N. M. Bogdanovich

Gas diffusion hindrances on Ni cermet anode in contact with Zr0.84Y0.16O1.92 solid electrolyte

The electrochemical behavior of Ni cermet electrode with CeO2 − x additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H2, H2O, CO2, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σlf) is characterized with the following regularities: (a) temperature dependence of σlf has a positive slope in Arrhenius coordinates; (b) σlf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σlfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σlf vs. 1/PCO2 (PCO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σlf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.

Effect of oxygen activity and water partial pressure to degradation rate of Ni cermet electrode contacting Zr0.84Y0.16O1.92 electrolyte

The time curves of full polarization resistance of Ni cermet electrode modified with CeO2 − δ additive were studied by means of impedance spectroscopy in binary gas mixtures x% H2 + (100 − x)% H2O, 10% CO + 90% CO2 and multicomponent gas mixtures H2 + CO2 + H2O + CO + Ar of various composition at the temperature of 900°C. The Ni cermet electrode degradation rate in binary gas mixtures H2 + H2O was shown to increase sharply at the partial water pressure over 45%. The Ni cermet electrode degradation rate in the mixture of 10% CO + 90% CO2 was significantly lower than that in 10% H2 + 90% H2O. The major changes in the electrode characteristics upon long exposure in working conditions were accounted for by changes in the high-frequency partial polarization resistance. In the course of long testing, the electrode microstructure was not significantly changed. In the presence of hydrogen-containing components (H2 and H2O), the carbon-containing components (CO and CO2) were shown to make an insignificant contribution to the current generation processes in Ni cermet electrode. It was suggested that strong degradation of Ni cermet electrode was caused by poisoning its reaction sites with strongly linked adsorption forms of water (hydroxyls) at the positive charge of electrode.

Single solid-oxide fuel cells with supporting ni-cermet anode

Single solid-oxide fuel cells (SOFCs) with a porous (36-41%) supporting Ni-cermet anode are manufactured and tested. The effect of the thickness of the supporting Ni-cermet anode on the electrochemical characteristics of single SOFCs is studied. It is shown that polarization losses on electrodes at the current density of 1.2 A/cm2 increase by about 2 times from 0.13 to 0.25 V at an increase in the thickness of the supporting Ni-cermet anode from 0.40 to 1.27 mm. The impedance spectroscopy method is used to identify relaxation processes responsible for the behavior of the fuel cell anode and cathode. It is found that a significant percentage of polarization losses on the anode is due to transport limitations in fuel supply to the three-phase nickel/electrolyte/gas phase interface and removal of the reaction products away from it.

Cathodes based on (La, Sr)MnO3 modified with PrO2 − x

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO3-(Zr, Sc)O2 (LSM-SSZ), modified with PrO2 − x additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO2 − x were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH4 and O2. In cells with (Ce, Sm)O2 (SDC) and (Zr, Y)O2 (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm2 and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm2, respectively, if YSZ or SDC electrolyte is used.

Single fuel cell with supported LSM cathode

Single fuel cells with bilayer supported cathodes are manufactured and tested. The cathodes consist of a high-porous La0.6Sr0.4MnO3 support with the thickness of approximately 1 mm and a functional composite layer with the thickness of 13–15 μm made of La0.75Sr0.2MnO3 and 8YSZ. Voltammetric and power characteristics of single fuel cells with a supported cathode, thin-film YSZ electrolyte, and platinum cathode are determined. The conclusion as to the significant contribution into the polarization overpotential losses on the cathode is made on the basis of the measurements of electric fuel cell characteristics. It decreases significantly as a result of the supported cathode modification by praseodymium oxide. At 850°C and voltage of 0.81 V, electric power density of a fuel cell was 1.65 W/cm2.

Options for adjustment of microstructure and conductivity of cathodic substrates of La(Sr)MnO3

The electrodes of solid-oxide fuel cells (SOFCs) must be characterized by high conductivity to decrease ohmic losses and sufficient porosity to provide high gas diffusion rate. In the cases, when the SOFC electrodes are substrates, they must be synthesized at the temperature above the temperature of formation of their solid-electrolyte coating. Herewith, manufacturing of supporting electrodes with the required micro-structure is rather complicated. The present paper studies the effect of the method of manufacturing of the initial La0.6Sr0.4MnO3 (LSM) powders, their degree of dispersion, introduction of sintering additives and pore agents on their microstructure, conductivity, and possibility of adjusting the temperature of SOFC cathodic substrate formation at which the required characteristics are reached. It is shown that sintering of cathodic substrates to the relative density of 65–70% can be carried out at the temperatures from 1050 to 1350–1400°C, which would allow obtaining electrolyte films of powders with different sintering ability on such substrates. The average pore size in cathodic substrates can be varied in the range of 0.4 to 2.5 μm by using the initial LSM powder with different dispersion degree and by employing graphite as a pore agent. At 900°C, conductivity of cathodic substrates of LSM grows at an increase in their relative density from 50% to 70% approximately from 50 to 100 S/cm and weakly depends on the dispersion degree of the initial powders.

