Increase In Interfacial Resistance Research Articles

Protective coating based on manganese–copper oxide for solid oxide fuel cell interconnects: Plasma spray coating and performance evaluation

A solid oxide fuel cell (SOFC) stack requires metallic interconnects to electrically connect unit cells, while preventing fuel from mixing with oxidant. During SOFC operations, chromia scales continue to grow on the interconnect surfaces, resulting in a considerable increase of interfacial resistance, and at the same time, gaseous Cr species released from the chromia scales degrades the cathode performance. To address these problems, in this study, protective Mn2CuO4 coatings are fabricated on metallic interconnects (Crofer 22 APU) via a plasma spray (PS) process. The PS technique involves direct spray deposition of molten Mn2CuO4 onto the interconnect substrate and leads to the formation of high-density Mn2CuO4 coatings without the need for post-heat-treatment. The thickness, morphology, and porosity of the PS-Mn2CuO4 coating are found to depend on the processing parameters, including plasma arc power, gas flow rate, and substrate temperature. The PS-Mn2CuO4 coating fabricated with optimized parameters is completely impermeable to gases and has high adhesion strength on the interconnect substrate. Furthermore, no resistive chromia scales are formed at the coating/substrate interface during the PS process. As a result, the PS-Mn2CuO4-coated interconnects show a very low area-specific resistance below 10 mΩ cm2 at 800 °C in air and excellent stability during both continuous operation and repeated thermal cycling. This work suggests that an appropriate combination of the material and coating process provides a highly effective protective layer for SOFC interconnects.

H2S Poisoning of Proton Conducting Solid Oxide Fuel Cell and Comparison with Conventional Oxide-Ion Conducting Solid Oxide Fuel Cell

The electrochemical response of proton conducting solid oxide fuel cells (PC-SOFC) with Ni-BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) cermet anode by low-ppm(v) level hydrogen sulfide (H2S) was studied using both electrolyte-supported cells and anode symmetrical cells at 750–450°C. Upon the introduction of H2S, both types of cells show small but observable increase in the electrode apparent interfacial resistance (Rai) with no change in ohmic resistance. In addition, secondary-ion mass spectroscopy (SIMS) analysis on Ni-BZCYYb composite pellets after exposure to H2S showed a strong correlation between the distributions of barium and sulfur species, yet both were complementary to the distribution of nickel species. The results indicate that PC-SOFC are still subjected to poisoning by low ppm(v)-level H2S at intermediate temperature. However, the H2S poisoning effect (in terms of relative increase in interfacial resistance) for PC-SOFC appears much smaller than for conventional oxide-ion conducting SOFC with Ni-yttria stabilized zirconia (Ni-YSZ) cermet anode and YSZ electrolyte. Such a difference in H2S poisoning behavior is hypothesized to be related to the different anode reaction process for PC-SOFC versus conventional oxide-ion conducting SOFC and the potential (electro-)catalytic role that proton conducting ceramic electrolyte might play in the anode reaction. The directions for future study are also pointed out.

