Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

Effects of magnesium doping on electronic conductivity and electrochemical properties of LiFePO4 prepared via hydrothermal route

Carbon free composites Li_1−xMg_xFePO₄ (x = 0.00, 0.02) were synthesized from LiOH, H₃PO₄, FeSO₄ and MgSO₄ through hydrothermal route at 180 °C for 6h followed by being fired at 750 °C for 6 h. The samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), flame atomic absorption spectroscopy and electronic conductivity measurement. To investigate their electrochemical properties, the samples were mixed with glucose as carbon precursors, and fired at 750 °C for 6 h. The charge–discharge curves and cycle life test were carried out at 23 ± 2 °C. The Rietveid refinement results of lattice parameters of the samples indicate that the magnesium ion has been successfully doped into the M1 (Li) site of the phospho-olivine structure. With the same order of magnitude, there is no material difference in terms of the electronic conductivities between the doped and undoped composites. Conductivities of the doped and undoped samples are 10⁻¹⁰ S cm⁻¹ before being fired, 10⁻⁹ S cm⁻¹ after being fired at 750 °C, and 10⁻¹ S cm⁻¹ after coated with carbon, respectively. Both the doped and undoped composites coated with carbon exhibit comparable specific capacities of 146 mAh g⁻¹ vs. 144 mAh g⁻¹ at 0.2 C, 140 mAh g⁻¹ vs. 138 mAh g⁻¹ at 1 C, and 124 mAh g⁻¹ vs. 123 mAh g⁻¹ at 5 C, respectively. The capacity retention rates of both doped and undoped samples over 50 cycles at 5 C are close to 100% (vs. the first-cycle corresponding C-rate capacity). Magnesium doping has little effects on electronic conductivity and electrochemical properties of LiFePO₄ composites prepared via hydrothermal route.     

Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation

Nitrogen and sulfur co-doped and N-doped TiO₂ anatase TKP 102 (Tayca) were prepared by manual grinding with thiourea and urea, respectively, and annealing at 400 °C. Both materials showed visible-light absorption as measured by Diffuse Reflectance Spectroscopy (DRS). Interstitial N-doping, anionic and cationic S-doping was found when the TiO₂ was doped with thiourea while TiO₂ doped with urea showed only the presence of interstitial N-doping as measured by X-ray Photo-electron Spectroscopy (XPS). The N content on the surface of N-doped TKP 102 photocatalyst was 2.85 at.% and higher than the N content in the N, S co-doped TiO₂ photocatalyst (0.6 at.%).The photocatalytic activity of the doped catalysts was tested using phenol and Escherichia coli as chemical and biological targets, respectively, using N, S co-doped, N-doped TiO₂, undoped Degussa P-25 and undoped TKP 102 powders under simulated solar light. It was found that undoped Degussa P-25 was the photocatalyst with the highest photocatalytic activity towards phenol oxidation and E. coli inactivation. N, S co-doped powders showed almost the same photocatalytic activity as undoped TKP 102 while N-doped TKP 102 was the less active photocatalyst probably due the N impurities on the TiO₂ acting as recombination centers.     

Pt/carbon nanofibers electrocatalysts for fuel cells: Effect of the support oxidizing treatment

Different Pt-based electrocatalysts supported on carbon nanofibers and carbon black (Vulcan XC-72R) have been prepared using a polymer-mediated synthesis. The electrocatalysts have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and cyclic voltammetry. The effect of carbon nanofibers treatment with HNO₃ solution on Pt particle size and electroactive area has been analyzed. Highly dispersed Pt with homogeneous particle size and an electroactive area around of 100 m² g⁻¹ is obtained in raw carbon nanofibers. The oxidizing treatment of the carbon nanofibers produces agglomeration of the platinum nanoparticles and an electroactive area of 53 m² g⁻¹. Durability studies indicate a decrease of 14% in the electroactive area after 90 h at 1.2 V in 0.5 M H₂SO₄ for platinum supported on raw carbon nanofibers and Vulcan XC-72R. The electrocatalyst supported on oxidized carbon nanofibers are stable under similar conditions.     

