Cathodic Oxygen Reduction Reaction Research Articles

Novel electrochemical advanced oxidation processes with H2O2 generation cathode for water treatment: A review

Recently, electrochemical generation of hydrogen peroxide (H2O2) using cathodic oxygen reduction reaction (ORR) has been conventionally implemented in electrochemical advanced oxidation processes (EAOPs) for water treatment. H2O2 could be activated using reagents like Fe2+ and O3 to produce the hydroxyl radicals (•OH), and further reduce organics. The conventional Fenton, Peroxone and Electro-Oxidation (EO) processes were enhanced using H2O2 generation cathode to execute Electro-Fenton, Electro-Peroxone and ORR-EO processes. In this study, the carbon-based materials as catalysts for H2O2 generation cathode were generally investigated. The development of cutting-edge EAOPs based on H2O2 generation cathode was introduced using three stories. The future viewpoint of EAOPs based on H2O2 generation cathode was discussed.

N and P dual heteroatom doped mesoporous hollow carbon as an efficient oxygen reduction reaction catalyst in alkaline electrolyte

Heteroatom is doped in hollow carbon materials could be an excellent approach for enhancing sluggish cathodic oxygen reduction reaction (ORR) in fuel cells. Herein, we proposed a facile one step synthesis of N and P dual heteroatom doped hollow mesoporous carbon sphere (NPHS) derived from dopamine hydrochloride (DA) and phytic acid (PA) precursors is inspected for the same. By altering PA addition time interval and its loading concentration we achieved NPHS with high surface area and excellent sphere morphology. We established that the addition of 0.4 g PA at an interval of 30 h into DA mixture found to be optimum condition for the formation of NPHS-0.4. Such conditions favored formation of high surface area (1120 m2 g−1), better shell thickness (31 nm), enriched pyridinic N, P–C and P–N sites thereby resulted to an exceptional ORR activity. Moreover, electrochemical studies proved that NPHS-0.4 material possess a moderate carrier concentration (2.3 × 1015 cm−3) and flat band potential (0.56 V vs RHE) leads to a positive onset potential (0.97 V), limiting current density (4.7 mA cm−2) by adopting a close to four electron ORR pathway. In addition, it shows a remarkable electrochemical stability and excellent methanol oxidation tolerance than the Pt/C catalyst. Thus, this interesting outcome absolutely provides a platform to develop a new type of active dual doped mesoporous carbon materials for various energy storage/conversion applications.

Dual optimization strategy to construct hierarchical reticulated porous framework with enriched Fe-NX active species for the highly efficient oxygen reduction reaction

Proton exchange membrane fuel cells (PEMFCs) are severely restricted by slow cathodic oxygen reduction reaction (ORR) kinetics and expensive noble metal platinum catalysts. In particular, the ORR serves as a key process in electrochemical conversion, which significantly impacts the overall performance of the related devices. Accordingly, we proposed a facile and novel dual-optimization strategy to construct abundant Fe-NX active site embedded hierarchical reticulated porous structure catalysts. Initially, an extensive encapsulated Fe and Fe–N complexes is created in Fe modified zeolitic-imidazolate-frameworks-8/graphene oxide (Fe-ZIF-8/GO) precursor, subsequently, the thermal melting and restructuring process fosters the structural transformation of the precursor to robust and open nano-porous frameworks. In the meanwhile, the encapsulated Fe in the pores of the precursors and the pre-existing Fe–N coordination structures are converted into new Fe-NX active sites. This approach realizes the dual optimization of Fe-NX active site density enhancement and utilization, making the optimal catalyst Fe and N-doped porous carbon/graphene nanosheets-1 (Fe-NPC/GNs-1) exhibits a half-wave potential (E1/2) of 0.771V under acidic medium, and E1/2 of 0.917 V toward the ORR in alkaline conditions. The activity and stability under alkaline conditions exceed even that of commercial Pt/C (with negligible potential decay after 10,000 cycles under alkaline conditions), and also showed its capacity to substitute platinum-based catalysts and the potential application in Zn−air batteries.

