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.     

Preparation of a porous carbon from ferrocene-loaded polyaniline and its use in hydrogen adsorption

A porous carbon made of polyaniline with different ferrocene loadings was prepared through carbonization and thermal chemistry activation with KOH. The ferrocene served as a pore-forming agent and a resource of iron nanoparticles. N₂ adsorption/desorption measurements showed that the specific surface area and pore volume ranged from 2681 to 3246 m² g⁻¹ and from 1.56 to 2.06 cm³ g⁻¹, respectively, with increasing ferrocene loadings. Similarly, hydrogen adsorption also increased from 5.3 to 6.2 wt% at 77 K/5 MPa and 0.6 wt% to 0.85 wt% at 293 K/8 MPa. Scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis showed that iron nanoparticles were embedded in the carbon matrix or dispersed on the surface. The large specific surface area and big pore volume improved the original hydrogen adsorption heat up to 7.2 kJ mol⁻¹ for the best sample.     

Solvent-induced porosity control of carbon nanofiber webs for supercapacitor

A simple and scalable method is reported for fabricating a porosity-controlled carbon nanofibers with a skin-core texture by electrospinning a selected blend of polymer solutions. Simple thermal treatment of the electrospun nanofibers from solution blends of various compositions creates suitable ultramicropores on the surface of carbon nanofibers that can accommodate many ions, removing the need for an activation step. The intrinsic properties of the electrode (e.g., nanometre-size diameter, high specific surface area, narrow pore size distribution, tuneable porosity, shallow pore depth, and good ionic accessibility) enable construction of supercapacitors with large specific capacitance (130.7 F g⁻¹), high power (100 kW kg⁻¹), and energy density (15.0 Wh kg⁻¹).     

Changes in hydrogen storage properties of carbon nano-horns submitted to thermal oxidation

The effect of thermal oxidation on the hydrogen storage properties of carbon nano-horns was investigated by gravimetric and electrochemical methods. The pristine nano-horn sample was oxidised at 673 K in air for different periods (15, 30 and 60 min) and the resulting materials were characterised. The N₂ adsorption experiments reveal a marked increase in the surface area, from 267 m² g⁻¹, for the pristine sample, up to 1360 m² g⁻¹ for the sample oxidised for the 60 min period, and a reduction in the average pore diameter. The gravimetric investigation, conducted at low temperature (77 K) showed an increase in the hydrogen storage, from 0.75 wt% for the pristine sample up to 2.60 wt% for the oxidised material. Reproducible and stable hydrogen storage was found for all the samples examined apart from the sample oxidised for 60 min. For the latter, a decrease in the amount of hydrogen stored between the first and second cycles was found. Electrochemical loading of hydrogen in the samples was performed at room temperature (298 K) in alkaline solution by the galvanostatic charge/discharge technique. The results obtained here however show a much lower hydrogen storage level by the samples as compared to the gas storage method, with a maximum value of 0.124 wt% H₂ and with very little dependence on the thermal oxidation treatment.     

Sea urchin-like mesoporous carbon material grown with carbon nanotubes as a cathode catalyst support for fuel cells

A sea urchin-like carbon (UC) material with high surface area (416 m² g⁻¹), adequate electrical conductivity (59.6 S cm⁻¹) and good chemical stability was prepared by growing carbon nanotubes onto mesoporous carbon hollow spheres. A uniform dispersion of Pt nanoparticles was then anchored on the UC, where the Pt nanoparticles were prepared using benzylamine as the stabilizer. For this Pt loaded carbon, cyclic voltammogram measurements showed an exceptionally high electrochemically active surface area (EAS) (114.8 m² g⁻¹) compared to the commonly used commercial E-TEK catalyst (65.2 m² g⁻¹). The durability test demonstrates that the carbon used as a support exhibited minor loss in EAS of Pt. Compared to the E-TEK (20 wt%) cathode catalyst, this Pt loaded UC catalyst has greatly enhanced catalytic activity toward the oxygen reduction reaction, less cathode flooding and considerably improved performance, resulting in an enhancement of ca. 37% in power density compared with that of E-TEK. Based on the results obtained, the UC is an excellent support for Pt nanoparticles used as cathode catalysts in proton exchange membrane fuel cells.     

