Comparison of the electrochemical oxidation of borohydride and dimethylamine borane on platinum electrodes: Implication for direct fuel cells

The electrochemical behaviour of dimethylamine borane and borohydride on platinum electrodes was investigated by cyclic voltammetry and polarization curves in discharges processes. Several overlapping peaks appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 V to −0.50 V versus Ag/AgCl, mainly with the borohydride. This behaviour is associated with the hydrolysis of BH₄⁻ or (CH₃)₂NHBH₃. As a consequence of secondary reactions the borohydride and dimethylamine borane oxidation in 3 M NaOH solution shows, respectively, a four- to six-electron process and a four- to five-electron process in direct fuel cells. The direct oxidation of the borohydride exhibits a peak at about −0.07 V versus Ag/AgCl, while the dimethylamine borane peak is at about −0.03 V versus Ag/AgCl. For the 0.04 M concentration the borohydride displays a power density of 31 W m⁻² which is 16% higher than that of the dimethylamine borane.     

Investigation of hydrogen generation from sodium borohydride using different cobalt catalysts

Effective Co/Cu, CoB/Cu, and CoBM (M = Mo,Zn,Fe)/Cu catalysts were prepared on the copper surface by a simple electroless deposition method using a morpholine borane as a reducing agent in the glycine solution. The activity of the deposited catalysts was investigated for hydrogen generation from an alkaline sodium borohydride solution. It was determined that these synthesized catalysts demonstrated the catalytic activity for the hydrolysis reaction of NaBH₄. The lowest obtained activation energy (E_A) of the hydrolysis reaction of NaBH₄was 27 kJ mol^?1 for the CoBMo/Cu catalyst. The hydrogen generation rate of 15.30 ml min^?1 was achieved using CoBMo/Cu catalysts at 313 K and it increased ~3.5 times with the increase of temperature to 343 K. The highest hydrogen generation rate obtained by CoBMo/Cu films may be related to the hierarchical cauliflower-shaped 3D structures and the high roughness surface area. Moreover, the CoBMo/Cu catalyst showed an excellent reusability.     

Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells

A novel Pd electrocatalyst is developed by self-assembly of Pd nanopartilces on phosphomolybdic acid (HPMo)-poly(diallyldimethylammonium chloride) (PDDA)-functionalized multiwalled carbon nanotubes supports (Pd/HPMo-PDDA-MWCNTs). The as-synthesized Pd/HPMo-PDDA-MWCNTs were characterized by TEM, EDS mapping, Raman spectra, X-ray photoelectron spectroscopy, electrochmeical CO stripping and cyclic voltammetry techniques. Pd nnaoparticles deposited on HPMo-PDDA-MWCNTs are in the range of 3.1 nm with uniform distributon. Pd/HPMo-PDDA-MWCNT catalysts have lower overpotential for CO_ad oxidation manifested as lower peak and onset potentials as compared to acid-treated MWCNTs supported Pd (Pd/AO-MWCNTs) and carbon supported Pd catalysts (Pd/C). Pd/HPMo-PDDA-MWCNTs catalysts also exhibit a much higher electrocatalytic activity and stability for formic acid oxidation reaction as compared to that on Pd/AO-MWCNTs and Pd/C. The high electrocatalytic activities of Pd/HPMo-PDDA-MWCNTs catalysts are most likely related to highly dispersed and fine Pd nanoparticles as well as synergistic effects between Pd and HPMo immobilized on PDDA-functionalized MWCNTs.     

Pd nanoparticles supported on MIL-101 as high-performance catalysts for catalytic hydrolysis of ammonia borane

Well dispersed ultrafine Pd NPs have been immobilized in the framework of MIL-101, and tested for the catalytic hydrolysis of ammonia borane. The powder XRD, N₂ adsorption–desorption, TEM, and ICP-AES were employed to characterize the Pd@MIL-101 catalyst. The as-synthesized Pd@MIL-101 exhibit the highest catalytic activity toward hydrolysis of AB among the Pd-based nano-catalysts ever reported, with the TOF value of 45 mol H₂ min⁻¹ (mol Pd)⁻¹.     

Review of methane catalytic cracking for hydrogen production

Methane catalytic cracking is a process by which carbon monoxide-free hydrogen can be produced. Despite the fact that hydrogen produced from methane cracking is a pure form of hydrogen, methane cracking is not used on an industrial scale for producing hydrogen since it is not economically competitive with other hydrogen production processes. However, pure hydrogen demand is increasing annually either in amount or in number of applications that require carbon monoxide-free hydrogen. Currently, hydrogen is produced primarily via catalytic steam reforming, partial oxidation, and auto-thermal reforming of natural gas. Although these processes are mature technologies, CO is formed as a by-product, and in order to eliminate it from the hydrogen stream, complicated and costly separation processes are required. To improve the methane catalytic cracking economics, extensive research to improve different process parameters is required. Using a highly active and stable catalyst, optimizing the operating conditions, and developing suitable reactors are among the different areas that need to be addressed in methane cracking. In this paper, catalysts that can be used for methane cracking, and their deactivation and regeneration are discussed. Also, methane catalytic cracking kinetics including carbon filament formation, the reaction mechanisms, and the models available in the literature for predicting reaction rates are presented. Finally, the application of fluidized beds for methane catalytic cracking is discussed.     

