Transition metal catalysts for porous carbon air-electrodes in neutral chloride electrolytes

Porous carbon air-electrodes activated by CoPC, FePHP, PdPC, PtPC and metal-free PC were studied in 2 mol dm^–3 NaCl electrolyte. The activity of heat-treated carbon activated by CoPC was compared to carbon activated byin situ formed Co oxide. All the results show that the main catalytic activity of the catalyst used comes from the central metal atom used, while the chelate or oxide structures serve predominantly to keep the metal in the stable form at the carbon surface. Metal-free phthalocyanine does not show any catalytic activity for oxygen reduction.     

Carbon-free dry reforming of methane to syngas over NdCoO3 perovskite-type mixed metal oxide catalyst

CoNdO_x (Co/Nd = 1) is a highly promising catalyst for the carbon-free CO₂ reforming of methane. Influence of the Co/Nd ratio on the catalyst performance in the CO₂ reforming and also on the crystalline phases and reduction by temperature programmed reduction (TPR) of the CoNdO_x catalyst has also been investigated. The CoNdO_x (CoNd = 1.0) catalyst consisted of mainly NdCoO₃ perovskite-type mixed metal oxide and it showed not only a high resistance to carbon formation at different process conditions (viz. temperature = 750–900 °C and gas hourly space velocity (GHSV) = 10000–50000 cm³ g^–1 h^–1) but also high activity and selectivity in the CO₂ reforming process. The high resistance to carbon formation for this catalyst is attributed mostly to strong metal (Co°)–support (Nd₂O₃) interactions.     

Modification of Porous Silica with Activated Carbon and its Application for Fixation of Yeasts

Mixtures of small silica particles and activated carbon were heated at 1250–1450°C in an inert atmosphere to make nano- and macro-sized porous silica for incorporating yeast in the porous strucrure. Without activated carbon, porous silica of 45–60% porosity and 15–30 m pore diameter was produced with a specific surface area below 1 m²/g. By the addition of 8 wt% of activated carbon granules, the surface area of porous silica increased to 100 m²/g at 1250°C. It was confirmed that there were micropores(1.2 nm) and mesopores(4.0 nm) due to activated carbon granules in porous silica when granule type activated carbon was used. However, in the case of activated carbon fiber, its micro- and mesoporous structure was destroyed in the firing process. The fixation of Z. rouxii yeasts was promoted on the porous silica with activated carbon.     

Durability test of SOFC cathodes

The durability of solid oxide fuel cell (SOFC) composite cathodes of lanthanum strontium manganite and yttria stabilised zirconia was investigated. The cathodes were kept at constant, realistic operating conditions (–300 mA cm^–2 at 1000 °C in air) for up to 2000 h. After the 2000 h test the increase in electrode overvoltage exceeded 100% of the initial value. Nominally identical cathodes kept for 2000 h at 1000 °C in air without current load for comparison showed little or no degradation. Thus, the current load of –300 mA cm^–2, rather than the operation temperature of 1000 °C, was responsible for the degradation. Structural analysis showed an increase in the porosity at the electrode interfaces, when the electrode had been polarised. No such structural changes were found for electrodes tested without current load. The degradation is primarily ascribed to pore formation in the electrode material induced by an electric field.     

Performance and thermodynamic properties of Na-Sn and Na-Pb molten alloy electrodes for alkali metal thermoelectric converter (AMTEC)

