Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

Non-Pt catalyst as oxygen reduction reaction in microbial fuel cells: A review

Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.     

H2-induced thermal treatment significantly influences the development of a high performance low-platinum core-shell PtNi/C alloyed oxygen reduction catalyst

In the purpose of maximizing the utilization of noble metal Pt in oxygen reduction catalysts, we illustrate a synthesis method of preparing the low-platinum PtNi/C alloyed oxygen reduction reaction (ORR) catalyst, which is developed through the H₂-induced treatment to a glucose reduced PtNi/C alloy. After post-treatment with H₂/N₂ mixture gases, this catalyst displays excellent ORR catalytic activity and durability for the synergetic influences of electronic and geometry effects on catalysts during the alloying. Specifically, the as-prepared PtNi/C (350°C-6 h) sample delivers preponderant ORR activity with only 53.5% Pt usage than the commercial Pt/C. The specific activity and mass activity are corresponding 7.49 times and 3.5 times to the commercial Pt/C. This catalyst exhibits excellent ORR catalytic activity after 10 000 potential cycles in acid, which benefits from the well alloyed core-shell structure of PtNi/C. H₂-induced thermal treatment has significant effects on the development of high performance low-platinum PtNi/C alloy catalyst, and plays the significant role in the formation of well-alloyed core-shell structures. The lowered d-band center is believed to facilitate ORR catalysis through weakening the adsorption of intermediate oxygen species on the alloyed Pt surface. Therefore, PtNi/C(350°C-6 h) alloyed catalyst possesses outstanding ORR catalytic activity with much lower Pt loading.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

Nitrogen-doped carbon xerogel as high active oxygen reduction catalyst for direct methanol alkaline fuel cell

A novel catalyst based on nitrogen-doped carbon xerogel for oxygen reduction reaction (ORR) was prepared via a sol–gel process, following by the subsequent pyrolysis under ammonia atmosphere. The catalytic activity in alkaline media was optimized by tuning the metal (cobalt) ratio to the gel precursor. Sample with the optimum activity was characterized by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis and electrochemical measurements. Results show that the catalyst possesses an amorphous microstructure with nitrogen doped on the surface. The nitrogen-doped carbon xerogel displays comparable ORR activity and superior methanol tolerance than Pt/C in alkaline medium, demonstrating its promising application in direct methanol alkaline fuel cells as non-precious cathode catalyst.     

Microwave assisted,facile synthesis of Pt/CNT catalyst for proton exchange membrane fuel cell application

The present study aims at developing a high performing Pt/CNT catalyst for ORR in PEM fuel cell adopting modified chemical reduction route using a mixture of NaBH₄ and ethylene glycol (EG) as reducing agent. In order to select the most suitable reduction conditions to realize high performing catalyst, heating of the reaction mixture is done following two methods, conventional heating (CH) or microwave (MW) irradiation. The synthesized Pt/CNT catalysts were extensively characterized and evaluated in-situ as ORR catalyst in PEM fuel cell. A comparison of their performance with the standard, commercial Pt/C catalyst was also made. The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher SSA for Pt/CNT-MWH compared to Pt/CNT-CH. In-situ electrochemical characterization studies revealed higher ESA, lower charge transfer resistance, lower activation over-potential loss and higher peak power density compared to the cathode with Pt/CNT-CH and Pt/C. This study suggests the viability of MW assisted, metal particle deposition as a simple, yet effective method to prepare high performing Pt/CNT catalyst for ORR in PEM fuel cell.     

