in the presence of iron- or cobalt chloride and nitrogen source. For the physicochemical characterisation of the catalysts, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) was used. The results obtained by rotating disk electrode (RDE) method showed an extraordinary electrocatalytic activity of these catalysts towards oxygen reduction reaction (ORR) in neutral media, which was also confirmed by the MFC results. The Co-N-CNT and Fe-N-CNT cathode catalysts exhibited maximum power density of 5.1 W m^?3 and 6 W m^?3, respectively. Higher ORR activity and improved electric output in the MFC could be attributed to the formation of the active nitrogen-metal centers. All findings suggest that these materials can be used as potential cathode catalysts for ORR in MFC to replace expensive noble-metal based materials.     

Thin film nanocrystalline Ba0.5Sr0.5Co0.8Fe0.2O3: Synthesis, conductivity, and micro-solid oxide fuel cells

We report on single phase polycrystalline Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ thin films grown on (1 0 0) (Y₂O₃)_0.08(ZrO₂)_0.92 substrates by radio frequency sputtering. Detailed studies on electrical conductivity as a function of temperature up to 500 °C are carried out for films in the 20-100 nm thickness range. Free-standing thin film micro-solid oxide fuel cells utilizing nanostructured Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ cathodes are fabricated and tested for the first time. A maximum power density of 35 mW cm⁻² at 520 °C was obtained with Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃/(Y₂O₃)_0.08(ZrO₂)_0.92/Pt micro-solid oxide fuel cells. These results indicate the significance of microstructure on electrical properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ and present the first successful thin film micro-solid oxide fuel cells integrating Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. We anticipate these results to be of relevance in advancing micro-solid oxide fuel cells for reduced temperature operation with dense oxide cathodes.