Kinetic study of formic acid oxidation on carbon supported Pd electrocatalyst

The oxidation of formic acid at the Pd/C catalyst electrode is a completely irreversible kinetic process with the reaction order of 1.0. The oxidation rate of formic acid is increased with increasing the concentration of formic acid and is decreased with increasing H⁺ concentration. The apparent negative reaction order with respect to H⁺ is about −0.18 or −0.04 in H₂SO₄ or HClO₄ solution respectively, because bisulfate anions would inhibit formic acid oxidation at some extent. The kinetic parameters, charge transfer coefficient and the diffusion coefficient of formic acid were obtained under the quasi steady-state conditions.     

Conversion of hydrogen/carbon dioxide into formic acid and methanol over Cu/CuCr2O4 catalyst

Cu/CuCr₂O₄ catalysts were prepared by impregnation method at various calcination temperatures (300, 400, and 500 °C) and then reduced in H₂ stream. The aggregated particles and decreasing surface area/pore volumes of the deactivated catalysts during HCOOH and CH₃OH formations were also observed. Particularly, the EXAFS data showed that first shells of Cu atoms transforms from Cu–O to Cu–Cu after catalytic reactions, their bond distances and coordination numbers are quite different, respectively. It revealed that metallic Cu atoms are one of the important active species over catalyst surface at different reaction temperatures having many unoccupied binding sites for HCOOH and CH₃OH formations. Additionally, the optimal calcination temperature for Cu/CuCr₂O₄ catalysts was demonstrated at 400 °C that attributed to its strongest acidity and basicity. The catalytic reactions in the duration of HCOOH and CH₃OH preparation were proposed that were composed of HCOOH formation, CH₃OH formation, and CH₃OH decomposition happening at CuCr₂O₄, Cu, and CuO active sites, respectively. The highest CO₂ conversion (14.6%), HCOOH selectivity/yield (87.8/12.8%), and TON/TOF values (4.19/0.84) were obtained at 140 °C and 30 bar in 5 h, respectively. Optimal rate constant (2.57 × 10^?2 min^?1) and activation energy (16.24 kJ mol^?1) of HCOOH formation were evaluated by pseudo first-order model and Arrhenius equation, respectively.     

Electro-oxidation of formic acid on carbon supported Pt-Os catalyst

Pt-Os (3:1)/C catalyst was prepared through the route of thermal decomposition of metallic carbonyl cluster. TEM image showed Pt-Os nanoparticles were well dispersed on carbon substrate with an average particle size of 2.2 ± 0.9 nm. XRD pattern indicated Pt-Os has a face-centered cubic crystal structure. Characterized by cyclic voltammetry and chronoamperometry, Pt-Os (3:1)/C catalyst shows a superior electro-catalytic activity to formic acid oxidation in comparison with Pt/C catalyst. This improved electro-catalytic performace was mainly due to the fine dispersion of Pt-Os nano particles and bi-functional effect.     

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.     

Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation

Carbon nanofiber (CNF) supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as stabilizing agent and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles are controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.85 to 3.62 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the further increasing of the pH value to 6.5 leads to an increased particle size and the formation of PdO phase in the synthesized Pd/CNF catalyst. The Pd/CNF catalyst synthesized at the pH value of 6.0 exhibits superior catalytic activity and stability for formic acid electrooxidation due to its small particle size and uniform size distribution.     

Pd nanoparticles supported on copper phthalocyanine functionalized carbon nanotubes for enhanced formic acid electrooxidation

The hydrothermal synthesis of a novel Pd electrocatalyst using copper phthalocyanine-3,4′,4″,4′″-tetrasulfonic acid tetrasodium salt (TSCuPc) functionalized multi-walled carbon nanotubes (MWCNTs) composite as catalyst support for Pd nanoparticles is reported. The prepared nanocomposites were characterized by UV–vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical tests. It is found that Pd nanoparticles are uniformly deposited on the surface of TSCuPc-MWCNTs, and their dispersion and electrochemical active surface area (ECSA) are significantly improved. Studies of cyclic voltammetry and chronoamperometry demonstrate that the Pd/TSCuPc-MWCNTs exhibits much higher electrocatalytic activity and stability than the Pd/AO-MWCNTs catalyst for formic acid oxidation. This study implies that the as-prepared Pd/TSCuPc-MWCNTs will be a promising candidate as an anode electrocatalyst in direct formic acid fuel cell (DFAFC).     

