Nb-TiO2 supported Pt cathode catalyst for polymer electrolyte membrane fuel cells

The Nb-doped TiO₂ nanostructure (Nb-TiO₂) was prepared as a support of metal catalyst in polymer electrolyte membrane fuel cells. Using the Nb-TiO₂ nanostructure support, we prepared the Nb-TiO₂ supported catalyst. The Nb-TiO₂ supported Pt catalyst (Pt/Nb-TiO₂) showed the well dispersion of Pt catalysts (∼3 nm) on the Nb-TiO₂ nanostructure supports (∼10 nm). The Pt/Nb-TiO₂ showed an excellent catalytic activity for oxygen reduction compared with carbon supported Pt cathode catalyst. The enhanced catalytic activity of Pt/Nb-TiO₂ in electrochemical half cell measurement may be mainly due to well dispersion of Pt nanoparticles on Nb-TiO₂ nanosized supports. In addition, from XANES spectra of Pt L edge obtained with the supported catalysts, the improved catalytic activity of Pt/Nb-TiO₂ for oxygen reduction may be caused by an interaction between oxide support and metal catalyst.     

Nanostructured ultrathin catalyst layer with ordered platinum nanotube arrays for polymer electrolyte membrane fuel cells

Fabrication of novel electrode architectures with nanostructured ultrathin catalyst layers is an effective strategy to improve catalyst utilization and enhance mass transport for polymer electrolyte membrane fuel cells (PEMFCs).Herein,we report the design and construction of a nanostructured ultrathin catalyst layer with ordered Pt nanotube arrays,which were obtained by a hard-template strategy based on ZnO,via hydrothermal synthesis and magnetron sputtering for PEMFC application.Because of the crystallographically preferential growth of Pt (111) facets,which was attributed to the structural effects of ZnO nanoarrays on the Pt nanotubes,the catalyst layers exhibit obviously higher electrochemical activity with remarkable enhancement of specific activity and mass transport compared with the state-of-the-art randomly distributed Pt/C catalyst layer.The PEMFC fabricated with the as-prepared catalyst layer composed of optimized Pt nanotubes with an average diameter of 90(±10) nm shows excellent performance with a peak power density of 6.0W/mg~(Pt) at 1 A/cm~2,which is 11.6%greater than that of the conventional Pt/C electrode.     

介孔结构Fe/N/C作为质子交换膜燃料电池氧还原催化剂

燃料电池具有高效、低排放等优势,非常有希望作为未来电动汽车的能源转化装置.目前,燃料电池的商业化受制于昂贵的铂基催化剂,特别是动力学迟缓的阴极氧还原反应(ORR)铂催化剂. Fe/N/C被认为是最有潜力的ORR非贵金属催化剂,但其活性仍远低于Pt催化剂,必须依靠增加载量来弥补其与Pt催化剂的活性差距.然而,较厚的催化层(~100mm)会降低阴极传质速率.因此,改善Fe/N/C阴极的传质是提高电池性能的重要途径.
　　本文选择高N含量的2-氨基苯并咪唑(ABI)为氮源,通过水热聚合包覆在碳黑表面,然后掺入FeCl3,经高温热解/酸洗制备了Fe/N/C-ABI催化剂,并与基于间苯二胺的微孔型Fe/N/C催化剂(Fe/N/C-PmPDA)进行比较. Ar等温吸附-脱附结果表明, Fe/N/C-ABI催化剂具有较高的比表面积(662 m2/g)和丰富的双级孔结构(微孔和介孔);透射电镜表征显示Fe/N/C-ABI催化剂具有中空结构,介孔孔径大约为10–25 nm.而Fe/N/C-PmPDA催化剂具有相当的比表面积(656 m2/g),但以微孔为主,基本不含介孔.旋转环圆盘电极(RRDE)测试表明,在0.1 mol/L H2SO4溶液中, Fe/N/C-ABI催化剂的起始还原电位为0.92 V,在0.8 V电位下质量电流密度可达9.21 A/g;而Fe/N/C-PmPDA催化剂具有相近的起始电位,但具有更高的催化活性,质量电流密度为13.4 A/g.氢氧燃料电池(PEMFC)系统测试结果表明, Fe/N/C-ABI催化剂在1个背压和80oC测试条件下的最大功率密度达710 mW/cm2,高于Fe/N/C-PmPDA催化剂(616 mW/cm2).燃料电池与RRDE测试活性顺序的差异归结于Fe/N/C-ABI的中空球状结构. PEMFC工作时阴极会产生大量的水,很容易堵塞氧气传输通道. Fe/N/C-ABI的介孔结构可以作为水的产生和排除的缓存空间,也有利于提高O2传质,从而提高燃料电池性能.本文为具有高传质速率的Fe/N/C催化剂研制提供了一种新思路.     

