| 1. |
- Villa, M., et al.
(författare)
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The apparent capacitance of the electrode "charging" process
- 2009
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Ingår i: Physical and Analytical Electrochemistry General Session - 215th ECS Meeting. - San Francisco, CA : The Electrochemical Society. ; , s. 47-59
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We investigate simple models of the electrode charging (discharging) at constant current i, where the potential (E) is recorded as a function of time, or charge, q = i·t. Our goal is to assess the advantages of a representation in terms of capacitance, C = dq / dE ,vs. E. Such a plot is related with cyclic voltammetry (c.v.) plot (ic.v. vs E) at a constant rate (β = dE / dt) since, under appropriate circumstances, we should have C = ic.v. / β = dq / dE. We will discuss the following items: a) origins and suppression of the singularities in the capacitance pilots, b) effects of the finite charging currents, c) comparison between c.v. and electrode charging experiments.
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| 2. |
- Caetano de Souza, Antonio Carlos, et al.
(författare)
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A low cost & safe system of hydrogen production utilizing NaBH4 and CoO catalysis
- 2007
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Ingår i: 2nd International Congress University-Industry Cooperation (UNIDU07).
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Konferensbidrag (refereegranskat)abstract
- The objective of this study was to evaluate the hydrogen production through hydrolysisof sodium borohydride (NaBH4) utilizing catalysts containing CoO. The reactant is safe and stable(when dry) at room temperature. Few works and studies have presented results of investigationsutilizing catalysts containing cobalt; however utilizing catalysts containing CoO were not found yet.In this work simple and cheap hydrogen generation system was developed having reactions atnormal conditions of temperature and pressure. A solution containing a gravimetric composition of10% wt. NaOH, 10% wt. NaBH4 and 80% wt. H2O was utilized. The reaction was carried out atvarious times using the same catalyst to evaluate its performance. This catalyst presented highrates of hydrogen production, especially at its start-up (about 99% of the theoretical hydrogenvolume was produced) at room temperature. After start-up, e.g., when more solution was put, rateof hydrogen production decreased having its production performance also decreased. Probablythis fact occurred due to the formation of the solid phase products such as NaBO2 which might fillthe porous catalyst structure; decreasing the catalytic area. This catalyst is suggested in situationswhere high production rates are necessary such as start-up of fuel cells.
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| 3. |
- Caetano de Souza, Antonio Carlos, et al.
(författare)
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Hydrogen production through hydrolysis of NaBH4: The use of catalysts containing Pt and Pt-Ru
- 2007
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Ingår i: 2nd International Congress University-Industry Cooperation (UNIDU07).
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Konferensbidrag (refereegranskat)abstract
- Several works about hydrolysis of NaBH4 utilizing various catalysts (such as catalysts containing Pt or Ru) are available in the literature. Investigations involving NaBH4 has increased due to the possibility to produce hydrogen using simple and safe systems, even at room temperatures with very high efficiencies. A solution containing a gravimetric composition of 10%wt. NaOH, 10%wt. NaBH4 and 80%wt. H2O was utilized and the reaction was initiated immediately as soon as this solution was put in the chosen catalysts, in this case, catalysts containing Pt and mixtures of Pt-Ru. Catalysts containing Pt and Pt-Ru presented high yields of hydrogen after the solution being inserted in the reaction vessel several times. In this study it was found out that the rates of hydrogen production were increased with catalysts containing Pt and Pt-Ru (99 and 96% of theoretical hydrogen production respectively). The catalysts containing Pt presented higher production rate, while the catalysts containing the mixed Pt-Ru presented a quasi-linear production, e.g., stable production rate.
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| 4. |
- Kiros, Yohannes, et al.
(författare)
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Low energy consumption in chlor-alkali cells using oxygen reduction electrodes
- 2008
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Ingår i: International Journal of Electrochemical Science. - 1452-3981. ; 3:4, s. 444-451
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Tidskriftsartikel (refereegranskat)abstract
- An air/oxygen gas diffusion electrode for use as a cathode to replace the traditional hydrogen-evolving electrode in chlor-alkali electrolysis was assessed. Attempts to stabilize the cathode have been addressed in order to circumscribe problems associated with "flooding" or "wetting-in" properties. Variation of the hydrophobic concentration in the gas diffusion layer had a significant effect on the electrochemical tests of both the half-cell and electrolysis of the chlor-alkali cells. Life-tests as well as performance characteristics for both types of cells have shown encouraging results at NaOH concentration levels of ca 8M NaOH and temperatures of 70 and ca. 80 degrees C, respectively. Though cell voltages of about 2V were achieved and thereby reducing the energy consumption by 30-35% compared to the state-of-the-art membrane cell, the contributions of overvoltages were still high compared to the equilibrium potential of about 1.23V. Efforts to limit the individual parts of overvoltages as well as maintenance of the zero-gap cell at least on the anode side have been carried out. Two different kinds of cation exchange membranes have been used for the electrolysis cell. However, the cation exchange membrane with hydrophilic properties having high initial performances showed tendencies of blister formations.