Highly active carbon nanotube-supported Pd electrocatalyst for oxidation of formic acid prepared by etching copper template method

In this work, a carbon nanotube-supported Pd nano-catalyst (Pd/MCNTs) is prepared by the etching copper template strategy. Cu nanoparticles (NPs) are formed onto MCNTs first as the template and Pd NPs are then obtained through a galvanic displacement reaction between Pd ions and Cu. TEM, XRD, and XPS characterizations show the crystalline of Pd NPs with a typical diameter of 2–5 nm is homogeneously decorated onto MCNTs without aggregation. Electrochemical characterizations reveal that the Pd/MCNTs materials exhibit much higher catalytic activity for the formic acid oxidation than both conventional Pd/MCNTs and commercial Pd/Vulcan catalysts do. The improved activity is mainly attributed the fact that no surfactant is required in synthesis of the catalyst, eliminating the possible passivation of catalytic sites associated with the use of surfactant in conventional synthesis methods. In addition, the narrower distribution and better dispersion of catalyst particles, as well as no defects of MCNTs are also beneficial for the improvement in the catalytic activity. Another feature of the present synthesis method is the loading of Pd can be adjusted by varying the amount of Cu ions.     

Synthesis of highly dispersed and active palladium/carbon nanofiber catalyst for formic acid electrooxidation

Highly dispersed and active palladium/carbon nanofiber (Pd/CNF) catalyst is synthesized by NaBH₄ reduction with trisodium citrate as the stabilizing agent. The obtained Pd/CNF catalyst is characterized by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The results show that the Pd nanoparticles with an average particle size of ca. 3.8 nm are highly dispersed on the CNF support even with a small ratio of citrate to Pd precursor, which is believed to be due to the pH adjustment of citrate stabilized colloidal Pd nanoparticles. The cyclic voltammetry and chronoamperometry techniques show that the obtained Pd/CNF catalyst exhibits good catalytic activity and stability for the electrooxidation of formic acid.     

Platinum-macrocycle co-catalysts for electro-oxidation of formic acid

In this paper, it was found that the electrocatalytic activity of a Pt electrode for the electro-oxidation of formic acid could be dramatically enhanced with the modification of macrocycle compounds, such as iron-tetrasulfophthalocyanine (FeTSPc). The electro-oxidation of formic acid on a modified Pt electrode with FeTSPc occurs mainly through a direct pathway. A series of macrocycle compounds were also investigated as modifiers and exhibited a promotion effect similar to the Pt electrode. Therefore, platinum-macrocycle co-catalysts can promote the electro-oxidation of formic acid through a highly effective route, and are potential catalyst materials for a direct formic acid fuel cell (DFAFC).     

Application of normal pulse voltammetry to the kinetic study of formic acid oxidation on a carbon supported Pd electrocatalyst

The kinetic parameters of formic acid oxidation on a carbon supported Pd electrode, such as the charge transfer coefficient (α) and apparent diffusion coefficient (D) are obtained by applying the technique of normal pulse voltammetry. The standard rate constant (k₀) of formic acid oxidation on a Pd/C electrode is estimated. The results show that formic acid oxidation is more sensitive to temperature at relatively high potential because the activation energy is significantly increased as the potential rose above 0.6 V.     