Enhancement of oxygen reduction activity with addition of carbon support for non-precious metal nitrogen doped carbon catalyst

An improved synthesis scheme of non-precious metal N-doped carbon catalysts for oxygen reduction reaction is reported. The non-precious metal N-doped carbon catalysts were prepared by pyrolysis of the mixture (phenol resin, Ketjen black carbon support and cobalt phenanthroline complex). The drastic improvement of distribution state of Ketjen black supported non-precious metal N-doped carbon catalysts was observed by means of transmission electron microscopy (TEM). In addition, the non-precious metal N-doped carbon catalyst synthesized with Ketjen black carbon support showed much higher oxygen reduction reaction (ORR) activity relative to unsupported non-precious metal N-doped carbon catalyst in O₂-saturated 0.5 mol l⁻¹ H₂SO₄ at 35 °C. Moreover, the highest ORR activity was obtained with addition of optimum amount of Ketjen black carbon support was thirtyfold compared to unsupported non-precious metal N-doped carbon catalyst at 0.7 V. Similarly, the performance of a polymer electrolyte fuel cell (PEFC) using the non-precious metal N-doped carbon catalyst as the cathode electrode catalyst was obviously better than that of the non-precious metal N-doped carbon catalyst before optimization. Microstructure, specific surface area and surface composition of the non-precious metal N-doped carbon catalysts were analyzed by XRD, XPS and BET measurement with nitrogen physisorption, respectively.     

A melamine formaldehyderesin route to in situ encapsulate Co2O3 into carbon black for enhanced oxygen reduction in alkaline media

Enhancing the activity and stability of the non-precious metal catalyst (NPMC) for oxygen reduction reaction (ORR) is vital for the commercialization of fuel cells. Herein, we put forward a method in which the melamine formaldehyderesin was used as a precursor to encapsulate in situ Co₂O₃ into carbon black to form Co₂O₃@MF-C catalysts. The prepared catalysts were characterized by TEM, XRD, XPS, BET, and Raman spectroscopy. The electrocatalytic activity was measured by using rotating disk electrode (RDE) voltammetry. The Co₂O₃@MF-Cs shows outstanding electrocatalytic activity in alkaline solution compared with the commercial Pt/C catalyst. The 20%Co₂O₃@MF-C-650 shows the highest activity for ORR and its initial reduction potential and half-wave potential reach 1.01 V and 0.94 V, respectively, in 0.1 M KOH solution. The prepared catalysts also follow the 4-electron pathway ORR process both in alkaline and in acid conditions.     

Non precious metal catalysts for the PEM fuel cell cathode

Low temperature fuel cells, such as the proton exchange membrane (PEM) fuel cell, have required the use of highly active catalysts to promote both the fuel oxidation at the anode and oxygen reduction at the cathode. Attention has been particularly given to the oxygen reduction reaction (ORR) since this appears to be responsible for major voltage losses within the cell. To provide the requisite activity and minimse losses, precious metal catalysts (containing Pt) continue to be used for the cathode catalyst. At the same time, much research is in progress to reduce the costs associated with Pt cathode catalysts, by identifying and developing non-precious metal alternatives. This review outlines classes of non-precious metal systems that have been investigated over the past 10 years. Whilst none of these so far have provided the performance and durability of Pt systems some, such as transition metals supported on porous carbons, have demonstrated reasonable electrocatalytic activity. Of the newer catalysts, iron-based nanostructures on nitrogen-functionalised mesoporous carbons are beginning to emerge as possible contenders for future commercial PEMFC systems.     

Solving Nafion poisoning of ORR catalysts with an accessible layer: designing a nanostructured core-shell Pt/C catalyst via a one-step self-assembly for PEMFC

A core-shell Pt/C@NCL300 catalyst with an accessible layer was designed to recover lost ORR activity and was constructed via a one-step self-assembly process in this paper. A thin porous layer derived from Nafion was first formed on the surface of Pt/C catalyst to create a shell. This first coating successfully separated the Nafion and Pt particles in the catalysts and reducing the negative impact of Nafion on ORR activity and enhancing the fuel cell performance. The newly fabricated Pt/C@NCL300 catalyst exhibited much higher specific activity than the original Pt/C catalyst in RDE tests under the same conditions and were comparable to the activity of Pt/C electrode without Nafion poisoning. Moreover, the fuel cell with Pt/C@NCL300 catalyst exhibited a higher power density without an obvious increase in proton transport and O₂ transport resistance compared to that of a Pt/C fuel cell with a low Pt loading. This result indicates that coating the Pt/C catalyst with a layer accessible for oxygen and protons is a promising way to effectively promote Pt-based catalysts that work under normal operating conditions.     

