Hydrogen transport through Pd-Ni alloy electrodeposited on Pd substrate

Complex capacitance analysis was done on the porous carbon electrode-electrolyte interface, where a minor leakage current is involved in addition to the dominant capacitor charging current. Based on the transmission line model, imaginary capacitance profiles ͑C im vs. log f͒ were theoretically derived for a cylindrical pore and multiple pore systems of nonuniform pore geometry. The parallel RC circuit was assumed for the interfacial impedance, where R is the charge-transfer resistance for leakage current and C the double-layer capacitance. The theoretical derivation illustrated that the resistive tail relevant to the leakage current appears in addition to the capacitive peak, which was in accordance with the experimental data taken on the porous carbon electrode. The electric double-layer capacitor ͑EDLC͒ parameters such as the extent of leakage current, total capacitance, and rate capability were visually estimated from the imaginary capacitance profiles. The more quantitative EDLC parameters were obtained by a nonlinear fitting to the experimental data. Electric double-layer capacitor ͑EDLC͒ utilizes the double layer formed at the electrode-electrolyte interface, where electric charges are stored on the electrode surface and ions of counter charge are arranged in the electrolyte side. The most demanding feature for EDLC electrodes is the ideally polarized behavior over a wide potential range. The practical EDLC electrodes, however, suffer from a self-discharge at the charged state that is caused by leakage currents. [1][2][3][4][5] This nonideally polarized behavior should thus be minimized to improve the charge-discharge efficiency and reliability of commercial cells.The leakage current appearing in EDLC electrodes can be analyzed using ac impedance spectroscopy. When impedance data are analyzed using the conventional Nyquist plot, the vertical line in the low-frequency region that is observed for ideally polarized electrodes becomes inclined with an increase in the leakage current. [6][7][8] The extent of leakage current can thus be estimated from the degree of inclination. The problem here, however, is that the inclination is also observed even in ideally polarized electrodes when they have a nonuniform pore geometry that is common in most practical porous EDLC electrodes. [9][10][11][12] In theory, the differentiation between the leakage current and nonuniform pore geometry for the cause of inclination is possible when the measurement is made at very low frequencies. In the extremely low-frequency region, ideally polarized electrodes with nonuniform pore geometry give a vertical line on the Nyquist plot, whereas the leakage current is visualized as a semicircle whose diameter reflects the charge-transfer resistance for leakage reaction. In practice, however, it is difficult to obtain data at such a low frequency due to an instrumental limitation and extremely long measuring time.In this work, the complex capacitance analysis is done on the leakage current involved in porous carbon electrode. From the imagi...

Section: Theorymentioning

confidence: 99%

mentioning

confidence: 99%

Complex Capacitance Analysis on Leakage Current Appearing in Electric Double-layer Capacitor Carbon Electrode

Jang

Yoon

et al. 2005

“…The migration length distribution might be more suitable rather than the migration area distribution to explain the CPE exponent, as reported in our previous communications 27,28 when the migration area distribution is quite narrow with respect to the cross-sectional area. In the case of the dense electrode where the YSZ structure is highly branched, however, the length distribution is hardly defined due to the frequent occurrence of overlapping in the migration path of ions.…”

Section: Resultsmentioning

confidence: 76%

Oxygen Reduction Kinetics at Dense (La[sub 0.85]Sr[sub 0.15])[sub 0.9]MnO[sub 3]–YSZ Composite Electrodes Investigated Using Potentiostatic Current Transient Method

