Metal-free Cathode Research Articles

A Comprehensive Investigation of Methanol Electrooxidation on Copper Anodes: Spectroelectrochemical Insights and Energy Conversion in Microfluidic Fuel Cells.

Here, we comprehensively investigated methanol electrooxidation on Cu-based catalysts, allowing us to build the first microfluidic fuel cell (μFC) equipped with a Cu anode and a metal-free cathode that converts energy from methanol. We applied a simple, fast, small-scale, and surfactant-free strategy for synthesizing Cu-based nanoparticles at room temperature in steady state (ST), under mechanical stirring (MS), or under ultrasonication (US). The morphology evaluation of the Cu-based samples reveals that they have the same nanoparticle (NP) needle-like form. The elemental mapping composition spectra revealed that pure Cu or Cu oxides were obtained for all synthesized materials. In addition to having more Cu2O on the surface, sample US had more Cu(OH)2 than the others, according to X-ray diffractograms and X-ray photoelectron spectroscopy. The sample US is less carbon-contaminated because of the local heating of the sonic bath, which also enhances the cleanliness of the Cu surface. The activity of the Cu NPs was investigated for methanol electrooxidation in an alkaline medium through electrochemical and spectroelectrochemical measurements. The potentiodynamic and potentiostatic experiments showed higher current densities for the NPs synthesized in the US. In situ FTIR experiments revealed that the three synthesized NP materials eletcrooxidize methanol completely to carbonate through formate. Most importantly, all pathways were led without detectable CO, a poisoning molecule not found at high overpotentials. The reaction path using the US electrode experienced an additional round of formate formation and conversion into carbonate (or CO2 in the thin layer) after 1.0 V (vs. Ag/Ag/Cl), suggesting improved catalysis. The high activity of NPs synthesized in the US is attributed to effective dissociative adsorption of the fuel due to the site's availability and the presence of hydroxyl groups that may fasten the oxidation of adsorbates from the surface. After understanding the surface reaction, we built a mixed-media μFC fed by methanol in alkaline medium and sodium persulfate in acidic medium. The μFC was equipped with Cu NPs synthesized in ultrasonic-bath-modified carbon paper as the anode and metal-free carbon paper as the cathode. Since the onset potential for methanol electrooxidation was 0.45 V and the reduction reaction revealed 0.90 V, the theoretical OCV is 0.45 V, which provides a spontaneous coupled redox reaction to produce power. The μFC displayed 0.56 mA cm-2 of maximum current density and 26 μW cm-2 of peak power density at 100 μL min-1. This membraneless system optimizes each half-cell individually, making it possible to build fuel cells with noble metal-free anodes and metal-free cathodes.

Electrochemical treatment coupled with solar light-driven photocatalytic approach: A challenging process in cascade for hydrogen production and wastewater remediation

This study presents an innovative approach for simultaneous hydrogen production and wastewater remediation, integrating electrochemical treatment with solar light-driven photocatalysis. The research focuses on the use of a noble metal-free cathode, based on a electrodeposited composite of Co2P and elemental P, for efficient hydrogen generation from simulated wastewater through water splitting. This composite is characterized, in its optimized form, by an overpotential equal to 133.6 mV (at 10 mA cm-2) and by a Tafel slope of 60.5 mV dec-1. Challenges like the high potential required for the Oxygen Evolution Reaction (OER) and the use of expensive noble metals in electrodes are addressed by employing earth-abundant compounds for electrode fabrication. Additionally, the study explores the degradation of diclofenac (DCF) in wastewater, demonstrating that electrochemical treatment alone is insufficient for organic matter removal. Therefore, a coupled process involving a first electrochemical treatment step followed by a photocatalytic process using BiOCl is proposed. Thanks to the exposure of the (110) active face, BiOCl possesses excellent photocatalytic performances even under solar light irradiation. This hybrid approach not only enhances the efficiency of DCF degradation (about 90%) and reaches an organic matter removal of 59%, but it also improves hydrogen production, offering a sustainable solution for energy generation and water purification in the face of increasing global industrialization and water scarcity.

