Electrochemical Approach for Analyzing Electrolyte Transport

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Electrochemical Approach for Analyzing Electrolyte Transport Properties and Their Effect on Protonic Ceramic Fuel Cell Performance Nikolay Danilov,†,‡ Julia Lyagaeva,†,‡ Gennady Vdovin,† Dmitry Medvedev,*,†,‡ Anatoly Demin,†,‡ and Panagiotis Tsiakaras*,†,‡,§ †

Laboratory of Electrochemical Devices Based on Solid Oxide Proton Electrolytes, Institute of High Temperature Electrochemistry, 620137 Yekaterinburg, Russia ‡ Ural Federal University, 620002 Yekaterinburg, Russia § Laboratory of Alternative Energy Conversion Systems, Department of Mechanical Engineering, School of Engineering, University of Thessaly, Pedion Areos, 383 34 Volos, Greece S Supporting Information *

ABSTRACT: The design and development of highly conductive materials with wide electrolytic domain boundaries are among the most promising means of enabling solid oxide fuel cells (SOFCs) to demonstrate outstanding performance across low- and intermediate-temperature ranges. While reducing the thickness of the electrolyte is an extensively studied means for diminishing the total resistance of SOFCs, approaches involving an improvement in the transport behavior of the electrolyte membranes have been less-investigated. In the present work, a strategy for analyzing the electrolyte properties and their effect on SOFC output characteristics is proposed. To this purpose, a SOFC based on a recently developed BaCe0.5Zr0.3Dy0.2O3−δ proton-conducting ceramic material was fabricated and tested. The basis of the strategy consists of the use of traditional SOFC testing techniques combined with the current interruption method and electromotive force measurements with a modified polarization-correction assessment. This allows one to determine simultaneously such important parameters as maximal power density; ohmic and polarization resistances; average ion transport numbers; and total, ionic, and electronic film conductivities and their activation energies. The proposed experimental procedure is expected to expand both fundamental and applied basics that could be further adopted to improve the technology of electrochemical devices based on proton-conducting electrolytes. KEYWORDS: solid oxide fuel cells, current interruption method, electromotive force measurements, protonic conductors, ohmic and polarization resistances

1. INTRODUCTION

target characteristics of PCFCs can be reduced due to the high electron transport, which is typical for the best-known protonconducting ceramic electrolytes.17 When operating in fuel cell mode, the parasitic electron current appearing in an electrolyte membrane can result in the deterioration of not only the power density and OCV parameters but also the fuel and electrical efficiency due to nonelectrochemical fuel oxidation.18−21 In detail, the thermodynamic voltage for wet H2, anode|electrolyte| cathode, wet air cells attains 1.151 and 1.135 V at 600 and 700 °C, respectively (relative humidity of air and hydrogen is 2%). In practice, cells exhibit much-lower OCV values. For example, OCVs drop down to 0.8 V at 600 °C for the Ba2In2O5-based PCFC22 and to 0.9 V for the LaNbO4-based one.23 For PCFCs

Protonic ceramic fuel cells (PCFCs) constitute a class of highly efficient, fuel-flexible, and environmentally friendly energy converters with the potential to operate across low and intermediate temperature ranges.1−4 PCFCs are promising devices due to their thermodynamic efficiency coming from the higher achievable open-circuit voltages (OCV) compared with those of traditional solid oxide fuel cells, SOFCs.5−9 Another advantage of PCFCs lies in the excellent ionic conductivity and relatively low corresponding activation energy of protonconducting electrolytes compared with oxygen-ionic systems below 600−700 °C.5,8,9 To date, intensive R&D efforts have resulted in the fabrication of laboratory prototypes with outstanding output properties, satisfying the concept of secondand third-generation SOFCs.10−15 Despite the use of electrochemically active electrode materials and highly conductive thin electrolytes,16 the actual © XXXX American Chemical Society

Received: May 26, 2017 Accepted: July 20, 2017

A

DOI: 10.1021/acsami.7b07472 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

