Metal-based cathode catalysts for electrocatalytic ORR in microbial fuel cells:A review

Microbial fuel cells(MFCs)have a simple structure and excellent pollutant treatment and power generation performance.However,the slow kinetics of the oxygen reduction reaction(ORR)at the MFC cathode limit power generation.The electrochemical performance of MFCs can be improved through electrocatalysis.Thus far,metal-based catalysts have shown astonishing results in the field of electrocatalysis,enabling MFC devices to demonstrate power generation capabilities comparable to those of Pt,thus showing enormous potential.This article reviews the research progress of meta-based MFC cathode ORR catalysts,including the ORR reaction mechanism of MFC,different types of catalysts,and preparation strategies.The catalytic effects of different catalysts in MFC are compared and summarized.Before discussing the practical application and expanded manufacturing of catalysts,we summarize the key challenges that must be addressed when using metal-based catalysts in MFC,with the aim of providing a scientific direction for the future development of advanced materials.

CuCr_2O_4@rGO Nanocomposites as High-Performance Cathode Catalyst for Rechargeable Lithium–Oxygen Batteries

Rechargeable lithium–oxygen batteries have been considered as a promising energy storage technology because of their ultra-high theoretical energy densities which are comparable to gasoline. In order to improve the electrochemical properties of lithium–oxygen batteries(LOBs), especially the cycling performance, a high-efficiency cathode catalyst is the most important component.Hence, we aim to demonstrate that CuCr_2O_4@rGO(CCO@rGO) nanocomposites, which are synthesized using a facile hydrothermal method and followed by a series of calcination processes, are an effective cathode catalyst. The obtained CCO@rGO nanocomposites which served as the cathode catalyst of the LOBs exhibited an outstanding cycling performance for over 100 cycles with a fixed capacity of 1000 mAh g^(-1) at a current density of 200 mA g^(-1). The enhanced properties were attributed to the synergistic effect between the high catalytic efficiency of the spinel-structured CCO nanoparticles, the high specific surface area, and high conductivity of the rGO.

Amorphous CoSnO_3@rGO nanocomposite as an efficient cathode catalyst for long-life Li-O_2 batteries

An amorphous CoSnO3@rGO nanocomposite fabricated using a surfactant‐assisted assembly method combined with thermal treatment served as a catalyst for non‐aqueous lithium‐oxygen(Li‐O2)batteries.In contrast to the specific surface area of the bare CoSnO3 nanoboxes(104.3 m2 g–1),the specific surface area of the CoSnO3@rGO nanocomposite increased to approximately 195.8 m2 g–1 and the electronic conductivity also improved.The increased specific surface area provided more space for the deposition of Li2O2,while the improved electronic conductivity accelerated the decomposition of Li2O2.Compared to bare CoSnO3,the overpotential reduced by approximately 20 and 60 mV at current densities of 100 and 500 mA g?1 when CoSnO3@rGO was used as the catalyst.A Li‐O2 battery using a CoSnO3@rGO nanocomposite as the cathode catalyst cycled indicated a superior cyclic stability of approximately 130 cycles at a current density of 200 mA g–1 with a limited capacity of 1000 mAh g–1,which is 25 cycles more than that of the bare amorphous CoSnO3 nanoboxes.

Pd cluster decorated free standing flexible cathode for high performance Li-oxygen batteries

As a promising candidate for the next generation energy storage system,rechargeable lithium-oxygen batteries(LOBs)still face substantial challenges caused by insulating discharge products that preclude their practical application.Exploring highly efficient cathode catalysts capable of facilitating formation/decomposition of discharge products is considered as an essential approach towards high performance LOBs.Herein,Pd decorated Te nanowires(Pd@Te NWs)were synthesized as advanced catalyst in LOBs to maximize Pd utilization and achieve synergistic effect,in which Pd clusters were uniformly grown on Te substrate though regulating the Pd:Te ratio.Meanwhile,Pd@Te nanowires assembled into an interpenetrating network-like structure by vacuum filtration and employed as flexible cathode,enabling LOBs achieved an ultralong 190 cycles stability and a superior specific capacity of 3.35 mAh·cm^(-2).Experimental studies and density functional theory(DFT)calculations reveal the excellent catalytic ability of Pd@Te and synergistic catalytic mechanism of Pd and Te,in which uniform electron distribution,extensive electron exchange,and large adsorption distance between Pd cluster and discharge products promote homogeneous adsorption/desorption of discharge products,while the high adsorption energy of Te substrate for Li species reduces the initial dynamical energy barrier during discharging process.The current work provides viable strategy to design composite catalysts for flexible cathode of LOBs with synergistic catalytic effects.