Synthesis and characterization of carbon nanotubes supported platinum nanocatalyst for proton exchange membrane fuel cells

Multi-walled carbon nanotubes (MWCNTs) were used as catalyst support for depositing platinum nanoparticles by a wet chemistry route. MWCNTs were initially surface modified by citric acid to introduce functional groups which act as anchors for metallic clusters. A two-phase (water-toluene) method was used to transfer PtCl₆²⁻ from aqueous to organic phase and the subsequent sodium formate solution reduction step yielded Pt nanoparticles on MWCNTs. High-resolution TEM images showed that the platinum particles in the size range of 1-3 nm are homogeneously distributed on the surface of MWCNTs. The Pt/MWCNTs nanocatalyst was evaluated in the proton exchange membrane (PEM) single cell using H₂/O₂ at 80 °C with Nafion-212 electrolyte. The single PEM fuel cell exhibited a peak power density of about 1100 mW cm⁻² with a total catalyst loading of 0.6 mg Pt cm⁻² (anode: 0.2 mg Pt cm⁻² and cathode: 0.4 mg Pt cm⁻²). The durability of Pt/MWCNTs nanocatalyst was evaluated for 100 h at 80 °C at ambient pressure and the performance (current density at 0.4 V) remained stable throughout. The electrochemically active surface area (64 m² g⁻¹) as estimated by cyclic voltammetry (CV) was also similar before and after the durability test.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Kinetics of oxygen reduction reaction on Corich core-Ptrich shell/C electrocatalysts

The nanostructured Co_{rich core}-Pt_{rich shell}/C electrocatalysts were prepared by combining the thermal decomposition and the chemical reduction methods. The particle size of homemade Co_{rich core}-Pt_{rich shell}/C analyzed by TEM was significantly greater than that of Pt grain size calculated from the XRD data due to the existence of Co in core. The mass activity and specific activity of oxygen reduction reaction (ORR) at the overpotential (η) of 0.1 V were 6.69 A g⁻¹ and 1.51 × 10⁻⁵ A cm⁻² for Pt/C, and 10.22 A g⁻¹ and 2.73 × 10⁻⁵ A cm⁻² for Co_{rich core}-Pt_{rich shell}/C in 0.5 M HClO₄ aqueous solution at 25 °C. The Tafel slopes of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C electrocatalysts were obtained as 64 and 67 mV dec⁻¹ at a lower η (50–100 mV), and 116 and 110 mV dec⁻¹ at a higher η (120–200 mV). The exchange current densities of ORR on Pt/C and Co_{rich core}-Pt_{rich shell}/C evaluated based on the higher Tafel slope regions were 6.76 × 10⁻⁵ and 9.21 × 10⁻⁵ A cm⁻², respectively. The experimental results indicated that the ORR on Co_{rich core}-Pt_{rich shell}/C electrocatalyst in 0.5 M HClO₄ aqueous solution was a four electron transfer mechanism and first order with respect to the dissolved oxygen.     

TiO2(B) as a promising high potential negative electrode for large-size lithium-ion batteries

Needle-like TiO₂(B) powder was obtained from K₂Ti₄O₉ precursor by ion exchange to protons, followed by dehydration. The charge and discharge characteristics of the TiO₂(B) powder were investigated as a high potential negative electrode in lithium-ion batteries. It had a high discharge capacity of 200–250 mAh g⁻¹ at around 1.6 V vs. Li/Li⁺, which was comparable with that of TiO₂(B) nanowires and nanotubes prepared via a hydrothermal reaction in alkaline solution. It showed very good cycleability, and gave a discharge capacity of 170 mAh g⁻¹ even in the 650th cycle. It also had a high rate capability, and gave a discharge capacity of 106 mAh g⁻¹ even at 10 °C.     

Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell

Thin film Pt/TiO₂ catalysts are evaluated in a polymer electrolyte electrochemical cell. Individual thin films of Pt and TiO₂, and bilayers of them, were deposited directly on Nafion membranes by thermal evaporation with varying deposition order and thickness (Pt loadings of 3–6 μg cm⁻²). Structural and chemical characterization was performed by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) polarization plots show that the presence of a thin TiO₂ layer between the platinum and the Nafion increases the performance compared to a Pt film deposited directly on Nafion. Based on the TEM analysis, we attribute this improvement to a better dispersion of Pt on TiO₂ compared to on Nafion and in addition, substantial proton conduction through the thin TiO₂ layer. It is also shown that deposition order and the film thickness affects the performance.     