Highly durable and active non-precious air cathode catalyst for zinc air battery

The electrochemical stability of non-precious FeCo-EDA and commercial Pt/C cathode catalysts for zinc air battery have been compared using accelerated degradation test (ADT) in alkaline condition. Outstanding oxygen reduction reaction (ORR) stability of the FeCo-EDA catalyst was observed compared with the commercial Pt/C catalyst. The FeCo-EDA catalyst retained 80% of the initial mass activity for ORR whereas the commercial Pt/C catalyst retained only 32% of the initial mass activity after ADT. Additionally, the FeCo-EDA catalyst exhibited a nearly three times higher mass activity compared to that of the commercial Pt/C catalyst after ADT. Furthermore, single cell test of the FeCo-EDA and Pt/C catalysts was performed where both catalysts exhibited pseudolinear behaviour in the 12-500 mA cm⁻² range. In addition, 67% higher peak power density was observed from the FeCo-EDA catalyst compared with commercial Pt/C. Based on the half cell and single cell tests the non-precious FeCo-EDA catalyst is a very promising ORR electrocatalyst for zinc air battery.     

Synthesis of cobalt and nitrogen co-doped carbon nanotubes and its ORR activity as the catalyst used in hydrogen fuel cells

In hydrogen fuel cells, the sluggish oxygen reduction reaction (ORR) requires the catalysts used. Unfortunately, the precious platinum based catalysts still exhibit the best ORR activity in the commercial hydrogen fuel cells. Therefore, developing non-precious metal catalysts ORR become an important aspect for the utilization of hydrogen energy by using hydrogen fuel cells to develop non-precious catalysts and understand their active sites of ORR, herein the cobalt and nitrogen co-doped CNTs, nitrogen-doped CNTs and cobalt doped CNTs were prepared, respectively, and their catalytic properties toward ORR were tested and compared. The surface composition, microstructure and ORR performance of the samples were examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), pore/specific surface analyzer and electrochemical methods. The results demonstrate that as the catalyst, the cobalt and nitrogen co-doped CNTs owns the highest ORR limiting current density, the most positive ORR onset potential and the largest transfer electron number close to four, and thus exhibits the better ORR catalytic performance compared to the other two samples of the nitrogen-doped CNTs and the cobalt doped CNTs. The good ORR performance of cobalt and nitrogen co-doped CNTs can be attributed to its active sites of nitrogen containing functional groups, cobalt or cobalt oxides, Co-N_x structure, and the synergistic effect of these sites on ORR.     

A study on novel pulse preparation and electrocatalytic activities of Pt/C-Nafion electrodes for proton exchange membrane fuel cell

To aim at reducing the platinum loading and increasing the utilization of platinum in PEMFC electrode, a new pulse electrodeposition technique for preparing proton exchange membrane fuel cell (PEMFC) electrodes has been developed in this paper. This method combines coating Pt seeds on the C-Nafion substrate and introducing polyethylene glycol (PEG) into the deposition solution. SEM images of the samples show that Pt seeds and PEG take an important role in the morphology of the Pt deposit. The surface area and average particle size of Pt were determined by charge integration under the hydrogen desorption peaks of cyclic voltammetry. The electrocatalytic activities of these electrodes towards oxygen reduction reaction (ORR) were investigated by using rotating disc electrode (RDE). The Pt catalyst which was prepared by Pt seeds and PEG, its active surface area and electrocatalytic activity towards ORR were improved remarkably. And the optimized electrode displayed higher catalytic activity than a conventional electrode made from commercial Pt/C catalyst. The possible reasons for the effects of Pt seeds and PEG on the higher catalytic activity of prepared Pt catalysts have been preliminarily discussed.     