good electrical properties and its affinity with the electrolyte in terms of thermal expansion.35,36 The synthesis of the YBCZ powder included two calcination steps (1050 °C for 5 h and 1100 °C for 5 h) and intermediate grindings. A pair of powders were used as anode materials. The first one was obtained by mixing the BCZD electrolyte, NiO, and starch (poreformer) in a weight ratio of 40:60:15; the second powder consisted of BCZD and NiO in a weight ratio of 45:55. These powders were prepared for the formation of supported and functional anode layers, respectively. 2.2. PCFC Fabrication. The single PCFC was fabricated by using a tape-calendering method (TCM) described in detail recently.35 Briefly, the films of electrolyte and anode layers were prepared by mixing the corresponding powders with an organic binder (nitrile butadiene rubber). These films were jointly co-rolled to obtain a planar half-cell controlling the thickness of the initial and obtained layers. This halfcell consisted of the electrolyte, functional anode, and supported anode layers having a thickness of ∼25, 40, and 500 μm, respectively. The half-cell was gradually heated at 900 °C (at a heating rate of 1 °C min−1) to burn the organic binder; subsequently, its co-sintering was carried out at 1450 °C (3 h) to reach the required densification level of the BCZD electrolyte. The YBCZ powder was thoroughly homogenized in a poly(vinyl alcohol) solution to prepare the cathode slurry, which was painted on the BCZD surface of the as-sintered halfcell. The single PCFC was prepared following calcination of the formed cathode layer at 1050 °C (1 h). 2.3. Characterization of Materials. The materials used were characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and four-probe conductivity measurements. The XRD analysis of the powders and their mixtures was performed using a Rigaku D/MAX-2200 diffractometer with CuKα radiation. The scans were carried out between 15° and 75° with a scan rate of 2° min−1 and a scan step of 0.02°. The morphology of the electrolyte surface and PCFC cross-section was evaluated by means of SEM analysis (JEOL JSM-5900 LV microscope). The thermal expansion was investigated using a DIL 402 C dilatometer (Netzsch GmbH) from 50 to 1000 °C with a heating and cooling rate of 2 °C min−1 in ambient air atmosphere. The average thermal expansion coefficient (TEC) was calculated on the basis of the dilatometry results under cooling. The electrical properties of BCZD were measured by means of fourprobe direct current (DC) conductivity measurements at 500−900 °C. A total of four platinum porous electrodes were deposited on the sample’s surface and then were connected with platinum wires. A certain current value was supplied through the external electrodes, while the voltage drop was measured between the internal electrodes. The bulk electrical conductivity was calculated, taking into account the slopes of the current−voltage (I−V) curves and the dimension parameters of the sample. Special emphasis in these measurements was paid to the determination of the ionic and electronic components of total conductivity, carrying out measurements as a function of oxygen partial pressure (pO2) in dry and wet atmospheres at 700, 800, and 900 °C. A Zirconia-318 microprocessor was used to collect electrical resistance (R) data as well as to continuously monitor and change the pO2 and temperature parameters. The pO2 values varied in a range of 10−10−0.21 atm. The water vapor partial pressures (pH2O) in amounts of 10−4 atm (dry atmosphere) and 0.03 atm (wet atmosphere) were set by passing the gases through a zeolite column and a water bubbler thermostated at 25 °C, respectively. Conductivity (σ) and activation energy (Ea) were calculated from the following equation:

with promising BaZrO3 electrolytes, the OCVs vary in the ranges of 0.96−1.06 V at 600 °C and 0.90−1.01 V at 700 °C.24 Finally, the BaCeO3-based cells are able to demonstrate higher OCVs reaching ∼1.13 V at 600 °C.14 One of most-promising ways of overcoming the abovementioned problem is to carry out research with new electrolytes having a lower electronic conductivity contribution. The applicability of this materials science approach may be rather limited because proton-conducting electrolytes having a wider electrolytic domain boundary exhibit lower ionic conductivity (as is the case with LaYO325,26 and CaZrO327,28 systems). Another possible strategy consists in the utilization of additional functional layers. This also results in higher achievable OCV values for SOFCs amd PCFCs on account of the electron-blocking effects in electrolytes.29−32 However, some technological challenges should be faced, including chemical compatibility of the electrolyte and introduced functional layers, the achievement of required adhesion between them, and similarity in thermal and chemical expansion coefficients. Therefore, the possibility of decreasing or eliminating the electronic transport in the electrolyte membrane of PCFCs is highly important as a basis for an effective operation of such devices. It should be noted that low OCV values of PCFCs can be caused by not only the electronic conductivity of electrolyte membranes but also the poor gastightness of the electrochemical systems and the insufficient electrochemical activity of electrodes. The details of conventional characterization methods of PCFCs do not allow the determining of the effect of each mentioned factor and their separation from each other. In an attempt to evaluate the electrolytic and electronic properties of proton-conducting electrolytes in fuel cell mode and establish which parameters affect their transport, we fabricated and tested a PCFC laboratory unit. This cell was based on a Dy-doped BaCeO3−BaZrO3 protonic electrolyte with high chemical stability, acceptable thermomechanical properties, and superior conductivity.33,34 The present study aims at developing a methodology for testing and analyzing solid oxide fuel cells based on proton-conducting materials using a combination of electrochemical techniques, such as volt−ampere measurements, current-interruption method, and transport-number determination by polarization correction.