Interlayer Polymerization to Construct a Fully Conjugated Covalent Organic Framework as a Metal-Free Oxygen Reduction Reaction Catalyst for Anion Exchange Membrane Fuel Cells.

Two-dimensional (2D) covalent organic frameworks (COFs) have a multilayer skeleton with a periodic π-conjugated molecular array, which can facilitate charge carrier transport within a COF layer. However, the lack of an effective charge carrier transmission pathway between 2D COF layers greatly limits their applications in electrocatalysis. Herein, by employing a side-chain polymerization strategy to form polythiophene along the nanochannels, a conjugated bridge is constructed between the COF layers. The as-synthesized fully conjugated COF (PTh-COF) exhibits high oxygen reduction reaction (ORR) activity with narrowed energy band gaps. Correspondingly, PTh-COF is tested as a metal-free cathode catalyst for anion exchange membrane fuel cells (AEMFCs) which showed a maximum power density of 176mWcm-2 under a current density of 533mAcm-2. The density functional theory (DFT) calculation reveals that interlayer conjugated polythiophene optimizes the electron cloud distribution, which therefore enhances the ORR performance. This work not only provides new insight into the construction of a fully conjugated covalent organic framework but also promotes the development of new metal-free ORR catalysts.

Construction of integrated oxygen-rich carbon-based metal-free cathode to simultaneous boost wastewater treatment performance and energy recovery in bio-electro-Fenton system

Fenton coupled microbial fuel cells (MFCs) used for antibiotic pollution purification are limited by energy recovery and metal leakage of electrode. Here, we constructed a two-compartment bio-electro-Fenton (MFC-Fenton) with biological anode and Fenton cathode, which using carbon nanotube based on carbon brushes (OCNT-80/CB) with active centers rich in oxygen-containing functional groups as metal-free cathode. The results show that C = O locked environmental oxygen in the cathode can in-situ use the electrons produced by the anode through two-electron oxygen reduction reaction (2e− ORR) synthesize H2O2. •OH is further generated under activation of H2O2, achieving effective degradation of tetracycline (TC) in the cathode chamber. This study breaks through the limitations of poor pH adaptability and environmental secondary pollution caused by metal dissolution. Moreover, with the advantageous features of oxygen-containing functional groups active centers, good conductivity, and cyclic stability, the OCNT-80/CB achieves a maximum power density increased by 79.4 % as compared to that of the CB control. More notably, Microbial analysis further indicated that OCNT-80/CB electrode could simultaneously enhance Proteobacteria and Firmicutes abundance in bioanode. Dominant bacteria can self-metabolize environmental wastewater generate and recycle green energy. This study proved that careful structural design of cathode could promote the electrode performance. MFC-Fenton can be a competitive technology to enhance wastewater treatment and energy recovery simultaneously.

Carbon Fiber Film with Multi-Hollow Channels to Expedite Oxygen Electrocatalytic Reaction Kinetics for Flexible Zn-Air Battery.

The high oxygen electrocatalytic overpotential of flexible cathodes due to sluggish reaction kinetics result in low energy conversion efficiency of wearable zinc-air batteries (ZABs). Herein, lignin, as a 3D flexible carbon-rich macromolecule, is employed for partial replacement of polyacrylonitrile and constructing flexible freestanding air electrodes (FFAEs) with large amount of mesopores and multi-hollow channels via electrospinning combined with annealing strategy. The presence of lignin with disordered structure decreases the graphitization of carbon fibers, increases the structural defects, and optimizes the pore structure, facilitating the enhancement of electron-transfer kinetics. This unique structure effectively improves the accessibility of graphitic-N/pyridinic-N with oxygen reduction reaction (ORR) activity and pyridinic-N with oxygen evolution reaction (OER) activity for FFAEs, accelerating the mass transfer process of oxygen-active species. The resulting N-doped hollow carbon fiber films (NHCFs) exhibit superior bifunctional ORR/OER performance with a low potential difference of only 0.60V. The rechargeable ZABs with NHCFs as metal-free cathodes possess a long-term cycling stability. Furthermore, the NHCFs can be used as FFAEs for flexible ZABs which have a high specific capacity and good cycling stability under different bending states. This work paves the way to design and produce highly active metal-free bifunctional FFAEs for electrochemical energy devices.