Pt and Pt–Ag nanoparticles supported on carbon nanotubes (CNT) for oxygen reduction reaction in alkaline medium

In this work, we investigated the effect of the carbon nanotubes (CNT) as alternative support of cathodes for oxygen reduction reaction (ORR) in alkaline medium. The Pt and Pt–Ag nanomaterials supported on CNT were synthesized by sonochemical method. The crystalline structure, morphology, particle size, dispersion, specific surface area, and composition were investigated by XRD, SEM-EDS, TEM, HR-TEM, N2 adsorption-desorption and XPS characterization. The electrochemical activity for ORR was evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in alkaline medium. The electrochemical stability was researched by an accelerated degradation test (ADT). Pt/CNT showed the better electrocatalytic activity towards ORR compared with Pt–Ag/CNT and Pt/C. Pt/CNT exhibited higher specific activity (1.12 mA cm−2Pt) than Pt/C (0.25 mA cm−2Pt) which can be attributed to smaller particle size, Pt-CNT interaction, and Pt load (5 wt%). The Pt monometallic samples supported on CNT and Vulcan showed higher electrochemical stability after ADT than Pt–Ag bimetallic. The ORR activity of all materials synthesized proceeded through a four-electron pathway. Furthermore, the EIS results showed that Pt/CNT exhibited the lower resistance to the transfer electron compared with conventional Pt/C and Pt–Ag/CNT.

Host-guest interactions promoted formation of Fe-N4 active site toward efficient oxygen reduction reaction catalysis

FeNC is the most promising material to replace the noble metal catalyst for cathodic oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, the practical performance of FeNC catalyst is significantly limited by its low active site (Fe-N4) density. Herein, we propose to promote the formation of Fe-N4 active sites in FeNC catalyst by strengthening the interaction of N precursors and Fe precursors during the carbonization synthesis. In our approach, ionic liquid (IL, [EMIM][NTf2]) with high nitrogen content and good thermal stability is caged in the pores of Fe-ZIF-8 through the host–guest interactions. These interactions are critical for the preservation of Fe and N species and formation of active sites during the synthesis. The optimal catalyst developed with this approach (Fe0.05NC/10) has a high density of accessible Fe-N4 sites (1.88*1019 sites g−1). Therefore, in both acidic and alkaline media, Fe0.05NC/10 showed excellent ORR activity comparable to commercial Pt/C catalyst. Moreover, PEMFC performance with a peak power density of 300 mW cm−2 was demonstrated with Fe0.05NC/10 under H2/O2 conditions. The synthetic approach reported herein may be used for tailoring of advanced catalyst with high intrinsic activity.

Electrocatalytic Hydrogen Oxidation in Alkaline Media: From Mechanistic Insights to Catalyst Design.

With the potential to circumvent the need for scarce and cost-prohibitive platinum-based catalysts in proton-exchange membrane fuel cells, anion-exchange membrane fuel cells (AEMFCs) are emerging as alternative technologies with zero carbon emission. Numerous noble metal-free catalysts have been developed with excellent catalytic performance for cathodic oxygen reduction reaction in AEMFCs. However, the anodic catalysts for hydrogen oxidation reaction (HOR) still rely on noble metal materials. Since the kinetics of HOR in alkaline media is 2-3 orders of magnitude lower than that in acidic media, it is a major challenge to either improve the performance of noble metal catalysts or to develop high-performance noble metal-free catalysts. Additionally, the mechanisms of alkaline HOR are not yet clear and still under debate, further hampering the design of electrocatalysts. Against this backdrop, this review starts with the prevailing theories for alkaline HOR on the basis of diverse activity descriptors, i.e., hydrogen binding energy theory and bifunctional theory. The design principles and recent advances of HOR catalysts employing the aforementioned theories are then summarized. Next, the strategies and recent progress in improving the antioxidation capability of HOR catalysts, a thorny issue which has not received sufficient attention, are discussed. Moreover, the significance of correlating computational models with real catalyst structure and the electrode/electrolyte interface is further emphasized. Lastly, the remaining controversies about the alkaline HOR mechanisms as well as the challenges and possible research directions in this field are presented.