Preparation and electrochemical properties of TiO2 hollow spheres as an anode material for lithium-ion batteries

TiO₂ hollow spheres are fabricated by a sol-gel process using carbon spheres as template. The diameter and the shell thickness of the TiO₂ hollow spheres are about 400-600 nm and 60-80 nm, respectively. The electrochemical properties of the hollow spheres are investigated by galvanostatic cycling and cyclic voltammetry (CV) measurements. The initial discharge capacity reaches 291.2 mAh g⁻¹ at a current density of 60 mA g⁻¹. The average discharge capacity loss is about 1.72 mAh g⁻¹ per cycle from the 2nd to the 40th cycles and the coulombic efficiency is approximately 98% after 40 cycles, indicating excellent cycling stability and reversibility.     

Comparison of solar cell performance of conducting polymer dyes with different functional groups

Conductive polymer precursors, including carboxylic acid, cyano groups, amino groups, 5,2′:5′,2″-terthiophene-3′-carboxylic acid (TTCA), 3′-cyano-5,2′:5′,2″-terthiophene (CTT), and 3′,4′-diamino-2,2′:5′,2″-terthiophene (DATT) are synthesized. Electrochemically polymerized films of the precursors on a nanocrystalline TiO₂ layer are examined as photo sensitizers, and the cell performance is compared. The photovoltaic cells are assembled with a polymer-coated TiO₂ layer treated with TiCl₄ as an anode and a Pt layer as a cathode in a propionitrile solution containing an iodide ion-based redox electrolyte. The charge-transfer processes of polymer-dyed cells are studied using impedance spectroscopy. The polymer dyes on the TiO₂ surfaces are characterized by scanning electron microscope (SEM), atomic force microscope (AFM), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). XPS results show that the conducting polymer dye, bearing a carboxylic acid group, is more strongly bound to the TiO₂ layer in comparison with other groups. Various experimental parameters affecting the cell efficiency are optimized, including the scan rate, number of potential cycles, and terthiophene monomer concentration. Of these polymers, the best cell efficiency is attained for poly-TTCA containing a carboxylic acid group. The optimized cell with the poly-TTCA dye shows a short-circuit current of 6.78 mA cm⁻², an open-circuit voltage of 0.54 V, and a fill factor of 63.6. An energy conversion efficiency of 2.32% is obtained with a cell area of 0.24 cm² under an air mass 1.5 solar simulated light irradiation of 100 mW cm⁻².     

Electrochemical corrosion of platinum electrode in concentrated sulfuric acid

We report on the electrochemical corrosion of a Pt electrode in strong sulfuric acid. The electrochemical measurements were conducted using a Pt-flag working electrode, Ag/Ag₂SO₄ reference electrode and Pt counter electrode at 25 °C. The measured cyclic voltammograms significantly changed in the H₂SO₄ concentration range of 0.5–18 mol dm⁻³, especially from 14 to 18 mol dm⁻³. After successive potential sweeps for 15 h in 16 mol dm⁻³ H₂SO₄, a weight loss of the Pt-flag electrode was realized. In contrast, a controlled potential electrolysis by cathodic polarization caused a weight gain, which was attributed to sulfur deposition by the H₂SO₄ reduction. The subsequent anodic polarization produced corrosion of the deposited sulfur. Consequently, the alternating polarization generated platinum corrosion, resulted in the production of platinum and sulfur composite particulates in the solution.     