Metal-free cathode rivals commercial battery performance

Metal-free cathode rivals commercial battery performance

Oxygen-doped carbon nanotubes with dual active cites to enhance •OH formation through three electron oxygen reduction

The electro-Fenton (EF) process generates H2O2 through the 2e- oxygen reduction reaction (ORR), which is subsequently activated to •OH by iron-based catalysts. To alleviate the potential risk of external Fe-based catalysts, along with metal dissolution in acidic or neutral environments, in this study we employed oxygen-doped carbon nanotubes (OCNT) as a bifunctional, metal-free cathode to establish a metal-free EF process for organic pollutant degradation. The results demonstrate that the metal-free electrode has excellent H2O2 accumulation (12 mg L−1 cm−1) and degrades sulfathiazole (STZ) with 97.05 % efficiency in 180 min with an explanation kinetic of 0.0189 min−1. For the first time, this enhancement came from the dual active site centers in OCNT: Ⅰ) -COOH and defects active sites were responsible for H2O2 production, Ⅱ) then -CO triggered H2O2 into •OH, avoiding the introduction of metal-based catalysts. These findings suggest that the EF system with in situ oxygen-doped cathodes have great potential for treating antibiotic wastewater.

Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•−) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O2•−) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h−1 gcatalyst−1. Experimental and theoretical studies revealed the mechanism of O2•− generation via oxygen activation, including one- and three-electron transfer pathways, and found that O2•− mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.

B-N Co-Doped Biphenylene as a Metal-Free Cathode Catalyst for Li-O2 Batteries: a Computational Study.

Lithium-oxygen batteries (LOBs) meet the growing demand for long-distance transportation over electric vehicles but face challenges because of the lack of high-performance cathode catalysts. Herein, using density functional theory calculations, we report a unique graphene allotrope, biphenylene, of which the doping structures exhibit great potential as metal-free catalysts for LOBs. Our modeling results demonstrate that the biphenylene nanosheets retain metallic properties after B doping, N doping, or B-N co-doping. Compared with the pristine biphenylene, the catalytic activity of the doped biphenylene is greatly improved due to charge redistributions. Notably, the overpotentials of the B-N co-doped biphenylene are as low as 0.19 and 0.18 V for the discharge and charge processes, respectively. Based on the electronic structure and bonding analysis, we identify two factors, i. e., Li-O bond strength and *Li2 O2 adsorption energy, that can influence the Li-O2 electrochemical reactions. This study not only proposes a promising cathode catalyst but also provides insights into optimizing cathode catalysts for LOBs.

Facile gas-steamed synthesis strategy of N, F co-doped defective porous carbon for enhanced oxygen-reduction performance in microbial fuel cells

The metal-free carbon-based catalyst with low cost and high oxygen reduction reaction (ORR) activity is urgently desired to satisfy the demands of microbial fuel cells (MFCs). However, it is still a great challenge to develop a facile and feasible strategy to construct efficient active sites of heteroatom doping for carbon-based electrocatalyst. Herein, we report a strategy based on an ammonium fluoride (NH4F) gas-steamed metal-organic frameworks (MOFs) to heighten structural defects and density of N, F active sites of metal-free catalyst. Oxygen temperature-programmed deposition and density functional theory results confirm that the NH4F gas-steamed process greatly enhances the adsorption affinity of O2 and oxygen intermediates on the catalysts. The resulted N and F co-doped porous carbon cage (FNC-15) demonstrates outstanding ORR catalytic activity and long-term stability in alkaline and neutral electrolytes. This work proposes a facile and efficient in situ gas-steamed strategy to develop metal-free cathode catalysts with superior performance.