Polyhedral Carbon Anchored on Carbon Nanosheet with Abundant Atomic Fe‐Nx Moieties for Oxygen Reduction

AbstractCarbon‐based single‐atom iron electrocatalysts with nitrogen coordination (CSAIN) have recently shown enormous promise to replace the costly Pt for boosting the cathodic oxygen reduction reaction (ORR) in fuel cells. However, there remains a great challenge to achieve highly efficient CSAIN catalysts for the ORR in acidic electrolytes. Herein, a novel CSAIN catalyst is synthesized by pyrolyzing a precursor mixture consisting of metal–organic framework and conductive polymer hybrid. After pyrolysis at a high temperature, the CSAIN with a structure of carbon nanosheet supported polyhedral carbon is achieved, where the unique structure endows CSAIN with expediting electron transfer and mass transport, as well as largely exposed surface to host atomically dispersed iron active sites. As a result, the optimal CSAIN catalyst shows a high ORR activity with its half‐wave potential of 0.77 V (vs RHE) and a Tafel slope of 74.1 mV dec–1, which are comparable to that of commercial Pt/C catalyst (0.80 V and 81.9 mV dec–1).

High-efficiency power amplification of microbial fuel cell by modifying cathode with iron-incorporated thermalized covalent organic framework

The low power output of microbial fuel cell (MFC) requires high-performance catalysts to facilitate cathodic oxygen reduction reaction (ORR). In this work, a series of electrocatalysts (Fe/TTF) are synthesized by thermalizing N-rich covalent triazine framework (CTF) with various amount of FeCl3·6H2O at different calcination temperatures for ORR catalysis in MFC. Electrochemical measurements reveal that Fe/TTF-0.5-900 synthesized with a mass ratio of 1:2 (FeCl3·6H2O: CTF) at 900 °C exhibits the minimum charge transfer resistance and optimal catalytic behavior in ORR process. The MFC with Fe/TTF-0.5-900 cathode displays a high maximum power density of 2617 mW·m−2, which surpasses those of other Fe/TTF electrocatalysts, thermalized covalent triazine framework (TTF) and the benchmark activated carbon (AC). The results indicate that TTF as a porous carbon matrix with favorable nitrogen doping promotes charge and mass transfer greatly, and the combination of iron carbide substantially benefits the improvement of MFC performance. Moreover, according to density functional theory (DFT) calculation, iron carbide is significantly preferable for ORR catalysis by accelerating the break of O-O bond in *OOH, which further reveals that the ORR on iron carbide follows four-electron pathway. Therefore, Fe/TTF-0.5-900 can be applied as an economical and highly active electrocatalyst in MFC to achieve high-efficiency energy conversion.

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

• A metal-free and efficient pH-universal ORR catalyst is prepared by simple pyrolysis. • ZIF-8#1000 exhibited ORR electrocatalytic activity, compared with that of 20% Pt/C. • ZIF-8#1000 showed high-performance in air–cathode MFCs. • Three single-chamber MFCs in series can light a light-emitting diode. Developing and utilization of clean and renewable energy is the key to alleviate energy crisis and environmental problems. As a biomass energy utilization, microbial fuel cell (MFC) suffers from low power generation performance caused by the sluggish electronic transfer kinetics of cathodic oxygen reduction reaction (ORR). It is of significance to develop efficient ORR catalysts to get a commercial-scale version in fuel cells. ZIF-8 was usually joined other metal elements (Pt, Fe, Co and etc) to improve ORR catalytic ability. Unfortunately, these catalysts are unstable under acidic conditions. Therefore, it is of great significance to develop an efficient pH-universal ORR catalyst. Herein, a kind of metal-free and efficient pH-universal ORR catalyst are fabricated via a simple method. The catalyst (ZIF-8#1000) exhibits excellent pH-universal ORR electrocatalytic activity and high-performance in single-chamber MFC (distinct output voltage 780 mV and maximum power density 600 mW m −2 ), comparable to those of 20% Pt/C. Interestingly, three single-chamber MFCs in series can light a light-emitting diode. Thus, the as-prepared ZIF-8#1000 has the potential to be alternative of Pt-based ORR catalysts.