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Powders having a purity of 99.0% (purchased from Sigma-Aldrich) were used for the preparation of all materials. The BaCe0.5Zr0.3Dy0.2O3−δ electrolyte material (BCZD) was synthesized using a citrate−nitrate combustion method. Stoichiometric amounts of Ba(NO3)2, Ce(NO3)3·xH2O, Dy(NO3)3·yH2O and ZrO(NO3)2·zH2O were dissolved in distilled water under intensive stirring. A preliminary determination of the water in the crystal hydrates was carried out using the thermogravimetric method. Citric acid was added to this solution, thus achieving a mole ratio of 2:1 between acid and metal cations. Then ammonia solution was added drop-by-drop, and the obtained neutral solution (pH ≈ 7) was continuously heated at 200 °C to ensure water evaporation, gel formation, and combustion. The as-combusted ash was first calcined at 700 °C (1 h) to burn organic residues and then at 1050 °C (5 h) to decompose the intermediate BaCO3 phase. Finally, the as-prepared powders were pressed into pellets at 300 MPa and then sintered at 1450 °C (5 h) for phase, microstructure, and electrical characterization. The YBaCo3.5Zn0.5O7+δ cathode material (YBCZ) was prepared from BaCO3 and the corresponding oxides using the solid-state synthesis method. The selection of this complex oxide was due to its

σ=

A ⎛ Ea ⎞ ⎟ exp⎜ − ⎝ RT ⎠ T

(1)

where A is the pre-exponential factor, T is the absolute temperature, and R is the universal gas constant. 2.4. Characterization of PCFC. The electrochemical characterization of the single PCFC was performed according to the scheme shown in Figure 1. A disk-shaped sample was pressed to the end face B


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numbers and to extract the electrodes overpotential (ηel) and the ohmic voltage drop in the electrolyte (ΔV) with the aid of the current interruption procedure.37 The obtained ηel and ΔV values were used for the calculation of the polarization (RP) and ohmic (Rohm) resistances. The refined transport numbers were determined by comparing the thermodynamic (theoretical) and experimental (measured) OCVs using the calculated RP and RO data (section 3.2).

3. THEORETICAL BACKGROUND 3.1. Electromotive Force Measurements. Transport properties of proton-conducting electrolytes can be assessed by considering the OCV or so-called electromotive force (EMF) of the concentration or chemical cells. Such a cell generates a certain electrical potential difference when oxygen, hydrogen, or water vapor gradients are generated on the opposite sides of the electrolyte membrane. By considering the electrochemical reaction and corresponding chemical potentials of ionic charge carriers arising due to the generated gradients, the Nernst-based equation can be obtained:38,39 E = −t j

⎛ ⎞ RT ⎜ pj′ ⎟ nF ⎜⎝ pj′′ ⎟⎠

(2)

where tj is the ionic transport number for spaces j (j = O − oxygen ions and j = H − protons), F is the Faraday constant, n is the number of electrons involved in the electrochemical reaction, and p′j and p′j ′ are the partial pressures of the potential-determined gas components. For a co-ionic electrolyte, which enables the oxygen and hydrogen charge to be conducted, the theoretical OCV (Eth) takes the following form: Eth = to

Figure 1. Principal scheme of the electrochemical cell for volt−ampere measurement and transport numbers evaluation: 1, unit PCFC; 2, YSZ tube; 3, glass sealant; 4, electrochemical sensor; 5, gas feed tubes; 6, thermocouple; 7 and 8, digital multimeters working as the voltmeter and ammeter; 9, electronic load; 10, oscilloscope; 11, mercury switch; and 12, fluoroplastic probes.

p′O2 RT p ′′ H 2 RT In + t H In = toEo + t HE H 4F p ′′ O2 2F p ′′ (3)

where EO and EH represent the thermodynamic values of oxygen and hydrogen concentration cells under created partial pressure differences of oxygen (p′O2 and p′′O2) and hydrogen (p′H2 and p′′H2), and tO and tH are the oxygen-ionic and protonic transport numbers. Consider the equilibrium of the following reaction:

of a tube made of YSZ electrolyte by means of a spring load (not shown). A special silicate glass sealant ((SiO2)0.67(Li2O)0.12(Na2O)0.09(CaO)0.08(Al2O3)0.04 with a melting temperature of 930 °C) was used to coat the junction of the YSZ and BCZD electrolytes. The electrochemical cell was heated up to 930 °C (without a dwell) and then slowly cooled to the required temperatures (600−800 °C) in ambient air atmosphere. When the system was glued, the wet forming gas (5% H2/N2) was fed to the external space of the electrochemical cell (anode side), and the wet air was fed into the internal space (cathode side) by the means of gasfeeding tubes. The forming gas was then changed for wet hydrogen in a stepwise process. The flow rates of hydrogen and air were about 50 and 150 mL min−1, respectively. The humidity of the gases (∼3 vol %) was regulated by passing them through a water bubbler thermostated at 25 °C. The pO2 value in the anode channel was measured using an YSZ-based electrochemical sensor (two platinum electrodes symmetrically deposited onto the internal and external surface of the YSZ tube). This sensor was located in close proximity to the tested PCFC. The temperature was measured by means of a Pt−Pt/Rh thermocouple placed near to the PCFC. In the traditional electrical circuit used for measuring volt−ampere characteristics (B7−77 digital multimeters), a electronic load (P33) for variable resistance setting and an oscilloscope (Rigol DS-1104) are inserted in parallel. With this connection arrangement by setting a variable resistance it is possible to evaluate the corrected transport

H 2O ↔ H 2 + 1/2O2

(4)

Eq 5 is straightforwardly derived from expression 3: Eth = t i

RT ⎛ p′O2 ⎞ RT ⎛ p ′′ H 2O ⎞ ln⎜ ⎟ + t H ln⎜ ⎟ 4F ⎝ p ′′ O2 ⎠ 2F ⎝ p′H 2O ⎠

= t iEO + t HE H2O

(5)

where EH2O is the thermodynamic value of the water concentration cell under the water vapor pressure difference (p′H2O and p′′H2O) and ti = tO + tH represents the ionic transport numbers. For the conductors possessing electronic transport along with the oxygen-ionic or protonic ones, the transport numbers of oxygen ions and protons can be calculated from the ratio between generated and Nernstian voltages:

tO = C

Eexp EO

(6) DOI: 10.1021/acsami.7b07472 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns, (b) thermal expansion behavior, and (inset of panel b) thermal expansion coefficients of the functional materials.

tH =

Eexp EH

appreciable influence of the electrodes polarizability. Therefore, the effect of the electrode polarization should be considered for a more correct estimation of t j.40−44 To the best of our knowledge, the influence of electrode polarization on the transport behavior of ionic conductors was revealed for the first time (in 1988) by Gorelov.44 The corrected average transport numbers can be found from the following expression:

(7)

As follows from the analysis of eqs 3 and 5, it is possible also to estimate the ti, tO, and tH values for triple-conducting materials (h•, H+, and O2−) when one of the potentialdetermined gas components does not create a partial pressure difference. As is well-known, in the classic EMF method, transport numbers are usually determined under a small chemical potential difference.38−41 This leads to slightly different atmospheres (in terms of their compositions: p′j ≈ pj′′) on the opposite sides of the electrolyte membranes. Therefore, the calculated tj values are referred to as apparent ones, corresponding approximately to certain thermodynamic parameters (T and pj = p′j ≈ p′j ′). The situation significantly changes when the partial pressure difference exceeds 1 order of magnitude and more (fuel cell or electrolysis cell modes). In this case, the cells also generate OCVs (Eexp), and the thermodynamic EO and EH values can also be easily calculated. However, the tj parameters acquire a different meaning and do not correspond to the same set of T and pj; instead, they can be considered to correspond to the average transport numbers (t j). As mentioned above, an estimation of the average transport numbers for SOFCs and PCFCs based on triple-conducting electrolytes poses certain difficulties. Nevertheless, setting the same level of moisture for fuel and oxidant (p′H2O = p′′H2O) and taking into account eqs 5−7, it is possible to determine the average transport numbers of ions and electrons using the fairly simple relations:

ti =

⎛ Rp ⎞ Eexp⎜1 + ⎟ = t iEO R i + Re ⎠ ⎝

where Rp is the polarization resistance of the electrodes, and Ri and Re are the ionic and electronic resistances of the electrolyte. As shown by the analysis carried out recently,37 eq 10 has three independent variables. To determine the values of these parameters, an electronic active load (Ract) is inserted in parallel with the cell, allowing one to use eq 11 from the consideration of a modified equivalent circuit. EO 1 1 − 1 = (R i + R p) + (R i + R p) Eexp Re R act

te = 1 −

(8)

ti = 1 −

Eexp EO

(11)