Boosting DMFC power output by adding sulfuric acid as a supporting electrolyte: Effect on cell performance equipped with platinum and platinum group metal-free cathodes

Direct methanol fuel cells (DMFCs) are promising electrochemical systems capable of producing electricity from the electrochemical oxidation of methanol and the reduction of oxygen. In this work, the effectiveness of the addition of sulfuric acid as a supporting electrolyte for methanol fuel composition was assessed. The results showed that the peak of power curve in DMFCs with Pt/C cathode electrocatalysts increased progressively from 70 mW cm−2 (0 mM of H2SO4) to 115 mW cm−2 with a concentration of 100 mM of H2SO4. These results underlined the positive effect of the addition of a supporting electrolyte in the methanol aqueous solution on the electrochemical output that was enhanced. Platinum group metal-free (PGM-free) electrocatalysts based on Fe-Nx-C type were also tested being insensitive to methanol crossover and oxidation at the cathode. DMFC with Fe–N–C cathode catalysts result in a performance of 21.5 mW cm−2. In these operating conditions, the addition of supporting electrolyte does not seem to bring excessive advantage. Short stability tests are presented and an overall assessment of the resistances within the system is also discussed.

Isolated Metalloid Tellurium Atomic Cluster on Nitrogen-Doped Carbon Nanosheet for High-Capacity Rechargeable Lithium-CO2 Battery.

Rechargeable Li-CO2 battery represents a sustainable technology by virtue of CO2 recyclability and energy storage capability. Unfortunately, the sluggish mass transport and electron transfer in bulky high-crystalline discharge product of Li2 CO3 , severely hinder its practical capacity and rechargeability. Herein, a heterostructure of isolated metalloid Te atomic cluster anchored on N-doped carbon nanosheets is designed (TeAC @NCNS) as a metal-free cathode for Li-CO2 battery. X-ray absorption spectroscopy analysis demonstrates that the abundant and dispersed Te active centers can be stabilized by C atoms in form of the covalent bond. The fabricated battery shows an unprecedented full-discharge capacity of 28.35mAhcm-2 at 0.05mAcm-2 and long-term cycle life of up to 1000h even at a high cut-off capacity of 1mAhcm-2 . A series of ex situ characterizations combined with theoretical calculations demonstrate that the abundant Te atomic clusters acting as active centers can drive the electron redistribution of carbonate via forming TeO bonds, giving rise to poor-crystalline Li2 CO3 film during the discharge process. Moreover, the efficient electron transfer between the Te centers and intermediate species is energetically beneficial for nucleation and accelerates the decomposition of Li2 CO3 on the TeAC @NCNS during the discharge/charge process.

Electrode Integration of Synthetic Hydrogenase as Bioinspired and Noble Metal-Free Cathodes for Hydrogen Evolution

Diiron complexes mimicking the H-cluster of [FeFe]-hydrogenases have been extensively studied as (electro-)catalysts for proton reduction under homogeneous conditions. The incorporation of such complexes as “active sites” within macromolecular scaffolds such as organic polymers is receiving increasing attention as this strategy allows controlling the environment, that is, the outer coordination sphere, around the molecular catalytic center, to tune its performance as well as its stability. Here, we report on the synthesis and characterization of a library of metallo-copolymers featuring a bioinspired diiron active site and internal proton relays based on a previous report [Brezinski et al. Angew. Chem. Int. Ed. 2018, 57, 11898–11902]. The polymers are further functionalized with various amounts of pyrene groups for efficient noncovalent anchoring onto multi-walled carbon nanotubes (MWNTs), enabling the preparation of molecularly engineered electrode materials. The addition of pyrene anchors resulted in improved activity and stability, with a pyrene loading of about ∼8% corresponding to an optimized balance between polymer hydrophilicity and surface affinity. The best material displayed an average turnover frequency (TOFH2) of 4.3 ± 0.6 s–1 and a conservative turnover number for H2 production (TONH2) of 3.1 ± 0.4 × 105 after 20 h of continuous bulk electrolysis in aqueous conditions at 0.39 V overpotential. Interestingly, comparing such activities with an analogous diiron site deprived from polymeric scaffold revealed that latter could only show TONH2 of ∼4 ± 2 × 103 and TOFH2 of 0.06 ± 0.02 s–1 in 20 h under the same conditions. Post operando analysis of the modified electrodes suggests that electrode inactivation occurs via leaching of the diiron core from MWNT. In addition, a life cycle assessment was carried out to evaluate the performance of the engineered electrode materials not only from a technical perspective but also from an environmental point of view.