Borohydride electrochemical oxidation on carbon-supported Pt-modified Au nanoparticles

The carbon-supported Pt-modified Au nanoparticles were prepared by two different chemical reduction processes, the simultaneous chemical reduction of Pt and Au on carbon process (A-AuPt/C) and the successive reduction of Au then Pt (B-AuPt/C) on carbon process. These two catalysts were investigated as the anode catalysts for a direct borohydride fuel cell (DBFC) and Au nanoparticles on carbon (Au/C) were also prepared for comparison. The DBFC with B-AuPt/C as the anode catalyst shows the best anode and fuel cell performance. The maximum power density with the B-AuPt/C catalyst is 112 mW cm⁻² at 40 °C, compared to 97 mW cm⁻² for A-AuPt/C and 65 mW cm⁻² for Au/C. In addition, the DBFC with the B-AuPt/C catalyst shows the best fuel utilization with a maximum apparent number of electrons (N_app) equal to 6.4 in 1 M NaBH₄ and 7.2 in 0.5 M NaBH₄ as compared to the value of N_app of 8 for complete utilization of borohydride.     

Effect of catalytic Pd coating on the hydrogen storage performances of ZrCo alloy by electroless plating method

The effects of Pd coating with different deposition concentration (PdCl₂ 0.2 g L^?1, 0.6 g L^?1, 1.0 g L^?1) on the surface morphology, microstructure and hydrogen storage performances of ZrCo alloy have been investigated. Results show that spherical Pd particles have been deposited on the surface of ZrCo alloy successfully, which transfer from sparse arrangement to continuous and compact film with increasing deposition concentration of PdCl₂. The hydriding kinetic property of all Pd coated alloys is improved compared with the bare alloy, which is due to the catalyst effect of Pd coating. The hydriding rate of the samples firstly increases and then decreases with increasing deposition concentration, which is closely related to the surface morphology and thickness of Pd coating. The hydriding kinetic property of the samples is greatly improved after 5 cycles, although Pd particles on the alloy surface peel off to some extent. This phenomenon indicates that the accumulated fresh surface during cycling makes a greater contribution to the improved hydriding kinetic property and the catalyst effect of Pd coating is weakened during cycling.     

Shape-controlled Pd nanostructure catalysts for highly efficient electrochemical power sources

We report polygonal Pd catalysts on carbon black synthesized by means of polyol process in the presence of poly(vinyl pyrrolidone) and NO₃⁻ ion. We find that the polygonal Pd/C has dominant {1 1 1} facets observed by X-ray diffraction method and transmission electron microscopy analysis. The current density for formic acid electrooxidation of polygonal Pd/C (1.908 mA cm⁻²) with controlled surface structures such as dominant {1 1 1} facets is much higher than Pd/C (0.478 mA cm⁻²) at 0 V. Furthermore, the polygonal Pd/C with controlled surface structures shows much improved performances in dye-sensitized solar cells (DSSCs) due to its highly thermal stability and enhanced catalytic activity for iodide reduction.     

Carbon-supported Pd nanocatalyst modified by non-metal phosphorus for the oxygen reduction reaction

A carbon-supported palladium catalyst modified by non-metal phosphorus (PdP/C) has been developed as an oxygen reduction catalyst for direct methanol fuel cells. The PdP/C catalyst was prepared by the sodium hypophosphite reduction method. The as-prepared Pd nanoparticles have a narrow size distribution with an average diameter of 2 nm. Energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) results indicate that P enters into the crystal lattice of Pd and forms an alloy. The PdP/C catalyst has an oxygen reduction reaction (ORR) activity comparable to the commercial Pt/C catalyst and a higher activity than the Pd/C catalyst synthesized by the conventional NaBH₄ reduction method. Its high catalytic activity can be attributed to its small size, lower relative crystallinity, and the formation of PdP alloy.     