Using this expression, it is possible to determine the variables by graphical analysis of linear dependences obtained the in (EO/Eexp − 1)/(1/Ract) coordinates. More precisely, the relation between the slope and its intersection with the ordinate axis gives the Re value. Thus, additionally measuring the total (ohmic, Rohm) resistance of the thin-layer electrolyte, it is possible to determine the average transport numbers of the ions as follows:

Eexp EO

(10)

R ohm Re

(12)

Here, the Rohm value can be calculated either by the impedance technique or by the oscilloscope method.37,45 The latter was used in the present work because it is quite simple and does not require a special mathematical analysis. As shown in eq 10, a correction of t j must be performed when (1) the electronic and ionic resistances of the electrolyte are comparable and (2) the ohmic resistance of the electrolyte and polarization resistances of the electrodes are also comparable. When the ionic conductors exhibit very low electronic conductivity or very high total resistance (electrolytesupported electrochemical cells), eqs 8 and 9 can be used for the evaluation of their transport nature. In conclusion this section, it should be pointed out that transport numbers correction procedure is subject to continuous modifications and improvements.41,45 However, the mentioned classical correc-

(9)

3.2. Assessment of Corrected Transport Numbers. As is well-known, the materials based on BaCeO3, BaZrO3, and BaCeO3−BaZrO3 solid solutions exhibit p-type electronic conductivity under oxidizing conditions at 500−900 °C.17 Because the cathode side of PCFCs is exposed to an air atmosphere, the electrolytes are also in contact with it, providing electron conductivity. When the electrochemical cells consist of reversible electrodes and the integrity of these cells is secured, the lower OCVs in comparison with the thermodynamic values can be explained only by undesired electron transport in the proton-conducting membranes. However, the calculation of average transport numbers for thin-layer electrolytes (eqs 8 and 9) is incorrect owing to the D


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Figure 3. Total conductivity of the BCZD electrolyte as a function of (a) temperature and (b) oxygen partial pressure. The oxygen pumping-out was carried out from dry and wet air atmospheres. (c) The electronic conductivity of the BCZD electrolyte is calculated by subtracting the ionic component from the total conductivity. x • H 2O + V •• O + OO ↔ 2OH O

tion can be used for an initial assessment of the transport behavior of the proton-conducting electrolyte under the fuel cell mode of operation.

(13)

in the first approximation, the concentration of protonic defects (OH•o) is assumed to be constant. The p-type electronic conductivity of the electrolyte at high pO2 values causes the observed differences between reducing and oxidizing atmospheres (Figure 3b,c). As can be seen, the level and contribution of electronic conductivity tend to grow with increasing temperature. Because conductivity measured in wet H2 is purely ionic and is close to that measured in wet air below 600 °C, the transport properties of BCZD across a low-temperature range in air can also be considered as ionic. The apparent Ea values support the above-mentioned suggestions. 4.2. Volt−Ampere Measurements. The electrochemical tests were carried out using the as-fabricated PCFC. This consists of a 20 μm thick BCZD electrolyte well-adhered to the other functional materials (Figure 4). Figure 5a,b and Table 1 present the main electrochemical data taken from such a cell. When comparing the

4. RESULTS AND DISCUSSION 4.1. Material Properties. Figure 2a shows the phase quality of the synthesized ceramic materials. The BCZD oxide crystallizes in a cubic-type perovskite structure with a space group of Pm3m. Its lattice parameter is equal to 4.3332(1) Å at room temperature, which closely fits the previously reported results.33 The layered YBCZ cobaltite exhibits a hexagonal swedenborgite-type structure with P63mc space group and 6.3032(2) and 10.2615(3) Å lattice parameters. The chemical compatibility between Zn-doped YBaCo4O7+δ and Ba(Ce,Zr)O3-based materials is satisfactory under co-sintering at 1100 °C for 10 h as well as at lower operation temperatures, as recent studies have demonstrated.35−37 This is due to the fact that the Zn doping causes a higher thermodynamic and chemical stability of YBCZ in comparison with the basic YBaCo4O7+δ oxide. No chemical interaction is found for the components of the anode materials. From Figure 2b, it is possible to conclude that cathode and electrolyte materials possess excellent thermal compatibility. More specifically, the average TEC values of YBCZ and BCZD are actually the same (10.79 × 10−6 and 10.83 × 10−6 K−1) across the entire studied temperature range. The TEC level for the material of the supported anode layer attains 13.9 × 10−6 K−1; however, the difference in thermal expansion for the unit cell is compensated by the intermediate functional anode layer having a lower TEC value (13.2 × 10−6 K−1). The total conductivity of the BCZD electrolyte in oxidizing and reducing atmospheres between 500 and 900 °C is depicted in Figure 3a. The moisture in air and hydrogen is maintained at the same level (pH2O = 0.03 atm). Therefore, when formally considering the quasi-chemical reaction of the interaction of water with oxygen vacancies (V•• o ),

