Fe-based catalysts for oxygen reduction in proton exchange membrane fuel cells with cyanamide as nitrogen precursor and/or pore-filler

Fe/N/C catalysts for oxygen reduction reaction were synthesized via impregnation or ballmilling. The role of cyanamide (CM) as nitrogen precursor and/or pore-filler for a highly microporous carbon (Black Pearls 2000) was investigated. The use of CM in this work resulted in two main differences compared with phenanthroline from our previous work; (i) ballmilling the precursors did not result in improved activity of the resulting catalysts, and (ii) the activity after the first pyrolysis in argon was relatively high, but did not increase after a second pyrolysis in NH₃. These differences may be explained by TGA measurements of both pore-fillers, where complete gasification of CM is observed at temperatures above 750 °C in Ar, while pyrolysis of phenanthroline in Ar results in 20 wt% residual carbon-based material. Consequently, when using CM as pore-filler with a highly microporous carbon support, the maximum microporous surface area and nitrogen content is reached after only a single pyrolysis in Ar. The most active catalyst prepared with CM was obtained by pyrolysing in Ar at 950 °C a catalyst precursor containing 1 wt% Fe, 80 wt% CM and Black Pearls 2000. This catalyst possessed about 1/6th the catalytic activity of best reported using phenanthroline as a pore-filler. Changing the carbon support had effects on the activity and stability of the catalysts. The catalysts made with a non-porous furnace black (N330) or carbon nanotubes as a carbon support were more stable but less performing than those using carbon supports having high microporous surface area like Black Pearls 2000 or Ketjenblack. The desirable properties for a pore-filler molecule used in the synthesis of Fe/N/C-catalysts by the pore-filling method are discussed.     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

聚吡咯在质子交换膜燃料电池中的应用

聚吡咯（polypyrrole,PPy）具有长链状共轭结构及多孔的载体形貌,且显示出高电导率、良好稳定性和无毒等优点,但PPy结构疏松且热稳定性和导电性不如碳材料。本文简述了PPy修饰载体后能为催化反应提供高效的电子和质子传导网络,并能通过改善载体表面形态更好地分散Pt,提高Pt的利用率。此外,本文还概述了聚吡咯类过渡金属复合催化剂在质子交换膜燃料电池（PEMFC）中表现出良好的氧还原反应（ORR）性能,且可通过优化合成条件、改变各成分的质量比、热处理或掺杂等方法提高此类非铂催化剂的性能。最后提出可利用M-PPy-C和Pt的协同效应,制备高活性和耐久性良好的Pt/M-PPy-C催化剂。     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.     

质子交换膜燃料电池催化剂性能增强方法研究进展

发展低成本、高性能的燃料电池催化剂是实现燃料电池商业化的关键。目前,铂基催化剂仍是动力燃料电池不可替代的主催化剂。本文综述了最近几年燃料电池催化剂增强方面的研究进展,探讨了新型催化剂材料的设计与制备以及提高催化剂活性或稳定性的方法,包括表面修饰、包覆、合金化、几何与电子结构以及晶体结构的调变、催化剂/载体相互作用等手段。开发高活性和高稳定性的非铂类催化剂是燃料电池催化剂的发展趋势和努力方向。其中,提高非铂燃料电池催化剂可靠性、稳定性和活性,迫切需要在燃料电催化理论、非铂催化剂理性设计、燃料电池水热管理、有序化膜电极等方面取得创新和突破。     

Enhanced performance of CO poisoned proton exchange membrane fuel cells via triode operation

The effect of triode operation on the performance of CO poisoned PEM fuel cells was investigated. In this mode of operation a third, auxiliary, electrode is introduced in addition to the anode and the cathode. Application of electrolytic current in the auxiliary circuit, comprising the cathode and the auxiliary electrode was found to significantly enhance the time-averaged power output of a state-of-the-art PEMFC unit operating with a 70 ppm CO in H₂ atmospheric pressure mixture. Both normal and triode operation were found to lead to self-sustained current and potential oscillations in the fuel cell circuit over wide ranges of external resistive load. The time averaged increase in power output was found to be typically a factor of three higher than the power output in conventional fuel cell operation and up to a factor of 1.32 larger than the power sacrificed in the auxiliary circuit. The mechanism of the enhanced anodic electrocatalysis was investigated via the use of two reference electrodes and the results are discussed together with a possible design for application of the triode concept in stacks.     

