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Electrochemical Impedance Spectroscopy of High-Efficiency Hydrogen Membrane Fuel Cells Based on Sputter-Deposited BaCe0.8Y0.2O3−δ Thin Films Yoshitaka Aoki,*,†,§ Shohei Kobayashi,‡ Tomoyuki Yamaguchi,‡ Etsushi Tsuji,† Hiroki Habazaki,† Keiji Yashiro,∥ Tetsuya Kawada,∥ and Toshiaki Ohtsuka† †

Faculty of Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628 Japan Graduate School of Chemical Science and Engineering, Hokkaido University, N13W8 Kita-ku, Sapporo, 060-8628 Japan § JST-PRESTO, 4-1-8 Honcho, Kawaguchi 332-0012, Japan ∥ Graduate School of Environmental Studies, Tohoku University, 6-6-01-A004 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan ‡

S Supporting Information *

ABSTRACT: A hydrogen membrane fuel cell (HMFC) consisting of a Pd solid anode, 1 μm thick BaCe0.8Y0.2O3−δ thin-film electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3 cathode was examined. A single-phase BaCe0.8Y0.2O3−δ thin film was successfully deposited by radio frequency cosputtering with BaCe0.8Y0.2O3 and Ce0.9Y0.1O2 double targets, as checked by X-ray diffraction, transmission electron microscopy, and wavelength dispersive X-ray analysis. The maximum power density reached 1.05 W cm−2 at 600 °C, and this value was higher than the champion data of the recently reported proton-conducting ceramic fuel cells (PCFCs). Electrochemical impedance analysis was performed to characterize the anode and cathode polarization behavior. The impedance responses of HMFC were explicable with an equivalent circuit built by a series connection of cathode charge-transfer elements and anode mass-transfer elements. The contribution of the mass transfer in Pd bulk was found to be relatively small in comparison to cathode polarization and ohmic loss in normal fuel cell atmosphere. Moreover, the cathodic charge-transfer resistance of HMFC was found to be 25 times smaller than those of the recent PCFC systems. The current results demonstrated that the HMFC retained relatively large gas−proton−electron triple-boundary zones near the interface between the BaCe0.8Y0.2O3−δ electrolyte and porous La0.6Sr0.4Co0.2Fe0.8O3−δ cathode.

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(BCY) thin film on a Pd foil and screen-printing of conventional cathode material La0.5 Sr 0.5 CoO 3 . 11,12 The HMFC has been reported to reach the maximum power density of 1.4 W cm−2 at 600 °C regardless of its simple cell strcuture.11 Accordingly, it is of fundamental and technological interest to investigate the cathode and anode interfacial polarization of HMFC to clarify the mechanism for the highefficiency power generation. Herein, we report on the electrochemical impedance analysis of the high-efficiency HMFC fabricated by radio frequency (rf) sputtering deposition of BCY thin films on a Pd solid anode and screen-printing La0.6Sr0.4Fe0.8Co0.2O3 (LSCF) cathode. It was demonstrated that the HMFC exhibited excellent performance, achieving the maximum power density of 1.05 W cm−2 at 600 °C. Electrochemical impedance analysis revealed that the superior performances of HMFCs originated from the significantly lowered cathode polarization resistances according to the formation of large gas−electrode−electrolyte triple-boundary zones.

INTRODUCTION Proton-conducting perovskite type oxides, such as BaZr0.8M0.2O3 or BaCe0.8M0.2O3 (M = Y, Gd, Yb, etc.), have been reported to exhibit higher ion conductivity than the oxide ion-conducting oxides based on zirconia, ceria, or lanthanum gallate,1−3 which are used as an electrolyte of solid oxide fuel cells (SOFCs). Therefore, recently, a growing interest has been driven toward proton-conducting ceramic fuel cells (PCFCs) for lower-temperature operation. Nonetheless, the current PCFC performance lags far behind the SOFC performance even at the intermediate temperature (IT) range of 400−600 °C. Two major reasons for the deteriorated performance are large cathodic interfacial polarization due to a lack of suitable cathode for PCFC operation4−7 and large grain boundary resistance owing to the deteriorated microstructural electrolyte.8−10 Hence, the maximum power density of the PCFC system is still far less than 1.0 W cm−2 at 600 °C even with the cell made of a micrometer scale thin electrolyte. Meanwhile, exceptionally high power output in the temperature range of 400−600 °C has been reported for hydrogen membrane fuel cells (HMFCs), which consist of a thin protonconducting ceramic electrolyte supported on a dense hydrogenpermeable metal anode.11−14 Ito et al. first proposed a cell designed by pulsed laser deposition (PLD) of BaCe0.8Y0.2O3 © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: December 24, 2015 Revised: January 31, 2016

A

DOI: 10.1021/acs.jpcc.5b12593 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION BaCe0.8Y0.2O3−δ (BCY) thin films were fabricated by rf sputtering with a single BaCe0.8Y0.2O3−δ target or cosputtering with BaCe0.8Y0.2O3−δ and Ce0.8Y0.2O2 double targets. The sputtering conditions of sputtering power, process atmosphere, target-substrate distance, substrate temperature, and postannealing temperature were optimized to prepare a highly crystalline film having the same composition as the target composition. The typical deposition conditions are listed in Table 1.