Electrochemical properties of cathodes made of (La,Sr)(Fe,Co)O3 containing admixtures of nanoparticles of cupric oxide and intended for fuel cells with a solid electrolyte based on ceric oxide

The electric and electrochemical characteristics of cathodes made of La0.6Sr0.4Fe0.8Co0.2O3−δ (LSFC) and intended for fuel cells with electrolytes based on ceric oxide are studied. Adding cupric oxide into the LSFC cathode is shown to exert a favorable effect of the properties of the LSFC-CuO/SDC electrode system, where SDC stands for the CeO2-Sm2O3 electrolyte. The effect produced by cupric oxide when added in the form of nanopowder is perceptibly greater than in the case of micropowdered CuO. Adding a mere 0.5 wt % of nanopowdered CuO reduces the LSFC cathode resistance nearly tenfold. The cathode’s adhesion to the electrolyte substantially improves as well, which makes it possible to lower the cathode’s firing temperature by 100°C. The maximum of electrochemical activity is intrinsic to cathodes containing 2 wt % CuO, with the caking temperature of 1000°C. According to a 2011-h life test of the LSFC-SDC composite cathodes containing nanopowdered CuO, temporal stability of their electrochemical characteristics improves with the SDC content. The time dependences of the polarization resistance of cathodes containing 40–50 wt % SDC look like decaying exponential curves. The steady-state polarization resistance, calculated on the basis of this, is equal to 0.1–0.2 ohm cm2. At an overvoltage of less than 100 mV, the cathodes may provide for a current density of 0.5–1.0 A cm−2.

Electroconductivity and transport numbers of solid electrolytes La10−x CaxA6O27−δ and La9.33+δA6−x AlxO26 (A = Si, Ge) with apatite-like structure

The ion-oxygen conductivity of apatite-like compounds based on lanthanum silicates and germanates La10A6O27 (A = Si, Ge), La10−xCaxSi6O27−δ (x = 0.25, 0.5, 1.0), La9.75Ca0.25Ge6O27−δ and La9.33+δSi6−xAlxO26(x=0.4, 0.8, 1.5) is studied in the interval of partial oxygen pressures pO2 extending from 10−16 to 105 Pa, at temperatures of 500–1000°C. The electroconductivity of undoped compounds La10A6O27 (A = Si, Ge) exceeds that of yttria-stabilized zirconia. The electroconductivity of lanthanum germanate (1.7 × 10−2 and 8.5 × 10−2S cm−1 at 700 and 900°C, respectively) is substantially higher than that of lanthanum silicate (9.8 × 10−3 and 3.5 × 10−2 S cm−1 at 700 and 900°C). Doping lanthanum germanate with calcium raises its electroconductivity (2.7 × 10−2 and 1.3 × 10−1 S cm−1 for La9.75Ca0.25Ge6O27−δ at 700 and 900°C). Conversely, doping lanthanum silicate with ions of calcium or aluminum reduces the conductivity. In the pO2 interval studied, the above compounds are ionic conductors and represent a class of solid electrolytes of promise for various electrochemical devices.

Cathodes based on rare-earth metal nickelate ferrites prepared from industrial raw materials for solid oxide fuel cells

Powders of composite materials based on lanthanum nickelate-ferrite with different contents of lanthanides were synthesized using Ln-Ni-Fe-O (LnNF) mischmetal. The phase composition of the powders was determined by XRD. The crystal structure of the main phase in LnNF obtained using a mischmetal with a high lanthanum content is perovskite-like, while the structure with a high cerium content is fluorite-like. The temperature coefficients of linear expansion of the synthesized materials were determined by dilatometry. The temperature dependences of electric conductivity of the compact samples and porous electrodes from these materials were studied by the DC four-probe method. The electric conductivity of compact samples from materials prepared using the mischmetal with the perovskite-like structure of the main phase exceeds the electric conductivity of the materials based on the mischmetal with a high cerium content by approximately three orders of magnitude. The temperature dependences of polarization conductivity of electrodes in cells with the Ce0.8Sm0.2O1.9 (SDC) electrolyte at 600–900°C in air were studied by impedance spectroscopy. The LnNF electrodes with a perovskite-like crystal structure of the main phase showed high electrochemical activity.

Nickel-cermet electrodes for high-temperature electrochemical devices made using nanomaterials

Characteristics of nickel-cermet anodes obtained from weakly aggregated NiO nanopowders made by wire electric blasting (NiO/WEB) were studied. Electrodes made of NiO/WEB nanopowders have low sheet resistance (<0.1 Ohm) and high electrochemical activity (Rη = 0.06–0.09 Ohm cm2 at 850–900°C). Prolonged studies of symmetric cells of the (0.9H2 + 0.1H2O) Ni-SSZ + CeO2/YSZ/CeO2 + Ni-SSZ (0.9H2 + 0.1H2O) type at the temperature of 850°C showed that the electrodes preserve sufficiently high activity (Rη < 0.1 Ohm cm2) for 1000 h. Using a NiO-WEB powder allows not performing presynthesis of nickel-cermet and decreasing the anode baking temperature to 1200°C.