质子交换膜燃料电池非铂电催化剂研究进展

质子交换膜燃料电池(PEMFCs)目前主要催化剂为贵金属Pt基催化剂。然而,Pt价格高、储量低等问题严重阻碍了PEMFCs的商业化进程。发展低成本、高性能的氧还原催化剂是解决铂资源短缺、降低燃料电池成本、实现燃料电池商业化的关键。结合本课题组的研究工作,综述了最近几年非铂催化剂在燃料电池阴极氧还原方面的研究进展,着重探讨了新型氮掺杂碳基纳米材料的设计与制备,并概述了非铂催化剂面临的困难以及未来发展方向。     

Nanostructured Pt-M/C (M = Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel cells

Nanostructured Pt-M/C (M = Fe and Co) catalysts have been synthesized by a microemulsion method and a high-temperature route. They have been characterized by cyclic voltammetry in 1 M H₂SO₄ and for oxygen reduction in proton exchange membrane fuel cells (PEMFC). The Pt-M alloy catalysts synthesized by the microemulsion method show higher electrochemical active surface area than those prepared by the high-temperature route, and some of them exhibit improved catalytic activity towards oxygen reduction compared to pure Pt. Among the various alloy catalysts investigated, the Pt-Co/C catalyst prepared by the microemulsion method shows the best performance with the maximum catalytic activity and minimum polarization loss. Mild heat treatment of the catalysts prepared by the microemulsion method at moderate temperatures (200 °C) in reducing atmosphere is found to improve the catalytic activity due to a cleaning of the surface and an increase in the electrochemical surface area.     

Development of gas diffusion layer using water based carbon slurry for proton exchange membrane fuel cells

The micro-porous layer of gas diffusion layers (GDLs) was fabricated with the carbon slurry dispersed in water containing sodium dodecyl sulfate (SDS), by wire rod coating process. The aqueous carbon slurry with micelle-encapsulation was highly consistent and stable without losing any homogeneity even after adding polytetrafluoroethylene (PTFE) binder for hundreds of hours. The surface morphology, contact angle and pore size distribution of the GDLs were examined using SEM, Goniometer and Hg Porosimeter, respectively. GDLs fabricated with various SDS concentrations were assembled into MEAs and evaluated in a single cell PEMFC under diverse operating relative humidity (RH) conditions using H₂/O₂ and H₂/air as reactants. The peak power density of the single cell using the GDLs with optimum SDS concentration was 1400 and 500 mW cm⁻² with H₂/O₂ and H₂/air at 90% RH, respectively. GDLs were also fabricated with isopropyl alcohol (IPA) based carbon slurry for fuel cell performance comparison. It was found that the composition of the carbon slurry, specifically SDS concentration played a critical role in controlling the pore diameter as well as the corresponding pore volumes of the GDLs.     

Boosting the electrocatalytic performance of Fe-based perovskite cathode electrocatalyst for solid oxide fuel cells

The development of cathode materials with excellent electrocatalytic activity and CO₂ tolerance is an important direction for the wide application of solid oxide fuel cells. Herein, the cobalt-free perovskite oxides Bi_0.5Sr_0.5Fe_1-xV_xO_3-δ (BSFVx, x = 0.025, 0.05 and 0.075) are developed as the efficient cathode electrocatalysts for SOFCs. The V-doping strategy is beneficial to improve the thermal stability, CO₂ tolerance and electrochemical performance of undoped Bi_0.5Sr_0.5FeO_3-δ. Among all samples, Bi_0.5Si_0.5Fe_0.95V_0.05O_3-δ (BSFV0.05) cathode presents excellent oxygen reduction reaction activity, achieving a lower polarization resistance of 0.076 Ω cm² and the peak power density of the single cell with the BSFV0.05 cathode reaches to 1.16 W cm⁻² at 700 °C, which can be comparable to those of the representative cobalt-based cathodes. Furthermore, the improved CO₂ tolerance of the BSFV0.05 cathode can be ascribed to the high acidity of the V⁵⁺ and the larger average bonding energy in the oxide.     