Development of Pd and Pd-Co catalysts supported on multi-walled carbon nanotubes for formic acid oxidation

Pd-Co and Pd catalysts were prepared by the impregnation synthesis method at low temperature on multi-walled carbon nanotubes (MWCNTs). The nanotubes were synthesized by spray pyrolysis technique. Both catalysts were obtained with high homogeneous distribution and particle size around 4 nm. The morphology, composition and electrocatalytic properties were investigated by transmission electron microscopy, scanning electron microscopy-energy dispersive X-ray analysis, X-ray diffraction and electrochemical measurements, respectively. The electrocatalytic activity of Pd and PdCo/MWCNTs catalysts was investigated in terms of formic acid electrooxidation at low concentration in H₂SO₄ aqueous solution. The results obtained from voltamperometric studies showed that the current density achieved with the PdCo/MWCNTs catalyst is 3 times higher than that reached with the Pd/MWCNTs catalyst. The onset potential for formic acid electrooxidation on PdCo/MWCNTs electrocatalyst showed a negative shift ca. 50 mV compared with Pd/MWCNTs.     

Influences of hydrogen evolution on the cell and stack performances of the direct borohydride fuel cell

To scale up power generation of the direct borohydride fuel cell (DBFC), 10-cell and 20-cell stacks have been constructed based on the single cell achievements. It has been found that the stacking loss of the DBFC is mainly caused by hydrogen evolution which leads to uneven fuel distribution in each cell of the stack. To reduce stacking loss, several efforts have been made to decrease hydrogen evolution influence on the stack performance. The anode preparation method has been modified from a dry-method to a wet-method. The influence of hydrogen evolution on stack performance can be alleviated by altering fuel supply manner. When hydrogen evolution is suppressed, an even distribution of cell voltage can be obtained and the maximum power of 10-cell stack reaches up to 229 W.     

A composite of borohydride and super absorbent polymer for hydrogen generation

To develop a hydrogen source for underwater applications, a composite of sodium borohydride and super absorbent polymer (SAP) is prepared by ball milling sodium borohydride powder with SAP powder, and by dehydrating an alkaline borohydride gel. When sodium polyacrylate (NaPAA) is used as the SAP, the resulting composite exhibits a high rate of borohydride hydrolysis for hydrogen generation. A mechanism of hydrogen evolution from the NaBH₄-NaPAA composite is suggested based on structure analysis by X-ray diffraction and scanning electron microscopy. The effects of water and NiCl₂ content in the precursor solution on the hydrogen evolution behavior are investigated and discussed.     

In situ growth of MoS2 on carbon nanofibers with enhanced electrochemical catalytic activity for the hydrogen evolution

Developing an effective and facile method to achieve mass production of MoS₂ nanostructures with abundant of edges may be the feasible way to meet the increasing demand for hydrogen evolution electrocatalysts. We developed a facile glucose-assisted hydrothermal method to in-situ grow MoS₂ nanosheets on the commercial carbon nanofibers (CNFs). The controlled growth of MoS₂ on CNFs (MoS₂@CNFs) is leveraged to reveal mass ratio- and structure-dependent catalytic activity in the hydrogen evolution reaction (HER). Due to the unique shell structure, abundant edges of the MoS₂ layer are exposed as active site, as well as the underlying CNFs effectively improves the conductivity, the resulting MoS₂@CNFs hybrid exhibited high electrocatalytic activity in HER. The catalyst demonstrated the lowest overpotential of 52 mV, the highest current density of 101.49 mA cm^?2 at ～200 mV overpotential and the smallest Tafel slope of 49 mV/decade, suggesting the Volmer–Heyrovsky mechanism for the MoS₂-catalyzed HER.     

Electrochemical oxidation of borohydride at nano-gold-based electrodes: Application in direct borohydride fuel cells

Nano-particulate gold-based materials along with commercial gold supported over carbon were investigated as possible alternative electrocatalysts for the oxidation of borohydride in alkaline media. Cyclic voltammetry experiments conducted on these materials show very high activity for the nano-particulate materials compared to the commercial materials despite a lower loading of gold (0.8 mg cm⁻² compared to 1.0 mg cm⁻²) and lower interface area in the nano-particulate materials. The presence of BH₄⁻ appears to have detrimental effect on the performances of the air-electrode for oxygen reduction. The current density recorded at −0.6 V versus Hg/HgO has decreased by a factor of six for silver nitrate AC65 while for MnO₂ a reduction in the current density by a factor of two only was observed. The implementation of the nano-particulate gold-based materials and the air-electrodes along with a low-cost anionic membrane in QinetiQ's tubular cell design has led to power density exceeding 28 mW cm⁻² obtained at ambient temperature.     