Synthesis and performance of a cathode catalyst derived from areca nut husk in microbial fuel cell

The role of the cathode catalyst is crucial in a single chamber Microbial Fuel Cell (MFC) to overcome the energy barrier. The present work aims todevelop a metal-free cathode catalyst from anagro-waste, areca nut husk and to evaluate its performance in MFC. Activated carbon with amorphous graphitic structure was synthesised at a pyrolysis temperature of 500 °C from the areca nut husk. The surface area of activated carbon is 1261.6 m2/g with an average particle size of 35.23 μm. The electrochemical characterisation of the cathode in oxygen saturated atmosphere reveals, a loading rate of 5 mg/cm2 possesses an equivalent conductivity to that of Pt catalyst. An Open Circuit voltage of 864 mV with a power density of 590 mW/m2 and a current density of 1.03517 A/m2 at 611.8 Ω was obtained. These results make the novel metal free catalyst a potential alternative to metal-based catalysts.

Half-cell electrode assessments of a crossover-tolerant direct methanol fuel cell with a platinum group metal-free cathode

In this work, a platinum group metal- free (PGM-free) Fe-N-C oxygen reduction reaction (ORR) catalyst derived from metal organic framework precursors (Fe-MOF) is evaluated at the cathode side of a direct methanol fuel cell (DMFC). A wide range of methanol concentrations were used, and the performance of the DMFC using air was evaluated and compared to a commercial Pt/C cathode. We conducted our tests with a custom, integrated H2 reference electrode that separately provided the anode and cathode overpotentials. This allowed us to perform a Tafel analysis on both electrodes and understand the dominant polarizations involved at the PGM-free cathode and the PtRu/C anode. Overall, we achieved a high cell performance using a Fe-N-C based catalyst under liquid methanol and air operation with a peak power density of 111 mW/cm2, where the cathode presented a markedly larger contribution to the voltage loss versus the anode. Additionally, we used nanoscale X-ray computed tomography (nano-CT) to image the Fe-N-C cathode and investigate the ionomer and pore size distributions allowing us to identify areas of improvement for catalyst layer fabrication.

First-Principles Plane-Wave-Based Exploration of Cathode and Anode Materials for Li- and Na-Ion Batteries Involving Complex Nitrogen-Based Anions

We present a first-principles study based on plane-wave-derived Löwdin population analysis and other local bonding descriptors to investigate cathode and anode materials for lithium- and sodium-ion batteries, with a special emphasis on complex nitrogen chemistry. By comparing the Löwdin charges of commonly used electrode materials to other phases such as salts of dicyanamide and nanoporous carbon-based compounds, new conclusions of an improved intercalation behavior of the latter are derived. In addition, we explore the stability of the dicyanamide salts upon Li and Na removal, some of them resulting in dimerized structures. In particular, having a look at the different kinds of bonds and the corresponding covalency indicators reveals insights into the bonding changes during dimerization. Considering the astonishing thermal stability of metal dicyanamide salts, which are solid at room temperature, their electrochemical activity, and non-toxicity of alkali metal-based compounds, these materials are potential alternatives to commercially available electrodes, particularly as they show some flexibility in exhibiting anodic and cathodic behavior and allow for transition metal-free cathode materials.