Effect of Dry and Humidified Gases at Porous Electrolyte NOx Sensors

Porous electrolyte NOx sensors promise greater NOx sensitivity in comparison to conventional dense electrolyte NOx sensors, and they are capable of detecting single digit ppm levels of NOx. This is important for monitoring and regulating diesel engine operation as diesel engine technology is resulting in substantially lower emissions. However, cross-sensitivity to H2O limits the feasibility of porous electrolyte NOx sensors and further investigation is needed to understand factors affecting sensor selectivity. It has been suggested in other studies that the electrolyte material may influence water behavior. Therefore, this study examined H2O cross-sensitivity in sensors composed of partially-stabilized zirconia (PSZ) and fully-stabilized zirconia (FSZ) porous electrolyte microstructures accompanied by Au wire electrodes. The data, based on impedancemetric sensor operation, indicated significantly lower H2O cross-sensitivity in PSZ electrolyte based sensors, in comparison to sensors containing FSZ. Standard ceramic process methods were used to fabricate NOx sensors containing 4.7 mol% Y2O3 – ZrO2 (PSZ, Advanced Ceramics) and 8 mol% Y2O3 – ZrO2 (FSZ, Tosoh Corp.). Polyvinyl butyral (B-76 Butvar) and ethanol were added to the electrolyte samples to form a mixture that was ball milled into a uniform slurry of which a portion was air dried to form a dry powder. The powder was uniaxially pressed under a load of 200 MPa to form pellets. Au wires were positioned over the electrolyte pellets and the remaining electrolyte slurry was coated over the electrodes. The PSZ and FSZ based sensors were fired at 1050˚C for 1 hour. Impedance studies were conducted using a Gamry Reference 600 for sensor operation over a temperature range of 600-700°C where NO and NO2 concentrations varied from 0 to 100 ppm for dry and humidified (3, 5, and 10% H2O) gas environments. Based on analysis of the impedance data the sensitivity, selectivity, oxygen dependence, activation energy, and response rate of the sensors was determined. The microstructure and morphology of the electrolytes were studied using SEM, BET, and MIP. The impedance response indicated the PSZ based sensors demonstrated less cross-sensitivity to H2O in comparison to the FSZ based sensors. This was observed from the impedance data where the interfacial resistance decreased by 12% for FSZ based sensors when 10% water was added to the gas stream, whereas, a slight increase in interfacial resistance of approximately 2% occurred for PSZ based sensors under the same conditions. The addition of water likely resulted in hydroxyl groups that effected the interfacial conductivity of the sensors. It is possible that reactions involving hydroxyl groups were greater for the FSZ based sensors such that an increase in interfacial conductivity reduced the resistance at the Au/FSZ interface. The formation of hydroxyl groups can result from oxygen ions traveling through the electrolyte. As the oxygen ion conductivity is lower for PSZ in comparison to FSZ, it is possible that the difference in the water behavior for the PSZ versus the FSZ based sensors was due to a lower rate of hydroxyl formation at the PSZ/Au interface. The impedance data also indicated the NOx sensitivity was greater for FSZ versus PSZ based sensors for both dry and humidified gas conditions; and the FSZ based sensors had a more rapid response. The activation energy of the sensors was approximately 1.0 and 1.2 eV for the PSZ and FSZ based sensors, respectively. The activation energy decreased slightly when NOx was present in the gas stream, and when water was added. Oxygen partial pressure measurements indicated dissociative adsorption was the dominant rate limiting for both dry and humidified gas environments for both PSZ and FSZ based sensors. The effect of electrolyte particle size on water behavior was considered by studying the behavior of sensors composed of PSZ electrolytes containing 1 micron and 2 micron sized particles. The magnitude of the impedance response was larger for the sensors containing the 2 micron sized particles, however, the water behavior was similar for by types of PSZ based sensors. A slight increase in the impedance was observed when water was added to the gas stream for sensors containing PSZ irrespective of the electrolyte particle size. Overall, the morphology and microstructure of the sensors did not appear to influence the behavior of the sensors in the presence of water. Rather, it is likely that the formation of hydroxyl groups and related reactions contributed to cross-sensitivity that was primarily observed for the FSZ based sensors. It is possible that the limitation of hydroxyl group reactions at PSZ based sensors restricted water cross-sensitivity, thereby improving the selectivity of the sensors to NOx.

Effects of cathode electrolyte interfacial (CEI) layer on long term cycling of all-solid-state thin-film batteries

All-solid-state lithium-ion batteries have the potential to not only push the current limits of energy density by utilizing Li metal, but also improve safety by avoiding flammable organic electrolyte. However, understanding the role of solid electrolyte – electrode interfaces will be critical to improve performance. In this study, we conducted long term cycling on commercially available lithium cobalt oxide (LCO)/lithium phosphorus oxynitride (LiPON)/lithium (Li) cells at elevated temperature to investigate the interfacial phenomena that lead to capacity decay. STEM-EELS analysis of samples revealed a previously unreported disordered layer between the LCO cathode and LiPON electrolyte. This electrochemically inactive layer grew in thickness leading to loss of capacity and increase of interfacial resistance when cycled at 80 °C. The stabilization of this layer through interfacial engineering is crucial to improve the long term performance of thin-film batteries especially under thermal stress.

