Comparative characteristics of cathodes with different catalytic systems in hydrogen–oxygen and hydrogen–air fuel cells with proton-conducting polymer electrolyte

The characteristics of low-temperature hydrogen–oxygen (air) fuel cell (FC) with cathodes based on the 50 wt % PtCoCr/C and 40 wt % Pt/CNT catalysts synthesized on XC72 carbon black and carbon nanotubes (CNT) are compared with the characteristics of commercial monoplatinum systems 9100 60 wt % Pt/C and 13100 70% Pt/C HiSPEC. It is shown that the synthesized catalysts exhibit a high mass activity, which is not lower than that of commercial Pt/C catalysts, a high selectivity with respect to the oxygen reduction to water, and a significantly higher stability. The characteristics of PtCoCr/C and Pt/CNT were confirmed by testing in the hydrogen—oxygen FCs. However, when air was used at the cathode, especially in the absence of excessive pressure, a voltage of FC with the cathode based on PtCoCr/XC72 is lower as compared with the commercial systems. Probably, this is associated with the transport limitations in the structure of trimetallic catalyst synthesized on XC72 carbon black due to the absence of mesopores. This drawback was eliminated to a large extent by raising the volume of mesopores as a result of application of mixed support (XC72 + CNT) and the use of only CNT for the synthesis of the monoplatinum catalyst. However, this did not eliminate another drawback, namely, a low platinum utilization coefficient in the cathode active layer as compared with that measured under the model conditions in the 0.5 M Н₂SO₄ solution. Therefore, further research is required to improve the structure of the catalytic systems, which are synthesized both on carbon black and nanotubes, while maintaining their high stability and selectivity.     

Kinetics and mechanism of oxygen electroreduction on PtCoCr/C catalyst containing 20–40 wt % platinum

It is shown that the mechanism of oxygen electroreduction on the PtCoCr/C systems in 0.5 M H₂SO₄ is similar to that proposed for the Pt/C catalyst. The activity of ternary catalysts is by two and more times higher than that of monoplatinum catalyst. The constant k ₁ is much larger than k ₂ (k ₁ and k ₂ are the rate constants of O₂ reduction to water and H₂O₂, respectively) for all catalysts studied. This indicates that the catalytic systems are selective with respect to O₂ reduction immediately to water in the practically important potential range from 1.0 to 0.6 V. The yield of H₂O₂ increases with a shift of potential in the cathodic direction (<0.7 V) and does not exceed 1%. The sum of rate constants of further conversion of hydrogen peroxide also increases with a shift of potential in the cathodic direction. After a corrosion attack (a treatment in the acid for 24 h), a ratio between the rate constants (k ₁/k ₂) for the PtCoCr/C catalysts increases. This is caused by a considerable increase in k ₁, which is 2.84 × 10⁻² cm/s for the catalyst containing 34 wt % Pt (against 1.5 × 10⁻² cm/s for the untreated catalyst). This can be explained by the reaction proceeding on the particle surface, which was enriched in platinum in the course of corrosion treatment. The properties of platinum clusters on the alloy surface differ from those of monoplatinum as a result of the ligand effect. The amount of oxygen chemisorbed from water on this surface is lower than on Pt/C catalyst. This is the main factor determining an increase in the activity and stability of ternary catalysts.     

Developing a membrane-electrode assembly with reduced Pt content for a hydrogen—air fuel cell based on a PtCoCr catalyst and F-950 membrane

Laboratory methods are developed for forming an active layer (AL) with a synthesized PtCoCr catalyst (20 wt % Pt) on the F-950 perfluorinated membrane. AL composition and the conditions for forming 3- and 5-layer membrane-electrode assemblies (MEA) are optimized. Reproducible, stable, and high-discharge characteristics are obtained for a hydrogen-air fuel cell (HAFC). At a current density of 0.5 A/cm², the voltage of an MEA with cathode based on a PtCoCr catalyst is 0.66–0.68 V, and the maximum power density is 500 mW/cm². Replacing the commercial HiSPEC 4000 catalyst (40 wt % Pt) with PtCoCr (20 wt % Pt) in the AL composition of the cathode makes it possible to reduce Pt consumption by a factor of two without decreasing MEA discharge characteristics. The parameters that characterize the catalytic activity of catalysts under model conditions and in the MEA cathode composition are shown to be correlated.     