Figure 4. Cross-section of PCFC prepared by the tape-calendering method. E


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°C) and then starts to drop (down to ∼230 mW cm−2 at 800 °C) with gradual growing temperature. The increment is associated with the well-known tendency composed of a decrease in both ohmic and polarization resistances (as shown below). However, a further deterioration is not expected. In an attempt to define the reasons negatively affecting OCV and Pmax parameters, the next sections aim at a thorough analysis of the transport behavior of PCFC, including (i) the calculation of the electronic resistance of the electrolyte membrane, (ii) the estimation of its ion transport numbers in fuel cell mode, (iii) the separation of ohmic and polarization resistances and (iv) the determination of the electrolytic properties of the protonconducting ceramic used. 4.3. Modified EMF Method. The Gorelov method of introducing the active load in the electrical circuit was successfully used in the present work. An electronic load with variable resistance (from 1 MΩ to 100 Ω) was used. The voltampere data depending on the resistance of the electronic load were obtained in a limited region of current density from 0 to 50 mA cm−2 at each temperature (see the initial section of U = f(I) dependences in Figure 5a). At these values, the transport properties of the electrolyte seem to be close to those measured under open-circuit conditions. Figure 6 demonstrates the data obtained for PCFC using the Gorelov modification of the EMF method. The lines fitted the Figure 5. (a) Volt−ampere and (b) power-density characteristics of the wet H2, Ni−BCZD|BCZD|YBCZ, and wet air cell at different temperatures.

Table 1. Thermodynamic Voltages for Wet H2−Wet Air Condition (EO), Reading of the YSZ Sensor (Esens), OpenCircuit Voltage Values (Eexp), and Maximal Power Densities for the PCFC (Pmax) T, °C

EO, B

Esens, V

Eexp, V

Pmax, mW cm−2

600 625 650 675 700 725 750 775 800

1.135 1.130 1.126 1.122 1.118 1.113 1.109 1.105 1.101

1.130 1.125 1.122 1.118 1.114 1.109 1.099 1.097 1.091

1.026 1.013 0.995 0.968 0.946 0.923 0.905 0.884 0.797

204 240 287 303 331 357 372 370 233

Figure 6. EO/Eexp dependence of the unit PCFC on reciprocal resistance of the electronic load at different temperatures.

experimental data follow a linear form, and therefore, they can be described by eq 11. As can be seen, the slope of these dependences decreases with increasing temperature, while the intersection value of the ordinate axis increases. The ratio of these values reflects the electronic resistance of the electrolyte, which is gradually diminishing. The quantitative calculation of the Re level is shown in Figure 7. It changes from 10.1 Ω cm2 at 600 °C to 1.6 Ω cm2 at 800 °C. This suggests an enhancement of electron conductivity and is in good qualitative agreement with the analysis of electrical properties (Figure 3b,c). To convert the absolute electronic resistance value to a relative value (eq 12), the total ohmic resistance of the electrolyte should be determined. This parameter is obtained using the current-interruption method. 4.4. Current Interruption Measurements. The current interruption method was used to determine the contribution of the ohmic and polarization resistances to the total resistance of the cell. As shown Figure 1, the traditional scheme was modified by inserting the mercury switch interrupting the current in the external circuit of the system and the oscilloscope continuously measuring the voltage response of PCFC. Under

thermodynamic and sensor voltage values, it can be noted that they are in close agreement. A possible small leakage in the system might explain the observed deviation. Nevertheless, the absolute difference between these parameters seems to be quite small (less than 10 mV). It is noteworthy that the presence of the oxygen sensor in the electrochemical cell is required for monitoring the cell’s integrity and the composition of the gas atmosphere. The BCZD electrolyte exhibits lower OCV values than the theoretical ones; the difference between them reaches about 100 mV at 600 °C and 300 mV at 800 °C. An electronic transport in the electrolyte membrane is the evident explanation for this fact because the imperfection of tightness causes only 10 mV of deviation from the theoretical level, as mentioned above. Considering the output characteristics, the maximal power density of PCFC first increases (up to ∼370 mW cm−2 at 750 F


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Figure 7. Electronic resistance of the BCZD electrolyte in fuel-cell mode as a function of temperature.