Reactions at the electrode/electrolyte interface of all-solid-state lithium batteries incorporating Li–M (M = Sn, Si) alloy electrodes and sulfide-based solid electrolytes

Reactions at the electrode/electrolyte interface of all-solid-state lithium batteries were studied for combinations of sulfide-based solid electrolytes with various Li4-xGe1-xPxS4 and Liy-M (M=Sn, Si) alloys as the negative electrodes, using ac impedance, X-ray diffraction and energy-dispersive X-ray spectroscopy. The solid electrolyte at the interfacial region was found to decompose with the application of a current through the cells, resulting in the formation of a solid electrolyte interphase (SEI) layer. Resistivity changes at the interface varied depending on the electrolyte composition and the redox potential (vs. Li/Li+) of the negative electrode material. Lower resistances were observed with lower Ge contents in the solid electrolyte and the use of a Li–M alloy with a higher redox potential due to the formation of an electrochemically stable SEI layer during battery operation. In contrast, a combination of higher Ge content and an alloy with a lower redox potential led to a rapid increase in the SEI resistance and increase in its thickness. The presence of a Li–P–S compound with low ionic conductivity in the interfacial region was found to be related with the increase of interfacial resistance, leading to poor cycling characteristics. The formation of a suitable SEI layer is an important factor in the future development of all-solid-state batteries and this study serves to clarify the relationships between the formation of the SEI phase, the redox potential of the electrode and the sulfide-based solid electrolyte composition.

Effects of Electrolyte Additives on the Suppression of Mn Deposition on Edge Plane Graphite for Lithium-Ion Batteries

To suppress the degradation of graphite negative-electrodes that is caused by the dissolution of Mn-containing positive-electrode materials, the effects of additives that should strongly interact with Mn ion in electrolyte solution were examined. The edge plane of highly oriented pyrolytic graphite (HOPG) was used as a model graphite electrode, and its electrochemical properties were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in Mn-ion-containing electrolyte solution. 100 ppm Mn ion dissolved in the electrolyte solution caused a significant decrease in redox currents due to the intercalation and de-intercalation reactions of Li+ at an edge plane HOPG electrode, and a remarkable increase in interfacial resistance consisting of surface film resistance and charge transfer resistance due to Li+ transfer at an interface between the electrode and electrolyte solution. This performance deterioration could be effectively suppressed by the addition of a certain type of cyclic ether to the electrolyte solution. The influence of the additive concentration on interfacial resistance was also investigated.

Electrochemical impedance spectroscopy analysis for lithium-ion battery using Li4Ti5O12 anode

The impedance of Li-ion battery using a Li4Ti5O12 (LTO) anode for high-power applications was measured at various depths of discharge and temperatures during charge–discharge cycles. The measured impedance was interpreted with an equivalent circuit made up of anode and cathode, in which the cathode component was composed of two particle size factors. The values obtained for equivalent circuit elements by modeling were in good agreement with the results measured by other techniques, and indicated that the capacity fade of this Li-ion battery due to cycling is mainly caused by the increase of interfacial resistance and a decrease in the capacity of the LiCoO2 cathode. These results suggest that the cyclability of this LIB was improved by using an LTO anode, and show the validity of the equivalent circuit for interpreting the causes of capacity fade.

Optimization of inverted tandem organic solar cells

Inverted tandem organic solar cells, consisting of two bulk heterojunction sub-cells with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM) active layer and a MoO 3/Ag/Al/Ca intermediate layer, have been presented and optimized. Indium tin oxide (ITO) modified by Ca acts as a cathode for electron collection and Ag is used as the anode for hole collection for the tandem device. A proper thickness of Ca (3 nm) forms a continuous layer, working as a cathode for the top sub-cell. MoO 3 as the anode buffer layer prevents exciton quenching and charge loss at the anode side, which could result in increase in interfacial resistance. The variance of sub-cell thickness adjusts the optical field distribution in the entire device, facilitating light absorption and good current matching in both sub-cells. The optimal inverted tandem device achieves a maximum power conversion efficiency of 2.89% with a short-circuit current density of 4.19 mA/cm 2, an open-circuit voltage of 1.17 V, and a fill factor of 59.0% under simulated 100 mW/cm 2 (AM 1.5G) solar irradiation.

Development of high-voltage and high-capacity all-solid-state lithium secondary batteries

We have previously indicated that the improvement of the interface degradation between a positive electrode and an ethylene oxide-based solid polymer electrolyte (SPE) is an important problem requiring a solution for all-solid-state lithium secondary batteries. Therefore, Li 3PO 4 thin-film on LiCoO 2 powder was prepared by the spray coating technique. The observed Li 3PO 4 film was so thin (approximately 20 nm thick) that its contribution to the increase in interfacial resistance was small. However, it played the role of an oxidation barrier for the SPE. A [Li 3PO 4-coated LiCoO 2|SPE|Li] cell exhibited reversibility up to 4.6 V with a 200 mAh g −1 discharge capacity. The energy density of 760 mWh g −1 is one of the highest values obtained for all-solid-state lithium secondary batteries.