Electrochemical processes on multi-component cathodic catalysts PtM and PtM<Subscript>1</Subscript>M<Subscript>2</Subscript> (M = Co,Ni, Cr): the effect of surface composition on the catalyst stability and its activity in O<Subscript>2</Subscript> reduction

The multi-component nanocatalysts based on platinum-transient metals alloys applied onto dispersed carbon material are considered as the most promising catalysts, which can be substituted for platinum in the fuel cell cathodes. The electrocatalytic activity of platinum in the PtM₁/C and PtM₁M₂/C alloys increases by several times with simultaneous increase in the stability. From the results obtained by structural and electrochemical methods, it is found that the synthesized binary and ternary catalysts are the metal alloys, whose surface is enriched in platinum as a result of surface segregation and subsequent chemical or electrochemical treatment. Under the corrosive attack, the less-noble metal, which has not entered into the alloy, dissolves, and the core-shell structures form. The properties of platinum in the shell differ from its properties in Pt/C due to the ligand effect of the core (metal alloy). As a result, the surface coverage with oxygen chemisorbed from water decreases in the binary and ternary systems. This causes an increase of the catalytic activity in the O₂ reduction reaction due to a decline in the effect of surface blocking against molecular oxygen adsorption and a decrease in the platinum dissolution rate, because the oxidation of platinum by water is the onset of corrosion process. For the catalytic systems studied, the mass activity decreases in the following order: 20% Pt in PtCoCr/C > 7.3% Pt in PtCo/C ≥ 7.3% Pt in PtCr/C and PtNi/C ≥ 40% Pt/C. The application of PtCoCr/C catalyst as the cathode in a low-temperature hydrogen-air fuel cell enabled one to reduce the platinum consumption by one half on retention of its performance.     

Highly active and stable catalysts based on nanotubes and modified platinum for fuel cells

Results on the development of new cathodic catalysts (monoplatinum and cobalt-modified platinum) applied on carbon nanotubes are shown. By means of a complex of electrochemical and structural techniques, it is shown that as regards their activity under model conditions and within membrane-electrode assembles, the catalysts synthesized by the polyol method are close to commercial monoplatinum systems with the same mass content of platinum (20 wt %) and their corrosion stability is double that of commercial catalysts. Platinum modified with cobalt is characterized by still higher stability, which allows considering these catalytic systems as the candidates to be used in fuel cells after the corresponding optimization.     

Degradation processes in hydrogen–air fuel cell as a function of the operating conditions and composition of membrane–electrode assemblies

The optimal composition of membrane–electrode assemblies and operating conditions of hydrogen–air fuel cells, which provide a high efficiency and stability of catalytically active cathode layers and the fuel cell as a whole are determined for commercial monoplatinum electrocatalysts on the highly dispersed carbon support containing 60–70 wt % Pt. The degradation processes in the Pt/C catalysts are studied by a complex of electrochemical methods and the methods of structural analysis.     

The structure and characteristics of a PtCoCr nanosized polymetallic cathode catalyst on a carbon carrier

The paper presents X-ray and transmission electron microscopy data characterizing the structure of trimetallic PtCoCr catalysts synthesized on a disperse carbon carrier (carbon black KhS 72) and the influence of the structure on electrocatalytic activity in the reduction of oxygen in 0.5 M H₂SO₄. The mechanisms of oxygen reduction on platinum and trimetallic catalysts were shown to be similar. A higher activity of platinum contained in the trimetallic catalyst was caused by smaller PtCoCr/C catalyst surface coverage by oxygen-containing particles formed from water and interfering with the adsorption of molecular oxygen, which was, in turn, determined by the electronic structure of trimetallic system nanoparticles.     

Synthesis and properties of a platinum catalyst supported on plasma chemical silicon carbide

A CO oxidation catalyst based on β–SiC and Pt nanoparticles has been synthesized and studied. The average size of Pt clusters on the surface of the plasma-chemical silicon carbide nanoparticles is close to 4 nm. It has been found that the rate of the CO oxidation reaction at low concentrations (100 mg/m³) in air at room temperature over the catalyst based on platinum and silicon carbide nanoparticles is 60–90 times that over a platinum black-based catalyst with a specific surface area of 30 m²/g. The Pt/SiC catalyst containing 12 wt % Pt has been found to provide the maximum CO oxidation rate.     