RESULTS Fabrication of BCY Thin Films. There have been many reports of fabrication of thin films of proton-conducting metal oxides by PLD methods.11,15−17 In this study, we attempt the fabrication by rf sputtering because it is important to confirm that the high-efficiency HMFC can be reproduced by various techniques. We first conducted a broad survey of the sputtering deposition with a single BCY target by changing the following parameters: (1) Ar and O2 mixing ratio in a sputtering gas, (2) substrate temperature, (3) target−substrate distance, (4) rf power, and (5) pressure. Based on the X-ray diffraction (XRD) and WDX measurements of the deposited film, the suitable conditions for parameters 2, 3, 4, and 5 were determined to be 500 °C, 70 mm, 70 mW, and 2.0 Pa, respectively, as listed in Table 1. Secondary phase of Y-doped CeO2 is always formed when the films are deposited in oxygen-containing atmosphere (Figure S1a). The films deposited in Ar atmosphere show a single XRD pattern which is identical to BaCe0.8Y0.2O3−δ (JCPDS No. 82-2372), and the peaks become more sharp when the deposited films are postannealed in the sputtering chamber at 700 °C for 1 h in O2 atmosphere (0.7 Pa) (Figure S1b). The chemical composition of the corresponding films is Ba/Ce/Y = 1.0/0.69/0.18 as checked by WDX, indicating that the sputtering rate of the Ce atom is slower than that of the other two metal atoms. Hence, cosputtering with BCY and Ce0.8Y0.2O2 (CY) double targets was conducted to compensate the deficiency of Ce by changing rf powers of the CY target. The composition can be adjusted to be close to the target composition (Ba/Ce/Y = 1.0/ 0.80/0.20) when the sputtering of CY is conducted at a rf power of 20 W (Figure S2). Consequently, a highly crystalline BCY thin film with ideal metal composition can be deposited in the conditions listed in Table 1 (Figure S1b). Figure 1 shows surface SEM and cross-sectional TEM images of the 1 μm thick BCY thin films deposited on Pd foils. The film is uniformly formed over a wide area of the Pd substrate. The films have “bamboo” like microstructures, in which pillar grains of hundreds of nanometers wide grow up perpendicular to the substrate and tightly adhered to each other (Figure 1b). Nanoparticles of a few tens of nanometers in diameter first precipitate on a bare Pd surface, and then the pillars grow up over the nanoparticle layer (Figure 1c). The SEM images reveal the pillar grains have a rectangular shape, and they are closely packed together without any microcracks or pinholes (Figure 1a). These microstructural features of our BCY thin films are similar to those of the corresponding film deposited by pulsed laser deposition.12,15 It is concluded that the densely packed BCY thin films can be prepared by the rf sputtering technique. Fuel Cell Test and Impedance Spectroscopy. I−V and I−P characteristics of HMFC consisting of a 1 μm thick BCY thin film were measured at 520, 550, 570, and 600 °C (Figure 2a). The cell performance is moderate at 520 °C, giving an open circuit voltage (OCV) of 1.03 V and a maximum power density of 200 mW cm−2. The performance is drastically improved at higher temperature; the cell can give maximum power density of 1050 mW cm−2 and OCV of 1.08 V at 600 °C. The anode reaction of HMFC is different from the corresponding one in normal hydrogen fuel cell and can be described by PdH → Pd + H+ + e−. It is speculated the increment of OCV by temperature may concern the thermodynamics of this anode reaction. The power output is

Table 1. Optimal Conditions for Radio Frequency Cosputtering Deposition of BaCe0.8Y0.2O3 Thin Films sputtering atmosphere substrate temperature target-substrate distance rf power pressure postannealing temperature postannealing atmosphere

Ar (50 cm3 min−1) 500 °C 70 mm BaCe0.8Y0.2O3, 70 W; Ce0.8Y0.2O2, 20 W 2.0 Pa 700 °C O2 (pO2 = 0.7 Pa)