Figure 9. Ohmic and polarization resistances of PCFC depending on temperature.

such interruption, the cell’s voltage undergoes a sharp change and then gradually tends to the value corresponding the steadystate open circuit voltage.46−48 If the ohmic losses give an immediate response (a few microseconds), the reversibility of the electrodes is significantly extended in time (up to 100 ms). Figure 8 illustrates an example of the voltage dynamic for the

using eq 12 and compared with those calculated by eq 8. The accounting of the electrodes polarization results in higher t i values, which amount about 0.92, 0.87, and 0.81 at 600, 700, and 800 °C, respectively (Figure 10). Nevertheless, both

Figure 8. Typical behavior of voltage response depending on time during interruption procedure. The dependence was obtained at 600 °C. The value of current density in the external circuit before interruption is ∼372 mA cm −2. The OCV value is shown by the upper dashed line.

Figure 10. Average ion transport numbers of the BCZD electrolyte in fuel cell mode (1) without and (2) with polarization corrections.

tendencies show a meaningful decrease in t i that agrees with the analysis of conductivity for the individual BCZD electrolyte. The conductivity of the electrolyte membrane (or so-called film conductivity) can be calculated because all of the required data (ohmic resistance, electrolyte thickness, and average iontransport numbers) are available:

PCFC tested at 600 °C. The interruption of current in the electrical circuit (i0 = 371.8 mA cm−2) results in an instantaneous increase of the cell’s voltage from 548 to 836 mV, while the rest of the voltage losses are attributed to the electrode overpotential. Therefore, the calculated ΔV and ηel values are equal to 288 and 190 mV, respectively; in terms of resistance, the Rohm and Rp values are equal to 0.77 and 0.51 Ω cm2, respectively. The trends of temperature changes of the Rohm and Rp parameters are depicted in Figure 9. The ohmic resistance gradually decreases in the entire temperature range (0.77, 0.49, and 0.30 Ω cm2 at 600, 700, and 800 °C, respectively), whereas the polarization resistance first decreases (from 0.51 Ω cm2 at 600 °C to 0.13 Ω cm2 at 750 °C) and then considerably grows (up to 0.30 Ω cm2 at 750 °C). If the electrolyte behavior is expected on account of the temperature-activated processes realized in ionic conductors, the data concerning nonmonotonicity of Rp are rather unexpected. To explain this result, let us discuss first the transport properties of the BCZD membrane in fuel-cell mode. 4.5. Transport Properties of the BCZD Electrolyte Membrane. Taking into account Re and Rohm data (Figures 7 and 9), the average transport numbers of ions were calculated

σfilm,total = σfilm,i = t i·

h R ohm

(14)

h R ohm

(15)

where σfilm,total and σfilm,i are the total and ionic film conductivity, and h is the electrolyte’s thickness. As in the case of t i , film conductivity (and partial constituents) is also a certain average value, which corresponds not to the specific conditions (for example, wet air or wet hydrogen) but to the gradient caused by the coexistence of both oxidizing and reducing atmospheres. The σfilm, total and σfilm,i parameters are useful because they refer to the thickness in contrast to Rohm and Pmax; this allows a comparison of SOFCs with different types of electrolytes and their dimensional characteristics. Figure 11 shows the film conductivity dependences on temperature in Arrhenius coordinates. It can be seen that the ionic conductivity increases from 2.4 mS cm−1 at 600 °C to 5.4 G


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associated with the change of electronic transport for electrolyte and electrode functional materials rather than an incidental loss of the cell’s tightness and integrity. 4.6. Improvement of Electrolytic Properties. The efficiency of PCFCs can be enhanced by extending the electrolytic boundary of the electrolyte membrane or, in other words, by blocking its electronic conductivity. When considering the defect structure of perovskite-related oxide materials,53,54 an increase of the atmospheric pH2O positively affects the protonic conductivity. Moreover, this leads to a decrease of the concentrations of holes and oxygen vacancies (and corresponding partial conductivities) in an oxidizing atmosphere when the pO2 value is assumed to be constant (Figure S1). This theoretical background is confirmed by many experimental data concerning the separation of total conductivity in its partial constituents,54−56 including the data for the studied BCZD material (Figure 3b,c). From the viewpoint of the defect model, water interacts with available oxygen vacancies of the materials (eq 13), creating a competition for the formation of electron holes (h•), as shown below:57

Figure 11. (1) Total and (2) ionic film conductivity of the BCZD membrane in fuel-cell mode.

