Gas Electrode Reactions in Molten Carbonate Media: IV . Electrode Kinetics and Mechanism of Hydrogen Oxidation in Eutectic

This research was performed to study the anode reaction in a molten carbonate fuel cell. Chronoamperometry experiments and impedance measurements were carried out at gold and nickel flag-type electrodes in a 62 mole percent (m/o) Li2COJ38 m/o K2CO3 eutectic carbonate mixture at 923 K as a function of gas composition (H2/CO2/H20), and as a function of temperature between 823 and 1073 K. The i -~ extrapolation technique, generally applied to evaluate chronoamperometry results, is shown to yield systematic errors for the hydrogen oxidation studies in molten carbonates. A direct fit of the Gerischer-Vielstich relation yields more reliable results for gold. The current responses on Ni cannot be described by the Gerischer-Vielstich relation. The high reaction orders for hydrogen (0.7 to 1.0), medium for carbon dioxide (order 0.5), and the low or even negative reaction order for water (0 to -0.25), indicate the hydrogen oxidation follows a Volmer-Heyrovsky-type mechanism on gold flag electrodes. The reaction rate at nickel electrodes was found to be too high to be studied accurately using impedance measurements and chronoamperometry on flag electrodes. In spite of the poor accuracy, reaction orders indicate that a final chemical react.ion is the rate-determining step in the reaction mechanism on nickel. The activation energy for the exchange current density was found to be in the order of 80 kJ/mol with a tendency to be higher for the gases richer in hydrogen. The activation energy for the Warburg coefficient (diffusion) was found to be about -15 kJ/mol. IntroductionThe molten carbonate fuel cell (MCFC) is a promising energy conversion system capable of producing electricity from hydrogen containing fuel gases with high efficiency. At the anode of an MCFC hydrogen oxidation takes place. The anode is made of porous Ni alloyed with Cr or A1.In this paper, the results of chronoamperometry and impedance measurements at gold and nickel electrodes are presented. The objective of these experiments was to study the reaction mechanism and kinetics of the hydrogen oxidation in molten carbonate. Gold was chosen as the electrode material to start with, because it has been reported that the kinetics of hydrogen oxidation on gold is relatively slow allowing a more accurate determination of the exchange current densities and hence the reaction orders. 1-5 Then experiments on Ni, being the actual anode material, were performed.

Section: Resultscontrasting

confidence: 67%

Section: I(t) = I(t = O) ~ -mentioning

confidence: 62%

Section: Introductionmentioning

confidence: 99%

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The Mechanism of Hydrogen Oxidation at Gold and Nickel Flag Electrodes in Molten Li/K Carbonate

Weewer

Hemmes

Wit

1995

“…8,15 The exchange current density (i 0 ), for a standard gas at 650°C in the present study is 70 A/m 2 , which is lower than other values given in the literature. 1,7,12,16 Here, n was set to 2, because this is the value obtained assuming the most likely reaction mechanisms occur. 14 Changing n will change the value of i 0 , although the partial pressure dependencies will still be the same.…”

Section: Kinetics Of the Hydrogen Reaction-mentioning

confidence: 99%

Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell

Lindbergh

Olivry

Sparr

2001

The electrochemical performance of the porous nickel anode in a molten carbonate fuel cell was experimentally investigated in this study. The electrode structure was studied by scanning electron microscope and Hg porosimetry. The effect of electrolyte filling, on overpotential at a constant current density, was also examined. Kinetics for the hydrogen reaction was studied by obtaining stationary polarization curves, for porous nickel anodes at varying temperatures and anode gas compositions. The slopes of these polarization curves were analyzed at low overpotentials, with the assumption that the porous anode was under kinetic control, and the corresponding exchange current densities were determined using a simplified porous electrode model. The obtained partial pressure dependencies of the exchange current density were high, and therefore, difficult to explain by the generally assumed mechanisms.The molten carbonate fuel cell ͑MCFC͒ operates at high temperatures ͑600-700°C͒, which gives it the potential to reach high overall efficiency in an integrated system. It is in general operated using natural gas as its fuel, although high concentrations of carbon monoxide and carbon dioxide can also be a part of the feed. This makes it very suitable for use with biogas, gasified coal, or gasified biomass feeds although the two latter fuels will result in much lower concentrations of hydrogen gas. Operation with a lower concentration of hydrogen increases the losses at the anode, providing a reason for investigating and optimizing the performance of the anode under such conditions. The overvoltage losses at the anode are considered to be relatively small, which is partly due to the relatively low current densities that present MCFC systems operate at. A significant increase in current density, which is desirable in order to improve the power density, would result in the increase of losses at the anode.A general procedure for elucidating the mechanism of the anode reaction is to determine the partial pressure dependency for the components, hydrogen, carbon dioxide, and water, that partake in the overall electrochemical reactionSeveral studies have previously been made to find out the reaction mechanism at the anode, 1-18 Table I. Most experiments were conducted on flag electrodes, which are quite different from porous electrodes, when regarding the electrode surface and mass transfer conditions. Despite this research, the reaction mechanism is still not well defined. One problem when evaluating the partial pressure dependency is the actual gas composition inside the cell, since different reactions can take place at the anode. One such reaction is the electrochemical reaction of carbon monoxideAnother possible reaction is the shift reaction, which is sum of Reactions 1 and 2According to Borucka and Appleby, 19 the reaction rate for direct carbon monoxide oxidation is extremely slow at a surface of 99.99% pure gold. They claimed that the most likely way for carbon monoxide to be utilized is through the shift reaction as that ...