Nanocomposite TiO2-f-MWCNTs as durable support for Pt in polymer electrolyte fuel cells

Journal of Solid State Electrochemistry - Durability is a major issue and has been the growing focus of research for the successful commercialization of polymer electrolyte fuel cells (PEFCs)....     

A cathode for solid polymer electrolyte fuel cells: Designing the optimal structure of the active layer

The complete computer simulation of the cathodic active layer with solid polymer electrolyte (Nafion) is carried out. The active layer structure can be described by 8 parameters. In designing the optimal structure, it is shown that to provide the high overall characteristics of the cathode and save the catalyst, 0.5 of the active layer volume should be set aside for the support grains (agglomerates of carbon particles covered with platinum and containing Nafion incorporations and microvoids). Protons and oxygen molecules must be supplied to the active layer by means of peculiar combined percolation clusters. The latter consist of a combination of support grains with either Nafion grains (to produce “protonic” clusters) or grains-voids (to afford “gas” clusters). The volume fractions of Nafion grains and grain-voids are assumed to be 0.25 and 0.25. The computer simulation of the support grain structure is also carried out. Their composition, i.e., the volume fractions of the carbon component (g _e), Nafion (g _ii), and microvoids (g _gg), is varied. The support grains play the key role in the active layer functioning. It is impossible to organize three full-value percolation clusters (electronic, protonic, and gas); hence, one has to have one or two combined clusters in the active layer. Thus the double load fells on the support grains. Their optimal structure should not only sustain the transport of protons and electrons in the active layer but also create the best conditions for the electrochemical process in each grain. The maximum current I _max (realized upon reaching the optimal active layer thicknesses Δ*) is calculated. The dependences of I _max and Δ* on the main parameters characterizing the support grains (g _e and g _ii) are analyzed. Here, two goals are sought: (1) to obtain the high currents, (2) to provide the low consumption of platinum per power unit. To solve the first problem, one has to work with high values of g _e. The second problem requires the opposite: the values of g _e must be minimal possible.     

Preparation of high catalyst utilization electrodes for polymer electrolyte fuel cells

We have developed a novel preparation procedure for an electrocatalyst layer with high utilization of catalyst for polymer electrolyte fuel cells. A commercial Pt catalyst supported on high surface area carbon black (Pt/CB) and Nafion ionomer solution was heated in an autoclave at 200 degrees C, followed by quenching to form the ink of the mixture. It was found that the cathode prepared with the new catalyst ink exhibited very high performance, i.e., high catalyst utilization and improved gas diffusivity. The microstructure analysis indicated that the autoclave treatment promoted an effective introduction of Nafion ionomer into primary pores of Pt/CB agglomerates, in which ca. 90% of Pt catalysts were supported. It was clearly observed by scanning transmission electron microscopy that Nafion ionomer was distributed more uniformly inside Pt/CB agglomerates, compared with those simply mixed with a ball mill in a conventional manner.     