Strategy optimization of proton exchange membrane fuel cell cold start

<p indent="0mm">Under the background of “carbon peaking and carbon neutrality” goals, the proton exchange membrane fuel cell (PEMFC) vehicle attracts extensive attention due to its high energy conversion efficiency, long range and zero emission. However, the challenge of cold start in low temperature environments becomes a large obstacle to its commercialization and application. The current cold start performance of typical commercial fuel cell vehicles cannot reach the optimal target yet. Several problems such as the lack of complete mechanical systems of water transport and phase change and the absence of simulation models for on-board fuel cells at system level still exist. Thus, researchers have carried out a great number of experiments and simulations to study the mechanism of degradation, water transport, phase change and heat transfer as well as start-up strategy optimization of PEMFC cold start. By measuring the output performance, it is found that the degradation of performance is attributed to the formation of ice which covers the reaction active region, blocks the gas channel and increases the electrical contact resistance. Researchers also believe that the reduction of durability is owing to the volume change of water-ice phase change which leads to the damage of the structure by characterizing the microstructure of the fuel cell. With the help of transparent cells, neutron imaging and simulations of multiphase PEMFC models, the distribution, transport and phase change of water and ice during cold start have been studied. The results indicate that the electrochemical product water first saturates the PEM and the catalyst layer (CL) as membrane water. Then the water remains liquid in the form of supercooled water at low temperatures, and finally flows out of the cell or freezes suddenly. By studying the heat transfer process both in plane and through plane during cold start, researchers find that at central regions of the single cell and in the middle part of the stack, the temperature rises fastest. They also find that the irreversible reaction heat generated by cathodic oxygen reduction reaction (ORR) is the main source of heat generation. Based on the cold start mechanism of PEMFCs, researchers have developed various self-start and auxiliary start strategies. The start-up mode and control strategies have been optimized. The internal structure and materials of each part have been improved. And a series of auxiliary startup strategies such as gas purging, heater heating, reaction heating, gas heating as well as coolant heating have been proposed. A large number of simulations and experiments are carried out to verify the advantages of these strategies. Through these excellent cold start strategies, the commercialization and promotion process of PEMFC vehicles can be accelerated.

Microscale simulations of reaction and mass transport in cathode catalyst layer of polymer electrolyte fuel cell

The resistance of the cathode oxygen reduction reaction in polymer electrolyte fuel cells must be reduced for improving the performance. Therefore, it is important to thoroughly understand the relationship between the heterogeneous structures and the cell performance. However, it is difficult to obtain such an understanding using experimental approaches and typical uniform porous simulations. In this study, numerical analysis was used to simulate a three-dimensional catalyst layer (CL) with carbon black (CB) aggregate structures and ionomer coating models, and a cathode reaction and mass transport simulation model incorporating the heterogeneous structure was developed. Moreover, the relationship between the electrode structure and the cell performance, including the reaction distribution and output performance, was examined. The current density distribution depended on the CB structure and ionomer adhesion shape. From the viewpoint of enhancing both the Pt utilization and the mass transport performance, an adequate heterogeneous pore structure in the CL is necessary. These results were used to determine the optimal material properties for the high performance cell. • Simulation model in cathode catalyst layer with 3D structure was developed. • Heterogeneous structure with carbon black and ionomer was simulated. • Ionomer adhesion affect mass transport, and it cause reaction distribution. • Local reaction on Pt surface was changed to bi-modal distribution by agglomerate. • Effective Pt utilization in cathode was estimated by this simulation.

Engineering heterostructured Co0.7Fe0.3@Co doped leaf-like carbon nanoplates from dual metal-organic frameworks for high-efficiency oxygen reduction reaction in microbial fuel cell

Microbial fuel cell (MFC) satisfies the needs of power generation and pollutants removal but still requires active electrocatalysts to accelerate cathodic oxygen reduction reaction (ORR). Metal-nitrogen-carbon materials (M − NCs) derived from Prussian blue analogues (PBAs), known for low cost and high yield, fail with satisfactory surface structure and their intrinsic activity requires to be further improved. Here, a MOF@MOF precursor (Co-ZIF-L@CoFe PBA-1, MOF: metal-Organic framework) is skillfully fabricated by utilizing two-dimensional (2D) leaf-like cobalt-based zeolitic-imidazolate framework (Co-ZIF-L) as the mother MOF to give birth to cobalt iron Prussian blue analogue (CoFe PBA) by adjusting reaction time and solvents with a ligand exchange method. After carbonization, Co-ZIF-L@CoFe PBA-1 transforms into a leaf-shaped carbon nanoplate with heterostructured CoFe alloy and Co (Co 0.7 Fe 0.3 @Co-NC-1), which exhibits outstanding ORR activity and high MFC performance (2486 ± 56 mW m -2 ) compared with Co-NC and Co 0.7 Fe 0.3 -NC derived from Co-ZIF-L and leaf-like CoFe PBA, respectively. The interfacial Co 0.7 Fe 0.3 @Co efficiently optimizes electronic structure for fast electrons transfer and the porous carbon matrix provides desirable surface structure for oxygen transfer, synergistically triggering a high ORR activity. Our work offers new insights for controllable preparation of PBAs-based dual-MOFs precursors and provides a feasible avenue for enhancing the activity of PBAs-based M − NCs catalysts. • Co-ZIF-L gives birth to Co-Fe PBA through a ligand exchange method. • The amount of Co-Fe PBA on Co-ZIF-L can be adjusted by reaction time and solvents. • The ORR activity is attributed to the integration of dual-MOF precursor. • The maximum power density of Co 0.7 Fe 0.3 @Co-NC-1 air cathode is 2486 ± 56 mW m 2 .