Highly polar CoP/Co_(2)P heterojunction composite as efficient cathode electrocatalyst for Li-air battery

In order to advance the commercialization of rechargeable Li-air batteries,it is of importance to explore cathode catalyst with efficient catalytic activity.Transition metal oxides have poor electrical conductivity,while cobalt phosphide has excellent electrical conductivity and large specific surface area.Nevertheless,its application in organic Li-air batteries has been much less studied,and the electrocatalytic activity desires to be further elevated.Here,CoP/Co_(2)P heterojunction composite with higher polarity was fabricated.The discharge product of high-polarity CoP/Co_(2)P had a new porous box-like morphology,which was easy to be decomposed and exposed more active sites.The highly polar CoP/Co_(2)P heterostructure composite had homogeneous pores,the synergistic effect existed between CoP and Co_(2)P,and the discharge product was porous box mixed with Li_(2)O_(2)and LiOH,which made CoP/Co_(2)P achieve high specific capacity of14632 m Ah/g and cycle stably 161 times when used as air electrode cathode catalyst.This work furnished a thought for the construction of cathode catalysts with efficient catalytic activity for Li-air batteries.

Co_3O_4 supported on N,P-doped carbon as a bifunctional electrocatalyst for oxygen reduction and evolution reactions

Noble metals, such as platinum, ruthenium and iridium‐group metals, are often used as oxygen reduction or evolution reaction （ORR/OER） electrocatalysts. To reduce the cost and provide an application of bifunctional catalysis, in this work, cobalt oxide supported on nitrogen and phospho‐rus co‐doped carbon （Co3O4/NPC） was fabricated and examined as a bifunctional electrocatalyst for OER and ORR. To prepare Co3O4/NPC, NPC was pyrolyzed from melamine and phytic acid support‐ed on carbon, followed by the solvothermal synthesis of Co3O4 on NPC. Linear sweep voltammetry was used to evaluate the activity for OER and ORR. For OER, Co3O4/NPC showed an onset potential of 0.54 V （versus the saturated calomel electrode） and a current density of 21.95 mA/cm2 at 0.80 V, which was better than both Co3O4/C and NPC. The high activity of Co3O4/NPC was attributed to a synergistic effect of the N, P co‐dopants and Co3O4. For ORR, Co3O4/NPC exhibited an activity close to commercial Pt/C in terms of the diffusion limited current density （–4.49 vs–4.76 mA/cm2 at–0.80 V）, and Co3O4 played the key role for the catalysis. Chronoamperometry （current versus time） was used to evaluate the stability, which showed that Co3O4/NPC maintained 46%current after the chronoamperometry test for OER and 95% current for ORR. Overall, Co3O4/NPC exhibited high activity and improved stability for both OER and ORR.

In situ decoration of nanosized metal oxide on highly conductive MXene nanosheets as efficient catalyst for Li-O2 battery

Combining nanomaterials with complementary properties in a well-designed structure is an effective tactic to exploit multifunctional, high-performance materials for the energy conversion and storage. Nonprecious metal catalysts, such as cobalt oxide, with superior activity and excellent stability to other catalysts are widely desired. Nevertheless, the performance of CoO nanoparticles as an electrode material were significantly limit for its inferior conductivity, dissolution, and high cohesion. Herein, we grow ultrafine cobalt monoxide to decorate the interlayer and surface of the Ti3C2 Txnanosheets via a hydrothermal method companied by calcination. The layered MXenes act as the underlying conductive substrate,which not only increase the electron transfer rate at the interface but also greatly improve the electrochemical properties of the nanosized Co O particles by restricting the aggregation of CoO. The resulting CoO/Ti3C2 Txnanomaterial is applied as oxygen electrode for lithium-oxygen battery and achieves more than 160 cycles and first cycle capacity of 16,220 mAh g-1 at 100 mA g-1. This work paves a promising avenue for constructing a bi-functional catalyst by coupling the active component of a transition metal oxide(TMO) with the MXene materials in lithium-oxygen battery.