Kinetics and mechanism of electrochemical oxygen reduction using Platinum/clay/Nafion catalyst layer for polymer electrolyte membrane fuel cells

This work demonstrates the use of amino functionalized Mg-phyllosilicate clay/Nafion nanocomposite film embedded with Pt nanoparticles (Pt/AC/N) for catalyzing oxygen reduction reaction (ORR) in sulphuric acid medium. Pt/AC/N nanocomposite films were surface characterized using transmission electron microscope. Cyclic and linear scan voltammetry studies were carried out under hydrodynamic conditions taking rotating-ring disc electrode (RRDE) as the working electrode. The effects of clay content, Pt mass loading, electrode rotation rate, and temperature on the ORR kinetics were studied. The Tafel slopes were found to vary between 118 and 126 mV dec⁻¹ indicating a good ORR kinetics. The exchange current density values calculated after mass transfer correction ranged from 5.8 × 10⁻⁷ to 2.4 × 10⁻⁶ A cm⁻². From the RRDE disc currents, Koutecky-Levich plots were constructed and the ORR mechanism was found to follow a four electron path with minimum H₂O₂ formation of ∼1.6%. The effect of temperature on ORR kinetics was found at 25, 40, and 50 °C. The energy of activation calculated to be 7.68 kJ mol⁻¹ and comparable to the standard Pt/C catalyzed ORR systems.     

A novel Pt/Au/C cathode catalyst for direct methanol fuel cells with simultaneous methanol tolerance and oxygen promotion

A novel Pt/Au/C catalyst was prepared by depositing the Pt and Au nanoparticles on the carbon support. The synthesized catalysts were characterized by energy-dispersive X-ray (EDX) and transmission electron microscopy (TEM), and electrochemically analyzed for activity towards oxygen-reduction reaction and methanol oxidation reaction. EDX and TEM results reveal that Pt nanoparticles supported on carbon supports were separated by Au nanoparticles. The electrochemical analysis indicate that the novel catalyst showed the enhanced methanol tolerance while maintaining a high catalytic activity for the oxygen-reduction reaction, which could be attributed to the less methanol adsorption on Pt/Au/C catalyst.     

Water management strategies for PGM-free catalyst layers for polymer electrolyte fuel cells

Platinum group metal–free (PGM-free) catalysts are promising candidates to catalyze the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs). Because of their low activity, larger loadings are used resulting in thicker catalyst layers. Transport, particularly water management, thereby becomes a more prominent performance factor. Currently, very few works attempted to understand water management in PGM-free catalyst layers, mainly because of other challenges that had to be overcome first, such as enhancing their activity and durability. The field has also been active in a hypothesis discussion of micropores flooding that led to the belief that poor stability of the PEFC performance is linked to active sites flooding within the micropores. We present here an overview of recent advances in understanding water management in the PGM-free catalyst layer for oxygen reduction reaction in PEFCs and provide an opinion on design guidance in optimizing catalyst layers to avoid flooding.     

Pt/WO_3/C nanocomposite with parallel WO_3 nanorods as cathode catalyst for proton exchange membrane fuel cells

Pt/WO3/C nanocomposites with parallel WO3 nanorods were synthesized and applied as the cathode catalyst for proton exchange membrane fuel cells(PEMFCs). Electrochemical results and single cell tests show that an enhanced activity for the oxygen reduction reaction(ORR) is obtained for the Pt/WO3/C catalyst compared with Pt/C. The higher catalytic activity might be ascribed to the improved Pt dispersion with smaller particle sizes. The Pt/WO3/C catalyst also exhibits a good electrochemical stability under potential cycling. Thus, the Pt/WO3/C catalyst can be used as a potential PEMFC cathode catalyst.     

Pt4ZrO2/C cathode catalyst for improved durability in high temperature PEMFC based on H3PO4 doped PBI