An alkali metal thermoelectric converter (AMTEC) testing cell was set up, and run with molten sodium-tin (Na-Sn) and sodium-lead (Na-Pb) alloy cathodes. The Na activity, the partial molar enthalpy and partial molar entropy of sodium in molten Na-Sn and Na-Pb alloys have been determined, using a Na concentration cell: Na(1) I beta-alumina Na-Me(1), where Me= Sn or Pb. The thermodynamic results of these investigations agree with those of other authors. The electric performance of these Na-Me alloy electrodes of different Na concentration and temperatures is described, measuring current-voltage characteristics and a.c. impedance in the AMTEC test cell. The power density of the AMTEC cell with molten alloy cathodes decreases with increasing Na concentration, with the Na concentrations in molten alloys varying from 0.5 to 15 mol%. Maximum power densities of 0.21 to 0.15 W cm^–2 at 700°C for Na-Sn molten electrodes, and 0.30 to 0.15 W cm^–2 for Na-Pb molten electrodes have been obtained. The a.c. impedance data demonstrated that the molten alloy electrodes have a smaller cell resistance, 0.3–0.35 S2 cm^–2 at 700°C after 10–20 h. Comparison with the sputtered thin, porous film electrodes, showed that the contact resistance between electrode and surface of beta-alumina plays an important role on enhancing cell power density. At 700°C the power density of an AMTEC cell with the molten Na-Pb alloy electrode can be raised to values of about 0.2 W cm eat current densities of 0.8 A cm^–2, but at cell voltages not exceeding 0.2V. A model for the theoretical efficiency of the AMTEC cell with molten Na metal electrodes is also presented.     

Development of a novel metal hydride–air secondary battery

A laboratory metal hydride/air cell was evaluated. Charging was via a bifunctional air gas-diffusion electrode. Mixed nickel and cobalt oxides, supported on carbon black and activated carbon, were used as catalysts in this electrode. At 30mAcm–2 in 6m KOH, the air electrode potentials were –0.2V (oxygen reduction) and +0.65V (oxygen evolution) vs Hg/HgO. The laboratory cell was cycled for 50 cycles at the C/2 rate (10mAcm–2). The average discharge/charge voltages of the cell were 0.65 and 1.6V, respectively. The initial capacity of the metal hydride electrode decreased by about 15% after 50 cycles.     

Review in Applied Electrochemistry. Number 54 Recent Developments in Polymer Electrolyte Fuel Cell Electrodes

Since the 1980s there has been a significant lowering of the platinum loading of polymer electrolyte fuel cell electrodes from about 4–10 mg cm^–2(platinum black) to about 0.4 mg cm^–2 or even less (carbon supported platinum), by the introduction of ionomer (liquid Nafion^®) impregnated gas diffusion electrodes, extending the three-dimensional reaction zone. From the 1990s to the present studies have been carried out to decrease the loss of performance during cell operation due both to the presence of liquid water causing flooding of the catalyst layer and mass transport limitations and to the poisoning of platinum by the use of reformed fuels. This review deals with the developments in electrode configuration going from dual layer to three layer electrodes. The preparation methods, the characteristics and the optimal composition of both diffusion and reactive layers of these electrodes are described. The improvement in the performance of both CO tolerant anodes and cathodes with enhanced oxygen reduction by Pt alloying is also discussed.     

New structures of thin air cathodes for zinc–air batteries

Thin composite cathodes for air reduction were manufactured using microfibre-based papermaking technology. The electrodes have a thin structural design, less than 0.15 mm in thickness. Composite cathode materials for oxygen reduction applications were fabricated by entrapping carbon particles in a sinter-locked network of 2–8 m diameter metal fibres. The thin structure not only results in electrodes that are 30–75% thinner than those commercially available, but also offers an opportunity for custom-built air cathodes optimized for high-rate pulse applications. Using a thin composite structure for the air cathode in a zinc–air battery that is part of a zinc–air/capacitor hybrid is likely to increase the pulse capability of the hybrid power system. The thin cathode structure provides a better, more efficient three-phase reaction zone. In a half-cell test, the ultrathin air cathode generated more than 1.0 V vs Zn/ZnO for a current of 200 mA cm^–2. Half-cell, full-cell and pulse-power tests revealed that thin composite cathodes have a better rate and pulse performance than the air cathodes commonly used.     