Innovative Fabrication of Pd/Pd4S Based Highly Active Electrocatalysts for ORR in a Primary Zn-Air Battery

On account of their high theoretical energy density and minimal cost, Zn-air batteries have gotten a lot of interest as a viable energy storage technology. However, the problem of a high overpotential at the cathode because of the slow oxygen reduction reaction (ORR) kinetics persists. Developing an efficient cathode ORR catalyst and exceeding in performance in comparison to the existing and widely utilized Pt-based catalysts is still a formidable challenge. Herein, this study focuses on the design of a Pt-free catalyst, precisely, Pd/Pd4S based electrocatalysts to enhance ORR performance of cathode. The simple encapsulated structure containing nitrogen-doped carbon (N-doped C) coated Pd/Pd4S and CeO2 nanoparticles (Pd-Pd4S/CeO2/N-C) fabricated by a practical and trouble-free approach has a powerful electrochemical ability. Pd-Pd4S/CeO2/N-C displays a virtually identical limited current density to the commercial 20% Pt/C catalysts and better durability under alkaline conditions. Moreover, this work involves the subsequent utilization of Pd-Pd4S/CeO2/N-C in a primary Zn-air battery as the air electrode, where it manifested comparable current density and power density at 0.6 V as furnished by the Pt/C catalyst. Along with this, the primary Zn-air battery system comprising Pd-Pd4S/CeO2/N-C exhibited constant discharge comprising several hours, a specific capacity of 748 mAh gZn −1, and stability at 10 mA cm−2.

Synthesizing Pt-Ni/C Nanoframes electrocatalyst using the solvothermal and in-house developed method for PEM fuel cells

As South Africa moves towards the production and storage of green energy sources, proton exchange membrane (PEM) fuel cells have been characterized as promising energy sources for transportation, heating, and power sources and have an efficient energy conversion that does not allow greenhouse gas emissions. However, to improve the energy efficiency and to reduce the system cost, and make it suitable for large-scale commercialization, precious metal catalyst needs to be developed with improved catalyst activities for PEM fuel cells. Due to the high cost of platinum, platinum alloy nanostructures have been investigated for use as an electrocatalyst in PEM fuel cells. Platinum-nickel alloy nanostructures in previous research studies have shown 36- and 22-times enhancement in mass and specific activity respectively, towards the cathodic oxygen reduction reaction (ORR) in PEM fuel cells and for the methanol oxidation reaction (MOR) in direct methanol fuel cell (DMFC) than the Pt/C catalyst. Therefore, this research focused on developing rich Pt-skin platinumnickel nanoframes which were synthesized using solvothermal and in-house developed methods. The intermediate products were etched to remove the interior using either a weak acid or an oxidative acid for comparison. The final product was supported by Vulcan XC-72 at a loading of 20 wt. % Pt-Ni. The properties of Pt-Ni/C will be characterized and evaluated to determine if the nanoframes are formed. The preliminary results for the X-ray diffraction pattern showed that the structure of Pt-Ni contracted and affected the catalyst properties. The catalytic activities were determined by electrochemical methods using thin-film RDE measurements, the results indicated that Pt-Ni as-synthesized has higher specific activity at 900 mV versus RHE. The specific and mass activity of the oxygen reduction reaction for Pt-Ni/C will be compared to the activities of the current high-performing Pt/C catalyst.