An Intermediate-temperature H_2S Fuel Cell with a Li_2SO_4-based Proton-conducting Membrane

A laboratory-scale intermediate-temperature H2S fuel cell with a configuration of H2S, (metal sulfide-based composite anode)/Li2SO4+Al2O3/(NiO-based composite cathode), air was developed and studied for production of power and for desulfurization of a fuel gas process stream. The cell was run at typical temperature (600—650℃) and ambient pressure, but its electrochemical performance may be limited by electrolyte membrane thickness. The membrane and its performance in cell have been characterized using scanning electron microscope (SEM) and electrochemical impedance spectrum (EIS) techniques. Composite anodes based on metal sulfides, Ag powder and electrolyte behaved well and stably in H2S stream, and composite cathodes based mainly on nickel oxide, Ag powder and electrolyte had superior per-formance to Pt catalyst. The maximum power density of up to 70mW?cm-2 and current density of as high as 250mA?cm-2 were obtained at 650℃. However, the long-term cell stability remains to be investigated.

A perspective on influences of cathode material degradation on oxygen transport resistance in low Pt PEMFC

A large-scale industrial application of proton exchange membrane fuel cells(PEMFCs)greatly depends on both substantial cost reduction and continuous durability enhancement.However,compared to effects of material degradation on apparent activity loss,little attention has been paid to influences on the phenomena of mass transport.In this review,influences of the degradation of key materials in membrane electrode assemblies(MEAs)on oxygen transport resistance in both cathode catalyst layers(CCLs)and gas diffusion layers(GDLs)are comprehensively explored,including carbon support,electrocatalyst,ionomer in CCLs as well as carbon material and hydrophobic polytetrafluoroethylene(PTFE)in GDLs.It is analyzed that carbon corrosion in CCLs will result in pore structure destruction and impact ionomer distribution,thus affecting both the bulk and local oxygen transport behavior.Considering the catalyst degradation,an eventual decrease in electrochemical active surface area(ECSA)definitely increases the local oxygen transport resistance since a decrease in active sites will lead to a longer oxygen transport path.It is also noted that problems concerning oxygen transport caused by the degradation of ionomer chemical structure in CCLs should not be ignored.Both cation contamination and chemical decomposition will change the structure of ionomer,thus worsening the local oxygen transport.Finally,it is found that the loss of carbon and PTFE in GDLs lead to a higher hydrophilicity,which is related to an occurrence of water flooding and increase in the oxygen transport resistance.

Hollow catalysts through different etching treatments to improve active sites and oxygen vacancies for high-performance Li-O_(2)battery

Li-O_(2)batteries are regarded as one of the most promising next-generation battery systems due to their high theoretical energy density,finding effective cathode catalysts with fine-tuned structure is a key way to improve the performance.Herein,based on the structure of cubic zeolitic imidazolate framework-67(ZIF-67),a series of hollow catalysts were synthesized by different chemical etching treatments.Firstly,from the perspective of metal,nickel nitrate is used for etching,hollow Ni ZIF is obtained through Kirkendall effect.Secondly,hollow TA-ZIF is obtained by adding tannic acid to replace the methylimidazole ligand.Hollow structures have larger surface areas,materials can expose more active sites,which can lead to better performance of Li-O_(2)batteries.On this basis,having more oxygen vacancies can also improve the battery performance.At the same time,further loading noble metal ruthenium on the synthesized cobalt-based catalyst can effectively reduce the overpotential of Li-O_(2)battery and improve the battery performance.For TA-ZIF with more stable hollow structure and more oxygen vacancies,the cycle performance reaches 330 cycles after loading Ru.Compared with the 64 cycles of solid Co_(3)O_(4),it has a great improvement.

2H-MoS_(2)Modified Nitrogen-Doped Hollow Mesoporous Carbon Spheres as the Efficient Catalytic Cathode Catalyst for Aprotic Lithium-Oxygen Batteries

Developing excellent cathode catalysts with superior catalytic activities is essential for the practical application of aprotic lithium-oxygen batteries(LOBs).Herein,we successfully synthesized nitrogen-doped hollow mesoporous carbon spheres encapsulated with molybdenum disulfide(MoS_(2))nanosheets as the cathode catalyst for rechargeable LOBs,and the relationship between the battery performance and structural characteristics was intensively researched.We found that the synergistic effect of the nitrogen-doped mesoporous carbon and MoS_(2)nanosheets endows superior electrocatalytic activities to the composite catalyst.On the one hand,the nitrogen-doped mesoporous carbon could enable fast charge transfer and effectively accommodate more discharging products in the composite skeleton.On the other hand,the thin MoS_(2)nanosheets could promote mass transportation to facilitate the revisable formation and decomposition of the Li2O2 during oxygen reduction reaction and oxygen evolution reaction,and the side reactions were also prevented,apparently due to their full coverage on the composite surfaces.As a result,the catalytic cathode loaded with 2H-MoS_(2)-modified nitrogen-doped hollow mesoporous carbon spheres exhibited excellent electrochemical performance in terms of large discharge-/charge-specific capacities with low overpotentials and extended cycling life,and they hold great promise for acting as the cathode catalyst for high-performance LOBs.