postannealing time

1h

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Phase purity and crystallinity were checked by X-ray diffraction patterns with a RIGAKU diffractometer (RIGAKU Rint2000). The scanning electron microscopy (SEM) analysis was carried out with a JEOL JSM-7100F instrument. The chemical composition of BCY thin films was examined by wavelength dispersive X-ray analysis (WDX) with a JEOL JXA8530F instrument. The scanning transmission electron microscopy (STEM) and energy dispersive X-ray fluorescent analysis (EDX) were carried out in a Hitachi HD-2000 instrument. The specimens for TEM observation were prepared by a focused ion beam microfabrication (FIB; Hitachi FB2100). The hydrogen membrane fuel cells were fabricated by depositing a BCY thin film of 1 μm thickness on a hydrogenpermeable Pd anode by rf sputtering in optimal conditions. The Pd foil (0.05 mm thickness, Tanaka Co.) was used as a solid anode. The foil (12 × 12) was polished with alumina particles (1.0 μm diameter) and was cleaned by sonication in acetone and pure water before deposition. La0.6Sr0.4Fe0.8 Co 0.2 O3 (LSCF) button electrode (5 mm ⌀) was deposited on the BCY films as a porous cathode by screen-printing with a commercial LSCF paste (NexTech) and subsequent heating with a heat gun for 2 min. The performance of the BCY thinfilm base HMFC was evaluated by measuring the current− voltage (I−V) relation and electrochemical impedance spectra at elevated temperature. The specimen was sealed in a specially designed sample holder with a mica gasket (NexTech). The surfaces of both cathode and anode were contacted with Pt mesh as a current collector. To obtain temperature data that was as accurate as possible, a thermocouple was placed in close proximity to the cell. Normally, dry H2 gas or dry H2/Ar mixed gas was fed to the Pd anode side of the sample sealed on a chamber at a flow rate of 100 cm3 min−1, and wet O2/Ar mixed gas was fed to the cathode side at a rate of 100 cm3 min−1. The wet gas (pH2 = 0.03 atm) was prepared by passing the gas through water at 25 °C. Impedance spectra were obtained by using a Solartron 1260/1287 system in the frequency range of 106 to 0.1 Hz with ac amplitude of 30 mV under several dc bias voltages. I−V relation was also recorded on the same apparatus. B


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increasing the DC current, while Sl does not depend on it. Sh is clearly increased by decreasing of oxygen partial pressure, pO2, in the cathode gas (Figure 3a). These features disclose that the main contribution to the Sh arc is the charge-transfer reaction in cathode−electrolyte interfaces. Meanwhile, Sl clearly responds to the hydrogen partial pressure pH2 in anode gas whereas Sh does not (Figure 3). The Sl arc is drastically increased with decreasing pH2, and then the arc at 0.5 atm pH2 is several times larger than the one at 1.0 atm pH2. In addition, a new arc, Sm, appears in the middle frequency region of 102−101 Hz when pH2 is lower than 0.8 atm. Based on these results, both Sm and Sl arcs can be assigned to the anodic polarization. The cell performance is deteriorated in diluted fuel conditions because of the large anode polarization, Sm and Sl. The peak power density is about 800 and 600 mW cm−2 at pH2 of 0.8 and 0.5 atm, respectively (Figure 4a). The Sl arc is drastically increased and Sm arc is more remarkable when the DC currents are larger in pH2 = 0.5 atm (Figure 4b). As a result, the anode polarization is predominant to the cathode polarization under DC currents of more than 400 mA cm−2. Equivalent Circuit Analysis. The fitting for the impedance spectra is carried out with an equivalent circuit described in Figure 5a,b. R is a resistance, and Q is a constant phase element representing a time-dependent capacitance by the following equation:18 Q−1 = Bq (jw)n

(1)

Here, Bq is a frequency-independent constant, j the square root of −1, and ω the angular frequency. The parallel-connected Ri and Qi are related to the capacitance Ci and relaxation frequency ϕi according to the following equations:19

Figure 1. (a) Surface SEM image and (b) cross-sectional TEM image of BCY thin film deposited on a Pd substrate by rf cosputtering. (c) Expansion of the region around BCY/Pd interface in panel b.

very stable, and the current remains at about 1300 mA cm−2 at 0.7 V for a few hours (Figure 2c). Figure 2b shows Nyquist plots of impedance response of the HMFC at various temperatures. Generally, in impedance spectra of fuel cells, high-frequency x-intercept is ohmic loss mainly due to the bulk resistance of electrolyte, and the following arcs are associated with electrode−electrolyte interfacial polarization. Both ohmic resistances and interfacial polarization become smaller by increasing temperatures. The HMFC exposes two distinct polarization arcs: large Sh arc in the high-frequency region at around 104−102 Hz and small Sl arc in the low-frequency region at around 101−10−1 Hz (see inset of Figure 2b). The Sh arc is asymmetric, having a shoulder in the low-frequency side, and the Sl arc has an elongated tail at the high-frequency side. These features presume the presence of a small arc in the middle-frequency region at around 102 Hz. In fact, the spectra measured in diluted fuel conditions can clearly give rise to the semi-arc in this frequency region, as mentioned below. The impedance spectra of HMFC are measured under various DC conditions at 600 °C (Figure 2d). Based on the results of Figure 3c, the drift of DC currents can be negligible during impedance measurements. The high-frequency intercepts, namely, ohmic losses, shift to the lower-impedance side by increasing DC current. This is probably due to the improvement of the electrolyte conductivity. Details are described in Discussion. Sh is systematically decreased with

C i = (R i·Q i)1/ ni R i−1

(2)

fi = 2p(R i·Q i)−1/ ni

(3)

Montella reported the theoretical model for the impedance responses of electrochemical hydrogen insertion into Pd membrane electrode.20−22 When the Pd is cathodically polarized, hydrogen atoms are inserted into Pd by the direct absorption reaction at the surface (x = 0) as shown in Figure 5c.23,24 ad ad

