Cathodic Oxygen Reduction Research Articles

Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction.

An approach to electrochemical oxygenation of indoles leading to isatins was developed by merging with a complementary cathode oxygen reduction reaction. The features of this green protocol include the use of molecular oxygen as the sole oxidant, it being free of an electron transfer mediator, and gram-scale preparation. Mechanistic studies suggested a radical process, and the two oxygen atoms in the isatins were both most likely from molecular oxygen. A detailed mechanism of the reaction utilizing density functional theory calculations was elucidated.

AgNPs@Fe-N-C oxygen reduction catalysts for anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) are hopeful clean energy equipment due to the mild reaction condition and the possibility of using Pt-free catalysts for cathodic oxygen reduction reaction (ORR). Developing excellent Pt-free ORR catalysts for AEMFC is still of great significance. Here, we construct a composite catalyst (AgNPs@Fe-N-C) of Ag nanoparticles (NPs) surrounded by the atomically dispersed Fe-N-C porous carbon. The AgNPs@Fe-N-C demonstrates outstanding ORR performance and enhanced durability, and its half-wave potential obviously exceeds AgNPs-N-C and Fe-N-C counterparts in alkaline condition. Impressively, the AgNPs@Fe-N-C-based AEMFC exhibits not only the ultrahigh peak power density of 0.848 W cm−2, but also long-term device durability of 200 h in H2/air (CO2-free) system, which is obviously better than that of the Fe-N-C-based AEMFC. Density functional theory (DFT) calculations further reveal the synergistic effect of Ag NPs and Fe-N-C for ORR. In short, this work provides a new route to develop high activity and robust Pt-free ORR catalysts for AEMFCs.

Cellular scaffolds based on multiwalled carbon nanotubes interpenetrating conductive metal-organic frameworks as efficient eelectrocatalysts in microbial fuel cells

Sluggish kinetics of the cathodic oxygen reduction reaction (ORR) is one of the major challenges hindering the development of microbial fuel cells (MFCs). In this work, a three-dimensional (3D) cellular scaffolds structure is fabricated by in-situ growth of conductive Ni/Co-catecholate bimetal metal-organic frameworks (MOFs) on multiwalled carbon nanotubes (NiCo-CAT/MWCNTs). Such specific structure successfully achieves favorable electronic channels and fast charge-transfer capacity and displays an ultra-low ohmic internal resistance of 7.27 Ω. Meanwhile, NiCo-CAT/MWCNTs exhibit a superior ORR half-wave potential (−0.442V vs. Hg/HgCl2), onset potential (−0.064V vs. Hg/HgCl2) and high limiting current density (8.25 mA cm−2), which exceed that of commercial Pt/C. In the MFC with the NiCo-CAT/MWCNTs cathode, the maximum power density is 130% higher than Pt/C cathode. The outstanding performance of MFC is mainly due to the optimized electronic structure and fully exposure of the Ni, Co synergistic catalytic active sites. In addition, oxygen permeability is greatly improved by electrospun polyvinylidene fluoride (PVDF) air diffusion layer replacing the traditional carbon cloth cathode. This study provides a new perspective for MFCs performance improvement by ORR conductive MOFs catalysts.

Stability of Platinum‐Group‐Metal‐Based Electrocatalysts in Proton Exchange Membrane Fuel Cells

AbstractProton exchange membrane fuel cells (PEMFCs) have attracted significant interest as a promising alternative technology to the combustion engine to power next‐generation vehicles with zero emission. Among the various technological targets, system cost and durability are currently identified as the main obstacles toward large‐scale implementation of PEMFCs. Platinum‐group‐metal‐based (PGM‐based) catalysts are the most efficient catalyst for the rate‐limiting cathodic oxygen reduction reaction in PEMFCs and take the highest share in the cost breakdown. Therefore, the stability of PGM‐based electrocatalysts plays a significant role in the development of PEMFCs. This review will summarize the recent progress made on the understanding of both the fundamental chemical and structural stability of PGM‐based catalysts, and their durability in operational PEMFC devices. It calls for systematic studies on the chemical, structural, and operational durability of the PGM‐based catalyst and the need to coin practical descriptors for catalyst design with both superior activity and durability for cost‐effective and high‐performance PEMFCs.