Pt nanoparticles deposited on TiO2 based nanofibers: Electrochemical stability and oxygen reduction activity

The electrochemical stability of Pt deposited on TiO₂ based nanofibers was compared with commercially available carbon supported Pt. Prior to the Pt deposition the TiO₂ material, which was either undoped or Nb doped, was air calcined. In one case the undoped TiO₂ was also reduced in a hydrogen atmosphere. XRD analysis revealed that the unreduced TiO₂ was present in the anatase phase, irrespective of whether the Nb dopant was present, whereas the rutile phase was formed due to reduction with H₂. The diameter of the TiO₂ fibers varied from 50 to 100 nm, and the average Pt particle diameter was approximately 5 nm. Pt supported on TiO₂ was more stable than Pt supported on C when subjected to 1000 voltammetric cycles in the range of 0.05-1.3 V vs. RHE. Nb doped TiO₂ showed the highest stability, retaining 60% of the electrochemically active surface area after 1000 cycles compared to the state after 100 cycles, whereas the carbon supported catalyst retained 20% of the active surface area. The commercial catalyst had the highest oxygen reduction activity due to its larger specific area (17.1 m² g⁻¹ vs. 5.0 m² g⁻¹ for Pt/TiO₂-Nb, measured after 100 cycles) and the higher support conductivity. The Pt supported on Nb doped or on H₂ reduced TiO₂ was more active than Pt supported on air calcined and otherwise unmodified TiO₂.     

High-frequency carbon supercapacitors from polyfurfuryl alcohol

Porous carbons with controllable and narrow pore-size distributions are prepared from the chemical activation of polyfurfuryl alcohol (PFA). High apparent BET surface areas, up to 2600 m² g⁻¹ (2611 m² g⁻¹ by Density Functional Theory (DFT)), and good electrical conductivities (up to ∼130 S cm⁻¹) are obtained. By varying the potassium hydroxide: carbon precursor ratio, the preparation of carbons with different proportions of micro- and fine mesoporosity (<5 nm) can be tailored to provide an ideal electronic and ionic pore structure for electrochemical energy-storage devices, such as electrical double-layer capacitors. High specific capacitance values are obtained up to 147 F g⁻¹ in a voltage window of 2.5 V using 1 M tetraethyl ammonium tetrafluoroborate in acetonitrile. Moreover, excellent high-current and high-frequency performance is demonstrated: 100 F g⁻¹ at 225 A g⁻¹ (10 Hz) and ∼30 F g⁻¹ at 100 Hz. When comparing the performance with commercial activated carbons (ACs) of similar textural properties, the PFA-derived ACs demonstrated better performance in terms of higher capacitance values and improved rate capabilities. There is a 125% increase in capacitance values at 1 kHz.     

Acetic acid catalysed carbon xerogels derived from resorcinol-furfural for hydrogen storage

Carbon xerogels were prepared from the sol-gel polymerisation of resorcinol with furfural followed by carbonisation and activation, and their hydrogen storage properties were studied. Acetic acid was used as a catalyst to adjust the initial pH value of the resorcinol-furfural solution. The acetic acid facilitates the condensation reaction by reducing the gelation time dramatically from a few days to several hours. The effect of precursor acidity on the final gels are chemically distinguishable (by 13C CPMAS NMR) from the acetic acid catalysed organic gels compared with uncatalysed gel. The connectivity of the primary carbon particles and particle size are related to the degree of acidity, as were found from SEM photographs. The pore structures were studied by small angle X-ray scattering and nitrogen adsorption techniques after carbonisation and activation in order to determine the effect of precursor pH variables on the resulting samples. A catalysed carbon xerogel prepared with pH ∼ 4.8 has a surface area of 1924 ± 26 m² g⁻¹, a micropore volume of 0.86 ± 0.08 cm³ g⁻¹ and a hydrogen uptake of 4.65 wt.% at 77 K and 3.9 MPa, showing it to be a promising material for hydrogen storage.     