H+ + e− + sPd → sPdH

(4)

+

H is the proton adsorbed on the Pd surface, and the superscript “s” denotes the surface species. Subsequently, absorbed hydrogen may diffuse inward; thus, H2 gas evolution proceeds at the opposite side of the membrane, namely x = L, as shown in Figure 5c. s

PdH → sPd + 1/2H 2

(5)

The equivalent circuit for such a hydrogen-permeable electrode is given by the model depicted in Figure 5d.20,21 Rct and Qel correspond to the charge-transfer resistance and double-layer capacitance, respectively.20 Zmt(ω) denotes the hydrogen masstransfer impedances, which is associated with the perturbation of hydrogen permeation flux across the Pd membrane, and is given by C


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Figure 2. (a) I−V (solid lines) and I−P (dashed lines) characteristics of HMFC measured at each temperature during operation in standard fuel cell atmosphere: H2(CH2 = 100%), Pd | BCY | LSCF, wet air. (b) Impedance spectra of HMFC under OCV conditions at each temperature. Inset is expansion of the spectra at 600 °C. (c) Current transient at 0.7 V cell voltage at 600 °C. (d) Impedance spectra of HMFC measured under various DC currents at 600 °C during operation in standard fuel cell atmosphere: H2(pH2 = 1.0 atm), Pd | BCY | LSCF, wet air (pO2 = 0.2 atm). The symbols (○) indicate the measured values, and the solid line indicates the fitting with an equivalent circuit model depicted in Figure 5b. The numbers show the decade of the frequency at each highlighted point.

⎛ C (0)F 2DH Zmt(ω) = ⎜ H ⎜ RTL ⎝

−1 ⎧ ⎛ ⎛ L2 ⎞⎞ ⎞ ⎪ ⎟ ⎟ ⎜⎜jω⎜ ⎟⎟ tanh⎨ ⎟ ⎪ D ⎝ ⎠ ⎝ H ⎠⎠ ⎩

⎛ ⎛ L2 ⎞⎞ ⎫ ⎪ ⎜⎜jω⎜ ⎟⎟⎟ ⎬ D ⎝ ⎠ ⎝ H ⎠⎪ ⎭

The anode reaction of HMFC must be replicated by the hydrogen-permeable electrode model (Figure 5d) because it is identical to the reverse reactions of reactions 4 and 5. Accordingly, the equivalent circuit for HMFC can be defined by the model of Figure 5a, where the electrolyte resistance, Rb, cathode polarization, (R c Q c ), and anode polarization, (Qa(RaZmt)), are connected in a series. In fact, the impedance spectra of HMFC are nicely fitted with this model in the case of pH2 < 1.0 atm (Figures 3b and 4b). (RcQc) elements clearly replicate the Sh arc. (RaQa) element for anodic charge transfer is allocated to the Sm arc. Zmt can replicate very well the Sl arc, in which the asymmetric shape of Sl arcs with a nose tail at the high-frequency side corresponds to the feature of Nernst element (Figure 2b,d). However, the spectra measured in pH2 = 1.0 atm are not fitted well with the aforementioned circuit. The Sm arc is relatively small compared to Sh in such a high pH2 condition, so it is difficult to converge Ra and Qa within the allowable error range. Therefore, the fitting to the spectra at pH2 = 1.0 atm is carried out with the simplified circuit model as shown in Figure 5b, in which the (RaQa) element associated with anodic charge transfer is omitted so as to exclude the relatively small contribution of the anode charge transfer. The spectra measured in 1.0 atm pH2 are nicely fitted with this simplified

(6)

Here, L is the thickness of membranes, DH the diffusion coefficient, F the Faraday constant, R the gas constant, and CH(0) the hydrogen concentration at x = 0.20 For fitting analysis, eq 6 is rewritten with an admittance, Ymt, and a time decay constant, Bmt. Zmt = (Y0 jω )−1 tanh{Bmt jω }

(7)

Bmt = L /DH 0.5

(8)

Equations 7 and 8 are called the Nernst circuit element, which is a physical model used to represent the finite-length diffusion in a planar system under non-zero DC conditions.25 If ω is fast enough, the “tanh” term is close to ±1/2 and Zmt is equivalent to Warburg diffusion element for infinite length diffusion because hydrogen atoms cannot see the finite thickness of the Pd membrane. However, if ω is slower, H atoms can move across the membrane within a period of applied AC, and finally Zmt becomes the DC resistance at ω → ∞. Hence, the corresponding Nyquist plots describe the linear part with a slope of 45° in the high-frequency regime and semiarc in the low-frequency regime (Figure 5c).22,25 D


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Figure 3. Impedance responses of HMFC operating at 200 mA cm−2 DC current by changing (a) pO2 in cathode gas and (b) pH2 in anode gas. In panel a, the pO2 is adjusted at 0.4 atm (red), 0.2 atm (green), 0.1 atm (blue), and 0.05 atm (black) while fixing the pH2 at 1.0 atm. In panel b, the pH2 is adjusted at 1.0 atm (blue), 0.8 atm (green), 0.65 atm (red), and 0.5 atm (black) while fixing the pO2 at 0.2 atm. The plots for pH2 = 0.8, 0.65, and 0.5 atm are shifted by 0.02, 0.04, and 0.06 Ω cm2 along the y-axis, respectively. In panels a and b, the symbols (○) indicate the measured spectra and the solid line indicates the fitting with an equivalent circuit model depicted in Figure 5a,b. The numbers show the decade of the frequency at each highlighted point.