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.

Visual signal sensor coupling to nitrification for sustainable monitoring of trichloroacetaldehyde and the response mechanisms

In this work, a toxicity monitoring microbial system (TMMS) with a nitrifying biofilm as a sensing element and cathode oxygen reduction as an electrical signal was successfully constructed for trichloroacetaldehyde (TCAL) detection. The current and nitrification rate showed a linear relationship with TCAL concentration from 0 to 100 μg/L (R2current = 0.9892, R2nitrification = 0.9858), indicating that the target substrate concentration can be directly obtained from an electrical signal without further sample concentration. High-throughput sequencing revealed that the TMMS was composed of autotrophic/heterotrophic nitrifying and denitrifying microorganisms. Further analysis via a symbiotic relationship network demonstrated that unclassified_Comamonadaceae and unclassified_Xanthobacteraceae were the core nodes for maintaining the interaction between autotropic and heterotrophic nitrifying bacteria. Kyoto Encyclopedia of Genes and Genomes analysis showed that the electron transfer process primarily relied on ferredoxin and cytochromes under TCAL stress, and the abundance of functional enzymes involved in the process of nitrification was decreased, resulting in changes in electrical signal output. This work explored a visual signal sensor combined with electrochemistry and autotrophic/heterotrophic nitrification, which provided new insights into recognition and response mechanisms for microbial monitoring of toxic substances.

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.

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.

Multi-Physical and Electrochemical Coupling Model for the Protonic Ceramic Fuel Cells with H+/e−/O2− Mixed Conducting Cathodes

A protonic ceramic fuel cell (PCFC) has great potential for medium temperature power generation. Its working process, however, is complicated and quite different from the traditional oxygen ionic solid oxide fuel cell (O2−-SOFC) and proton exchange membrane fuel cell (PEMFC). In this paper, a multi-physical model for the PCFC with H+/e−/O2− mixed conducting cathode is established, in which the fuel- and oxidant-diffusing processes; electron-, oxygen ion-, and proton-conducting processes; three electrochemical reactions; and their coupling working details are carefully considered. Taking Ni-BZCY/BZCY/BZCY-LSCF PCFC as an example, the validation of the model is well verified by good agreements with the experiment iop-Vop curves at different temperatures. The result shows that the cathodic electrochemical reactions will be concentrated to a small thickness near the electrolyte because of the greatly decreased ionic conductivity compared with the high electronic conductivity at an intermediate temperature. O2− within the PCFC cathode is only an intermediate transform substance between the electrons and protons. Thus, there is a peak oxygen ion current distribution within the composite cathode of PCFC. The cathodic oxygen reduction half reaction is found to be a key factor to dominate the total PCFC voltage loss at the intermediate temperature zone. The concentration polarization of anode-supported PCFC is small, due to the vapors that are generated in the cathode side instead of anode side.

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.

Dandelion-like titanium nitride supported platinum as an efficient oxygen reduction catalyst in acidic media

The main challenges currently existing in proton-exchange membrane fuel cells (PEMFCs) are (1) the slow chemical reaction kinetics at cathode oxygen reduction with high cost of precious metal as catalysts, (2) the degradation of precious metals, (3) the corrosion of carbon carriers. A dandelion-like structure of titanium nitride nanospheres with abundant dendritic structures on the surface was synthesized, This non-carbon material was used as a carrier for platinum nanoparticles, which were deposited on the carrier using an electrochemical pulse deposition method. At 0.9 V (in comparison to RHE), the catalysts perform admirably for oxygen reduction reaction at cathode, with a mass activity of 0.44 mA g −1 Pt and specific activity of 0.33 mA cm −2 . Under the same test conditions, the prepared catalysts demonstrate better stability than commercial Pt/C catalysts, maintaining a high level of activity (61% of the initial value) after 3000 consecutive cycles. This research provides a framework for the development of long-lasting non-carbon catalysts with low-Pt loading. • Dandelion-like TiN was synthesized to achieve a 3D porous structure. • The resulting TiN was used as support material with good corrosion resistance. • Platinum nanoparticles were loaded onto the TiN surface using a pulsed electrodeposition method. • The obtained catalyst exhibits high ORR performance and stability in acid media.

