Recent Advances in the Development of Noble Metal-Free Cathode Catalysts for Microbial Fuel Cell Technologies

Metal-free catalyst for efficient pH-universal oxygen reduction electrocatalysis in microbial fuel cell

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

In the past 20 years Microbial fuel cells (MFCs) have been extensively studied regarding the possibility of transforming organic waste directly into electricity. There are significant differences between MFCs and conventional low temperature Fuel Cells (FCs), which make MFCs attractive: biotic catalyst at the anode; the anodic fuel is complex organic waste; MFCs operate under mild reaction conditions (neutral pH, temperature and pressure), close to ambient levels as optimum. Like chemical fuel cells, MFCs are composed of anode and cathode. Oxygen is an ideal electron acceptor for MFCs because of its high redox potential, availability, and sustainability. However, the Oxygen Reduction Reaction (ORR) is kinetically sluggish, resulting in a large proportion of potential loss. Also, working conditions are quite different because of the type of complex media in which MFCs operate. In order to overcome these limitations, catalysts are often used to lower the overpotentials and accelerate the kinetics of the oxygen reduction reaction. One of the main challenges is the development of efficient and stable cathode catalysts for MFCs. By far, Pt and Pt-based catalysts (PGMs) have been extensively used, due to their catalytic efficiency in gas-diffusion electrodes. But the high cost and low durability have significantly lowered their utilization in MFCs. A variety of non-precious metal catalysts have been developed for MFC applications including carbon-based catalysts, carbon supported composite catalysts, Me-based catalysts and biocatalysts. It is supposed that the ORR catalyst used for wastewater treatment in MFCs is simple to synthesize, cost-effective, durable after long-term operation in wastewater, tolerant to poisoning and able to restore catalytic activity after cleaning. In this regard carbon-based catalyst may be the most promising candidate for practical applications. This study reviews different carbon-based ORR catalysts for MFC applications for wastewater treatment and energy recovery.

<i>Harvesting Energy using Biocatalysts</i>

Iron-rich nanoparticle encapsulated, nitrogen doped porous carbon materials as efficient cathode electrocatalyst for microbial fuel cells

Cobalt (iron), nitrogen and carbon doped mushroom biochar for high-efficiency oxygen reduction in microbial fuel cell and Zn-air battery

The paper presents the results of forecasting the dynamics of the depletion of conventional energy resources, including hydrocarbons (coal, oil, natural gas) and natural uranium, which currently form the mainstay of the energy supply of the economy. A balance model developed by the authors shows the dynamics of the growing shortage of exhaustible conventional energy resources after 2040 at various energy-consumption rates. For the time being, it is unclear which nonconventional primary energy sources are able to supply advanced commercially acceptable fast-growing energy systems (including capital investment) in order to equalize the rapidly growing primary energy shortages expected in the 1940s. The most advanced energy technology with almost unlimited resources that has reached the near-commercial stage of development can be nuclear fuel breeding.

Bimetallic organic framework-derived, oxygen-defect-rich FexCo3-xS4/FeyCo9-yS8 heterostructure microsphere as a highly efficient and robust cathodic catalyst in the microbial fuel cell

Zeolitic imidazolate framework-8 (ZIF-8) as robust catalyst for oxygen reduction reaction in microbial fuel cells

The fast depletion of conventional energy resources and the issue of global warming have encouraged researchers worldwide to come up with the best energy solution. Renewable energy resources such as wind and solar energy have been widely adopted as an alternative source of energy. In this work, an integrated solar and wind energy system were implemented aiming to produce the maximum possible output power from the available renewable energy resources such as solar irradiance and wind energy. The proposed system comprised two solar modules and horizontally rotating wind blades. An energy storage system plus a charge controller were also used aiming to improve the overall energy conversion efficiency. The results showed that this system demonstrated superior performance compared with the solar modules and wind system when they had worked individually. The proposed system was generating an average energy of 61.729 Wh daily. Therefore, it was estimated that the system can generate an annual output power of about 207.4 kWh. During the conducted experiments, the solar panels worked as the main source of the generated energy while the wind system acted as a secondary source of energy during the solar absent times. Moreover, the safety factor was calculated to be within the limits of 2 that shows the proposed system can work according to the industrial safety limits of Malaysia.