Figure 4. (a) I−V (solid lines) and I−P (dashed lines) characteristics of HMFC operating at 600 °C in diluted fuel conditions: H2/Ar (pH2 = 0.5 atm), Pd | BCY | LSCF, wet air. (b) Impedance spectra of HMFC operating at various DC currents at 600 °C in diluted fuel conditions. The numbers show the decade of the frequency at each highlighted point.

model (Figures 2d and 3a). The impedance fitting parameters are listed in Tables 2−4. The capacitance and relaxation frequency of each (RiQi) element are calculated by eqs 2 and 3, respectively. Zmt gives the mass-transfer resistance, Rmt; capacitance, Cmt; and relaxation frequency, ϕmt, by the following25

R mt = Bmt Ymt

−1

in BCY are increased at higher DC current. The concentration of pH2O in vicinity of cathode/electrolyte interface increases because of the increments of H2O products at higher current; thus, the association reaction between H2O molecules and oxygen vacancies V•• O might be shifted by

(9)

Cmt = YmtBmt

(10)

ϕmt = 0.4Bmt 2

(11)

× • OO + V •• O + H 2O → 2OH O

The diffusion coefficient of the Pd anode, DH, is determined by using eq 8 with L = 50 × 10−4 cm.

(12)

The Joule heating is also plausible for reducing electrolyte resistance because of the large DC current flow in the very thin layer. Hibino et al. reported that BaCe1−xYxO3−δ with x = 0.2 is a mixed proton and oxide-ion conductor under fuel cell conditions at temperatures around 600 °C, where the transport number of protons is about 0.8.26 The exfoliation of BCY electrolyte from Pd is not caused in HMFC; therefore, the Pd anode efficiently blocks the oxide ion migration so as to avoid H2O production at BCY−Pd interfaces. Cathode Polarization. ϕc and Cc for the cathodic interfacial polarization is 3−6 × 103 Hz and 10−4−10−3 F cm−2, respectively (Tables 2−4). The values of Cc are in agreement with the capacitances for the charge-transfer reaction at the

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DISCUSSION Parameters of Equivalent Circuit Analysis. Electrolyte Resistances. Resistances and capacitances determined by equivalent circuit fitting are summarized in Figure 6. Figure 6a1 shows variation of each resistance as a function of DC currents under standard fuel cell atmosphere (pO2 = 0.2 atm and pH2 = 1.0 atm). The ohmic resistance, Rb, is much larger than other polarization resistances, disclosing that major parts of the voltage losses in HMFC are caused by the ohmic loss at BCY thin films. The Rb apparently decreases in proportion to the DC currents. This may indicate the protonic carrier concentrations E


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1. Adsorption and dissociation of oxygen molecules on MIEC surface. 1/2O2 →

ads

O

(13)

2. Charge transfer at electrode−electrolyte interface, followed by surface diffusion of oxygen adatoms (adsO). O + 2e′ + 2OH• → H 2O + OO×

ads

(14)

According to the weak pO2 dependence and the capacitance values around 10−4 F cm−2, the cathode polarization could be identical to the interfacial charge-transfer process (eq 14). Anode Polarization. Anodic interfacial charge-transfer process in HMFC is distinct from the normal PCFCs because it proceeds at the Pd/BCY solid−solid heterointerface. het

PdH +

het

O2 − →

Pd + e− +

het

het

OH−

(15)

Here, the superscript “het” indicates the species is located at the heterointerface. hetO2− is the lattice oxygen of BCY located at the heterointerface. The corresponding Sm arcs are not remarkable in pH2 = 1.0 atm, indicating the contribution of reaction 15 to overall voltage losses is rather small in standard fuel cell atmosphere. However, Ra rapidly grows by reducing pH2; thus, Ra at 0.5 atm pH2 becomes three times larger than that at 0.8 atm pH2 (Figure 6c1). The related capacitance, Ca, jumps up by 1 order of magnitude when pH2 drops from 0.8 to 0.65 atm (Figures 6c2). Hence, Sm arcs appear at the relaxation frequency (ϕa) of 5−100 Hz (Table 3). Ra is an almost linear function of reciprocal of pH2 (Figure 6c1). This result may suggest the concentration of hetPdH is decreased in proportion to pH2 in anode gas so that reaction 15 is slower at lower pH2. The hydrogen mass-transfer impedance appears at around 5−0.1 Hz (ϕmt). The associated capacitances Cmt are very large (>10 F cm−2) in accordance with the general feature of the mass-transfer process. The hydrogen diffusion coefficient of Pd anode, DH, is calculated to be about 2 × 10−4 cm2 s−1 by eq 12 at 1.0 atm pH2, which is very close to the diffusion coefficient of pure Pd foils (3 × 10−4 cm2 s−1 at 600 °C)31 (Table 2). However, DH falls to 10−5 cm2 s−1 order at pH2 < 1.0 atm (Table 4), indicating hydrogen transfer in the Pd anode is limited by the interfacial process rather than the bulk one at low pH2. The Rmt exhibits pH2 dependence similar to that of Ra, both of which are in proportion to pH2−1.3 (Figure 6c1). Furthermore,