Numerical optimization of proton exchange membrane fuel cell cathodes

A fuel cell gradient-based optimization framework based on adaptive mesh refinement and analytical sensitivities is presented. The proposed approach allows for efficient and reliable multivariable optimization of fuel cell designs. A two-dimensional single-phase cathode electrode model that accounts for voltage losses across the electrolyte and solid phases and water and oxygen concentrations is implemented using an adaptive finite element formulation. Using this model, a multivariable optimization problem is formulated in order to maximize the current density at a given electrode voltage with respect to electrode composition parameters, and the optimization problem is solved using a gradient-based optimization algorithm. In order to solve the optimization problem effectively using gradient-based optimization algorithms, the analytical sensitivity equations of the model with respect to the design variables are obtained. This approach reduces the necessary computational time to obtain the gradients and improves significantly their accuracy when compared to gradients obtained using numerical sensitivities. Optimization results show a substantial increase in the fuel cell performance achieved by increasing platinum loading and reaching a Nafion mass fraction around 20-30 wt.% in the catalyst layer.     

The effect of nitrogen oxides in air on the performance of proton exchange membrane fuel cell

The effects of NO_x on the performance of proton exchange membrane (PEM) fuel cell were investigated through the introduction of a mixture containing NO and NO₂, in a ratio of 9:1, into the cathode stream of a single PEM fuel cell. The NO_x concentrations used in the experiments were 1480 ppm, 140 ppm and 10 ppm, which cover a range of three orders. The experimental results obtained from the tests of durability, polarization, reversibility and electrochemical impedance spectroscopy (EIS) showed a detrimental effect of NO_x on the cell performance. The electrochemical measurements results suggested that the impacts of NO_x are mainly resulted from the superposition of the oxygen reduction reaction (ORR), NO and HNO₂ oxidation reactions, and the increased cathodic impedance. Complete recovery of the cell performance was reached after operating the cell with clean air and then purging with N₂ for hours.     

Synthesis and characterization of Pd-Ni nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Carbon-supported Pd-Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol method, followed by heat treatment in a reducing atmosphere at 500-900 °C. The samples have been characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), rotating disk electrode (RDE) measurements, and single-cell proton exchange membrane fuel cell (PEMFC) tests for oxygen reduction reaction (ORR). XRD and TEM data reveal an increase in the degree of alloying and particle size with increasing heat-treatment temperature. XPS data indicate surface segregation with Pd enrichment on the surface of Pd₈₀Ni₂₀ after heat treatment at ≥500 °C, suggesting possible lattice strains in the outermost layers. Electrochemical data based on CV, RDE, and single-cell PEMFC measurement show that Pd₈₀Ni₂₀ heated at 500 °C has the highest mass catalytic activity for ORR among the Pd-Ni samples investigated, with stability and catalytic activity significantly higher than that found with Pd. With a lower cost, the Pd-Ni catalysts exhibit higher tolerance to methanol than Pt, offering an added advantage in direct methanol fuel cells (DMFC).     

Study on the growth of platinum nanowires as cathode catalysts in proton exchange membrane fuel cells

The platinum nanowires have been verified to be a promising catalyst to promote the performance of proton exchange membrane fuel cells.In this paper,accurately controlled growth of nanowires in a carbon matrix is achieved for reducing Pt loading.The effects of formic acid concentration and reaction temperature on the morphology and size of the Pt nanowires,as well as their electrochemical performances in a single cell,are investigated.The results showed that the increase in the formic acid concentration results in a volcano trend with the length of Pt nanowires.With increasing reduction temperature,the diameter of Pt nanowires increases while Pt particles evolve from one-dimensional to zero-dimensional up to 40°C.A mechanism of the Pt nanowires growth is proposed.The optimized Pt nanowires electrode exhibits a power density(based on electrochemical active surface area)79%higher than conventional Pt/C one.The control strategy obtained contributes to the design and control of novel nanostructures in nano-synthesis and catalyst applications.     