Surface-oxidized Fe–Co–Ni alloys anchored to N-doped carbon nanotubes as efficient catalysts for oxygen reduction reaction

The growing demand for sustainable energy and effective wastewater treatment has propelled the advancement of bio-electrochemical systems (BESs), particularly microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). These systems integrate bioelectricity generation with organic and inorganic pollutant degradation, offering a sustainable solution for environmental remediation. However, challenges such as high overpotential, reliance on noble metal electrodes, and inconsistent performance have necessitated innovative improvements. The incorporation of photocatalysis into BESs has led to the development of photo-bio-electrochemical systems (PBESs), including photo-microbial fuel cells (PMFCs) and photo-microbial electrolysis cells (PMECs), which leverage optical energy to enhance efficiency. Carbon-based electrode materials, owing to their high porosity, conductivity, and biocompatibility, have emerged as ideal candidates for improving PBES performance. Advanced carbon nanostructures, such as graphene, carbon nanotubes, and metal-graphitic carbon nitride composites, have demonstrated superior photocatalytic properties, promoting enhanced charge separation, CO2 reduction, hydrogen production, and wastewater treatment. PBES integrating light-activated semiconductor materials with BESs, further amplify pollutant degradation and energy conversion efficiency. Despite significant progress, optimizing electrode materials and improving charge transport remain key challenges for scalable and cost-effective deployment. This review highlights the latest advancements in carbon-based electrodes for PBESs, detailing their mechanisms, photocatalytic properties, and future prospects in sustainable energy production and environmental remediation. By addressing existing material limitations and exploring novel photocatalytic enhancements, this work aims to contribute to the development of next-generation PBESs, fostering circular economy practices and carbon-neutral energy solutions.

Nitrogen and phosphorus co-doped carbon networks derived from shrimp shells as an efficient oxygen reduction catalyst for microbial fuel cells

The efficient and durable oxygen reduction reaction (ORR) catalyst is of great significance to boost power generation and pollutant degradation in microbial fuel cells (MFCs). Although transition metal-nitrogen-codoped carbon materials are an important class of ORR catalysts, copper-nitrogen-codoped carbon is not considered a suitable MFC cathode catalyst due to the insufficient performance and especially instability. Herein, we report a three-dimensional (3D) hierarchical porous copper, nitrogen, and boron codoped carbon (3DHP Cu-N/B-C) catalyst synthesized by the dual template method. The introduced B atom as an electron donor increases the electron density around the Cu-Nx active site, which significantly promotes the efficiency of the ORR process and stabilizes the active site by preventing demetallization. Thus, the 3DHP Cu-N/B-C catalyst exhibited excellent ORR performance with the half-wave potential of 0.83 V (vs reversible hydrogen electrode (RHE)) in a 0.1 M KOH electrolyte and 0.68 V (vs RHE) in a 50 mM PBS electrolyte. Meanwhile, 3DHP Cu-N/B-C had satisfactory stability with 94.16% current retention after 24 h of chronoamperometry test, which is better than that of 20% Pt/C. The MFCs using 3DHP Cu-N/B-C not only showed a maximum power density of up to 760.14 ± 19.03 mW m-2 but also operating durability of more than 50 days. Moreover, the 16S rDNA sequencing results presented that the 3DHP Cu-N/B-C catalyst had a positive effect on the microbial community of the MFC with more anaerobic electroactive bacteria in the anode biofilm and fewer aerobic bacteria in the cathode biofilm. This study provides a new approach for the development of Cu-based ORR electrocatalysts as well as guidance for the rational design of high-performance MFCs.

In-situ gas foaming synthesis of N, S-rich co-doped hierarchically ordered porous carbon as an efficient oxygen reduction reaction catalyst