“…Lu and Selman 9 agreed with the AS mechanism, and they also noted that the hydrolysis equilibrium reaction played an important role in carbonate melts. Nishina et al 10 proposed a modification of the JS mechanism, including a split up of the rate-determining step. The intermediate species M-HCO 3 -was mentioned as plausibly rate-determining.…”

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confidence: 99%

Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in a Molten Carbonate Electrolysis Cell (MCEC)

Hu¹,

Lindbergh²,

Lagergren³

2015

The purpose of this study was to elucidate the kinetics of a porous nickel electrode for hydrogen production in a molten carbonate electrolysis cell. Stationary polarization data for the Ni electrode were recorded under varying gas compositions and temperatures. The slopes of these iR-corrected polarization curves were analyzed at low overpotential, under the assumption that the porous electrode was under kinetic control with mass-transfer limitations thus neglected. The exchange current densities were calculated numerically by using a simplified porous electrode model. Within the temperature range of 600-650 • C, the reaction order of hydrogen is not constant; the value was found to be 0.49-0.44 at lower H 2 concentration, while increasing to 0.79-0.94 when containing 25-50% H 2 . The dependence on CO 2 partial pressure increased from 0.62 to 0.86 with temperature. The reaction order of water showed two cases as did hydrogen. For lower H 2 O content (10-30%), the value was in the range of 0.47-0.67 at 600-650 • C, while increasing to 0.83-1.07 with 30-50% H 2 O. The experimentally obtained partial pressure dependencies were high, and therefore not in agreement with any of the mechanisms suggested for hydrogen production in molten carbonate salts in this study. Electrolysis in molten carbonate salts at high temperatures is a promising method for hydrogen and/or syngas (H 2 +CO) production, especially when combining it with renewable electricity resources such as solar energy and wind power. Due to the favorable thermodynamic and kinetic conditions, high-temperature electrolysis will attain higher overall efficiency and require lower applied voltage when compared to low-temperature electrolysis.1 Electrolysis in molten carbonates has been evidenced by some authors, mainly by converting CO 2 into CO.2-5 Peelen et al. 2 studied the electrochemical reduction of CO 2 into CO on a gold flag electrode in 62/38 mol% Li/K carbonate mixture in the temperature range 575-700• C. Kaplan et al.3-4 observed the conversion of CO 2 to CO by using a cell with a molten electrolyte consisting of lithium carbonate and lithium oxide, in which the electrodes were graphite (anode) and titanium (cathode), and the working temperature was 850-900• C. Chery et al. 5 presented thermodynamic calculations and experimental measurements on the reduction of CO 2 into CO on a gold flag or planar disk electrode with Li/K and Li/Na carbonate eutectics at temperatures from 575 to 650• C. However, most experiments were carried out on flag electrodes, which are different from porous electrodes when considering electrode surface and mass-transfer limitations. In our previous study 6 a cell operating at 650• C, with conventional molten carbonate fuel cell (MCFC) materials (Ni-based porous electrode and molten carbonate electrolyte), was investigated also for electrolysis. The cell was found to give lower polarization losses in MCEC mode than in MCFC mode, mainly due to the NiO electrode performing much better as anode in the electrolysis cell. Reversing...