A Review on Metal‐Free Doped Carbon Materials Used as Oxygen Reduction Catalysts in Solid Electrolyte Proton Exchange Fuel Cells

Within the last decade, metal‐free heteroatom doped carbon nanomaterials have gained attention as effective electrocatalysts for the oxygen reduction reaction (ORR) in many electrochemical systems. Since then, reports have stated that the ORR catalytic activity, onset potential, and H₂O production selectivity of these materials is similar to that of platinum‐based catalysts. These statements rely on cyclic voltammetry (CV) and rotating disc electrode (RDE) measurements in liquid alkaline electrolyte. However, fuel cell researchers aim to replace the costly platinum catalysts in the more prominent acidic solid electrolyte proton exchange fuel cell (PEFC). In this respect, there are only a few reports of unpromising activity, stability, and H₂O production selectivity. In addition, only few reports have been presented on the implementation of such materials in cathode catalyst layers of actual PEFC devices. This mini‐review aims to summarize and evaluate results of these reports. Material synthesis, cell power, open circuit voltage, stability properties, and proposed active sites are reviewed. To date, the highest reported PEFC power densities with guaranteed metal‐free heteroatom doped carbon cathode catalysts have reached up to 321 mW cm⁻²; which although a promising value is substantially short of values obtained for platinum based catalysts.     

Nitrogen‐Doped Carbon Materials Prepared by Ammoxidation as Solid Base Catalysts for Knoevenagel Condensation and Transesterification Reactions

Nitrogen‐doped carbon materials were prepared by ammoxidation of commercial carbon sources (carbon black and activated carbon) and applied as base catalysts for Knoevenagel and transesterification reactions. It was shown that these carbon materials were active and the activities were different depending on the ammoxidation conditions (temperature and ammonia concentration in air) and carbon sources used. The bulk, textural, and surface properties of the nitrogen‐doped carbon materials were examined by several methods to clarify possible factors determining their final catalytic activities. The activated carbon‐derived catalysts were more active than the carbon black‐derived ones. The surface area and porosity were not responsible for this difference between the two carbon sources but the difference in the reactivity with oxygen was important. The reactivity of carbon sources with oxygen should influence the doping of nitrogen onto their surfaces by ammoxidation with ammonia and air and the resulting activities as base catalysts. The catalytic activity increases with the amount of nitrogen doped and, therefore, the nitrogen doped should be responsible for the catalytic activities. In addition, the activities are maximal at a ratio of nitrogen to oxygen of around 1, suggesting the importance of cooperative functions of nitrogen and oxygen on the surface of carbons.     

Low-temperature synthesis of nitrogen/sulfur co-doped three-dimensional graphene frameworks as efficient metal-free electrocatalyst for oxygen reduction reaction

The development of metal-free catalyst for oxygen reduction reaction (ORR) is one of the most challenging tasks in fuel cells. Heteroatom doped graphenes have been recognized as the promising candidate. In this work, we have developed a one-pot hydrothermal approach towards three-dimensional nitrogen and sulfur co-doped graphene frameworks (N/S-GFs) employing graphene oxide and ammonium thiocyanate as the precursors. N/S-GFs manifest excellent catalytic behavior with mainly four electron transfer pathway in ORR in alkaline condition.     

氮化物对柴油加氢脱硫影响的研究进展

分别从催化剂,工艺条件等方面介绍了氮化物对柴油加氢脱硫反应(HDS)影响的机理。介绍了氮化物对柴油HDS反应影响的动力学研究。结果表明:氮化物对柴油HDS反应具有抑制作用;在不同催化剂活性中心上和不同的工艺条件下氮化物对柴油HDS反应的影响存在差异且符合于拟一级反应动力学。氮化物对柴油HDS的影响研究,可以指导高活性催化剂的开发,寻求最优柴油HDS反应工艺。     

Highly active nitrogen-doped few-layer graphene/carbon nanotube composite electrocatalyst for oxygen reduction reaction in alkaline media