NiCo2O4 spinel supported N-dopped porous Hollow carbon derived MOF functionalized SiO2 for efficient ORR electrocatalysis

The improvement of carbon-based metal-free electrocatalysts for the cathodic ORR that are environmentally benign, cost-effective, and highly durable is required for large-scale commercial applications of fuel cells. Herein, we developed the TiO₂ nanoparticles supported by a nitrogen-doped carbon matrix with high surface defects derived by the pyrolysis of NH₂-MIL-125 (MOF). The construction of controllable morphology, porosity, and particle size through the pyrolysis process can be obtained by introducing the SiO₂ template during the NH₂-MIL-125 synthesis. Moreover, the synergic covalent coupling between metal oxide spinel (AB₂O₄, especially NiCo₂O₄) and TiO₂/N- doped nanocarbon effectively enhanced the ORR catalytic activity, particularly with high content of nickel atoms in the spinel due to the various valences of metals in spinel which accelerated the electron transfer during ORR. As a highly effective ORR electrocatalyst with an onset potential of −0.14 V vs Ag/AgCl, the as-prepared N₃CO/Es-TiO₂/NC exhibited not only high activity but also good stability in an alkaline environment compared to the Pt/C catalyst. The less current change after continuous 1000 cycles of cyclic voltammetry or the methanol addition may be due to the protective effect of the N-doped carbon carrier and the corrosion resistance of TiO₂.     

Ultrafine iron oxide nanoparticles supported on N-doped carbon black as an oxygen reduction reaction catalyst

An oxygen reduction reaction (ORR) catalyst comprising ultrafine iron oxide nanoparticles supported on N-doped Vulcan carbon (FeO_1.4/N-C) was prepared via a two-step method. X-ray photoelectron spectroscopy revealed the iron oxide nanoparticles comprised Fe₂O₃ and FeO phases with a combined average oxidation state of 2.8. The FeO_1.4/N-C catalyst produced an ORR onset potential of −0.056 V and a half-wave potential of −0.190 V in alkaline media, which was comparable to that of commercial Pt/C catalyst. Moreover, FeO_1.4/N-C had higher methanol tolerance than Pt/C catalyst and thus affords a promising non-precious metal ORR catalyst for fuel cells.     

Zeolotic imidazolate frameworks (ZIFs) derived porous carbon: A review from crystal growth & green synthesis to oxygen reduction reaction activity

Oxygen reduction reaction (ORR) has slow reaction rate that decrease the chemical conversion productivity in proton exchange membrane fuel cells (PEMFCs) which has to be improved. Noble metals such as Pt nanoparticles supported on carbon (Pt/C) was considered the most essential catalyst in ORR despite their limitations including being rare and expensive, CO poisoning etc. In the past few years, nitrogen doped carbons (N–C) or zeolitic imidazole framework (ZIFs) derived (M-N-C) including single or bimetallic metals take attention due to their outstanding properties such as high surface area, excellent electrical conductivity, cost effectiveness, thermal and chemical stability which were used either as catalyst or supports for noble metal nanoparticles to improve the sluggish ORR in PEMFC cathode. This review briefly outlines conventional crystal preparation and activation of porous carbons derived from ZIFs and their green synthesis methods, followed by modern synthesis methods of nanostructured MNP/MOF composites and recently their ORR activity evaluation in PEMFC. Particular attention was given to the porous carbon supports derived from two kind of frameworks such as ZIF-8 and ZIF-67 which are the most frequently reported ORR electrocatalysts and/or supports in the literature.     

Synthesis of nano-tube CoNi/N–C complex and its catalytic properties for highly efficient oxygen reduction