Study on the catalytic synthesis of diethyl carbonate from CO and ethyl nitrite over supported Pd catalysts

A vapor phase synthesis of diethyl carbonate (DEC) from carbon monoxide and ethyl nitrite (EN) was studied in a continuous flow micro fixed-bed reactor at atmospheric pressure. PdCl₂–CuCl₂/AC (activated carbon) catalyst exhibited better catalytic activity compared with other binary catalyst systems. The suitable Pd-loading is about 2.0 wt%, and some additives (LaCl₃, CeCl₃, PrCl₃) are benefit for the DEC yield and selectivity. Influences of various reaction parameters on the DEC yield and selectivity were tested. The optimum reaction temperature lies in 378–388 K and the suitable gas hourly space velocity (GHSV) range is 2500–3000 h⁻¹ considering both factors of DEC production and CO conversion. An optimum CO/C₂H₅ONO mole ratio exists for catalytic activity, which is about 1/1. The stability of PdCl₂–CuCl₂/AC catalyst and PdCl₂–CuCl₂–CeCl₃/AC catalyst was also investigated. The possible reason of the deactivation behavior of catalysts was discussed with the help of XRD.     

Copper(II) formate cathodes for seawater batteries

The preparation of copper(II) formate cathodes, their discharge in magnesium seawater cells, and the discharge reactions are described. This soluble active material could be discharged at high voltages (1.4–1.0 V) with high efficiencies (80–95%) in magnesium cells, when polystyrene solutions were used as the binder to make the cathode plates. For single cells energy densities in the range 70–120 Whkg^–1 (based on the dry state) were obtained. The Cu(HCOO)₂/Mg cell would meet many applications at low or moderate discharge rates as a substitute for the AgCl/Mg cell.     

Study of a high power density sodium polysulfide/bromine energy storage cell

Nickel catalyst supported on carbon was made by reduction of nickelous nitrate with hydrogen at high temperature. Ni/C catalyst characterization was carried out by XRD. It was found that the crystal phase of NiS and NiS₂ appeared in the impregnated catalyst. Ni/C and Pt/C catalysts gave high performance as the positive and negative electrodes of a sodium polysulfide/bromine energy storage cell, respectively. The overpotentials of the positive and negative electrodes were investigated. The effect of the electrocatalyst loading and operating temperature on the charge and discharge performance of the cell was investigated. A power density of up to 0.64 W cm^–2 (V = 1.07 V) was obtained in this energy storage cell. A cell potential efficiency of up to 88.2% was obtained when both charge and discharge current densities were 0.1 A cm^–2.     

Influence of carbon support on the performance of platinum based oxygen reduction catalysts in a polymer electrolyte fuel cell

Novel carbons from the Sibunit family prepared via pyrolysis of hydrocarbons [Yermakov YI, Surovikin VF, Plaksin GV, Semikolenov VA, Likholobov VA, Chuvilin AL, Bogdanov SV (1987) React Kinet Catal Lett 33:435] possess a number of attractive properties for fuel cell applications. In this work Sibunit carbons with BET surface areas ranging from ca. 20 to 420 m² g⁻¹ were used as supports for platinum and the obtained catalysts were tested as cathodes in a polymer electrolyte fuel cell. The metal loading per unit surface area of carbon support was kept constant in order to maintain similar metal dispersions (∼0.3). Full cell tests revealed a strong influence of the carbon support texture on cell performance. The highest mass specific activities at 0.85 V were achieved for the 40 and 30 wt.% Pt catalysts prepared on the basis of Sibunit carbons with BET surface areas of 415 and 292 m² g⁻¹. These exceeded the mass specific activities of conventional 20 wt.% Pt/Vulcan XC-72 catalyst by a factor of ca. 4 in oxygen and 6 in air feed. Analysis of the I–U curves revealed that the improved cell performance was related to the improved mass transport in the cathode layers. The mass transport overvoltages were found to depend strongly on the specific surface area and the texture of the support.     

Deep oxidation of aqueous organics over a silicon carbide-activated carbon supported platinum–ruthenium bimetallic catalyst

In previous work, we developed a highly active bimetallic platinum–ruthenium catalyst supported on a very high surface area activated carbon substrate. In fixed bed reactors, this catalyst proved capable of the continuous long-term deep oxidation of a variety of aqueous organic contaminants associated with spacecraft wastewater streams at 121 °C. This work was extended to the mineralization of more typical environmental contaminants, including halocarbons and aromatics. The primary weakness of this catalyst was the tendency toward relatively high rates of chemical decomposition. To overcome this limitation, methods were developed for the production of a silicon carbide coating over the surface of the activated carbon, yielding a reasonable trade-off between increasing resilience and decreasing surface area. Here we report the catalytic decomposition of dissolved organic contaminants at 130 °C using this silicon carbide/activated carbon supported bimetallic catalyst.     