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.

A novel electrochemical sensor based on nitrite-oxidizing bacteria for highly specific and sensitive detection of nitrites

Real-time nitrite control in water is necessary for environmental safety and human health, and has triggered the research and development of novel detection methods. Previous studies have made great progress on enzyme-free and enzyme electrochemical sensors. However, enzyme-free sensors have low selectivity and a complex preparation process, and enzyme sensors have short lifetimes, and these issues need to be addressed. In this work, we proposed for the first time a highly specific and sensitive biofilm sensor based on nitrite-oxidizing bacteria (NOB) for the bio-electrochemical detection of nitrite in water. The mechanism of nitrite detection was attributed to the competition of oxygen between aerobic respiration of the NOB and the cathode oxygen reduction on the carbon felt electrode, resulting in a decrease in current. This decrease in current (ΔI) had a linear relationship with the nitrite concentration in the range of 0.1 to 1 mg L−1 and 1 to 10 mg L−1, which was corresponding to the sensitivities of 48.62 and 2.24 μA mM−1 cm−2, respectively. And the limit of detection (LOD) was calculated to be 0.033 mg L−1 (2.39 μM) with a signal-to-noise ratio of 3. Moreover, several common interfering ions had no effect on the nitrite detection owing to the functional microbial species (NOB) and weakly electrochemical behavior of electrode at the low potential of −0.1 V, showing high specificity for nitrite detection of biofilm sensor. Therefore, the actual nitrified wastewater was well detected by the biofilm sensor. In addition, allylthiourea (ATU) took good effect on the resistance of the influence of ammonia oxidizing bacteria (AOB) in the biofilm sensor, maintaining the high selectivity of biofilm sensor in case the biofilm sensor was fouled with AOB. The biofilm sensor in our work showed good selectivity, sensitivity and stability in long-term detection.

A novel electrochemical sensor based on autotropic and heterotrophic nitrifying biofilm for trichloroacetic acid toxicity monitoring

Trichloroacetic acid (TCA), a toxic substance produced in the disinfection process of wastewater treatment plants, will accumulate in the receiving water. The detection of TCA in the water can achieve the purpose of early warning. However, currently there are few reports on microbial sensors used for TCA detection, and the characteristics of their microbial communities are still unclear. In this work, a toxicity monitoring microbial system (TMMS) with nitrifying biofilm as a sensing element and cathode oxygen reduction as a current signal was successfully constructed for TCA detection. The current and nitrification rate showed a linear relationship with low TCA concentration from 0 to 50 μg/L (R2current = 0.9892, R2nitrification = 0.9860), and high concentration range from 50 to 5000 μg/L (R2current = 0.9883, R2nitrification = 0.9721). High-throughput sequencing revealed that the TMMS was composed of autotrophic/heterotrophic nitrifying and denitrifying microorganisms. Further analysis via symbiotic relationship network demonstrated that Arenimonas and Hyphomicrobium were the core nodes for maintaining interaction between autotropic and heterotrophic nitrifying bacteria. Kyoto Encyclopedia of Genes and Genomes analysis showed that after adding TCA to TMMS, the carbon metabolism and the abundance of the tricarboxylic acid cycle pathway were reduced, and the activity of microorganisms was inhibited. TCA stress caused a low abundance of nitrifying and denitrifying functional enzymes, resulting in low oxygen consumption in the nitrification process, but more oxygen supply for cathode oxygen reduction. This work explored a novel sensor combined with electrochemistry and autotrophic/heterotrophic nitrification, which provided a new insight into the development of microbial monitoring of toxic substances.

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 .