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| 5. |
- Kiros, Yohannes
(författare)
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Metal porphyrins for oxygen reduction in PEMFC
- 2007
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Ingår i: International Journal of Electrochemical Science. - 1452-3981. ; 2:4, s. 285-300
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Tidskriftsartikel (refereegranskat)abstract
- A short literature review on alternative catalysts for the cathodic oxygen reduction in acid (H2SO4) and polymer electrolyte membrane fuel cell ( PEMFC) with special emphasis on pyrolyzed macrocycles and precursor materials from metals, organic molecules and N-containing elements has been conducted. Furthermore, various catalytic materials comprising two different concentrations of iron and cobalt tetramethoxyphenyl porphyrins and perovskites were prepared by both the impregnation and precipitation reactions. Screening tests of the individual catalytic materials and their mixtures were carried out in half-cell measurements using the rotating disc electrode (RDE) in 0.5M H2SO4 and at room temperature. Cyclic voltammograms were recorded at a scan rate of 10 mV s(-1) both with and without rotations. The peak potential at 0 rpm was used to study and compare the catalytic activities towards oxygen reduction reaction (ORR). Concentrations of 30wt% Fe and Co/TMPP have shown increased performance characteristics, while those with lower or increased contents, acid-treated and mixed with perovskite have displayed lower activities. Polarization data for the catalyst containing 30wt% FeTMPP was also obtained. Acid leaching of the pyrolysis products has resulted in substantial decrease of the metals from the pyrolysis products supported on carbon. TEM, BET-surface area and EDX analyses on the samples have shown high aggregation of the metals with crystalline structure, surface areas depending on the compositions of the catalysts and increased surface concentration of the metals with absence of nitrogen on the moiety.
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| 6. |
- Kiros, Yohannes, et al.
(författare)
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Oxygen reduction electrodes for electrolysis in chlor-alkali cells
- 2006
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 51:16, s. 3346-3350
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen reduction electrodes, containing non-noble metal catalysts supported on high surface area carbon and wet-proofed with PTFE were tested under reaction conditions for the chlor-alkali electrolysis. Double-layer gas diffusion electrodes were prepared by rolling of an active layer and diffusion layer on a nickel wire screen, compressed and sintered at 300 degrees C. Electrochemical measurements for substantiating the activity and stability of the half-cells were conducted in 8 M NaOH by supplying oxygen at a cell temperature of 70 degrees C and a constant current load of 300 mA cm(-2). An electrolysis cell with a dimensionally stable anode (DSA) and double-layered cathode was assembled, where 4.8 M NaCl and oxygen were supplied, respectively, for the production of chlorine and NaOH. The cell performances as well as stability of the electrodes were investigated at about 80 degrees C. This study shows that by replacing the high voltage consuming hydrogen-evolving reaction and by adopting highly active electrocatalysts as cathode materials, energy savings of more than 30% could be realized.
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| 7. |
- Kiros, Yohannes, et al.
(författare)
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Preparation of high surface area La0.1Ca0.9MnO3 and its electrochemical activities
- 2007
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Ingår i: THERMEC 2006, Pts 1-5. - 9780878494286 ; , s. 1361-1366
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Konferensbidrag (refereegranskat)abstract
- The use of perovskites as potential heterogeneous catalysts in a number of chemical and electrochemical reactions have attracted the research interest due to high surface reactivity and low cost. Since catalysis is strongly dependent on surface areas, texture and structural homogeneity, the development of effective modes of preparation of perovskites is of utmost importance for the achievement of high performances, especially for electrochemical applications in fuel cells and metal/air batteries. The precipitation reaction method for the synthesis of La0.1Ca0.9MnO3 at low temperatures in the presence of alkaline support electrolyte has shown significant gain in the surface and electrochemical properties. Enhanced BET-surface area of this production method as compared to the dry pyrolysis method was obtained. Multifunctional application for cathodic reactions in alkaline fuel cell (AFC) and intermediate temperature solid oxide fuel cell (IT-SOFC) as well as for bifunctional systems of both oxygen reduction reaction (orr) and oxygen evolution reaction (oer) has been investigated. Parameter optimisations of the ingoing materials and analyses of the solid-state properties of the catalyst pertaining to structural, chemical and surface characteristics have also been carried out.