In this paper, Pt₄ZrO₂/C was prepared and compared with commercial Pt/C (46.6 wt.% TKK) in terms of the durability as cathode catalyst in a high temperature proton exchange membrane fuel cell (PEMFC) based on H₃PO₄ doped polybenzimidazole (PBI) by a potential sweep test. The catalysts before and after the potential sweep test were characterized by rotating disk electrode (RDE), X-ray diffraction (XRD), transmission electron microscopy (TEM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). After 3000 cycles potential sweep test, the overall performance loss of the Pt₄ZrO₂/C membrane electrode assembly (MEA) was less than that of the Pt/C MEA. The RDE results demonstrated that the oxygen reduction reaction (ORR) activity of the as-prepared Pt₄ZrO₂/C is nearly the same as TKK-Pt/C. The XRD and TEM results showed that Pt₄ZrO₂/C catalyst presented higher sintering resistance than commercial Pt/C catalyst during the potential sweep test. This may be attributed to the addition of ZrO₂, which acts an anchor to inhibit the adjacent platinum particles to agglomerate. The ICP-AES results of Pt₄ZrO₂/C cathode catalyst before and after the potential sweep test showed that the composition of Pt and Zr were very near the nominal atomic ratio of Pt:Zr, which reflected that Pt₄ZrO₂/C catalyst had a good stability during the potential sweep test. In brief, the preliminary results indicate that Pt₄ZrO₂/C catalyst is a good candidate of Pt/C catalyst in high temperature PEMFC based on H₃PO₄ doped PBI for achieving longer cell life-time and higher cell performance.     

Assessment of transport-limited catalyst utilization for engineering of ultra-low Pt loading polymer electrolyte fuel cell anode

In this study, transport-limited catalyst utilization in polymer electrolyte fuel cell (PEFC) anodes is assessed via an agglomerate model with a broader view of designing ultra-low Pt loading, high performance anode. The model accounts for electrical and chemical potential-driven transport of electrons/protons and dissolved hydrogen, respectively and multi-component gas-phase transport in the catalyst layer. The model employs the kinetics of hydrogen oxidation reaction based on dual-pathway reversible reaction mechanism reported recently [J.X. Wang, T.E. springer, R.R. Adzic, J. Electrochem. Soc. 153 (2006) A1732]. The model predictions show that for conventional, randomly-structured catalyst transport limitations exist at two levels. At single-agglomerate level, the catalyst utilization is restricted by dissolved hydrogen diffusivity limiting the reaction to occur primarily in the outer shell of the agglomerate. At the catalyst layer level, the catalyst utilization is limited primarily by poor protonic conductivity. However, significant electronic potential gradients can exist in the catalyst layer thereby effectively reducing the available overpotential. Simulation results also show that by engineering the catalyst layer to overcome the transport limitations and, thereby, improving the effective catalyst utilization, high performance can be achieved in a PEFC anode at ultra-low Pt loading of 0.0225 mg/cm².     

Nano-architecture platinum catalyst layer prepared by electrophoresis deposition for PEM fuel cells

A Pt-loaded carbon black electrode was prepared by pulsed electrophoresis deposition in a Pt colloid solution as a plating bath to overcome the growth problem of a Pt catalyst during deposition in an electrochemical process. This method is a promising technique for preparing Pt catalyst layers at the polymer electrolyte/electrode interface. The particle size of the Pt catalyst loaded by electrophoresis deposition was the same as that of Pt nanoparticles (3–4 nm) in a colloid and the particle size was maintained even during deposition. The loading of the Pt catalyst was controlled by the pH of the Pt colloid and deposition time. The Pt nanoparticles were deposited on a carbon black electrode to a depth of 2.5 μm.     

Pt supported on carbon nanofibers as electrocatalyst for low temperature polymer electrolyte membrane fuel cells

Carbon nanofibers synthesized via the thermo catalytic decomposition of methane were investigated for the first time as an electrocatalyst support in PEMFC cathodes. Their textural and physical properties make them a highly efficient catalyst support for cathodic oxygen reduction in low temperature PEMFC. Tests performed in MEAs showed that Pt supported on carbon nanofibers exhibited an enhancement of ca. 94% in power density at 0.600 V, in comparison with a commercial catalyst supported on conventional carbon black, Pt/Vulcan XC-72R.     