Special Section on Mechanics of Electrochemical Energy Storage and Conversion

The increasing population growth, depletion of natural resources, and rising energy demand have sparked enormous research endeavors in electrochemical energy storage and conversion. For example, rechargeable lithium-ion batteries are ubiquitous in everyday life. Mechanics plays a critical role in designing a wide range of energy technologies. The emerging field of electro-chemo-mechanics, the interplay of mechanics and electrochemistry, is crucial for understanding the coupled physiochemical processes. The electrochemical phenomena can govern the mechanical response such as stress generation, deformation, fracture initiation/propagation, elasticity, and plasticity. Similarly, mechanical phenomena also influence the electrochemical properties such as device reliability and durability. Therefore, the in-depth mechanical study of electrochemical systems is urgently necessary for fundamental science and technological applications. Over the past few years, there has been significant progress in modeling, theories, and experimental characterizations of mechanical aspects of energy storage and conversion. This timely special issue addressed some recent advances in electro-chemo-mechanics. We have selected eight papers covering a wide range of issues in batteries and fuel cells such as (i) deformation, microstructural changes, creep, overcharge detection and prevention, optimization of structural parameters in batteries and (ii) temperature and load variations, metal-free cathode catalyst in fuel cells. The selected papers cover a gamut of electrochemical-mechanics centric research in energy storage and conversion.In this issue, Adams et al. used ex-situ X-ray microtomography to study cycling-induced microstructural changes in alloy anodes for Lithium-ion Batteries. They showed that besides mechanical degradation of electrodes, excessive volume expansion could also influence transport networks in the active materials and supporting phases of the electrode. Dienemann et al. measured the time- and temperature-dependent mechanics of two thicknesses of commercial lithium anodes inside a dry industrial room, where battery cells are manufactured at high volume. Furthermore, they examined the anisotropic microstructure of lithium anodes and its effect on bulk creep mechanics. Zaidi et al. proposed a robust passivity-based control scheme for 1.26 KW proton exchange membrane fuel cell under temperature and load variations. Li et al. performed experiments to study overcharge detection and prevention with temperature monitoring of Li-ion batteries. Furthermore, they developed a linear regression-based machine learning model to validate the experimental results. Nguyen et al. experimentally studied the deformation behavior of single prismatic battery cell cases and cell assemblies loaded by internal pressure. They demonstrated that the use of nominal cell casting characteristics significantly underestimates the resistance provided by the cell case to counter swelling of the active battery components. Bao et al. performed combined experiments and computation (computational fluid dynamics and multi-objective optimization method) to study structural parameters of the air-cooled system for the lithium battery pack. Woodcox et al. performed density functional theory (DFT) calculations to investigate 19 CaxSn1−x structures (six bulk materials and 13 alloys) as potential battery anodes. They found that for stable/metastable compositions of CaxSn1−x, those sharing the same chemical composition (stoichiometry) also share remarkably similar material properties, indicating that such materials would be advantageous for uses in battery anodes. Miao et al. developed a two-dimensional, two-phase transport model to investigate the transport characteristics in direct methanol fuel cells (DMFCs) using platinum group metal (PG)-free cathode catalysts. To achieve the projected performance target of 300 mW/cm2 peak power density, catalytic activities of both the anode and cathode catalysts need to be improved by one order of magnitude compared with the state-of-the-art commercial catalysts.We believe that this special issue will be a valuable contribution to the energy storage and conversion literature and open new frontiers for the researchers. The interdisciplinary field of electro-chemo-mechanics is rapidly evolving. Our special issue will motivate many researchers to implement novel techniques such as artificial intelligence, digital twins, and automated advanced manufacturing in this field and initiate new collaborations between experimentalists, theorists, and modelers. We would like to thank all contributors to this special section issue for submitting their latest high-impact work addressing the mechanics aspects of electrochemical energy storage. Also, special thanks would go out to the invited reviewers for helping us further enhance the quality of the articles. A special issue of ASME Applied Mechanics Reviews is also being planned for the second half of 2022 and developed in collaboration with the ASME Journal of Electrochemical Energy Conversion and Storage, dedicated to state-of-the-art reviews that highlight contributions of the discipline of electrochemical energy conversion and storage to research in applied mechanics.