Pristine and graphitized-MWCNTs as durable cathode-catalyst supports for PEFCs

Long-term deterioration in the performance of PEFCs is attributed largely to reduction in active area of the platinum catalyst at cathode, usually caused by carbon-support corrosion. Multi-walled carbon-nanotubes (MWCNTs) as cathode-catalyst support are found to enhance long-term stability of platinum catalyst (Pt) in relation to non-graphitic carbon. In addition, highly graphitic MWCNTs (G-MWCNTs) are found to be electrochemically more stable than pristine MWCNTs. This is because graphitic-carbon-supported-Pt (Pt/MWCNTs) cathodes exhibit higher resistance to carbon corrosion in-relation to non-graphitic-carbon-supported-Pt (Pt/C) cathodes in PEFCs during accelerated stress-test (AST) as evidenced by chronoamperometry and carbon dioxide studies. The corresponding change in electrochemical surface area (ESA), cell performance, and charge-transfer resistance are monitored through cyclic voltammetry, cell polarization, and impedance measurements, respectively. The extent of crystallinity, namely amorphous or graphitic nature of the three supports, is examined by Raman spectroscopy. X-ray diffraction and transmission electron microscopy studies both prior and after AST suggest lesser deformation in catalyst layer and catalyst particles for Pt/G-MWCNTs and Pt/MWCNTs cathodes in relation to Pt/C cathodes, reflecting that graphitic carbon-support resists carbon corrosion and helps mitigating aggregation of Pt particles. It is also found that with increasing degree of graphitization, the electrochemical stability for MWCNTs increases due to the lesser surface defects.     

Activity, stability and degradation of multi walled carbon nanotube (MWCNT) supported Pt fuel cell electrocatalysts

Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m(2) g(-1) and with a comparable Pt mean particle size were subjected to electrode potential cycling in a "lifetime" stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a "start-up" stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the "lifetime" regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the "start-up" regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10,000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.     

Remarkably Enhanced Hydrogen Oxidation Reaction Activity of Carbon-supported Pt by Facile Nickel Modification

Developing high activity catalysts for hydrogen oxidation reaction(HOR)under alkaline condition remains a challenge in the exchange membrane fuel cell(AEMFC).Herein,we report that the activity of carbon-supported platinum(Pt/C)towards the hydrogen oxidation reaction(HOR)in alkaline media can be remarkably enhanced by simple immersion of Pt/C in nickel chloride solution.The adsorption of hydrogen on the catalyst surface is weakened by modification of nickel.The HOR activity on the Pt/C after immersion possesses an excellent mass current density of 33.4 A/gmetal,which is 18%higher than that(28.3 A/gmetal)on Pt/C.     

Hydrogenation of Anthracene and Dehydrogenation of Perhydroanthracene on Pt/C Catalysts

The hydrogenation of anthracene on a heterogeneous catalyst containing 3 wt % Pt/C (Aldrich) at 215, 245, and 280°C and the pressures of 40 and 90 atm is studied. The hydrogenation of anthracene to a completely hydrogenated product is considered in detail. The final product (perhydroanthracene) consists of five conformational isomers with total selectivity of more than 99%. The ratio of perhydroanthracene isomers in the end product is shown to be determined by the conditions (P, T) of hydrogenation. The rate of hydrogenation is found to slow upon an increase in the degree of benzene ring saturation. A mixture of perhydroanthracene isomers is dehydrogenated in an autoclave at 260?325°C on 3 wt % Pt/C catalyst (Aldrich) and in a flow reactor at 300–360°C on 3 wt% Pt/Sibunit catalyst. The reactivity of perhydroanthracene isomers in dehydrogenation is shown to differ.     

Study of degradation of membrane-electrode assemblies of hydrogen-oxygen (air) fuel cell under the conditions of life tests and voltage cycling

The degradation processes of HiSPEC 9100 (60% Pt/C) and 13100 (70% Pt/C) cathodic monoplatinum catalysts, which were tested under the model conditions and in the composition of membrane-electrode assemblies (MEA) of hydrogen-air and hydrogen-oxygen fuel cells, are studied. It is shown that, in all cases, the main reason for a decrease in the catalyst activity was a decrease in its surface area, which was caused by the coarsening of platinum nanoparticles, irreversible oxidation of a fraction of active centers, and the destruction of the catalyst due to the carbon support oxidation. The results of electrochemical measurements are supplemented with the structural investigations by the methods of transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that the degradation processes of MEA in the accelerated stress tests (AST) are similar to those in the long-term life tests. With respect to a decrease in the catalyst active surface area, the application of 2500 cycles in the voltage range of 0.6 to 1.2 V in the AST is equivalent to the life tests for 1010 h. During the fuel cell operation, the destruction of polymer electrolyte proceeds along with the catalyst degradation. This leads to a decrease in the ion-exchange capacity of the membrane and ionomer in the composition of cathode active layer.     