Tight-binding study of hydrogen adsorption on palladium decorated graphene and carbon nanotubes

In this work we report a theoretical study on the atomic and molecular hydrogen adsorption onto Pd-decorated graphene monolayer and carbon nanotubes by a semi-empirical tight-binding method. We first investigated the preferential adsorption geometry, considering different adsorption sites on the carbon surface, and then studied the evolution of the chemical bonding by evaluation of the overlap population (OP) and crystal orbital overlap population (COOP). Our results show that strong C–Pd and H–Pd bonds are formed during atomic hydrogen adsorption, with an important role in the bonding of C 2p_z and Pd 5s, 5p_z and 4d_z² orbitals. The hydrogen storage mechanism in Pd-doped carbon-based materials seems to involve the dissociation of H₂ molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface.     

Evaluation of carbon nanotubes produced from toluene steam reforming as catalyst support for selective catalytic reduction of NOx

There is current interest in developing low cost, effective catalysts for the low temperature selective catalytic reduction (SCR) of nitrogen oxides (NO_x). In this work, we have applied carbon nanotubes (CNTs), produced as a by-product of hydrogen production from the steam reforming of toluene (as a representative hydrocarbon), as a catalyst support for a V₂O₅–WO₃ catalyst for SCR of NO_x. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis showed well dispersed metals on the surface of the CNTs. The V₂O₅–WO₃/CNT catalyst has exhibited NO_x reduction efficiency higher than 95% at reaction temperatures between 340 and 400 °C. However, there was a low NO_x reduction at SCR reaction temperature of less than 200 °C which is suggested to be due to the lack of Lewis acid sites, as determined from NH₃-TPD (temperature program desorption) analysis. Future work to lower the SCR reaction temperature with high NO_x efficiency is suggested.     

Pd-C powder and thin film catalysts for hydrogen production by hydrolysis of sodium borohydride

In this work we study the hydrogen generation by catalytic hydrolysis of alkaline _NaBH4${\mathrm{NaBH}}_4$ solution employing Pd-supported on carbon powder (Pd/C) as well as in form of Pd and Pd–C thin films synthesized by pulsed laser deposition (PLD). Two sets of samples were prepared: (1) pure Pd catalyst films which were bombarded with Ar⁺${\mathrm{Ar}}^{+}$ ions at different ions fluence in order to increase the surface roughness; (2) highly irregular C film were deposited by using different Ar pressure in the PLD chamber prior to deposition of the Pd film to further increase the surface area for the active Pd catalyst. Surface morphology was studied by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) while compositional analysis was performed by using energy dispersive spectroscopy (EDS). Cone like structure on the surface of the Pd film developed by Ar⁺${\mathrm{Ar}}^{+}$ ion bombardment was not efficient to enhance the catalytic activity of the Pd. Pd/C films showed higher catalytic activity in comparison to Pd/C powders when the same amount of catalyst is used. The results are discussed in relation to the morphology of the C-films.     

Platinum supported catalysts for carbon monoxide preferential oxidation: Study of support influence

The aim of this work is to study the influence of the addition of different oxides to an alumina support, on surface acidity and platinum reducibility in platinum-based catalysts, as well as their effect on the activity and selectivity in CO preferential oxidation, in presence of hydrogen. A correlation between surface acidity and acid strength of surface sites and metal reducibility was obtained, being Pt-support interaction a function of the acid sites concentration under a particular temperature range. In platinum supported on alumina catalysts, CO oxidation follows a Langmuir-Hinshelwood mechanism, where O₂ and CO compete in the adsorption on the same type of active sites. It is noteworthy that the addition of La₂O₃ modifies the reaction mechanism. In this case, CO is not only adsorbed on the Pt active sites but also on La₂O₃, forming bridge bonded carbonates which leads to high reactivity at low temperatures. An increase on temperature produces CO desorption from Pt surface sites and favours oxygen adsorption producing CO₂. CO oxidation with surface hydroxyl groups was activated producing simultaneously CO₂ and H₂.     

Hydrogen production by catalytic partial oxidation of methane over staged Pd/Rh coated monoliths: Spatially resolved concentration and temperature profiles

The catalytic partial oxidation (CPOX) of methane is studied over staged palladium/alumina and rhodium/alumina coated monoliths by an in-situ sampling technique to resolve the axial species concentration and temperature profiles. A molar C/O ratio of unity, which is stoichiometric for the formation of synthesis gas (CO/H₂), and short residence times are chosen for this autothermal process. The profiles of the staged monoliths are compared with profiles of single-sliced palladium/alumina and rhodium/alumina coated monoliths. The investigations clearly show two zones inside the catalytic channel. In the staged catalyst with the Pd stage being on the upstream side as well as in the single-sliced Pd catalyst, hydrogen is not formed before oxygen is almost completely consumed, i.e. a total oxidation zone is observed in front of a steam reforming zone. In the Rh catalyst, in the first 2 mm zone, total oxidation and steam reforming occur as prevalent reactions followed by a reforming zone with steam reforming as dominating reaction. The observations are interpreted in terms of the indirect and direct route towards hydrogen formation in CPOX of methane.