Preparation and characterization of carbonaceous materials containing nitrogen as electrochemical capacitor

Carbonaceous materials containing nitrogen (C/N materials) were prepared by a pyrolysis of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (CAN). A C/N material prepared by the pyrolysis of CAN at 1070 K (CAN-1070 K) had a C/N atomic ratio of 3.0 and a non-crystalline carbonaceous structure with a BET surface area of 880 m² g⁻¹. The material CAN-1070 K showed large capacitances of 160–180 F g⁻¹ and 110–120 F cm⁻³ in case of current density of 10 mA cm⁻² (2 A g⁻¹) by using three-electrode cell in 1 M H₂SO₄ aqueous solution, in comparison with that of activated carbon (160 F g⁻¹ and 55 F cm⁻³) having BET surface area of 2300 m² g⁻¹. ESCA study indicated that pyridinic and quarternary nitrogen atoms existed in the C/N materials, which could result in producing a pseudo-capacitance in addition to the electric double layer capacitance. Also introduction of nitrogen into the carbonaceous material could enhance the wettability of material, which might also improve the capacitance.     

Hydrogen generation from catalytic hydrolysis of alkaline sodium borohydride solution using attapulgite clay-supported Co-B catalyst

An attapulgite clay-supported cobalt-boride (Co-B) catalyst used in portable fuel cell fields is prepared in this paper by impregnation-chemical reduction method. The cost of attapulgite clay is much lower compared with some other inert carriers, such as activated carbon and carbon nanotube. Its microstructure and catalytic activity are analyzed in this paper. The effects of NaOH concentration, NaBH₄ concentration, reacting temperature, catalyst loadings and recycle times on the performance of the catalysts in hydrogen production from alkaline NaBH₄ solutions are investigated. Furthermore, characteristics of these catalysts are carried out in SEM, XRD and TEM analysis. The high catalytic activity of the catalyst indicates that it is a promising and practical catalyst. Activation energy of hydrogen generation using such catalysts is estimated to be 56.32 kJ mol⁻¹. In the cycle test, from the 1st cycle to the 9th cycle, the average hydrogen generation rate decreases gradually from 1.27 l min⁻¹ g⁻¹ Co-B to 0.87 l min⁻¹ g⁻¹ Co-B.     

Application of mesoporous carbon to counter electrode for dye-sensitized solar cells

The mesoporous carbons were prepared by the carbonation of the triblock copolymer F127/phloroglucinol-formaldehyde composite self-assembled in an acid medium and employed as the catalyst for triiodide reduction in dye-sensitized solar cells (DSCs). The characteristics of mesoporous carbon were analyzed by scanning electron microscopy, transmission electron microscopy, N₂ sorption measurement and X-ray diffraction. The mesoporous carbon with low crystallinity exhibited Brunauer-Emmett-Teller surface area of 400 m² g⁻¹, pore diameter of 6.8 nm and pore volume of 0.63 cm³ g⁻¹. The photovoltaic performances of DSCs with mesoporous carbon counter electrode were improved by increasing the carbon loading on counter electrode due to the charge-transfer resistance of mesoporous carbon counter electrode decreasing with the increase of the carbon loading. However, further carbon loading increase has no obvious effect on the photovoltaic performance of DSCs with carbon electrode when carbon loading exceeds 300 μg cm⁻². The overall conversion efficiency of 6.18% was obtained by DSCs composed of mesoporous carbon counter electrode with the carbon loading of 339 μg cm⁻². This value is comparable to that of DSCs with conventional platinum counter electrode.     

Electrochemical properties of graphene nanosheets/polyaniline nanofibers composites as electrode for supercapacitors

Graphene nanosheets/polyaniline nanofibers (GNS/PANI) composites are synthesized via in situ polymerization of aniline monomer in HClO₄ solution. The PANI nanofibers homogeneously coating on the surface of GNS greatly improve the charge transfer reaction. The GNS/PANI composites exhibit better electrochemical performances than the pure individual components. A remarkable specific capacitance of 1130 F g⁻¹ (based on GNS/PANI composites) is obtained at a scan rate of 5 mV s⁻¹ in 1 M H₂SO₄ solution compared to 402 F g⁻¹ for pure PANI and 270 F g⁻¹ for GNS. The excellent performance is not only due to the GNS which can provide good electrical conductivity and high specific surface area, but also associate with a good redox activity of ordered PANI nanofibers. Moreover, the GNS/PANI composites present excellent long cycle life with 87% specific capacitance retained after 1000 charge/discharge processes. The resulting composites are promising electrode materials for high-performance electrical energy storage devices.     