The enhancement of microbial fuel cell performance by anodic bacterial community adaptation and cathodic mixed nickel–copper oxides on a graphene electrocatalyst

BackgroundAlthough microbial fuel cells (MFCs) represent a promising technology for capturing renewable energy from wastewater, their scaling-up is significantly limited by a slow-rate cathodic oxygen reduction reaction (ORR) and the development of a resilient anodic microbial community. In this study, mixed transition metal oxides of nickel and copper (Ni and Cu), supported on a graphene (G) (NiO–CuO/G) electrocatalyst, were synthesized and tested as a cost-effective cathode for ORR in MFCs. Electrochemical measurements of electrocatalyst were conducted using a rotating disk electrode (RDE) and linear sweep voltammetry (LSV) in a neutral electrolyte, and compared with a benchmark Pt/C catalyst. Furthermore, the long-term performance of the as-synthesized electrocatalyst was evaluated in a single-chamber MFC by measuring organic matter removal and polarization behavior. The successful enrichment of electroactive biofilm was also monitored using transmission electron microscopy and the Vitek2 compact system technique. ResultsWhen compared with the benchmark platinum cathode, the NiO–CuO/G electrocatalyst exhibited high selectivity toward ORR. The rotating disk electrode (RDE) experiments reveal that ORR proceeds via a 4-electron ORR mechanism. Furthermore, the NiO–CuO/G electrocatalyst also exhibited a high power density of 21.25 mW m−2 in an air-cathode MFC, which was slightly lower than that of Pt/C-based MFC (i.e., 50.4 mW m−2). Biochemical characterization of the most abundant bacteria on anodic biofilms identified four genera (i.e., Escherichia coli, Shewanella putrefaciens, Bacillus cereus, and Bacillus Thuringiensis/mycoides) that belonged to Gammaproteobacteria, and Firmicutesphyla. ConclusionsThis study demonstrates that the NiO–CuO/G cathode had an enhanced electrocatalytic activity toward ORR in a pH-neutral solution. This novel mixed transition metal oxide electrocatalyst could replace expensive Pt-based catalysts for MFC applications.

Realizing Efficient Catalytic Performance and High Selectivity for Oxygen Reduction Reaction on a 2D Ni2SbTe2 Monolayer.

One of the immediate challenges for the large-scale commercialization of hydrogen-based fuel cells is to develop cost-effective electrocatalysts to enable cathodic oxygen reduction reaction (ORR). Herein, we focus on the potential of the two-dimensional (2D) ternary chalcogenide Ni2SbTe2 monolayer as a high-performance electrocatalyst for the ORR using density function theory. Our computed results reveal that there are an obvious hybridization and electron transfer between the O 2p and Te 5p orbitals, which can activate the adsorbed oxygen and trigger the whole ORR process, with an overpotential as low as 0.33 V. In addition, the adsorption capacity of the monolayer surface for oxygen molecules can be effectively enhanced by doping with Fe or Co atoms. The Ni2SbTe2 monolayers doped with Fe or Co atoms not only maintain their original excellent ORR catalytic activity but also improve selectivity toward the four-electron (4e) reduction pathway. We highly anticipate that this work can provide excellent candidates and new ideas for designing low-cost and high-performance ORR catalysts to replace noble metal Pt-based catalysts in fuel cells.

Catalytic advancements in carbonaceous materials for bio-energy generation in microbial fuel cells: a review.

Microbial fuel cells (MFCs) are a sustainable alternative for wastewater treatment and clean energy generation. The efficiency of the technology is dependent on the cathodic oxygen reduction reaction, where the sluggish reaction kinetics hampers its propensity. Carbonaceous materials with high electrical conductivity have been widely explored for oxygen reduction reaction (ORR) catalysts. Here, incorporating transition metal (TM) and heteroatom into carbon could further enhance the ORR activity and power generation in MFCs. Nitrogen (N)-doped carbons have also been a popular research hotspot due to abundant active sites formed, resulting in superior conductivity, stability, and catalytic activity over carbons. This review summarizes the progress in the carbon-based materials (primary focus on the cathode) for ORR and their utilization in MFCs. Furthermore, we discussed the conceptualization of MFCs and carbonaceous materials to instigate the ORR kinetics and power generation in MFC. Furthermore, prospects of carbon-based materials for actual application in bio-energy generation have been discussed. Carbonaceous catalysts and biomass-derived carbons exhibit good potential to replace precious Pt catalysts for ORR. M-N-C catalysts were found to be the most suitable catalysts. Electrocatalysts with MNx sites are able to achieve excellent activity and high-power output by taking advantage of the active site exposure and rapid mass transfer rate. Moreover, the use of biomass-derived carbons/self-doped carbons could further reduce the overall cost of catalysts. It is anticipated that the research gaps and future perspectives discussed will show new avenues to develop excellent electrocatalysts for better performance and transformation of technology to industrial applications.

