Platinum-based ternary catalysts for low temperature fuel cells: Part II. Electrochemical properties

The development of high performance electrode materials is currently one of the main activities in the field of the low temperature fuel cells, fuelled with H₂/CO or low molecular weight alcohols. A promising way to attain higher catalytic performance is to add a third element to the best binary catalysts actually used as anode and cathode materials. In Part I of this review an overview of the preparation and structural characteristics of Pt-based ternary catalysts was presented. This part of the review deals with the electrochemical properties of these catalysts regarding their CO tolerance and electrocatalytic activity for methanol and ethanol oxidation in the case of anode materials, and their activity for oxygen reduction and stability in fuel cell conditions when used as cathode materials.     

Performance of highly dispersed Pt/C catalysts for low temperature fuel cells

A novel technique based on intermittent microwave heating (IMH) is used to prepare highly dispersed Pt/C catalysts. It has been proved that more than 60% Pt on carbon can be prepared by one-step procedure. The average Pt clusters on carbon are less than 5 nm with very narrow size distribution. The catalysts prepared by the present method show better performance in low temperature fuel cells.     

Upgrading of diesel fuels and mixtures of hydrocarbons by means of oxygen low pressure plasmas: a comparative study

The oxidation of n-heptane, 1-octene, toluene, cis-decahydronaphthalene, mixtures of them, 4-phenyl-1-butene, 1,2,3,4-tetrahydronaphthalene, and three commercial diesel fuels, all in the liquid phase, by means of low pressure high-voltage oxygen plasmas was studied. Oxygen pressure was 0.2 mbar, applied power was 35 watts and reaction times ranged from 1 min to 23 h. Both individually and forming part of mixtures, olefins were the most reactive with ground-state atomic oxygen, O(³P). Olefinic double bonds reacted ca. 150 times faster than C-H bonds. Products were: epoxides and aldehydes for olefins; alcohols and ketones for alkanes; phenols for aromatics. Addition of 4.7-7.8% wt of oxygen was achieved for the diesels, depending on the particular composition, those with higher content of olefins being favoured, followed by those with higher content of alkanes.     

PEM fuel cells as membrane reactors: kinetic analysis by impedance spectroscopy

Proton exchange membrane fuel cells (PEMFC) are considered as electrochemical reactors, performances of which are regarded in the context of the various effects influencing FC output, such as mass transports, kinetic of electrode reactions and charge transfer in polymer electrolyte membrane (PEM). An experimental approach, involving the employment of impedance spectroscopy (IS), which allows a deep insight into the nature of these effects, is discussed and its applications to the different aspects of PEMFC functioning are reported. As examples of the use of IS in PEMFC studies, the investigations of the membrane conductivity and in situ studies of the anode and the cathode processes during FC operation are presented.     

Preparation of platinum-based electrode catalysts for low temperature fuel cell

We have studied systematically the effects of synthesis parameters in both precipitation and colloidal methods to obtain highly dispersed Pt/carbon catalyst and compared the characteristics of prepared catalysts with commercial ones. The average Pt particle size at optimum condition for 10–60 wt.% Pt/carbon was in the range 1.7–3.8 nm which was about 70–80% of the commercial catalysts at the same Pt loading. The Pt surface area was also 20–40% higher than those of the commercial catalysts. The activities of prepared catalysts, measured by a single cell unit, were comparable with those of commercial ones.     

Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells

A ceramic composite anode composed of (La_0.8Sr_0.2) (Cr_0.5Mn_0.5)O_3-δ (LSCM), Ba(Zr_0.75Y_0.15)O_3-δ (BZY), and catalysts was applied in hydrocarbon fuels for protonic ceramics fuel cells. LSCM and BZY served as an electronic conductor and a protonic ceramic, respectively. The single phase of LSCM, a promising electronically conductive ceramic, could be obtained by performing calcination when exposed to air and hydrogen reduction at 973 K, which was much lower than the conventional calcination temperature (approximately 1273 K). The LSCM-BZY composite anode was fabricated successfully at such a low temperature using the infiltration method. By testing the composite electrodes at different temperatures, namely 973, 873, 1223, and 1373 K, the effect of the calcination temperature of LSCM on anode performance in hydrogen and methane fuels was successfully investigated. The composite anode with LSCM calcined at 973 and 1073 K showed three-fold improved performance in H₂ fuel and two-fold in CH₄ fuel than that of the composite anode with LSCM calcined at 1373 K.     

Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with “high conductivity” honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected).     

Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part II. Electrocatalytic activity

The methods developed and described in paper—part I are employed to prepare nanometer size Pt-Ru particles on a Vulcan^® XC72R substrate with controlled metal loading. Transmission Electron Microscopy (TEM) confirmed uniform particles size (average diameter 2 nm) and homogeneous dispersion of the particles over the substrate. Energy Dispersive X-ray absorption (EDX) analysis confirmed the compositional homogeneity. The catalytic activity of these supported nanoparticles with regard to methanol electrooxidation is investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO-stripping voltammetry techniques at temperatures between 25 and 60 °C. Such investigation concerns supported catalysts prepared with ca. 10 and 18 wt.% overall metal loading (Pt + Ru) onto the Vulcan^® XC72R substrate. Comparative testing of our catalysts and a commercial Pt-Ru/Vulcan reveals markedly superior activity for our catalysts. In fact, we observe for the latter a five-fold increase of the oxidation current as compared to a commercial Pt-Ru/Vulcan with equal metal loading. One of the reasons for the greater activity is found to be the very high dispersion of the metals over the substrate, i.e. the large surface area of the active phase. Other reasons are plausibly ascribable to the varied Pt/Ru composition and/or reduced presence of contaminants at the catalyst surface.     

Photocatalytic activity of TiO2-based materials for the oxidation of propene and benzene at low concentration in presence of humidity

This paper complements previous studies devoted to the photocatalytic oxidation of a low-concentration propene gaseous stream in absence of humidity using agglomerated TiO₂-based materials. These prepared agglomerated materials have shown very good oxidation activities and complete selectivity towards CO₂. The present paper analyses the role of humidity on propene oxidation, which has not been studied before, and extends the use of the prepared agglomerated photocatalysts to low concentration benzene oxidation, both in absence and presence of humidity. The obtained results have shown that humidity must be totally avoided, or kept as low as possible, to achieve high propene conversions. In the case of benzene, some insight to controversial published results is given; very low conversions together with benzene cracking on the surface of the photocatalyst occur in absence of humidity. However, the introduction of humidity leads to high conversions and avoids benzene cracking. The performance of the agglomerated photocatalyst containing a high surface area activated carbon (TiO₂/C1) must be underlined in terms of activity.     

Progress of vanadium phosphorous oxide catalyst for n-butane selective oxidation

The utilization of lighter alkanes into useful chemical products is essential for modern chemistry and reducing the CO₂ emission. Particularly, n-butane has gained special attention across the globe due to the abundant production of maleic anhydride (MA). Vanadium phosphorous oxide (VPO) is the most effective catalyst for selective oxidation of n-butane to MA so far. Interestingly, the VPO complex exists in more or less fifteen different structures, each one having distinct phase composition and exclusive surface morphology and physiochemical properties such as valence state, lattice oxygen, acidity etc., which relies on precursor preparation method and the activation conditions of catalysts. The catalytic performance of VPO catalyst is improved by adding different promoters or co-catalyst such as various metals dopants, or either introducing template or structural-directing agents. Meanwhile, new preparation strategies such as electrospinning, ball milling, hydrothermal, barothermal, ultrasound, microwave irradiation, calcination, sol–gel method and solvothermal synthesis are also employed for introducing improvement in catalytic performance. Research in above-mentioned different aspects will be ascribed in current review in addition to summarizing overall catalysis activity and final yield. To analyze the performance of the catalytic precursor, the reaction mechanism and reaction kinetics both are discussed in this review to help clarify the key issues such as strong exothermic reaction, phosphorus supplement, water supplement, deactivation, and air/n-butane pretreatment etc. related to the various industrial applications of VPO.     

Behavior of SiC at high temperature under helium with low oxygen partial pressure

The behavior of SiC at high temperature under helium with low oxygen partial pressure is a key factor for its application as structural material in Gas-cooled Fast Reactors (GFR). After a literature study on the active–passive transition in the oxidation of SiC, a numerical study reproducing environments of the future reactors was realized with GEMINI software to evaluate SiC behavior under helium with low oxygen partial pressure (0.5–3500 Pa). It was found that increasing the partial pressure of oxidant pushes the passive to active transition to higher temperature and suppresses the vaporization of SiC. These results are in agreement with the calculation using the Wagner model. Experimental tests at high temperature (1300–2000 K) on massive SiC samples (sintered α and β CVD), coupled to SEM, XPS and XRD analyses before and after oxidation tests are presented. They show that the level of oxidizing species has an important impact on the physico-chemical behavior of SiC as was also predicted by thermodynamic calculation. In addition, for the mass loss with time, the crystallographic structure is an important factor. Silicon carbide is a promising structural material for Generation IV nuclear reactors. Because of its mechanical and physico-chemical properties, it maintains its structural integrity at high temperature in helium environment with low oxygen partial pressure.     