In this work the electrocatalysis of oxygen reduction on nitrogen-doped few-layer graphene/multi-walled carbon nanotube (FLG/MWCNT) composite catalyst has been investigated. These composite materials were prepared from different nitrogen precursors, acid-treated MWCNTs and graphene oxide (GO), which was synthesised from graphite by the modified Hummers’ method. Urea and dicyandiamide were used as nitrogen precursors and the doping was achieved by pyrolysing the mixture of GO and MWCNTs in the presence of these nitrogen-containing compounds at 800 °C. The N-doped composite catalyst samples were characterised by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy, the latter method revealed successful nitrogen doping. The oxygen reduction reaction (ORR) was studied in 0.1 M KOH on glassy carbon electrodes modified with N-doped FLG/MWCNT electrocatalysts employing the rotating disk electrode (RDE) method. The RDE results indicated that these metal-free nitrogen-doped nanocarbon catalysts possess remarkable electrocatalytic activity towards the ORR in alkaline media similar to that of commercial Pt/C catalyst. The results obtained in this work are particularly important for the development of non-Pt cathode catalysts for alkaline membrane fuel cells.     

A Comparative Study of Organic Cobalt Complex Catalysts for Oxygen Reduction in Polymer Electrolyte Fuel Cells

Reduction of platinum catalysts loading is a central issue in polymer electrolyte fuel cells. As alternatives for platinum, some organic metal chelate compounds are tested as cathode catalysts, such as cobalt aza-complexes or cobalt complexes possessing aminophenyl moieties featured as Co-N₄ or Co-N₂O₂ chelate structures. The way of immobilization of catalysts on the graphite surface influences their stability as well as the performance of oxygen reduction. Heat-treated catalysts supported on graphite at 600°C show much better oxygen reduction abilities than as-received metal complexes. The original chemical structure of metal complexes affects crucially the catalytic ability, though initial structures of molecules are no more intact after the heat treatment. The catalytic activity of these complexes may originate from the central chelate unit CoN₄ on the carbon substrate, and this unit is assumed to constitute the basic coordination site for an oxygen molecule. Electropolymerized catalysts impart a high level of oxygen reduction ability, probably due to the improved molecular orientation for oxygen coordination and formation of good chelate sites on the graphite surface.     

Fe/N/C catalysts systhesized using graphene aerogel for electrocatalytic oxygen reduction reaction in an acidic condition

Graphene aerogel was modified with polyaniline and Fe precursors to produce Fe/N/C catalysts for electrocatalytic oxygen reduction reaction in the acidic condition. The graphene aerogel was produced by a simple hydrothermal treatment of graphene oxide dispersion with a high surface area. Aniline was polymerized with the graphene aerogel powder, and the pyrolysis of the resulting material with FeCl₃ produced Fe/N/C catalyst. The loading amount on the electrode and the catalyst ink concentration was carefully selected to avoid the mass transfer limitation inside the catalyst layer. The pyrolysis temperature affected the states of nitrogen sites on the catalyst; the sample prepared at 900 °C presented the highest mass activity. The sulfur was also doped with various amounts of FeSO₄ with enhanced mass activity of up to 2.1 mA/mg at 0.8 V in 0.5 M H₂SO₄ solution. Its durability was also tested by repeating cyclic voltammetry in a range of 0.6–1.1 V 5000 cycles. This graphene-aerogel-based carbon catalysts showed improved activity and durability for the oxygen reduction reaction in the acidic condition.     

Multifunctional graphene‐based nanostructures for efficient electrocatalytic reduction of oxygen

Graphene derivatives have been used extensively as a functional support for nanoparticle catalysts in diverse applications, in particular, oxygen reduction reactions (ORR) at fuel cell cathodes. This review summarizes recent progress in this area of research, where the catalytic performance is evaluated within the context of stabilization of metal nanoparticles against sintering/aggregation and metal–substrate interactions that manipulate the electronic properties of metal nanoparticles and hence the bonding interactions with reaction intermediates. Also discussed are the latest breakthroughs of heteroatom‐doped graphene derivatives as effective metal‐free catalysts for oxygen reduction. In addition, the review includes a perspective on the development of effective ORR catalysts with a focus on a further understanding of the ORR mechanism as well as on other two‐dimensional layered nanostructures such as MoS₂ that have been observed to exhibit electrocatalytic activity for oxygen reduction. Leading mechanistic models are discussed to account for the electrocatalytic activity. © 2015 Society of Chemical Industry     