以切短多壁碳纳米管为载体制备高活性Pt/SCNT及PtRu/SCNT燃料电池催化剂

采用乙醇为助磨剂,利用球磨的方法将5-15μm长的多壁碳纳米管切短成长度约为200nm,并且分布较为均匀的短碳纳米管(SCNT).以SCNT为载体,采用有机溶胶法制得了含铂20%(w)的Pt/SCNT及PtRu/SCNT催化剂.实验发现:对于甲醇的阳极电氧化过程,以切短碳纳米管为载体的Pt/SCNT催化剂具有比相同条件制得的Pt/CNT催化剂高得多的催化活性,前者甲醇氧化峰电流密度是后者的1.4倍,并且远远高于商品的Pt/C催化剂.同时我们发现添加了钌的PtRu/SCNT具有比不含钌的催化剂更好的活性.采用X射线衍射(XRD)、透射电镜(TEM)、比表面积分析(BET)等方法对催化剂进行表征,结果表明,切短碳纳米管的晶相结构并未改变,但Pt/SCNT和PtRu/SCNT催化剂的比表面积和电化学活性得到了显著的提高.     

层级多孔碳载铂催化剂的制备及可达性

层级多孔碳作为氧还原铂基催化剂载体的选择之一, 简单的旋转圆盘电极(RDE)验证此类催化剂具有较高的氧还原活性, 但几乎都缺少膜电极(MEA)性能验证, 实用性无法保证. 本文设计制备了基于聚苯胺的层级多孔碳（NHPC）载铂催化剂（Pt/NHPC850）, 研究了其氧还原活性、 MEA质子传输和氧传输特性. RDE测试研究表明, Pt/NHPC850催化剂在低I/C（离聚物与碳载体质量比）时的面积活性低于实心碳载铂催化剂（Pt/XC-72）, 但当I/C增大到与膜电极中一致时, 由于Nafion树脂对Pt催化剂的毒化作用增强, 其面积活性反而优于 Pt/XC-72. Pt/NHPC850催化剂的高Pt分散性及其优异的抗Nafion毒化性能, 使其在I/C为0.8时的质量活性为Pt/XC-72催化剂的1.34倍. MEA质子传输研究表明, 即使在高加湿条件下, Pt/NHPC850质子电阻率仍高达72.6 mΩ·cm², 为Pt/XC-72的3倍. Pt/NHPC850制备的膜电极极化曲线在500 mA/cm²电流密度下性能迅速下降, Pt/NHPC850的氧增益电压达到144.4 mV, 比Pt/XC-72高56.7 mV. 表明Pt/NHPC850膜电极的质子传输和氧传输性能较差. 对比Pt/NHPC850催化剂的RDE和MEA的测试结果, 说明以层级多孔碳为载体的铂碳催化剂虽然耐Nafion毒化能力提高, 但是质子和氧气的氧传输性较差, 此类层级多孔碳还需进一步优化其结构, 才有可能满足低铂质子交换膜燃料电池（PEMFC）的应用需求.     

水溶性含钌-铂双金属催化剂催化卤代芳香硝基化合物选择性加氢

考察了水溶性Ru/Pt-TPPTS双金属催化剂催化卤代芳香硝基化合物的加氢性能.实验结果表明,在Ru-TPPTS中添加铂或钯后,反应活性明显提高,尤其是Ru/Pt-TPPTS双金属催化剂更表现出显著的双金属协同效应.在pH₂=1.0MPa,70℃,反应2h的条件下,双金属催化剂0.50Ru/0.50Pt-TPPTS催化对-氯硝基苯加氢生成对-氯苯胺的反应转化率达到100%.对于取代基和取代位置不同的一些卤代硝基苯加氢,该双金属催化剂也表现出很高的催化活性和生成卤代苯胺的选择性,脱卤反应的程度很小.     

The influence of plasma chemical treatment of a platinum catalyst on its activity in the dehydrogenation of cyclohexane

The influence of plasma chemical treatments on the catalytic activity of 0.64 wt % Pt/SiO₂ and 1.0 wt % Pt/SiO₂ platinum catalysts in the dehydrogenation of cyclohexane was studied. The state of the surface of the catalysts was examined using X-ray photoelectron spectroscopy. Temperature hysteresis caused by the formation of active carbon was observed in flow experiments. It was shown that the reaction on the initial catalysts occurred on neutral and positively charged Pt particles, and that the active centers contained carbon. After catalyst treatment with a high-frequency plasma in H₂, its activity increased by many times because of the formation of a large number of low-activity centers on positively charged platinum particles also containing carbon. Glow discharge plasma in Ar sharply decreased catalytic activity, and the reaction then predominantly occurred on centers localized on neutral Pt particles, whereas centers on positive Pt particles were blocked. The state of the substrate (silica gel) did not change under the action of plasmas of both kinds.     