Ultrafine RuO_(2) nanoparticles/MWCNTs cathodes for rechargeable Na-CO_(2) batteries with accelerated kinetics of Na_(2)CO_(3) decomposition

Na-CO_(2) batteries have attracted extensive attention due to their high theoretical energy density(1125 Wh/kg),efficient utilization of CO_(2),and abundant sodium resources.However,they are trapped by the sluggish decomposition kinetic of discharge products (mainly Na_(2)CO_(3)) on cathode side during the charging process.Here we prepared a series of nano-composites composed of RuO_(2) nanoparticles in situ loaded on activated multi-walled carbon nanotubes (RuO_(2)@a-MWCNTs) through hydrolyzing reaction followed by calcination method and used them as cathode catalysts to accelerate the decomposition of Na_(2)CO_(3).Among all catalysts,the RuO_(2)@a-MWCNTs with appropriate ratio of RuO_(2)(49.7 wt%) demonstrated best stability and rate performance in Na-CO_(2) batteries,benefiting from both high specific surface area (160.3 m^(2)/g) and highly dispersed RuO_(2) with ultrafine nanostructures (~2 nm).At a limited capacity of 500 mAh/g,Na-CO_(2) batteries could afford the operation of over 120 cycles at 100 mA/g,and even at the current density to 500 mA/g,the charge voltage was still lower than 4.0 V after 40 cycles.Further theoretical calculations proved that RuO_(2) was the catalytically active center and contributed to the decomposition of Na_(2)CO_(3) by weakening the C=O bond.The synergetic functions of high specific surface(CNTs) and high catalytic activity (RuO_(2)) will inspire more progress on metal-CO_(2) batteries.

Exploration of the oxygen transport behavior in non-precious metal catalyst-based cathode catalyst layer for proton exchange membrane fuel cells

High cost has undoubtedly become the biggest obstacle to the commercialization of proton exchange membrane fuel cells(PEMFCs),in which Pt-based catalysts employed in the cathodic catalyst layer(CCL)account for the major portion of the cost.Although nonprecious metal catalysts(NPMCs)show appreciable activity and stability in the oxygen reduction reaction(ORR),the performance of fuel cells based on NPMCs remains unsatisfactory compared to those using Pt-based CCL.Therefore,most studies on NPMC-based fuel cells focus on developing highly active catalysts rather than facilitating oxygen transport.In this work,the oxygen transport behavior in CCLs based on highly active Fe-N-C catalysts is comprehensively explored through the elaborate design of two types of membrane electrode structures,one containing low-Pt-based CCL and NPMCbased dummy catalyst layer(DCL)and the other containing only the NPMC-based CCL.Using Zn-N-C based DCLs of different thickness,the bulk oxygen transport resistance at the unit thickness in NPMC-based CCL was quantified via the limiting current method combined with linear fitting analysis.Then,the local and bulk resistances in NPMC-based CCLs were quantified via the limiting current method and scanning electron microscopy,respectively.Results show that the ratios of local and bulk oxygen transport resistances in NPMCbased CCL are 80%and 20%,respectively,and that an enhancement of local oxygen transport is critical to greatly improve the performance of NPMC-based PEMFCs.Furthermore,the activity of active sites per unit in NPMCbased CCLs was determined to be lower than that in the Pt-based CCL,thus explaining worse cell performance of NPMC-based membrane electrode assemblys(MEAs).It is believed that the development of NPMC-based PEMFCs should proceed not only through the design of catalysts with higher activity but also through the improvement of oxygen transport in the CCL.