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| 8. |
- Nylén, Linda
(författare)
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Critical potential and oxygen evolution of the chlorate anode
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. This increases the mass transport of the different species in the chlorate electrolyte. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency. One of the aims of his work was to understand how the chlorate process has to be run to avoid exceeding the critical anode potential (Ecr) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At Ecr anodic polarisation curves in chlorate electrolyte bend to higher Tafel slopes, causing increasing potential losses and accelerated ageing of the anode. Therefore the impact on the anode potential and on Ecr of different electrolyte parameters and electrolyte impurities was investigated. Additionally, the work aimed to investigate the impact of an addition of chromate on oxygen evolution and concentration profiles under conditions reminiscent of those in the chlorate process (high ionic strength, 70 °C, ruthenium based DSA, neutral pH), but without chloride in order to avoid hypochlorite formation. For this purpose a model, taking into account mass transport as well as potential- and concentration-dependent electrode reactions and homogeneous reactions was developed. Water oxidation is one of the side reactions considered to decrease the current efficiency in chlorate production. The results from the study increase the understanding of how a buffer/weak base affects a pH dependent electrode reaction in a pH neutral electrolyte in general. This could also throw light on the link between electrode reactions and homogeneous reactions in the chlorate process. It was found that the mechanism for chloride oxidation is likely to be the same for potentials below Ecr as well as for potentials above Ecr. This was based on the fact that the apparent reaction order as well as αa seem to be of the same values even if the anode potential exceeds Ecr. The reason for the higher slope of the polarisation curve above Ecr could then be a potential dependent deactivation of the active sites. Deactivation of active ruthenium sites could occur if ruthenium in a higher oxidation state were inactive for chloride oxidation. Concentration gradients of H+, OH-, CrO4 2- and HCrO4 - during oxygen evolution on a rotating disk electrode (RDE) were predicted by simulations. The pH dependent currents at varying potentials calculated by the model were verified in experiments. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments a reaction for the chromate buffering has been proposed. Under conditions with bulk pH and chromate concentration similar to those in the chlorate process, the simulations show that the current density for oxygen evolution from OH- would be approximately 0.1 kA m-2, which corresponds to about 3% of the total current in chlorate production.
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| 9. |
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| 10. |
- Spets, J.-P, et al.
(författare)
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Direct-Mode Glucose Fuel Cells with Near-Neutral-State Electrolytes : Anode Electrode Studies with Different Catalysts and Electrolytes
- 2009
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Ingår i: The Open Fuels & Energy Science Journal. - : Bentham Open. - 1876-973X. ; 2, s. 82-86
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, a direct-mode glucose fuel cell with a neutral-state and near-neutral-state aqueous electrolytes is studied. The near-neutral state electrolytes are important for two reasons. Firstly, the pH of the electrolytes would be near the pH of liquid in living cells. Secondly, the neutral electrolyte would enable good corrosion resistance of catalyst materials. Three different catalyst materials, i.e. Pt-Pd, Raney-Ni and Ni-porphyrin complex, are tested in an anode half-cell configuration with one neutral-state (battery water) and with two near-neutral-state aqueous electrolytes, i.e. modified Krebs-Ringer (K-R) and phosphate, both buffered to a pH value of 7.4. Pt-Pd catalyst in the aqueous K-R electrolyte maintains the negative voltage of the anode half cell with higher current densities that the nickel catalysts do. To estimate the operation of the direct-mode glucose fuel cell, the K-R electrolyte from the anode half-cell tests is tested also in the cathode half-cell with combined catalyst of cobalt porphyrin complex and of spinel. The open circuit voltages and polarisation curves are measured. Also, preliminary results and oxidation degrees of glucose in the tests are shown. Based on our half cell measurements, there are high development demands for the electro-catalysts, which could work efficiently in the near-neutral-state electrolytes.
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| 11. |
- Spets, J.-P, et al.
(författare)
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Starch and cellulose as fuel sources for low temperature direct mode fuel cells
- 2008
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Ingår i: The Open Fuel Cells Journal. - : Bentham Open. - 1875-9327. ; 1, s. 1-3
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Tidskriftsartikel (refereegranskat)abstract
- This paper is a study about a direct mode fuel cell with a near-neutral-state and alkaline electrolytes. The aim of study was to develop a fuel cell, which operates directly by mixing the fuel with the electrolyte. This arrangement helps to avoid inserting membranes and additional bacterial cultures in fuel cell. The target is also to create a fuel cell with a capacity of few mWcm-2 with the starch as a fuel. Also, glucose and sorbitol have been tested as fuel for the fuel cell.
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| 12. |
- Spets, J.-P, et al.
(författare)
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The Progress in the Ongoing Development Work : Enhancement of GlucoseElectro-Oxidation in Direct-Mode Fuel Cells - An Update
- 2009
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Ingår i: The Open Fuel Cells Journal. - : Bentham Open. - 1875-9327. ; 2, s. 11-14
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with the R&D regarding the direct glucose fuel cell with a capacity of increasing the power density with glucose as a fuel. The direct-mode fuel cell in which the fuel and the alkaline electrolyte are mixed with each other is tested at room temperature. The direct-mode fuel cell is exposed to an externally generated electromagnetic field with 4 GHz sine signals between electrodes to cause both the splitting of the fuel molecule and the electrochemical oxidation. As a result from the use of the higher frequency signals, a maximum current density of 15 mAcm-2 has been achieved with the total voltage of 0.5 V.
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