Platinum/mesoporous WO3 as a carbon-free electrocatalyst with enhanced electrochemical activity for methanol oxidation

A new type of carbon-free electrode catalyst, Pt/mesoporous WO3 composite, has been prepared and its electrochemical activity for methanol oxidation has been investigated. The mesoporous tungsten trioxide support was synthesized by a replicating route and the mesoporous composties with Pt loaded were characterized by using X-ray diffraction (XRD), nitrogen sorption, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) techniques. Cyclic voltammetry (CV), line scan voltammetry (LSV) and chronoamperometry (CA) were adopted to characterize the electrochemical activities of the composites. The mesoporous WO3 showed high surface area, ordered pore structure, and nanosized wall thickness of about 6-7 nm. When a certain amount of Pt nanoparticles were dispersed in the pore structure of mesoporous WO3, the resultant mesostructured Pt/WO3 composites exhibit high electro-catalytic activity toward methanol oxidation. The overall electro-catalytic activities of 20 wt % Pt/WO3 composites are significantly higher than that of commercial 20 wt % Pt/C catalyst and are comparable to the 20 wt % PtRu/C catalyst in the potential region of 0.5-0.7 V. The enhanced electro-catalytic activity is attributed to be resulted from the assistant catalytic effect and the mesoporous structure of WO3 supports.     

Catalysts of ethanol anodic oxidation for ethanol-air fuel cell with a proton-conducting polymer electrolyte

Synthesis techniques for binary PtSn, PdM (M = Sn, V, Mo) and ternary PtSnNi, PtRuSn catalysts of ethanol electrooxidation on highly dispersed carbon materials are suggested. The highest activity in the 0.5 M H₂SO₄ solution containing 1 M C₂H₅OH corresponds to the system of PtSn (3: 1, 40 wt % Pt) with the particle size of 2–4 nm and tin content in the alloy with platinum of about 6%. It was shown that the catalyst efficiency as regards ethanol oxidation depth decreases in the series of Pt > PtRu ≈ PtSn, and the catalyst activity by current forms the series of PtSn > PtRu > Pt. The membrane-electrode assembly (MEA) with the anodes on the basis of the PtSn (3: 1, 40 wt % Pt) catalyst had stable characteristics for 220 h at the current density of ∼50 mA/cm².     

Hydrocracking vegetable oil on borate-containing catalysts: Effect of nature of support

The structure, texture, and acid properties of platinum catalysts on oxide (Al₂O₃, ZrO₂, ZrO₂–Al₂O₃) and borate-containing supports (B₂O₃–Al₂O₃, B₂O₃–ZrO₂) are studied. The catalysts are tested in the process of hydrocracking sunflower-seed oil at 380°C, 4.0 MPa, and a weight stock feed rate of 1.0 h^–1. It has been found that aluminum oxide (A) contains the γ-Al₂O₃ phase, zirconium dioxide (Z) includes 85 and 15 rel. % of the monoclinic (M) and tetragonal (T) phases, respectively, while zirconium dioxide with the addition of 2.5 wt % Al₂O₃ (ZA) comprises 14 and 86 rel. % of the M–ZrO₂ and T–ZrO₂ phases, respectively. The B₂O₃–Al₂O₃ (BA) and B₂O₃–ZrO₂ (BZ) systems modified with boron oxide (20 wt %) are X-ray amorphous. A Pt/BA catalyst differs from a Pt/A catalyst, while a Pt/BZ catalyst has a larger specific surface area and acidity than Pt/Z and Pt/ZA catalysts and contains Bronsted acidic centers (BACs) along with Lewis acidic centers (LACs). Only LACs are present on the surface of Pt/A, Pt/Z, and Pt/ZA catalysts. The LAC/BAC ratio in Pt/BA and Pt/BZ catalysts is 0.3 and 1.0, respectively. All the catalysts provide complete oil conversion to give C₅₊ hydrocarbons with a yield of 81.7–87.3 wt %. Pt/A catalyzes mainly decarboxylation and hydrogenation–dehydration reactions, while Pt/Z and Pt/ZA provide decarboxylation. The yield of diesel fraction reaches 71.8–73.9 wt % with an n-alkane content of 94.0–95.9 wt %. One-stage oil hydrocracking with the prevalence of hydrodecarbonylation and hydrogenation–dehydration reactions occurs on Pt/BA and Pt/BZ catalysts for 20 h to give the yield of the diesel fraction of at least 81.4 and 74.4 wt % and the total content of iso-alkanes and cycloalkanes of at least 28.3 and 60.7 wt %, respectively.