Passive direct formic acid microfabricated fuel cells

This paper reports on microscale silicon-based direct formic acid fuel cells (Si-DFAFCs) in which the fuel and the oxidant are supplied to the electrodes in a passive manner. Passive delivery of fuel and oxidant eliminates the need for ancillary components and associated parasitic losses. In this Si-DFAFC, an aqueous solution of formic acid is in direct contact with a Pd- or Pt-based anode and a Pt-based cathode is exposed to either a forced oxygen stream or quiescent air. In the presence of a forced oxygen flow on the cathode side the cell with Pd catalyst on the anode delivers a maximum power density of about 30 mW cm⁻² at room temperature, limited mostly by mass transfer at the anode, while in an all-passive mode (quiescent air on the cathode side) a maximum power density of 12.3 mW cm⁻² is obtained, limited by oxygen transport. This all-passive Si-DFAFC is fabricated using processes that are post-CMOS compatible, and thus can be integrated directly with envisioned MEMS applications, such as small sensors and actuators.     

Electrochemical impedance studies of electrooxidation of methanol and formic acid on Pt/C catalyst in acid medium

Cyclic voltammetry (CV), amperometric i − t experiments, and electrochemical impedance spectroscopy (EIS) measurements were carried out by using glassy carbon disk electrode covered with the Pt/C catalyst powder in solutions of 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ containing 0.5 mol L⁻¹ HCOOH at 25 °C, respectively. Electrochemical measurements show that the activity of Pt/C for formic acid electrooxidation is prominently higher than for methanol electrooxidation. EIS information also discloses that the electrooxidation of methanol and formic acid on the Pt/C catalyst at various polarization potentials show different impedance behaviors. The mechanisms and the rate-determining steps of formic acid electrooxidation are also changed with the increase of the potential. Simultaneously, the effects of the electrode potentials on the impedance patterns were revealed.     

Catalytic reduction of CO2 by Al–Sn-CNTs composite for synthesizing formic acid

Electrochemical reduction and photocatalytic reduction of CO₂ have attracted more and more attention, but they also face the problem of low utilization efficiency of electricity and solar energy. In this study, a new strategy of applying a novel Al–Sn-CNTs composite in electrochemical corrosion process was proposed to reduce CO₂ without additional electricity and light. In the Al–Sn-CNTs/CO₂ system, micro galvanic cell with Al as anode and Sn or CNTs as cathode was formed, and CO₂ was reduced to formic acid on the cathode surface. The cumulative formic acid concentration of the Al–Sn-CNTs/CO₂ system achieved 21.18 mg/L within 60 min under the conditions of initial pH 9.0, Cl⁻ concentration 10 mmol/L, and Al–Sn-CNTs composite dosage 2 g/L. Based on the morphology, crystal structure and electrochemical test results of the Al–Sn-CNTs composite, a possible mechanism of CO₂ reduction to formic acid in the Al–Sn-CNTs/CO₂ system was proposed.     

Microstructure and activity of Pd catalysts prepared on commercial carbon support for the liquid phase decomposition of formic acid

In this work, a series of Pd catalysts supported on commercially available activated carbon (Norit ®) were prepared by employing different metal precursors (Pd(NO₃)₂ and Na₂PdCl₄) by the impregnation-reduction method at different pH. Catalysts were tested for the liquid phase decomposition of formic acid to generate hydrogen. The best results, in terms of small particle size and high catalytic activity were achieved for the Pd/C sample prepared by using Pd(NO₃)₂ salt impregnated at pH = 2.5, and reduced with sodium borohydride. The particle size of the best Pd/C catalyst is (4.1 ± 1.4) nm with initial TOFs of 2929 and 683 h^?1 at 60 and 30 °C respectively and an apparent activation energy of 40 kJ mol^?1. Samples prepared by using Na₂PdCl₄ precursor, consisted of particles with higher size and thus lower activity than the ones prepared with Pd(NO₃)₂. Regardless the Pd precursor employed, the best results in terms of particle size and activity were achieved at the point of zero charge of the support when the Pd species and the carbon surface were both neutral. The impregnation pH not only determines the particle size, but also the nature of the reducing agent does. The catalytic activity was shown to be size-dependent and it was shown that a mixture of surface Pd⁰ and Pd^II oxidation states is beneficial for the activity. When comparing with literature catalysts with similar composition, we found that our best catalyst is competitive enough and that Norit ® support could be promising for future studies on this reaction.     