Platinum nanoparticles supported on electrospinning-derived carbon fibrous mats by using formaldehyde vapor as reducer for methanol electrooxidation

The Pt nanoparticles have been well dispersed on electrospinning-derived carbon fibrous mats (CFMs) by using formaldehyde vapor as reducer to react with H₂PtCl₆·6H₂O adsorbed on the CFMs at 160 °C. The prepared electrodes of Pt-CFMs have been characterized by using scanning electron microscopy, transmission electron microscopy and X-ray diffraction spectroscopy, and the performance of the electrodes for methanol oxidation has been investigated by using cyclic voltammetry, chronoamperometry, quasi-steady state polarization and electrochemical impedance spectroscopy techniques. The results demonstrate that Pt-CFMs electrodes exhibit peak current density of 445 mA mg⁻¹ Pt, exchange current of 235.7 μA cm⁻², charge transfer resistance of 16.1 Ω cm² and better stability during the process of methanol oxidation, which are superior to the peak current density of 194 mA mg⁻¹ Pt, exchange current of 174.7 μA cm⁻² and charge transfer resistance of 39.4 Ω cm² obtained for commercial Pt/C supported on CFMs. It indicates that the novel process in which formaldehyde vapor is used as reducer to prepare Pt catalyst with high performance can be developed.     

Effects of anatase TiO2 with different particle sizes and contents on the stability of supported Pt catalysts

Pt/TiO₂-C catalyst with TiO₂ and carbon black as the mixed support has been synthesized by the microwave-assisted polyol process (MAPP). Effects of anatase TiO₂ with different particle sizes and contents on the stability of supported Pt catalysts have been systematically studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammograms (CV), and accelerated potential cycling tests (APCT) have been carried out to present the influence degree. The experimental results indicate that the original electrochemically active specific surface areas (ESA) of the catalysts decrease with the increase of mean particle sizes of TiO₂ and TiO₂ contents. However, the activity of Pt/TiO₂-C-20 is very close to that of Pt/TiO₂-C-5 and the stability of Pt/TiO₂-C-20 is the best after 1000 cycles APCT, illustrating that the optimized particle size of TiO₂ in Pt/TiO₂-C catalyst is 20 nm. Furthermore, the stability of the catalysts increase with the increase of TiO₂ contents in the mixed support. Taking into account both the activity and stability of various Pt/TiO₂-C catalysts, the optimized particle size of TiO₂ is 20 nm and the optimal TiO₂ content existed in the mixed support is 40%.     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.     

Doping effects on complex perovskite Ba3Ca1.18Nb1.82O9−δ intermediate temperature proton conductor

In this work, the effects of Ce doping on the Ca and Nb ions in complex perovskite Ba₃Ca_1.18Nb_1.82O_9−δ (BCN18) proton conductor have been evaluated. It has been found that cerium ions can be doped into both the Ca and Nb sites to form a single-phase complex perovskite structure when the sintering temperature is 1550 °C. Ce ions substituted with Nb ions enhances the electrical conductivity, especially the grain boundary conductivity. The highest conductivity has been obtained for a composition of Ba₃Ca_1.18Nb_1.62Ce_0.2O_9−δ, possessing a conductivity of 2.69 × 10⁻³ S cm⁻¹ at 550 °C in wet H₂, a 78% enhancement compared with BCN18 (1.51 × 10⁻³ S cm⁻¹). The chemical stability tests show that Ce-doped BCN18 samples remain single phase after treated either in boiling water for 7 h or in pure CO₂ for 4 h at 700 °C. This work has demonstrated a new direction in developing intermediate temperature proton conducting materials that possess both high conductivity and good stability.     

Inkjet printing of carbon supported platinum 3-D catalyst layers for use in fuel cells

We present a method of using inkjet printing (IJP) to deposit catalyst materials onto gas diffusion layers (GDLs) that are made into membrane electrode assemblies (MEAs) for polymer electrolyte fuel cell (PEMFC). Existing ink deposition methods such as spray painting or screen printing are not well suited for ultra low (<0.5 mg Pt cm⁻²) loadings. The IJP method can be used to deposit smaller volumes of water based catalyst ink solutions with picoliter precision provided the solution properties are compatible with the cartridge design. By optimizing the dispersion of the ink solution we have shown that this technique can be successfully used with catalysts supported on different carbon black (i.e. XC-72R, Monarch 700, Black Pearls 2000, etc.). Our ink jet printed MEAs with catalyst loadings of 0.020 mg Pt cm⁻² have shown Pt utilizations in excess of 16,000 mW mg⁻¹ Pt which is higher than our traditional screen printed MEAs (800 mW mg⁻¹ Pt). As a further demonstration of IJP versatility, we present results of a graded distribution of Pt/C catalyst structure using standard Johnson Matthey (JM) catalyst. Compared to a continuous catalyst layer of JM Pt/C (20% Pt), the graded catalyst structure showed enhanced performance.     