硫氮化合物在磷改性NiW/Al2O3加氢催化剂上的吸附行为研究

采用等体积浸渍法制备了一系列以γ-Al₂O₃及磷改性γ-Al₂O₃为载体,Ni、W为活性金属组分的加氢催化剂,以N₂物理吸附-脱附、XRD、NH₃-TPD、Py-IR等技术对Al₂O₃及P/Al₂O₃系列催化剂进行了表征,考察了磷改性对加氢催化剂理化性质的影响,探究了喹啉、吲哚和二苯并噻吩（DBT）吸附行为与催化剂理化性质以及吸附质本身性质的关系。研究发现,喹啉最易于吸附在Al₂O₃及P/Al₂O₃系列催化剂上,吲哚和DBT的吸附能力较为接近;磷的引入会降低催化剂的比表面积和孔体积,但是能够提高喹啉、吲哚及DBT的吸附能力;硫氮化合物在催化剂上的吸附能力随着催化剂表面酸性的增强或酸中心数量的增多、活性金属分散度的增大以及硫氮化合物杂原子电子云密度或分子极性的增大而增大。     

A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction

This paper reviews over 100 articles related to heat-treated Fe- and Co-N/C catalysts for the oxygen reduction reaction. The literature shows that through several decades’ effort in the development of non-noble catalysts such as heat-treated Fe- and Co-N/C catalysts, tremendous progress has been made in catalyst synthesis methodologies and the understanding of the mechanism. A heat-treatment step has been identified as necessary for catalyst activity and stability improvement. The enhanced performance of the catalysts is strongly dependent on the carbon support, the source of metal and nitrogen, and the thermal treatment conditions. The metal content in these catalysts also plays an important role in their activity and stability. A saturated metal content has been identified as a major limiting factor for further improvement of catalyst activity. The nitrogen content and the presence of a disordered or heterogeneous phase on the carbon-support surface seem to be the main requirements for an effective catalyst. The mechanisms by which activity and stability are enhanced after the heat treatment of these Fe- and Co-N/C catalysts are not fully understood yet. It is necessary to answer the question of whether or not the metal is part of the active catalytic site, as well as to identify the nature of the catalytic site. A more fundamental understanding will be of great help in designing alternative and innovative routes for catalyst synthesis. In general, the catalytic activity and stability of Fe- and Co-N/C catalysts are still below those of a Pt-based catalyst. However, under the strong driving force of fuel cell commercialization, Pt-free cathode catalysts with methanol tolerance, such as Fe- and Co-N/C, are attractive candidates for solving the problem of the cost of fuel cell catalysts.     

非铂燃料电池电催化剂研究进展

商业铂碳催化剂价格高昂,开发非铂材料是推进燃料电池商业化的关键一步。本文首先介绍了燃料电池氧还原反应电催化剂的研究背景,接着分别介绍了非贵金属、非金属以及复合材料的催化剂,并对各类催化剂的活性位点和催化机理进行了简要的评述。其中,过渡金属的氮碳化物成本低廉,具有较高的催化活性以及优异的稳定性,是最有望替代贵金属Pt的一类催化剂。杂原子的掺杂能够改变碳材料的表面电荷分布,提升碳材料的催化活性。将过渡金属的氮碳化物和特殊结构的碳材料有效结合,可以设计出具有双功能的复合材料。最后,针对非铂催化剂存在的问题进行了分析并提出了今后工作的几个方向,为今后非铂电催化剂的研究提供参考。高活性高稳定性的非铂催化剂是未来该领域的重点研究方向。     

Highly Durable Platinum based Cathode Electrocatalysts for PEMFC Application using Oxygen and Nitrogen Functional Groups Attached Nanocarbon Supports

The combined effect of oxygen and nitrogen functional groups on highly crystalline carbon supports like multiwalled carbon nanotubes (MWCNT) and MWCNT‐few layer graphene hybrid structures (MWCNT+FLG) have been investigated towards oxygen reduction reaction (ORR) performance and carbon corrosion durability in polymer electrolyte membrane fuel cell (PEMFC) applications. The pristine carbon supports were modified with oxygen and nitrogen functionalities by treating with concentrated mineral acids and subsequent nitrogen plasma treatment assisted with R.F. magnetron sputtering. Pt nanoparticles were dispersed over these chemically modified carbon supports by polyol reduction method. The physicochemical properties of as synthesized electrocatalysts were studied by different techniques such as XRD, TEM, FTIR, Raman and XPS. Electrochemical properties were investigated by cyclic voltammetry and linear sweep voltammetry in 0.1M HClO₄ medium. Compared to commercial Pt/C catalysts, durability show ∼30 % enhancement for the as prepared electrocatalysts due to the presence of large amount of pyrrolic nitrogen and highly oriented graphitic nature of the catalyst supports. The ORR performance were comparable with Pt/C (TEC10E30E) in terms of MSA, 259, 270, 252 A g⁻¹ for Pt/C, Pt/N‐f‐MWCNT, Pt/N‐f‐(MWCNT+FLG) respectively.     