Kim

Pyun

Shin

et al. 2008

The oxygen reduction kinetics was investigated at dense ͑La 0.85 Sr 0.15 ͒ 0.9 MnO 3 ͑LSM͒-yttria-stabilized zirconia ͑YSZ͒ composite electrodes as a function of YSZ content of 20-40 wt % by using the potentiostatic current transient ͑PCT͒ technique combined with ac-impedance spectroscopy. From the discrete Fourier transformation analysis of impedance spectra, the individual reaction steps were successfully differentiated and the distribution of relaxation time for ion migration was quantitatively estimated for oxygen ion migration as well. The cathodic PCTs were characterized by decay and buildup transients, which corresponded to the oxygen reduction current at the three-phase boundaries ͑TPBs͒ I TPB and the current at the LSM surface I V o •• superimposed on the steady-state current at the TPBs I TPB st , respectively. As the YSZ content increased, the equilibrium exchange flux at the LSM surface j o decreased and the difference between the steady-state currents at the TPBs and at the LSM surface ⌬I st increased in absolute value. These results strongly indicate that when the YSZ content increases, the contribution of the facile ion migration to j o is suppressed by the prevailing contribution of the sluggish charge transfer reaction at the LSM surface and oxygen reduction at the TPBs predominantly contributes to the overall oxygen reduction than that reduction at the extended active sites.During the solid oxide fuel cell ͑SOFC͒ operation, the overall losses are mainly controlled by the cathodic oxygen reduction reaction ͑ORR͒. 1,2 In this respect, among the various cathode materials that can be used in the SOFC, the ͑La 1−x Sr x ͒ 1−y MnO 3 ͑LSM͒-yttriastabilized zirconia ͑YSZ͒ composite electrodes have been studied extensively and have been used commercially as well, because the three-phase boundaries ͑TPBs͒ are three-dimensionally extended to the electrode beyond the electrode/electrolyte interface. [3][4][5][6][7] Because the YSZ particle of this composite electrode provides the ionic pathway necessary for oxygen reduction and increases the length of TPBs, the kinetics of oxygen reduction is significantly influenced by the YSZ content. Furthermore, due to the distribution of particle connectivity, the YSZ content is expected to affect the resistance-capacitance ͑RC͒ time constant distribution for oxygen ion migration through the composite electrode. 8,9 Thus, the YSZ content in the composite electrode is a significant parameter in the oxygen reduction kinetics.For the case of cathode materials composed of LSM, it is generally reported 10-13 that during a large cathodic polarization, the LSM phase undergoes the partial reduction of Mn 3+ to Mn 2+ , leading to the formation of oxygen vacancies at the TPBs. These oxygen vacancies participate in the ORR, and thus, the performance of the electrode is improved effectively during the cathodic polarization. Much attention 12,14-16 has been paid to the improvement of cathodic performance by the activation processes under cathodic polarization only for the pure LSM e...

“…14 All the carbon electrodes were cooled in an Ar gas atmosphere after heat-treatment in order to hinder the formation of the surface acidic functional groups, which seriously influence the electrochemical performance of the carbon electrodes. 22,23 Pore fractality for four kinds of micro-and meso/macroporous electrodes was previously investigated using the nitrogen gas adsorption method. 13,14 The resulting values of d F,pore , r min , and r max for four kinds of carbon electrodes are listed in Table I.…”

Section: Methodsmentioning

confidence: 99%

“…The RC time constant distribution is closely related to the pore structure parameters such as pore size, pore size range, and standard deviation of PSD, and hence it determines the depth of the ion penetration into the pores during double-layer charging/discharging. [16][17][18][19][20][21][22] Thus, in order to find practical applications of the porous carbon electrode for the purpose of high power, it is necessary to investigate the effect of geometric heterogeneity on kinetics of double-layer charging/discharging.…”

mentioning

confidence: 99%

Experimental Study on Ion Penetration into Pores with Pore Fractality at Carbon Electrode for EDLC

Lee

Pyun

2007

Ion penetration into pores during double-layer charging/discharging has been investigated by analyses of potentiostatic current transients ͑PCTs͒ and cyclic voltammograms ͑CVs͒ experimentally measured on porous carbon electrodes with pore fractality. From the analyses of the cathodic PCTs measured on microporous electrodes, it was found that the PCTs showed ideal behavior with a slope of −0.5 in the earlier stage, indicating that the change in the resistance-capacitance ͑RC͒ time constant distribution produced by pore fractal dimension in the micropore size range is too small to affect the shape and value of the PCT. The cathodic PCTs measured on meso/macroporous electrodes exhibited anomalous behavior with a slope lower than −0.5 in the earlier stage due to the considerably wider RC time constant distribution. From the analyses of the cathodic PCTs measured on multifractal electrodes, it is strongly suggested that the current decays more rapidly with increasing relative volume fraction of larger-sized pores. From the analyses of the measured CVs, it is experimentally confirmed that the higher relative volume fraction of larger-sized pores enhances the rate capability ␥, which yields us the higher power density, but it reduces the double-layer capacitance, which provides us the lower energy density.