Iron/carbon-black composite nanoparticles as an iron electrode material in a paste type rechargeable alkaline battery

Iron/carbon-black composite nanoparticles were synthesized by chemically reducing the iron salt mixing with carbon black by adding NaBH₄ in the aqueous solution. Carbon-black particles, with a mean particle size of approximately 40 nm, function as the nucleation cores for iron deposition. Additionally, core-shell iron composite particles are observed to be 30-100 nm with spherical sharp. At the first time discharge, the iron/carbon-black composite nanoparticle discharged 1200 mAh g⁻¹(Fe) at plateau one and 400 mAh g⁻¹at plateau two at a high current density of 200 mA g⁻¹(Fe). The capacity is larger than the theoretical value, which is attributed to the formation of iron hydride (FeH_x) while the iron was reduced by NaBH₄, followed the hydrogen reaction as an active material while the battery discharge occurs. In further cycles, the iron/carbon-black composite iron electrode shows a good reversibility of about 600 mAh g⁻¹(Fe) when the nickel-iron battery operated between 1.65 and 0.8 V. XRD analysis results indicate that the carbon black in the core of the iron/carbon-black composite enhances the reduction/oxidation reactions of iron, as achieved by the carbon black forming an enhanced electronic conductive network while iron is discharged as the insulator species such as Fe(OH)₂ and Fe₃O₄. SEM images reveal that the iron/carbon-black composite keeps particle sizes smaller than 300 nm during the electrode cycling, indicating that carbon black also acts as the nucleation cores for the dissolution-deposition of iron.     

Dye-sensitized solar cells with a micro-porous TiO2 electrode and gel polymer electrolytes prepared by in situ cross-link reaction

The ionic conductivities and performances of dye-sensitized solar cells (DSSCs) of gel polymer electrolytes (GPEs) prepared by in situ cross-link reaction with different cross-linkers were investigated. The poly(imidazole-co-butylmethacrylate)-based GPE containing the 1,2,4,5-tetrakis(bromomethyl)benzene (B4Br) cross-linker showed a higher ionic conductivity than that containing cross-linkers with a linear structure, due to the formation of micro-phase separation that resulted in an increase in ion transport paths in the GPE. Moreover, the co-adsorbent ((4-pyridylthio) acetic acid, PAA) co-adsorbed with N3 dye on the TiO₂ electrode not only reduced dye aggregation, but also reacted with the cross-linkers in the GPE at the TiO₂/GPE interface. A decrease in the charge transport resistance at the TiO₂/GPE interface was noted after forming the gel; thus the value of J_SC significantly increased from 7.72 to 10.00 mA cm⁻². In addition, in order to reduce the ionic diffusion resistance within the TiO₂ electrode, incorporation of monodispersed PMMA in the TiO₂ paste was considered. With the optimal weight ratio of PMMA/TiO₂ (w/w=3.75), the TiO₂ electrode exhibited larger pores (ca. 350 nm) uniformly distributed after sintering at 500 °C, and the ionic diffusion resistance within the TiO₂ film could significantly be reduced. The cell conversion efficiency increased from 3.61% to 5.81% under illumination of 100 mW cm⁻², an improvement of ca. 55%.     

Electrochemical lithium storage of TiO2 hollow microspheres assembled by nanotubes

TiO₂ hollow microspheres with the shell consisting of nanotubes have been successfully synthesized via a template-free hydrothermal process and subsequent treatments. The electrochemical properties of the anatase sample have been investigated by cyclic voltammetry and galvanostatic method. The initial Li insertion/extraction capacity at a current density of 0.2 C reach 290 and 232 mAh g⁻¹ respectively. Moreover, as-prepared TiO₂ delivers a reversible capacity of ca. 150 mAh g⁻¹ after 500 cycles at 1 C, and it also shows superior high rate performance (e.g., 90 mAh g⁻¹ at 8 C) without any modification.