Cable‐Shaped Lithium–Sulfur Batteries Based on Nitrogen‐Doped Carbon/Carbon Nanotube Composite Yarns

The research on flexible and wearable devices has attracted extensive attention in the last few years. Lithium–sulfur (Li‐S) batteries are regarded as a promising option because of their high theoretical capacity and energy density. Here, cable‐shaped Li‐S batteries are developed based on a nitrogen‐doped carbon/carbon nanotube/sulfur (NCNT/S) composite cathode and lithium metal anode. The carbon nanotube (CNT) yarns with high conductivity and an appropriate amount of doped nitrogen are synthesized by wet‐spinning followed by a carbonization process, and further act as a self‐supported conductive backbone for the active material. The NCNT/S yarns exhibit a high initial capacitance of 1001 mAh g^?1 and excellent cyclic stability with 87% capacity retention after 200 cycles at 0.5 C. Furthermore, the assembled cable‐shaped Li‐S batteries by NCNT/S yarns present good ability to light up the LEDs for more than 8 h under normal and bending states at various angles, indicating that the cable‐shaped Li‐S batteries could be a prospective candidate for application in wearable electronics.     

The use of nitrogen-doped graphene supporting Pt nanoparticles as a catalyst for methanol electrocatalytic oxidation

Nitrogen-doped graphene (N-G) was prepared by thermal annealing of graphene oxide in ammonia at different temperatures. The resultant N-G was used as a conductive support for Pt nanoparticles (Pt/N-G) and the electrocatalytic activity of the Pt/N-G catalysts towards methanol oxidation was examined. To investigate the microstructure and morphology of the synthesized catalysts, X-ray diffraction, scanning and transmission electron microscopy and X-ray photoelectron spectroscopy were used. The catalytic activity of the catalysts towards the oxidation of methanol was evaluated by cyclic voltammetry. Compared to a control catalyst of Pt loaded on undoped graphene, the Pt/N-G materials show higher electrochemical activity towards methanol oxidation. The excellent electrochemical performance of Pt/N-G is mainly attributed to the nitrogen doping and the uniform distribution of Pt particles on the doped graphene support. These results indicate that N-doped graphene has great potential as a high-performance catalyst support for fuel cell electrocatalysis.     

Confined space reduced graphite oxide doped with sulfur as metal-free oxygen reduction catalyst

Reduced GO in confined space of silica gel nanopores doped with sulfur shows high catalytic activity for oxygen reduction reaction (ORR) in alkaline medium and exhibits a superior tolerance to the presence of methanol. Even though the partially reduced GO with hydroquinone structures is a good catalysts for ORR, it shows instability in KOH. The good performance of S-doped material is linked to the coexistence of sulfur and oxygen on the surface in equal atomic quantities and a unique porosity being the replica of the silica pores. The former leads to the positive charge on the carbon atoms, which are the reaction sites. Hydrophobicity of the surface and small pores enhance adsorption of O₂.     

Fe-based electrocatalysts made with microporous pristine carbon black supports for the reduction of oxygen in PEM fuel cells

Fe-based catalysts for the oxygen reduction reaction at the cathode of polymer electrolyte membrane (PEM) fuel cells have been prepared using several highly microporous (defined as pores having a size <2 nm) carbon supports. The aim is to produce better performing catalysts as it is known that catalytic sites are hosted in the micropores of the carbon supports. All catalysts were loaded with a nominal Fe content of 0.2 wt% and were obtained by heat-treatment at 950 °C in pure NH₃ atmosphere. It is demonstrated, however, that the use of highly microporous carbon supports does not lead to improved catalytic activity, as originally expected, since the surface of these micropores is devoid of the nitrogen functionalities necessary to build the catalytic sites. Also, it is shown that for these microporous carbon supports, it is only the new micropores, i.e. those created during NH₃ etching at high temperature, that are capable of hosting catalytic sites.     