Selective Electrocatalytic Reduction of Nitrate to Ammonia with Nickel Phosphide.

Liquid ammonia is considered a sustainable liquid fuel and an easily transportable carrier of hydrogen energy; however, its synthesis processes are energy-consuming, high cost, and low yield rate. Herein, we report the electrocatalytic reduction of nitrate (NO3-) (ERN) to ammonia (NH3) with nickel phosphide (Ni2P) used as a noble metal-free cathode. Ni2P with (111) facet was grown in situ on nickel foam (NFP), which was regarded as a self-supporting cathode for ERN to synthesis NH3 with high yield rate (0.056 mmol h-1 mg-1) and superior faradaic efficiency of 99.23%. The derived atomic H (*H), verified by a quenching experiment and an electron spin resonance (ESR) technique, effectively enhanced the high selectivity for NH3 generation. DFT calculations indicated that *NO3 was deoxygenated to *NO2 and *NO, and *NO was subsequently hydrogenated with *H to generate NH3 with an energy releasing process (ΔG < 0). OLEMS also proved that NO was the merely gas intermediate. NFP exhibited the unique superhydrophilic surface, metallic properties, low impedance, and abundant surface sites, favorable for adsorption of NO3-, generation of *H, and then hydrogenation of NO3-. Hence, NFP cathode showed high selectivity for NH3 (89.1%) in ERN. NFP with long-term stability and low energy consumption provides a facile strategy for synthesis of NH3 and elimination of NO3- contamination.

N/B Co-doped carbon as metal-free cathode catalyst for high-performance asymmetric neutral-alkaline microbial fuel cell

Microbial fuel cells (MFCs) is a device that uses microorganisms as biocatalysts to directly convert the chemical energy of waste organic into electrical energy. The sluggish kinetics and the high cost of oxygen reduction reaction (ORR) are the major factors hindering the development of MFCs, especially in neutral electrolyte that is widely used in common MFCs. Therefore, it is highly desirable either to explore cheap cathode ORR catalysts with high catalytic activity and stability, or to develop newly MFCs system that can significantly enhance cathode kinetic. In this study, we develop a metal-free N/B-co-doped carbon-based catalyst (denoted as PANI/B-8) by pyrolysis of mixtures of polyaniline and boric acid, which show remarkably improved kinetic and activity toward ORR in alkaline electrolyte relative to neutral electrolyte, inspiring us to develop an asymmetric neutral-alkaline microbial fuel cell (ANA-MFCs) with microbial as neutral anode catalysts and PANI/B-8 as alkaline cathode catalysts, between the two chambers are separated by a proton exchange membrane (PEM) to prevent mixing of anolyte and catholyte while to ensure ion conductivity. The present ANA-MFCs notably deliver an output power density twice higher than that of the symmetric MFCs, thanks to the remarkably enhanced ORR kinetic in alkaline cathode reaction.

Highly efficient metal-free electro-Fenton degradation of organic contaminants on a bifunctional catalyst

Heterogeneous electro-Fenton (e-Fenton) is a promising technology for the treatment of persistent organic pollutants, in which H2O2 is produced via 2e- oxygen reduction and is simultaneously activated into •OH by the iron-based catalyst. This system often suffers from the inevitable metal dissolution in the acidic or even neutral environment, leading to poor pH adaptation and low stability. In this work, we designed a metal-free e-Fenton system, using O-doped carbon nanotubes (OCNTs) as the bifunctional metal-free cathode for the degradation of organic pollutants. The system showed the excellent e-Fenton performance under neutral conditions with the phenol degradation kinetic constant of 0.071 min−1, and the corresponding TOC removal was 76.6% within 300 min. It also exhibited excellent performance for actual coking wastewater treatment with the specific energy consumption of 7.4 kW h kg−1 COD−1, which was lower than that reported heterogeneous electro-Fenton system (9.2–14.4 kW h kg−1 COD−1). The in-situ metal-free e-Fenton system could be regarded as a promising strategy for actual wastewater treatment.