质子交换膜燃料电池研究进展

质子交换膜燃料电池（proton exchange membrane fuel cell, PEMFC）因具有效率高、功率密度大、排放产物仅为水、低温启动性好等多方面优点,被公认为下一代车用动力的发展方向之一。然而,目前PEMFC在耐久性和成本方面距离商业化的要求还存在一定差距。为攻克上述两大难题,需要燃料电池全产业链的共同努力和进步。本文回顾了近年来质子交换膜燃料电池从催化剂、膜电极组件、电堆到燃料电池发动机全产业链的研究进展和成果,梳理出单原子催化剂、非贵金属催化剂、特殊形貌催化剂、有序化催化层、高温质子交换膜、膜电极层间界面优化、一体化双极板-扩散层、氢气系统循环等研究热点。文章指出,催化层低铂/非铂化、质子交换膜超薄化、膜电极组件梯度化/有序化、燃料电池运行高温化、自增湿化是未来的发展趋势,迫切需要进一步的创新与突破。     

质子交换膜燃料电池系统低温启动技术研究进展

质子交换膜燃料电池电动汽车具有绿色环保、续航里程长等优点,但在温度较低的环境下存在启动困难甚至失败的问题,这一问题严重制约了质子交换膜燃料电池电动汽车的发展。研究调查了质子交换膜内部结冰的原理,简述了0℃以下低温环境下启动过程对质子交换膜本身、催化层、气体扩散层以及膜电极整体带来不同程度的损伤,重点分析了质子交换膜燃料电池电动汽车低温启动的策略,可大致分为三类：停机吹扫的控制策略、外部辅助加热和无辅助加热。分析表明每种方法都有其各自的优点与缺点,但总的来说单一的启动方法对质子交换膜燃料电池电动汽车低温启动的效果不如多种方法混合使用的效果理想,未来燃料电池电动汽车的低温启动技术将会朝着多种方法共同协助的趋势发展。     

Calculation of reversible electrode heats in the proton exchange membrane fuel cell from calorimetric measurements

A calorimeter was used to measure the heat production in polymer electrolyte membrane (PEM) fuel cells operated on hydrogen and oxygen at 50 °C and 1 bar. Two cells were examined, one using a 35 μm thick Nafion membrane and a catalyst loading of 0.6/0.4 mg Pt cm⁻², for the cathode and anode layer, respectively, the other using a 180 μm thick Nafion membrane and loading of 0.4/0.4 mg Pt cm⁻². The cells investigated thus had different membranes and catalyst layers, but identical porous transport layers and micro-porous layers. The calorimeter is unique in that it provides the heat fluxes out of the cell, separately for the anode and the cathode sides. The corresponding cell potential differences, ohmic cell resistance and current densities are also reported. The heat fluxes through the current collector plates were decomposed by a thermal model to give the contributions from the ohmic and the Tafel heats to the total heat fluxes. Thus, the contributions from the reversible heat (the Peltier heats) to the current collectors were determined. The analysis suggests that the Peltier heat of the anode of these fuel cell materials is small, and that it is the cathode reaction which generates the main fraction of the total heat in a PEM fuel cell. The entropy change of the anode reaction appears to be close to zero, while the corresponding value for the cathode is near −80 J K⁻¹ mol⁻¹.     

微型质子交换膜燃料电池设计与性能分析

This study describes a novel micro proton exchange membrane fuel cell (PEMFC) (active area, 2.5 cm2). The flow field plate is manufactured by applying micro-electromechanical systems (MEMS) technology to silicon substrates to etch flow channels without a gold-coating. Therefore, this investigation used MEMS technology for fabrication of a flow field plate and presents a novel fabrication procedure. Various operating parameters, such as fuel temperature and fuel stoichiometric flow rate, are tested to optimize micro PEMFC performance. A single micro PEMFC using MEMS technology reveals the ideal performance of the proposed fuel cell. The optimal power density approaches 232.75 mW•cm－1 when the fuel cell is operated at ambient condition with humidified, heated fuel.     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.