Figure 5. (a) Equivalent circuit for HMFC at pH2 = 1.0 atm. Rb is electrolyte resistance. Rc and Qc are the resistance and constant phase element (CPE) related to cathode polarization, respectively. Ra, Qa, and Zmt the resistance, CPE, and mass-transfer impedance associated with anode polarization, respectively. (b) Simplified model for HMFC at pH2 < 1.0 atm. (c) Scheme of the electrochemical hydrogen adsorption by Pd membrane electrode (thickness of L)20 and (d) the corresponding equivalent circuit. (e) Schematic representation of Nyquist plots of Zmt given by eq 6. Details are described in the main text.

interface between LSCF porous cathode and BCY bulk electrolyte.27,28 Rc is drastically decreased with increasing DC current (Figure 6a1). This behavior is consistent, because the charge-transfer resistances must be decreased to generate large DC current. Rc is dependent on oxygen partial pressure in a manner of pO2−0.24 (Figure 6b1). The elementary steps of cathode reaction on mixed ion electron-conducting (MIEC) cathodes, such as LSCF, can be shown as follows.29,30

Table 2. Fitting Parameters of Impedance Spectroscopy of HMFC (pH2 = 1.0 atm) at 600°C under Various DC Conditionsa 2

Rb (Ω cm ) Rc (Ω cm2) nc Cc (F cm−2) ϕc (Hz) Ymt (S s−0.5) Bmt (s−0.5) Rmt (Ω cm2) Cmt (F cm−2) ϕmt (Hz) DH (cm2 s−1) a

OCV

400 mA cm−2

600 mA cm−2

800 mA cm−2

1 A cm−2

0.25 0.15 0.75 2.8 × 10−4 3.7 × 103 0.42 0.42 0.040 16 2.3 1.4 × 10−4

0.22 0.085 0.79 6.2 × 10−4 3.0 × 103 0.44 0.39 0.035 18 2.7 1.6 × 10−4

0.21 0.060 0.82 8.7 × 10−4 3.0 × 103 0.49 0.39 0.032 20 2.6 1.6 × 10−4

0.20 0.042 0.85 1.1 × 10−3 3.5 × 103 0.48 0.36 0.031 20 3.0 1.9 × 10−4

0.19 0.039 0.86 1.4 × 10−3 2.9 × 103 0.51 0.40 0.031 20 2.4 1.5 × 10−4

The original spectra are shown in Figure 2d. F


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Table 3. Fitting Parameters of Impedance Spectroscopy of HMFC Measured at 600°C under 200 mA cm−2 DC Conditions by Changing pO2 in Cathode or pH2 in Anodea pO2 (kPa) Rb (Ω cm2) Rc (Ω cm2) nc Cc (F cm−2) ϕc (Hz) Ra (Ω cm2) na Ca (F cm−2) ϕa (Hz) Ymt (S s−0.5) Bmt (s−0.5) Rmt (Ω cm2) Cmt (F cm−2) ϕmt (Hz) DH (cm2 s−1) a

pH2 (kPa)

40

20

10

5

101*

80

65

50

0.27 0.075 0.86 3.1 × 10−4 6.8 × 103 − − − − 0.47 0.33 0.028 20 3.7 2.3 × 10−4

0.27 0.089 0.83 3.3 × 10−4 5.5 × 103 − − − − 0.44 0.32 0.029 19 3.9 2.4 × 10−4

0.26 011 0.77 4.2 × 10−4 3.6 × 103 − − − − 0.38 0.34 0.036 16 3.5 2.2 × 10−4

0.26 0.12 71 6.2 × 10−4 2.1 × 103 − − − − 0.34 0.36 0.043 14 3.0 1.9 × 10−4

0.27 0.089 0.83 3.3 × 10−4 5.5 × 103 − − − − 0.44 0.32 0.029 19 3.9 1.9 × 10−4

0.26 0.083 0.89 3.5 × 10−4 5.5 × 103 0.028 0.78 0.050 110 0.49 0.40 0.033 19 2.5 1.6 × 10−4

0.27 0.084 0.88 3.5 × 10−4 5.4 × 103 0.035 0.77 0.16 28 0.44 0.48 0.044 16 1.7 1.1 × 10−4

0.27 0.090 0.87 3.2 × 10−4 5.6 × 103 0.051 0.87 0.21 7.8 0.44 0.75 0.069 13 0.70 4.4 × 10−5

The original spectra are shown in Figure 3a,b.