Synthesis of Pt and Pd nanosheaths on multi-walled carbon nanotubes as potential electrocatalysts of low temperature fuel cells

Pt and Pd nanosheaths are successfully synthesized on multi-walled carbon nanotubes (MWCNTs) using the non-covalent poly(diallyldimethylammonium chloride) (PDDA) functionalization and seed-mediated growth methods. In this method, negatively charged Pt or Pd metal precursors are self-assembled with positively charged PDDA-functionalized MWCNTs, forming uniformly distributed Pt or Pd nanoseeds on MWCNTs supports. The contiguous and highly porous Pt and Pd nanosheath structured catalysts are then formed by the seed-mediated growth in corresponding metal precursors using ascorbic acid as the reducing agent. The essential role of uniformly dispersed Pt and Pd nanoseeds on PDDA-MWCNTs is demonstrated. The results indicate that both Pt and Pd nanosheaths show an enhanced catalytic activity for the methanol and formic acid oxidation reaction in acid solution, respectively, as compared with conventional Pt/C and Pd/C catalysts. The enhanced activities are most likely due to the reduced oxophilicity, which results in a weakened chemisorption energy with oxygen-containing species such as CO_ad, and the increased reactive sites due to the large number of grain boundaries of the Pt and Pd nanosheath structured electrocatalysts.     

Platinum-based ternary catalysts for low temperature fuel cells: Part I. Preparation methods and structural characteristics

Pt-based ternary catalysts have been proposed as electrode materials for low temperature fuel cells. Pt–Ru-based ternary catalysts were tested as anode materials with improved CO tolerance or enhanced activity for methanol or ethanol oxidation. Ternary catalysts based on platinum alloyed with first row transition metals were tested as cathode materials with improved activity for the oxygen reduction. This paper presents an overview of the preparation methods and structural characteristics of these ternary catalysts.     

凹凸棒石低温SCR脱硝催化剂的研究进展

选择性催化还原（SCR）脱硝技术是目前主流的氮氧化物脱除技术,其核心是催化剂。凹凸棒石成本低廉,性能优越,适合用作SCR催化剂的载体,而且以凹凸棒石为载体的催化剂显示出良好的低温选择性和稳定性,具有很好的应用前景。本文总结了凹凸棒石低温SCR脱硝催化剂的研究进展,阐述了活性组分、制备方法、前体物种、活性组分负载量、煅烧温度、元素掺杂等因素对催化剂脱硝活性的影响,同时简要介绍了导致此类催化剂失活的原因以及失活催化剂的再生方法,并指出在凹凸棒石负载型低温脱硝催化剂上进行的SCR脱硝反应遵循E-R机理,最后指出此类催化剂的未来研究方向主要是进一步提高现有催化剂的低温催化活性和抗中毒能力,实现工业化应用。     

The influence of activated carbon as an additive in anode materials for low temperature solid oxide fuel cells

The effects of activated carbon (AC) as an additive in multi-oxide nano composite LiNiCuZn–O for application as anode in solid oxide fuel cell (SOFC) is reported. The composite was synthesized using solid state reactions method with varying content of AC in range 0.1%–0.9% for use as anode in the cell. The cell was composed of the synthesized composite as anode, LiNiCuZn–O as cathode and Samaria doped ceria (SDC) as electrolyte. The prepared composites were characterized for morphology and crystal structure by scanning electron microscope (SEM) and x-ray diffraction (XRD) respectively. Furthermore, the crystallite sizes of LiNiCuZn–O and LiNiCuZn–O with AC as an additive have been found in the range from 50 nm to 70 nm. The prepared composite materials were observed porous and the porosity of the sample having 0.5% additive was found highest. The conductivity and power density of the SOFC were studied at temperature of 600 °C. The maximum value of conductivity was found as 4.79 S/cm for the composite containing 0.5% AC as measured by using 4-probe method. The maximum value of power density of the fuel cell with anode comprising of 0.5% AC along with the mentioned cathode and the electrolyte was 455 mW/cm². Therefore, out of the compositions studied, the composite comprising of LiNiCuZn–O with 0.5% AC offered best performance for anode in the cell. This oxide composite is reported as a potential candidate for use as anode in low temperature SOFCs.     

Electrochemical and gas phase parameters of cathodes for intermediate temperature solid oxide fuel cells

A series of cyclic voltammetry, chronoamperometry and electrochemical impedance experiments have been carried out in order to investigate the effect of cathode composition and porosity on the electrochemical characteristics of strontium-doped lanthanum, praseodymium and gadolinium cobaltite cathodes. The impedance responses at different electrode potentials of the half cell and symmetric single cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cell cathode and anode reactions contributions based on the results of simultaneous analysis of half cells and symmetric single cells has been made by differential impedance real part vs. ac frequency plot analysis method. Noticeable influence of cathode chemical composition, meso-porosity and macro-porosity on the electrochemical activity of the oxygen electroreduction has been demonstrated. Seeming activation energy values have been calculated and discussed.     