Electrochemical energy storage systems such as fuel cells and metal air cells can be used as clean energy. In these systems, an essential reaction on the cathode is the oxygen reduction reaction (ORR). As ORR catalyst, non noble metal based catalyst is an important substitute for commercial Pt/C catalyst due to its rich reserves, low cost, good stability and high catalytic activity. Herein, CoNi alloy supported on nitrogen doped carbon, CoNi/N–C nanotubes, prepared by hydrothermal method and high-temperature pyrolysis method, shows excellent ORR catalytic activity and stability in 0.1 M KOH solution. In particular, the obtained CoNi/N-C-800 demonstrates the highest ORR activity of the prepared samples, with a half wave potential of 0.81V, which was equivalent to the commercially available Pt/C (0.82V). At the same time, it exhibits approximate 4e^- pathway with a comparable electron transfer number to the commercial Pt/C, and is much higher tolerant in methanol than the latter. Co and Ni alloying can induce the internal electron interaction of the catalyst, thus exposing more active sites. Furthermore, nano-tube CoNi exhibits appropriate size and hollow geometry, and its large surface area and strong conductivity improve its catalytic activity. The results may possibly provide a new impetus to the rational design of non noble metal based nanocomposite catalysts. Moreover, it is also of great significance to improve the performance of electrocatalysis and energy storage applications.     

The stability of Pt–M (M = first row transition metal) alloy catalysts and its effect on the activity in low temperature fuel cells: A literature review and tests on a Pt–Co catalyst

Carbon supported platinum metal alloy catalysts (Pt–M/C) are widely used in low temperature fuel cells. Pt alloyed with first-row transition elements is used as improved cathode material for low temperature fuel cells. A major challenge for the application of Pt–transition metal alloys in phosphoric acid (PAFC) and polymer electrolyte membrane (PEMFC) fuel cells is to improve the stability of these binary catalysts. Dissolution of the non-precious metal in the acid environment can give rise to a decrease of the activity of the catalysts and to a worsening of cell performance. The purpose of this paper is to provide a better insight into the stability of these Pt–M alloy catalysts in the PAFC and PEMFC environments and the effect of the dissolution of the non-precious metal on the electrocatalytic activity of these materials, in the light of the latest advances on this field. Additionally, the durability of a PtCo/C cathode catalyst was evaluated by a short test in a single PEM fuel cell.     

Methanol tolerant core-shell RuFeSe@Pt/C catalyst for oxygen reduction reaction

Pt decorated RuFeSe/C catalyst is prepared by reduction of Pt precursor on pre-formed RuFeSe/C for oxygen reduction reaction (ORR). The catalyst is characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalyst particles are found to disperse on the carbon support with an average particle size of 2.8 nm. Physical characterizations and electrochemical tests confirm that Pt is deposited on the surfaces of RuFeSe particles and RuFeSe@Pt/C catalyst has a core-shell structure. The as-prepared catalyst has high durability and shows high ORR activity through a four-electron transfer process. RuFeSe@Pt/C exhibits 1.3-fold greater specific activity and 1.4-fold greater mass activity for ORR than Pt/C. More importantly, it has excellent tolerance to methanol. Consequently, RuFeSe@Pt/C may be used as fine cathode catalyst in direct methanol fuel cells (DMFCs).     

Facile synthesis of efficient core-shell structured iron-based carbon catalyst for oxygen reduction reaction

Controlled synthesis of efficient core-shell non-precious metal catalysts for oxygen reduction reaction (ORR) is undoubtedly crucial but challenging for the extensive application of fuel cells and metal-air batteries. Herein, we prepared a core-shell structured Fe/FeC_x nanoparticles and porous carbon composited catalyst (Fe/FeC_x@NC) via a facile two-step heat treatment strategy. The Fe/FeC_x@NC-800_?0.5 prepared with secondary anneal at 800 °C for 0.5 h exhibits superior ORR performance to the commercial Pt/C in terms of comparable onset potential, higher half-wave potential, and outstanding long-term durability in alkaline media. Through combining the physical and electrochemical characterizations of Fe/FeC_x@NC-T_?t with different anneal temperature and precursors, the outstanding ORR performance of Fe/FeC_x@NC-800_?0.5 is caused by the synergistic effect between Fe/FeC_x core and enriched pyridinic N- and graphitic N-doped carbon shell as well as porous carbon with large specific surface area. The structure-activity relationship of core-shell structured Fe–N–C catalysts for ORR provides directions for the development of advanced nonprecious metals catalysts.