Performance of catalyst layers of polymer electrolyte fuel cells: exact solutions

Performance of a catalyst layer of polymer electrolyte fuel cell under the assumptions of ideal transport of reactants and Tafel kinetics of electrochemical reaction is considered. Explicit expressions for the profiles of basic parameters (proton current density, overpotential and reaction rate) across the catalyst layer are obtained and a new conservation law is found. Exact expression for voltage–current curve of the catalyst layer is derived and simplified in the limiting cases of small and large current densities. The physics of transition from small to large currents is discussed.     

Effects of ionomer coverage on agglomerate effectiveness in catalyst layers of polymer electrolyte fuel cells

A large proportion of voltage losses in polymer electrolyte fuel cells (PEFCs) originates in cathode catalyst layers. Catalyst utilization and performance of conventional catalyst layers depend largely on their ionomer content and distribution. The present study explores effects of agglomerate size and ionomer distribution on reaction rate distributions and effectiveness factor of Pt utilization. To study the oxygen reduction reaction, we have developed an agglomerate model, which consists of coupled relations for proton and oxygen transport, metal charging behavior, and interfacial charge transfer kinetics. The model is considered under steady state conditions. Results show that higher effectiveness factor is attained for agglomerates with smaller size and larger oxygen partial pressure on the surface. In addition, low to medium coverage of the ionomer skin layer is beneficial in view of high effectiveness factors due to the optimized interplay of oxygen and proton supply.     

Effect of inert gas flow on hydrogen underpotential deposition measurements in polymer electrolyte fuel cells

The effect of inert gas flow rate on hydrogen underpotential deposition (H_upd) measurements in polymer electrolyte fuel cells (PEFCs) was investigated using a novel experimental technique. The method combines local voltammetric measurements in PEFCs with the use of sectioned electrodes. The results give experimental proof that the high inert gas flow rate usually employed in voltammetric measurements in PEFCs at the working electrode results in high hydrogen reduction currents in both the cathodic and the anodic sweep, which hampers an accurate determination of the electrochemically active surface area (ECA). Strong spatial inhomogeneities occur at low potentials as a consequence of formation and accumulation of molecular hydrogen along the flow field. The results show that the flow of inert gas should be minimized or even stopped during a measurement to allow molecular hydrogen to accumulate at the working electrode and to provide uniform conditions along the flow field.     

燃料电池聚合物电解质膜的研究进展

本文根据聚合物电解质膜燃料电池操作温度、使用的电解质和燃料的不同,将其分为高温质子交换膜燃料电池、低温质子换膜燃料电池、直接甲醇燃料电池和阴离子交换膜燃料电池,综述了它们所用电解质膜的最新进展.第一部分简要介绍了这4种燃料电池的优点和不足.第二部分首先介绍了Nafion膜的结构模型,并对平行柱状纳米水通道模型在介观尺度上进行了修正;接着分别对应用于不同燃料电池的改性膜的改性思路作了分析;最后讨论了用于不同燃料电池的新型质子交换膜的研究,同时列举了性能突出的改性膜和新型质子交换膜.第三部分介绍了阴离子交换膜的研究现状.第四部分对未来聚合物电解质膜的研究作了展望.     

Numerical simulation of the ordered catalyst layer in cathode of proton exchange membrane fuel cells

A steady-state, one-dimensional numerical model based on cylindrical electrode structure is presented to analyze the performance of the ordered cathode catalyst layer in Proton Exchange Membrane Fuel Cells. The model equations account for the Tafel kinetics of oxygen reduction reaction, proton migration, oxygen diffusion in the cylindrical electrolyte and the gas pores, oxygen distribution at the gas/electrolyte interface. The simulation results reveal that ordered catalyst layers have better performance than conventional catalyst layers due to the improvements of mass transport and the uniformity of the electrochemical reaction rate across the whole width of the catalyst layer. The influences of oxygen diffusivity in gas phase and electrolyte, and the proton conductivity have been shown. The limitation by oxygen diffusion in gas phase drives the active region of the catalyst layer to the catalyst layer/gas diffuser interface. The limitation by proton migration confines the active region of the catalyst layer to the membrane/catalyst layer interface. The limitation due to oxygen diffusion in electrolyte film maintains the uniform distribution of the active region throughout the ordered catalyst layer.