Fischer–Tropsch synthesis over cobalt dispersed on carbon nanotubes-based supports and activated carbon

Carbon nanotubes (CNTs) and the ones grown on MgO and alumina are used as supports for cobalt catalyst in Fischer–Tropsch (FT) synthesis. Carbon nanotubes were synthesized by chemical vapor deposition of methane on 5.0 wt.% iron on MgO or alumina at 950 °C. The carbon nanotubes were characterized by SEM and TEM microscopy and Raman spectroscopy. Cobalt nitrate was impregnated onto the supports by impregnation, and the samples were dried and reduced in-situ at 400 °C for 12 h, and then FT synthesis was carried out in a fixed-bed reactor. The catalysts were characterized by BET surface area measurement, TPR and TPD. The effect of carbon nanotubes as cobalt support on CO conversion, product selectivity, and olefin to paraffin ratio of FT synthesis was investigated and compared with activated carbon as well as Al₂O₃, as a traditional support. The results revealed that the activity of the Co/CNT catalyst was improved by 22%, compared to the conventional Co/alumina catalysts. Also the cobalt supported on CNTs grown on MgO (Co/CNT–MgO) shows the highest selectivity to C₅₊ as the most desired FTS products. The C₅₊ selectivity enhancement was about 37, 34, 17, and 77% as compared to the Co/CNT, Co/alumina, Co/CNTs-alumina, and Co/activated carbon, respectively. Also the olefin/paraffin ratio on the Co/CNTs-MgO catalyst is about 7.7 times higher than the conventional cobalt catalysts. It seems that the degree of reduction of cobalt is higher when supported on CNTs than on alumina. This leads to higher FTS activity. Also, the particle size distribution of the cobalt is affected by the CNT–MgO support leading to higher C₅₊ selectivity.     

Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chlor-alkali electrolysis

Application of a gas-diffusion type oxygen cathode will contribute to energy saving in chlor-alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80 °C and 1 atm O₂, but loading of 2 mg cm^–2 Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four-electron reduction to produce OH^–, while H₂O₂ is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H₂O₂ has been demonstrated by conducting CV measurements to detect H₂O₂ in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor-alkali electrolysis.     

Graphitic Carbon as Durable Cathode‐Catalyst Support for PEFCs

Long‐term deterioration in the performance of PEFCs is attributed largely to reduction in active area of the platinum catalyst at cathode, usually caused by carbon‐support corrosion. It is found that the use of graphitic carbon as cathode‐catalyst support enhances its long‐term stability in relation to non‐graphitic carbon. This is because graphitic‐carbon‐supported‐Pt (Pt/GrC) cathodes exhibit higher resistance to carbon corrosion in‐relation to non‐graphitic‐carbon‐supported‐Pt (Pt/Non‐GrC) cathodes in PEFCs during accelerated stress test (AST) as evidenced by chronoamperometry and carbon dioxide studies. The corresponding change in electrochemical surface area (ESA), cell performance and charge‐transfer resistance are monitored through cyclic voltammetry (CV), cell polarisation and impedance measurements, respectively. The degradation in performance of PEFC with Pt/GrC cathode is found to be around 10% after 70 h of AST as against 77% for Pt/Non‐GrC cathode. It is noteworthy that Pt/GrC cathodes can withstand even up to 100 h of AST with nominal effect on their performance. X‐ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy and cross‐sectional field‐emission scanning electron microscopy (FE‐SEM) studies before and after AST suggest lesser deformation in catalyst layer and catalyst particles for Pt/GrC cathodes in relation to Pt/Non‐GrC cathodes, reflecting that graphitic carbon‐support resists carbon corrosion and helps mitigating aggregation of Pt‐particles.     