MOF-template derived hollow CeO_(2)/Co_(3)O_(4) polyhedrons with efficient cathode catalytic capability in Li-O_(2) batteries

Li-O_(2) batteries(LOBs) have been perceived as the most potential clean energy system for fast-growing electric vehicles by reason of their environmentally friendlier,high energy density and high reversibility.However,there are still some issues limiting the practical application of LOBs,such as the large gap between the actual capacity level and the theoretical capacity,low rate performance as well as short cycle life.Herein,hollow CeO_(2)/Co_(3)O_(4) polyhedrons have been synthesized by MOF template with a simple method.And it is was further served as a cathode catalyst in Li-O_(2) batteries.By means of the synergistic effect of two different transition metal oxides,nano-sized hollow porous CeO_(2)/Co_(3)O_(4) cathode obtained better capacity and cycle performance.As a result,excellent cyclability of exceeding 140 and 90 cycles are achieved at a fixed capacity of 600 and 1000 mAh/g,respectively.The successful application of this catalyst in LOBs offers a novel route in the aspect of the synthesis of other hollow porous composite oxides as catalysts for cathodes in LOBs systems by the MOF template method.

Three-in-one Fe-porphyrin based hybrid nanosheets for enhanced CO_(2)reduction and evolution kinetics in Li-CO_(2)battery

Efficient cathode-catalysts with multi-functional properties are essential for Li-CO_(2)battery,while the construction of them with simultaneously enhanced CO_(2)reduction and evolution kinetics is still challenging.Here,a kind of hybrid nanosheets based on Ru nanoparticles,Fe-TAPP and grapheme oxide(GO)has been designed through a one-pot self-assembly strategy.The Ru,Fe-porphyrin and GO based hybrid nanosheets(denoted as Ru/Fe-TAPP@GO)with integrated multi-components offer characteristics of ultrathin thickness(~4 nm),high electro-redox property,uniformly dispersed morphology,and high electrical conductivity,etc.These features endow Ru/Fe-TAPP@GO with ultra-low overpotential(0.82 V)and fully reversible discharge/charge property with a high specific-capacity of 39,000 m Ah/g within 2.0-4.5 V at 100 m A/g,which are much superior to Ru@GO and Fe-TAPP@GO.The achieved performance was presented as one of the best cathode-catalysts reported to date.The synergistically enhanced activity originated from the integrated hybrid nanosheets may provide a new pathway for designing efficient cathode-catalysts for Li-CO_(2)batteries.

A self-assembled nanoflower-like Ni_(5)P_(4)@NiSe_(2) heterostructure with hierarchical pores triggering high-efficiency electrocatalysis for Li-O_(2) batteries

The remarkably high theoretical energy densities of Li–O_(2) batteries have triggered tremendous efforts for next-generation conversion devices.Discovering efficient oxygen reduction reaction and oxygen evolution reaction(ORR/OER)bifunctional catalysts and revealing their internal structure-property relationships are crucial in developing high-performance Li–O_(2) batteries.Herein,we have prepared a nanoflower-like Ni_(5)P_(4)@NiSe_(2) heterostructure and employed it as a cathode catalyst for Li–O_(2) batteries.As expected,the three-dimensional biphasic Ni_(5)P_(4)@NiSe_(2) nanoflowers facilitated the exposure of adequate active moieties and provide sufficient space to store more discharge products.Moreover,the strong electron redistribution between Ni_(5)P_(4) and NiSe_(2) heterojunctions could result in the built-in electric fields,thus greatly facilitating the ORR/OER kinetics.Based on the above merits,the Ni_(5)P_(4)@NiSe_(2) heterostructure catalyst improved the catalytic performance of Li–O_(2) batteries and holds great promise in realizing their practical applications as well as inspiration for the design of other catalytic materials.

Nano-Morphology of a Polymer Electrolyte Fuel Cell Catalyst Layer Imaging, Reconstruction and Analysis

The oxygen reduction reaction （ORR） in the cathode catalyst layer （CCL） of polymer electrolyte fuel cells （PEFC） is one of the major causes of performance loss during operation. In addition, the CCL is the most expensive component due to the use of a Pt catalyst. Apart from the ORR itself, the species transport to and from the reactive sites determines the performance of the PEFC. The effective transport properties of the species in the CCL depend on its nanostructure. Therefore a three-dimensional reconstruction of the CCL is required. A series of two-dimensional images was obtained from focused ion beam- scanning electron microscope （FIB-SEM） imaging and a segmentation method for the two-dimensional images has been developed. The pore size distribution （PSD） was calculated for the three-dimensional geometry. The influence of the alignment and the anisotropic pixel size on the PSD has been investigated. Pores were found in the range between 5 nm and 205 nm. Evaluation of the Knudsen number showed that gas transport in the CCL is governed by the transition flow regime. The liquid water transport can be described within continuum hydrodynamics by including suitable slip flow boundary conditions.