Table 4. Fitting Parameters of Impedance Spectroscopy of HMFC (pH2 = 0.5 atm) at 600°C under Various DC Conditionsa 2

Rb (Ω cm ) Rc (Ω cm2) nc Cc (F cm−2) ϕc (Hz) Ra (Ω cm2) na Ca (F cm−2) ϕa (Hz) Ymt (S s−0.5) Bmt (s−0.5) Rmt (Ω cm2) Cmt (F cm−2) ϕmt (Hz) DH (cm2 s−1) a

OCV

200 mA cm−2

400 mA cm−2

0.29 0.13 0.84 2.1 × 10−4 6.3 × 103 0.063 0.72 0.82 3.1 0.82 0.87 0.043 22 0.53 4.8 × 10−5

0.27 0.090 0.87 3.2 × 10−4 5.6 × 103 0.051 0.87 0.21 7.8 0.44 0.75 0.069 13 0.70 4.4 × 10−5

0.26 0.063 0.91 5.1 × 10−4 5.0 × 103 0.048 0.91 0.069 48 0.30 0.65 0.082 9.2 0.94 5.9 × 10−5

600 mA cm−2 0.25 0.045 1.0 6.7 × 5.3 × 0.038 0.94 0.037 1.1 × 0.27 0.66 0.099 8.2 0.92 5.7 ×

10−4 103

102

10−5

1 A cm−2 0.23 0.040 0.92 1.1 × 10−3 3.7 × 103 0.037 0.91 0.046 61 0.18 0.58 0.13 5.8 1.2 7.4 × 10−5

The original spectra are shown in Figure 4b.

cathode−electrolyte interfaces in PCFCs. Recently, some authors proposed proton−oxide ion−electron triple conducting oxides as a new PCFC cathode because the partial proton conductivity enables progressing the charge-transfer reaction on the cathode surface.5,6,33 Others reported the composite cathodes consisting of electrocatalyst and electrolyte mixtures to elongate TPB lengths.5,32,34,35 The highest record of the maximum power density of the recent PCFCs is, in our search, about 700 mW cm−2 at 600 °C.33 Apparently, the maximum power density of our HMFC is higher than this champion data. This result indicates that HMFC possesses extremely low electrolyte resistances and/or interfacial polarization resistances in comparison to the anodesupported PCFCs. The cathodic polarization resistances of HMFC are significantly reduced as much as the recent cathodes with designed ionic conductivity and microstructures although the former is just applied by the conventional LSCF cathode. Rc of HMFC is 0.15 Ω cm2 at 600 °C under OCV in standard fuel cell atmosphere (Table 2). This value is smaller than most of

Rmt is also dependent on pO2 in cathode gas in a way similar to that of Rc, changing by pO2−0.2 (Figure 6b1). Under equilibrium, the hydrogen flux (mol cm−2 s−1) in Pd anode must be equivalent to the flux of the proton-transfer current at Pd/ BCY and LSCF/BCY interfaces, so that hydrogen transfer in Pd anode is correlated with the kinetics of interfacial chargetransfer processes in HMFC. Comparison to Recent PCFCs. For direct comparison, electrochemical performances of some anode-supported PCFCs reported in recent studies are summarized in Table 5. These basically use BaZr x Ce 0.8‑xY 0.2 O 3−δ (BZCY) 37 or BaZrxCe0.8‑xY0.1Yb0.1O3−δ (BZCYYb)38 as an electrolyte because it can satisfy both superior proton conductivity and tolerance to CO2 atmosphere. Their anodes are applied by Nielectrolyte cermets. In principle, the cathode charge-transfer reaction on PCFC can progress only at gas−electrode− electrolyte triple phase boundary (TPB) because of mismatch of ionic carriers between mixed oxide ion electron-conducting electrodes and proton-conducting electrolytes.4,5 Therefore, numerical efforts have been made to extend the TPB length of G


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Figure 6. Various electrochemical parameters determined by equivalent circuit analysis of impedance spectra. Resistances: Rb (●), Rc (▲), Ra ( × ), and Rmt (■). Capacitances: Cc (△), Ca (+), and Cmt (□). p0 = 1.0 atm. Variation of resistances versus DC currents at 600 °C during fuel cell operation (a1) in standard fuel cell atmosphere (H2, Pd | BCY | LSCF, wet air) and (a2) in diluted fuel conditions (H2/Ar (pH2 = 0.5 atm), Pd | BCY | LSCF, wet air). The resistances are determined by fitting of the impedance spectra in Figures 2d and 4b. Variation of (b1) resistances and (b2) capacitances versus pO2, under 200 mA cm−2 DC conditions. The parameters are determined by fitting of the impedance spectra in Figure 3a. Variation of (c1) resistances and (c2) capacitances versus pH2, under 200 mA cm−2 DC conditions. The parameters are determined by fitting the impedance spectra in Figure 3b.