Increasing the activity of Fe/N/C catalysts in PEM fuel cell cathodes using carbon blacks with a high-disordered carbon content

Fe/N/C catalysts for the reduction of oxygen in PEM fuel cells were prepared by pyrolyzing three series of iron acetate-impregnated developmental carbon blacks at 950 °C. The carbon supports used were derived from the N234, N330, and N650 commercial furnace grades. In this study, we tried to increase the performance of Fe/N/C-based cathode of PEM fuel cells by using the following two approaches: (1) increasing the number of catalytic sites on the carbon black either by optimizing the structural parameters of the pristine carbon supports or by increasing the initial metal content above 0.2 wt% Fe on the carbon support; (2) increasing the catalyst loading in the cathodic layer of a PEM fuel cell. For (1), we show, on the one hand, that optimizing the structural parameters of the pristine carbon support, in order to increase the number of catalytic sites, has its limits and that these limits have been reached for the present synthesis method of Fe/N/C catalysts. On the other hand, increasing the initial metal content above 0.2 wt% Fe leads to a decrease in catalytic activity. For (2), it is shown that increasing the catalyst loading per cm² of cathode well improves the performance of a cathode based on Fe/N/C catalysts in the kinetic region of the polarization curve. At lower potentials, a large improvement in the performance of these non-precious metal cathodes would occur if the mass transport properties in these electrodes were significantly increased.     

Electrochemical reduction of carbon dioxide in methanol at ambient temperature and pressure

The electrochemical reduction of carbon dioxide was studied in methanol-based supporting electrolytes on various metal electrodes at ambient temperature and pressure. The ionophore of the catholyte was benzalkonium chloride, [RN(CH3)2CH2C6H5]⁺Cl^–, where R=C8–C18, the chain length being distributed around C14. A divided H-type cell was used, the supporting electrolytes were 10–2moldm–3 benzalkonium chloride in double distilled methanol (catholyte) and a 10–1moldm–3 aqueous KHCO3 solution (anolyte). Nine different, high purity (>99.5%) metal electrodes were used: Ti, Fe, Co, Ni, Pt, Ag, Au, Zn and Sn. Carbon monoxide, methane and ethane were the main organic products. Silver, Au, Zn and Sn cathodes allowed for the best faradaic yields of CO production, the maximum amount of CO (71%, 185 mmol) being formed on the Ag electrode. Methane evolved on each of the nine tested electrodes, with current yields in the range from 0.2 to 3.0%. Ethane and ethylene were produced on the nickel electrode, with low faradaic efficiencies, 0.5 and 0.3%, respectively. No dimerization products were detected. This research can contribute to large-scale manufacturing of useful organic products from a readily available and cheap raw material: CO2-saturated methanol from industrial absorbers (the Rectisol process).     

Effect of the oxygen reduction catalyst loading method on the performance of air breathable cathodes for microbial fuel cells

This paper presents three different methods of hydrothermal (HT), microwave (MW), and cyclic voltammetry (CV) used to load a catalyst on a cathode surface. In the HT and MW methods, a multiwall carbon nanotube (MWCNT) is used as a support material to fix the catalyst, while Nafion solution is used as a binder to load the catalyst on the cathode surface. For the third option, the CV method is used to directly load the catalysts on the cathode surface without any support material. The performances of the three cathodes are tested in an air breathable batch microbial fuel cell (MFC) and compared to that of a commercial carbon cloth cathode with platinum (Pt). The maximum power density of the MFC with the HT cathode is measured as 833 mW m^?2, which is higher than those of the CV and MW cathodes and slightly smaller than the MFC with the Pt cathode. The open circuit voltage of the MFC with the HT cathode is 610 mV, which is higher than those of MFCs with other cathodes, while the power density is higher than the MFCs of the MW and CV cathodes. In the case of the HT cathode, a conductive MWCNT network is well formed and entangled with the catalyst nanostructure of the cathode surface while the small ohmic and activation resistances of the HT cathode contribute to the good MFC performance.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.