Structure and electrochemical properties of activated polyacrylonitrile based carbon fibers containing carbon nanotubes

Solution spun polyacrylonitrile (PAN), PAN/multi-wall carbon nanotube (MWCNT), and PAN/single-wall carbon nanotube (SWCNT) fibers containing 5 wt.% carbon nanotubes were stabilized in air and activated using CO₂ and KOH. The surface area as determined by nitrogen gas adsorption was an order of magnitude higher for KOH activated fibers as compared to the CO₂ activated fibers. The specific capacitance of KOH activated PAN/SWCNT samples was as high as 250 F g⁻¹ in 6 M KOH electrolyte. Under the comparable KOH activation conditions, PAN and PAN/SWCNT fibers had comparable surface areas (BET surface area about 2200 m² g⁻¹) with pore size predominantly in the range of 1–5 nm, while surface area of PAN/MWCNT samples was significantly lower (BET surface area 970 m² g⁻¹). The highest capacitance and energy density was obtained for PAN/SWCNT samples, suggesting SWCNT advantage in charge storage. The capacitance behavior of these electrodes has also been tested in ionic liquids, and the energy density in ionic liquid is about twice the value obtained using KOH electrolyte.     

On-board hydrogen storage and production: An application of ammonia electrolysis

On-board hydrogen storage and production via ammonia electrolysis was evaluated to determine whether the process was feasible using galvanostatic studies between an ammonia electrolytic cell (AEC) and a breathable proton exchange membrane fuel cell (PEMFC). Hydrogen-dense liquid ammonia stored at ambient temperature and pressure is an excellent source for hydrogen storage. This hydrogen is released from ammonia through electrolysis, which theoretically consumes 95% less energy than water electrolysis; 1.55 Wh g⁻¹ H₂ is required for ammonia electrolysis and 33 Wh g⁻¹ H₂ for water electrolysis. An ammonia electrolytic cell (AEC), comprised of carbon fiber paper (CFP) electrodes supported by Ti foil and deposited with Pt-Ir, was designed and constructed for electrolyzing an alkaline ammonia solution. Hydrogen from the cathode compartment of the AEC was fed to a polymer exchange membrane fuel cell (PEMFC). In terms of electric energy, input to the AEC was less than the output from the PEMFC yielding net electrical energies as high as 9.7 ± 1.1 Wh g⁻¹ H₂ while maintaining H₂ production equivalent to consumption.     

A novel synthesis of mesoporous carbon microspheres for supercapacitor electrodes

Mesoporous carbon microspheres (MCMs) with the diameters of 0.5-2.0 μm, main mesopore sizes of 2.6-4.0 nm and specific surface areas of 449-1212 m² g⁻¹ are synthesized by a novel hydrothermal emulsion-activated method. The typical MCMs as electrode materials have a specific capacitance of 157 F g⁻¹ at a high current density of 10.0 A g⁻¹ in 6 M KOH aqueous solution. The resultant MCMs electrode materials with high current charge and discharge capability in 6 M KOH aqueous solution provide important prospect for electrode materials in supercapacitors which could offer high power density for electric vehicles.     

Growth of polyaniline nanowhiskers on mesoporous carbon for supercapacitor application

Vertically aligned polyaniline nanowhiskers (PANI-NWs) doped with (1R)-(−)-10-Camphorsulfonic acid (L-CSA) have been successfully synthesized on the external surface of ordered mesoporous carbon (CMK-3) by chemical oxidative polymerization. The specific surface area of the PANI-NWs/CMK-3 nanocomposite remains as high as 497 m² g⁻¹ by removing mesoporous silica template after the polymerization of aniline. Structural and morphological characterizations of the nanocomposite were further investigated by XRD, FTIR and FE-SEM measurements. The result shows that the nanocomposite with 40 wt% PANI applying in supercapacitor devices possesses a large specific capacitance of 470 F g⁻¹ and good capacitance retention of 90.4% is achieved after 1000 cycles at a current density of 1.0 A g⁻¹. The synergistic effect of small PANI nanowhisker arrays and well-ordered mesoporous carbon endows the composite with high electrochemical capacitance and good cycling stability.     