Effects of addition of different carbon materials on the electrochemical performance of nickel hydroxide electrode

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. The capacity of nickel-metal hydride (Ni-MH) batteries depends on the specific capacity of the positive electrode and utilization of the active material because of the Ni(OH)₂/NiOOH electrode capacity limitation. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH)₂ particle with the current collector. As β-Ni(OH)₂ is a kind of semiconductor, the additives are necessary to improve the conductivity between the active material and the current collector. In this study the effect of adding different carbon materials (flake graphite, multi-walled carbon nanotubes (MWNT)) on the electrochemical performance of pasted nickel-foam electrode was established. A method of production of MWNT special type of catalysts had an influence on the performance of the nickel electrodes. The electrochemical tests showed that the electrode with added MWNT (110-170 nm diameter) exhibited better electrochemical properties in the chargeability, specific discharge capacity, active material utilization, discharge voltage and cycling stability. The nickel electrodes with MWNT addition (110-170 nm diameter) have exhibited a specific capacity close to 280 mAh g⁻¹ of Ni(OH)₂, and the degree of active material utilization was ∼96%.     

Investigation of direct carbon conversion at the surface of a YSZ electrolyte in a SOFC

Fuel cells offer a promising way to produce electricity efficiently. In this work, a direct carbon fuel cell (DCFC) based on a solid oxide fuel cell (SOFC) has been investigated, in which solid carbon has been used as fuel in form of a pellet. The DCFC is an interesting technology because it offers the possibility to use, as fuel source, available and abundant raw materials with only minor pretreatment. Moreover, the thermodynamic efficiency slightly exceeds 100% in a wide temperature range due to the positive near-zero value of reaction entropy change. As pure carbon dioxide is produced at the anode, it can be easily captured and sequestered. Direct carbon conversion is competed by the Boudouard reaction, which produces carbon monoxide at high operating temperatures. This reaction is endothermic and leads to a fuel loss. The present paper relates to the contribution of both reactions by a long-term run over about 12 h with a non-porous anode layer.     

Pd nanoparticles deposited on vertically aligned carbon nanotubes grown on carbon paper for formic acid oxidation

Vertically aligned carbon nanotubes (VACNTs) grown on carbon paper were obtained by the spray pyrolysis method and highly dispersed Pd nanoparticles were deposited on VACNTs by the wet chemical method. For comparison, the entangled carbon nanotubes (ECNTs) and Vulcan XC-72 based electrodes were fabricated by brush painting the corresponding Pd/ECNTs and Pd/XC-72 catalysts on carbon paper, respectively. Compared with Pd deposition on the entangled carbon nanotubes (ECNTs) and Vulcan XC-72 electrodes, the VACNTs electrode exhibited higher activity for formic acid oxidation, which is mainly due to the three-dimensional structure and better conductive paths in the VACNTs electrode, as well as higher Pd utilization.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

On the global warming problem due to carbon dioxide

The subject of global warming due to the increased use of fossil fuels is analyzed using a modification of the predator prey equations. The results of the calculation indicate that both the fossil fuels and civilization will both become extinct as time increases.     

Enhanced formic acid oxidation on Cu-Pd nanoparticles

Developing catalysts with high activity and high resistance to surface poisoning remains a challenge in direct formic acid fuel cell research. In this work, copper-palladium nanoparticles were formed through a galvanic replacement process. After electrochemically selective dissolution of surface Cu, Pd-enriched Cu-Pd nanoparticles were formed. These particles exhibit much higher formic acid oxidation activities than that on pure Pd nanoparticles, and they are much more resistant to the surface poisoning. Possible mechanisms of catalytic activity enhancement are briefly discussed.