Corrosion behavior of austenitic stainless steels as a function of pH for use as bipolar plates in polymer electrolyte membrane fuel cells

Stainless steels (types 304 and 310S) were employed as bipolar plates for polymer electrolyte membrane fuel cells. For the cell operation, the decayed cell voltage was approximately 22 mV for the type 310S stainless steel after 1000 h operation, while that for type 304 stainless steel was about 46 mV. Corrosion products appeared on the cathode side bipolar plate for the type 304 stainless steel, while trace of corrosion was barely detected for type 310S stainless steel. In order to follow the pH on the bipolar plates during fuel cell operation, polarization tests were carried out for the type 310S stainless steel in synthetic solutions (0.05 M SO₄²⁻ (pH 1.2-5.5) + 2 ppm F⁻) as a function of pH (1.2-5.5) at 353 K. We also examined the contact resistance between the stainless steel and carbon diffusion layer before and after polarization. X-ray photoelectron spectroscopic (XPS) analyses were carried out for comparison of the surface states of the steels after the polarization tests and cell operation. In the synthetic solutions with lower pHs (≤3.3), the films were thinner and were mainly composed by enriched with chromium oxide. Whereas, they mainly consisted of relatively thick iron oxide when the solution pH was higher (≥4.3). XPS analyses for the bipolar plate of type 310S stainless steel on cathode side after cell operation demonstrated pH gradient on the plate, that is, the thicker iron-rich surfaces presented relatively higher pH from the gas inlet to center area, and the thinner chromium-rich surface appeared with lower pH around the gas outlet.     

Electrochemical oxidation of boron containing compounds on titanium carbide and its implications to direct fuel cells

Electrochemical oxidation of sodium borohydride (NaBH₄) and ammonia borane (NH₃BH₃) (AB) have been studied on titanium carbide electrode. The oxidation is followed by using cyclic voltammetry, chronoamperometry and polarization measurements. A fuel cell with TiC as anode and 40 wt% Pt/C as cathode is constructed and the polarization behaviour is studied with NaBH₄ as anodic fuel and hydrogen peroxide as catholyte. A maximum power density of 65 mW cm⁻² at a load current density of 83 mA cm⁻² is obtained at 343 K in the case of borhydride-based fuel cell and a value of 85 mW cm⁻² at 105 mA cm⁻² is obtained in the case of AB-based fuel cell at 353 K.     

The influence of CO on the current density distribution of high temperature polymer electrolyte membrane fuel cells

In this work the poisoning effect of carbon monoxide (CO) on the performance of high temperature polymer electrolyte membrane (PEM) fuel cell is reported. The poisoning of the anode is assessed at 160 °C and 180 °C based on the transient behavior of the fuel cell potential and current density distribution. The current density distribution at similar cell potential and global current density is also critically compared for CO-free hydrogen feed and for CO-contaminated hydrogen feed. Furthermore, the current–cell potential (I–V) and power density curves and impedance spectra are obtained.The presence of CO causes a performance loss which is aggravated for higher CO concentrations and higher current densities and for lower temperatures. The transient behavior of the fuel cell potential and current density distribution show that the poisoning effect of carbon monoxide at the anode is very fast.The use of CO contaminated hydrogen at the anode yields an anisotropic distribution of carbon monoxide, which is accentuated for higher carbon monoxide concentrations and current densities.     

Electrochemical performance and stability of thin film electrodes with metal oxides in polymer electrolyte fuel cells

Thin film electrodes are prepared by thermal evaporation of nanometer thick layers of metal oxide and platinum on a gas diffusion layer (GDL), in order to evaluate different metal oxides’ impact on the activity and stability of the platinum cathode catalyst in the polymer electrolyte fuel cell. Platinum deposited on tin, tantalum, titanium, tungsten and zirconium oxide is investigated and the morphology and chemistry of the catalysts are examined with scanning electron microscopy and X-ray photoelectron spectroscopy. Cyclic sweeps in oxygen and nitrogen are performed prior and after potential cycling degradation tests. Platinum seems to disperse better on the metal oxides than on the GDL and increased electrochemically active surface area (ECSA) of platinum is observed on tin, titanium and tungsten oxide. A thicker layer metal oxide results in a higher ECSA. Platinum deposited on tungsten performs better than sole platinum in the polarisation curves and displays higher Tafel slopes at higher current densities than all other samples. The stability does also seem to be improved by the addition of tungsten oxide, electrodes with 3 nm platinum on 3, 10 and 20 nm tungsten oxide, performs better than all other electrodes after the accelerated degradation tests.