Table 5. Summary of the Maximum Power Density (Pmax), Electrolyte Resistance, and Cathode Polarization Resistance Reported in the Recent Literature for PCFCs Operating at 600°C Using H2 as Fuel and Air as Oxidant electrolyte

L (μm)

electrolyte resistance (Ω cm−2)

BZCY BZCYYb BZCY

65 15 10

− 0.13 −

BZCYYb BZCY BZCY

15 30 25

0.3 0.5 0.6

cathode Sm0.5Sr0.5CoO3−BZCY composite NdBa0.5Sr0.5Co1.5Fe0.5O5 Ba0.5Sr0.5Co0.5Fe0.5O3−BZCY composite BaCo0.4Fe0.4Zr0.1Y0.1O3 Sm0.5Sr0.5CoO3−BZCY composite Ba0.5Sr0.5Zn0.2Fe0.8O3

cathode polarization resistance (Ω cm−2)

Pmax @600 °C (mW cm−2)

ref

0.17 0.28 −

445 690 550

32 33 34

0.09 0.52 0.35

640 344 280

5 35 36

PCFCs. Nonetheless, activation energy, Ea, is quite similar to each other (about 1.3 eV). These results indicate that the relatively large active TPB zones are formed along LSCF−BCY interface in HMFC. The detailed formation mechanism will be reported in a future contribution. Figure 7b shows Arrhenius plots of proton conductivity, σ, of BCY electrolyte, calculated from Rb. σ of our BCY thin films is 4 × 10−4 S cm−1 at 600 °C, which is 2 orders of magnitude smaller than that of the sintered pellets (ca. 1 × 10−2 S cm−1).2 The Ea of BCY film, 0.98 eV, is much larger than the Ea

the recent PCFC with composite cathodes (Table 5) and is comparable to the polarization resistance of triple-conducting BaCo0.4Fe0.4Zr0.1Y0.1O3−δ cathode on BZCYYb cell, 0.09 Ω cm−2 at 600 °C.5 Figure 7a shows Arrhenius plots of Rc under OCV conditions in standard fuel cell atmosphere. The polarization resistances reported for the LSCF cathode on BCYb base PCFCs27 are also shown in the same figure, in which the cathode reaction has been also rate-limited by the interfacial charge-transfer process (eq 14).27 Rc of HMFC is 25 times smaller than the cathode polarization resistances of the H


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demonstrated that the superior performance can mainly be attributed to the formation of extended gas−electrode− electrolyte triple-boundary zones near the cathode−electrolyte interface, which can efficiently decrease the cathode polarization resistances as much as the current PCFCs having the proton−oxide ion−electron triple conducting cathodes. The current results motivate a more detailed investigation of the mechanism forming the extraordinarily large TPB zone at the cathode in the HMFC configuration.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12593. XRD patterns and elemental analysis of the sputterdeposited BCY thin films (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-11-706-6752. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by JST, PRESTO, “Creation of innovative core technology for manufacture and use of energy carriers from renewable energy” program. This work was conducted at Hokkaido University, supported by “Nanotechnology Platform” Program of the MEXT Japan.

Figure 7. Arrhenius plots of (a) cathode polarization resistance, Rc, and (b) proton conductivity, σ, of BCY thin films. In panel a, symbols (○) indicate this work. The solid red line is the data reported for LSCF on BCYb base PCFC.27 In panel b, symbols (○) are this work and the solid red line is the data reported for the BCY sintered pellets.2

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ABBREVIATIONS BCY, BaCe0.8Y0.2O3; HMFC, hydrogen membrane fuel cell; PCFC, proton-conducting ceramic fuel cell; CY, Ce0.8Y0.2O2; LSCF, La0.6Sr0.4Co0.2Fe0.8O3

reported for the pellets, 0.3−0.45 eV2,39,40 (Figure 7b). Shima et al. reported that Ba-rich Ba1.1CeO3 phase reveals low σ of 10−4 S cm−1 and Ea of about 1.2 eV for proton conduction in wet air atmosphere.41 This is comparable to the observed Ea of BCY films. It is plausible that such a poorly conducting phase is segregated in the grain boundary or film−substrate interface region so as to hinder the fast proton migration across films. As mentioned before, most of the voltage losses of HMFC are involved by the ohmic loss at the electrolyte (Figure 6a1). The Rb of our BCY thin films is 0.27 Ω cm−2, and this value is comparable to the electrolyte resistances of the recent PCFCs, 0.1−0.5 Ω cm2 (Table 5), even though the thickness of our film is less than one-tenth of the thickness of the later (10−50 μm). This reveals that advantages of the reduced thickness in terms of electrolyte resistances are quite few in the HMFC. It is concluded that the superior performance of HMFC originates from the significantly lowered cathode polarization resistances.

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REFERENCES

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CONCLUSION A single-phase BCY thin film can be fabricated by rf cosputtering deposition with BCY and CY double targets. HMFCs are constructed by depositing 1 μm thick BCY thin film on a Pd foil anode and screen-printing of a porous LSCF cathode on the deposited films. The proton conductivity of the BCY thin films is 2 orders of magnitude smaller than the values reported for BCY bulk pellets; therefore, the ohmic resistance is not sufficiently lowered as is expected from the reduced thickness. Nonetheless, the HMFC provides the maximum power density of 1.05 W cm−2 at 600 °C, and this value is higher than the champion data of the recent PCFC. It is I


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