石墨烯用于烷烃氧化脱氢体系中的研究进展

简述了石墨烯的结构和性质,对石墨烯的制备方法进行总结,重点论述其用于烷烃氧化脱氢体系中的研究进展。氧化石墨烯经过还原形成石墨烯,大部分羟基和环氧官能团可能被除去,但仍存在一些含氧官能团以及一定的缺陷位,边缘或缺陷处的羰基和醚基团都可以作为氧化脱氢的活性位,石墨烯复合非金属催化剂对烷烃氧化脱氢体系表现出较好的烯烃选择性。指出石墨烯复合材料在氧气气氛中不稳定性,需要探索出更好的方法来提高稳定性和寿命。     

Carbon as catalyst and support for electrochemical energy conversion

Carbon has unique characteristics that make it an ideal material for use in a wide variety of electrochemical applications ranging from metal refining to electrocatalysis and fuel cells. In polymer electrolyte fuel cells (PEFCs), carbon is used as a gas diffusion layer, electrocatalyst support and oxygen reduction reaction (ORR) electrocatalyst. When used as electrocatalyst support, amorphous carbonaceous materials suffer from enhanced oxidation rates at high potentials over time. This drawback has prompted an extensive effort to improve the properties of amorphous carbon and to identify alternate carbon-based materials to replace carbon blacks. Alternate support materials are classified in carbon nanotubes and fibers, mesoporous carbon, multi-layer graphene (undoped and doped with metal nanoparticles) and reduced graphene oxide. A comparative review of all these supports is provided. Work on catalytically active carbon hybrids is focused on the development of non-precious metal electrocatalysts that will significantly reduce the cost without sacrificing catalytic activity. Of the newer electrocatalysts, nitrogen/metal-functionalized carbons and composites are emerging as possible contenders for commercial PEFCs. Nitrogen-doped carbon hybrids with transition metals and their polymer composites exhibit high ORR activity and selectivity and these catalytic properties are presented in detail in this review.     

A theory of ultradeep hydrodesulfurization of diesel in stacked‐bed reactors

Hydrodesulfurization catalysts have two types of active sites for hydrogenation and hydrogenolysis reactions. While hydrogenation sites are more active for desulfurizing refractory sulfur species, they are more susceptible to organonitrogen inhibition than hydrogenolysis sites. In contrast, hydrogenolysis sites are more resistant to organonitrogen inhibition but are less active for desulfurizing refractory sulfur species. This dichotomy is exploited to develop an ultradeep hydrodesulfurization stacked‐bed reactor comprising two catalysts of different characteristics. The performance of such a catalyst system can be superior or inferior to that of either catalyst alone. A mathematical model is constructed to predict the optimum stacking configuration for maximum synergies between the two catalysts. The best configuration provides the precise environment for the catalysts to reach their full potentials, resulting in the smallest reactor and minimum hydrogen consumption. Model predictions are consistent with experimental results. A selectivity‐activity diagram is developed for guiding the development of stacked‐bed catalyst systems. © 2017 American Institute of Chemical Engineers AIChE J, 64: 595–605, 2018     

Physical and electrochemical characterization of catalysts for oxygen reduction in fuel cells

The cathode catalysts in low temperature fuel cells are associated with major cell efficiency losses, because of kinetic limitations of the oxygen reduction reaction. Additionally, methanol oxidation at the cathode leads to significant lowering of the efficiency in direct methanol fuel cells, which can be alleviated by use of methanol-tolerant catalysts. In this work, alternative carbon-supported platinum-alloy catalysts were investigated by physical methods. Second, methanol-tolerant ruthenium-selenide catalysts were characterized by physical and electrochemical methods. Besides V–i characteristics and electrochemical impedance spectroscopy as electrochemical methods, physical methods such as X-ray photoelectron spectroscopy, nitrogen adsorption, porosimetry by mercury intrusion and temperature programmed reduction are used to characterize the catalysts. The electrochemical characterization yields information about properties and behavior of the catalyst. In contrast to platinum a significantly different hydrophobic behavior of the RuSe/C catalysts is found. Low open circuit voltage values measured for RuSe/C indicate an effect on both electrodes. The anode reaction was also influenced by the different cathode catalysts. As a result of the formation of H₂O₂ at the cathode, which passes through the membrane from cathode to anode side, a mixed anode potential is formed. By comparing RuSe/C catalysts before and after electrochemical stressing, changes of the catalysts are determined. Postmortem surface analysis (by X-ray photoelectron spectroscopy) revealed that catalyst composition and MEA structure changed during electrochemical stressing. During fuel cell operation selenium oxide is removed from the surface of the catalysts to a large extent. Additionally, a segregation effect of selenium in RuSe to the surface is identified.