A highly ordered Fe-N-C nanoarray as a non-precious oxygen-reduction catalyst for proton exchange membrane fuel cells

A highly ordered Pt-free Fe-N-C catalyst is synthesized through a hydrogen bonding-assisted self-assembly route. The catalyst has a porous structure with an average pore size of 5.5 nm and a large surface area of 416 m² g⁻¹, making it highly active in oxygen reduction. Cells assembled with the synthesized catalyst perform significantly better than those assembled with amorphous Fe-N-C cathode catalysts. The maximum powers of cells assembled from the highly ordered and amorphous catalysts are 252 and 60 mW cm⁻², respectively.     

The gasification of wet biomass using Ca(OH)2 as CO2 absorbent: The microstructure of char and absorbent

Aimed at the hydrogen production by gasification of wet biomass coupled CO₂ absorption, the microstructures of chars produced from gasification of wet/pre-dried biomass and several possible stages of absorbent were studied with N₂ adsorption, XRD and SEM. The results show that all the chars examined have essentially microporous structures of pore size less than 2 nm, and the char obtained from gasification of wet biomass at 923 K exhibits larger surface area (A_BET) of 430.87 m² g⁻¹ and total pore volume (V_pore) of 0.188 m³ g⁻¹ than those (368.15 m² g⁻¹ and 0.157 m³ g⁻¹) from pre-dried biomass at the same temperature. The great change of microstructures in the absorbents was observed in the gasification process. The CaO generated from the dehydration of Ca(OH)₂ has larger A_BET and V_pore, than its former Ca(OH)₂, and the increase of pore mainly belongs to mesopores ranges (pore size between 2 nm and 50 nm). However, the generated CaO suffers from a jam of mesopores with the formation of CaCO₃ which will cause incomplete utilization. Although the burning of SR sample can realize CaO generation, the regenerated CaO with a totally deteriorated pore networks suggests poor reactivity. With the increase of temperature the CO₂ absorption by CaO is weakened gradually except in low temperature range of less than 973 K, meaning that the maximum temperature of the process should not exceed the decomposition temperature of CaCO₃.     

High activity Pt/C catalyst for methanol and adsorbed CO electro-oxidation

A high active Pt/C(b) catalyst was prepared by chemical reduction. The experimental results showed that the Pt/C(b) catalyst formed by reduction of hexachloroplatinic acid with formic acid has excellent catalytic properties for methanol and adsorbed CO(CO_ad) electro-oxidation. The electrocatalytic activity of the catalyst was characterized as having a specific surface activity of 33.38 mA cm⁻² at 0.6 V (versus Ag-AgCl). The Pt in the catalyst was well dispersed on carbon with an electrochemically-active specific surface area (ESA) of 84.16 m² g⁻¹ and a BET specific surface area of 192.34 m² g⁻¹ and an average particle size of about 2.6 nm. The catalyst showed a very good stability for 12 h.     

Electrochemical performance of mesoporous TiO2 anatase

Mesoporous TiO₂ was prepared via a sol–gel method from an ethylene glycol-based titanium-precursor in the presence of a non-ionic surfactant at pH 2. Only the anatase structure was detected after annealing, while the BET specific surface area was measured as being 90 m² g⁻¹ with a rather monomodal pore diameter close to 5 nm. Electrochemical performances were investigated by cyclic voltammetry and galvanostatic techniques. Mesoporous TiO₂ exhibits excellent rate capability (184 mAh g⁻¹ at C/5, 158 mAh g⁻¹ at 2C, 127 mAh g⁻¹ at 6C, and 95 mAh g⁻¹ at 30C) and good cycling stability.