Perovskite-Based Nanocomposite Electrocatalysts: An Alternative to Platinum ORR Catalyst in Microbial Fuel Cell Cathodes

Microbial fuel cells (MFCs) are biochemical systems having the benefit of producing green energy through the microbial degradation of organic contaminants in wastewater. The efficiency of MFCs largely depends on the cathode oxygen reduction reaction (ORR). A preferable ORR catalyst must have good oxygen reduction kinetics, high conductivity and durability, together with cost-effectiveness. Platinum-based electrodes are considered a state-of-the-art ORR catalyst. However, the scarcity and higher cost of Pt are the main challenges for the commercialization of MFCs; therefore, in search of alternative, cost-effective catalysts, those such as doped carbons and transition-metal-based electrocatalysts have been researched for more than a decade. Recently, perovskite-oxide-based nanocomposites have emerged as a potential ORR catalyst due to their versatile elemental composition, molecular mechanism and the scope of nanoengineering for further developments. In this article, we discuss various studies conducted and opportunities associated with perovskite-based catalysts for ORR in MFCs. Special focus is given to a basic understanding of the ORR reaction mechanism through oxygen vacancy, modification of its microstructure by introducing alkaline earth metals, electron transfer pathways and the synergistic effect of perovskite and carbon. At the end, we also propose various challenges and prospects to further improve the ORR activity of perovskite-based catalysts.

Recent Advancements in the Synthesis and Application of Carbon-Based Catalysts in the ORR

Fuel cells are a promising alternative to non-renewable energy production industries such as petroleum and natural gas. The cathodic oxygen reduction reaction (ORR), which makes fuel cell technology possible, is sluggish under normal conditions. Thus, catalysts must be used to allow fuel cells to operate efficiently. Traditionally, platinum (Pt) catalysts are often utilized as they exhibit a highly efficient ORR with low overpotential values. However, Pt is an expensive and precious metal, posing economic problems for commercialization. Herein, advances in carbon-based catalysts are reviewed for their application in ORRs due to their abundance and low-cost syntheses. Various synthetic methods from different renewable sources are presented, and their catalytic properties are compared. Likewise, the effects of heteroatom and non-precious metal doping, surface area, and porosity on their performance are investigated. Carbon-based support materials are discussed in relation to their physical properties and the subsequent effect on Pt ORR performance. Lastly, advances in fuel cell electrolytes for various fuel cell types are presented. This review aims to provide valuable insight into current challenges in fuel cell performance and how they can be overcome using carbon-based materials and next generation electrolytes.

Exploration of carbon additives to the synthesis of Cu2Mo6S8 structures and their electrocatalytic activity in oxygen reduction reaction

Catalytic processes are contemplated as break point in generating alternative and sustainable energy platforms. The cathodic oxygen reduction reaction (ORR) is an important catalytic system, mainly finding practice in fuel cell and metal-air battery technologies. This work presents the synthesis, structural characterization and electrocatalytic properties of three different Cu2Mo6S8 structures as alternative ORR electrocatalysts. The effect of different carbon additives during synthesis was studied and no positive influence of the carbon addition was indicated. Our findings show that only the bare Cu2Mo6S8 enhances the ORR electro-performance to class with the state-of-the-art ORR catalysts. Excellent stability of 10,000 consecutive ORR cycles, a superior onset potential of 0.894 V and half-wave (E1/2) potential of 0.641 V vs. reversible hydrogen electrode (RHE) increase the noteworthiness of the Cu2Mo6S8 electrodes. Aside from experimental investigations, density functional theory calculations deliver profound knowledge on the structural and electronic properties (electronic band structure, partial density of states and electron density) of Cu2Mo6S8.