mS cm−1 at 800 °C with an activation energy of 0.40 eV. Moreover, the total conductivity value amounts to about 2.6 mS cm−1 at 600 °C and 6.7 mS cm−1 at 800 °C. However, its increase is found to be nonmonotonic. Considered in detail, the activation energy of the total film conductivity is almost the same as that of the ionic one at relatively low-temperature intervals (600−750 °C); however, it is 50% higher at a hightemperature range. This is due to the electronic conductivity, which typically possesses higher activation energies compared with those for the oxygen-ionic and protonic conductivities.17 The comparative analysis demonstrates that the attained total film conductivity level of BCZD is consistent with those for the highly conducting electrolytes of close chemical composition: 2.8 mS cm−1 for BaCe0.5Zr0.3Y0.2O3−δ,35 1.5 mS cm−1 for BaCe0.5Zr0.3Y0.2O3−δ,49 1.5 mS cm−1 for BaCe 0.5 Zr 0.3 Y 0.16 Zn 0.04 O 3−δ , 50 and 2.5 mS cm −1 for BaCe0.7Zr0.1Y0.2O3−δ,51 all at 600 °C. However, it is three to five times lower than the ionic conductivity determined by the four-probe DC method for the bulk sample (wet H2, Figure 3a). A possible explanation is related with the fact that only the electrolyte properties are involved in electrotransfer when using the four-probe DC measurement. However, the contact resistance of the electrolyte−electrode interface is also significant for the PCFC testing.52 In terms of activation energy, the Ea level for the total film conductivity (0.43 eV) obtained for the wet air−wet H2 gradient is between 0.30 and 0.55 eV for wet hydrogen and wet air, respectively. This confirms the realization of a certain intermediate state of transport nature of the electrolyte membrane in comparison with the boundary conditions. Returning to Figure 9, it is possible to explain the anomaly observed in the temperature dependence of Rp and Pmax. A part of the BCZD electrolyte (in the subsurface region, near with the electrolyte−cathode interface) exhibits a mixed ionicelectronic conductivity. When the level and contribution of the electronic conductivity is rather high (800 °C), this electrolyte part acts as a virtual electrode system. However, due to the very low electrochemical activity of the BCZD subsurface (on account of its dense microstructure and much-lower electronic conductivity in contrast to that of the YBCZ material), the apparent polarization resistance starts to increase up to a value corresponding to Rp at 625−650 °C. This causes the corresponding change of Pmax. It should be noted that the PCFC demonstrates close OCV and Pmax characteristics compared with the initial obtained ones when the electrochemical system is cooled below 750 °C. Therefore, the mentioned deterioration observed at higher temperatures is

x • 1/2O2 + V •• O ↔ OO + 2h

(16)

This well-established fact supports a possible strategy to improve the electrolytic properties of the BCZD membrane operating in fuel cell mode. Experimentally, the same PCFC was tested under highly moisturized oxidizing and reducing atmospheres (p′H2O = p′′H2O = 0.10 atm). The same pH2O on the opposite sides of the membrane allows a simplified calculation to be used for the evaluation of average ion transport numbers. Figure 12 presents the comparison of volt−ampere and performance dependences for two the same cells tested under

Figure 12. Volt−ampere and power-density characteristics for wet H2, Ni−BCZD|BCZD|YBCZ, and wet air at 700 °C, for which (1) p′H2O = p′′H2O = 0.03 atm and (2) p′H2O = p′′H2O = 0.10 atm (2).

different values of pH2O. As can be distinguished, the small differences between EO and Esens for both experiments indicate the tightness and integrity of the electrochemical system. The theoretical voltage decreases by ∼5% in the second case owing to thermodynamic reason (a decrease in the oxygen partial pressure gradient; eq 5). However, the OCV value is found to increase by ∼6%, respectively, for the conditions with lower pH2O in both gases. This very interesting and unexpected result can only be related to the fact that proton transport in the BCZD electrolyte starts to dominate over the ionic one.58 It might be expected that the humidification of fuel and oxidant should result in higher PCFC performance. However, this is contradicted from the experimental results. A possible H


Research Article


Table 2. Electrochemical Data for the Cells of Wet H2, Ni−BCZD|BCZD|YBCZ, and Wet Air at 700 °C Depending on Humidity of Gases: (1) p′H2O = p′′H2O = 0.03 atm and (2) p′H2O = p′′H2O = 0.10 atma

a

Green and red triangles indicate positive and negative effects, respectively.

compromise between the efficiency-determined parameters (OCVs, t i ) and output-determined parameters (Rohm, Rp, and Rtotal) of PCFCs. Therefore, the results of these experiments allow the proposing of further challenges for development of PCFCs with both high-output characteristics and high efficiency: the search of the rational operation conditions of PCFCs by the means of the pH2O variation and the selection of electrodes exhibiting a positive response with air and fuel humidification.

explanation is discussed below, involving the estimation of the average ion transport numbers and the determination of the ohmic and polarization resistances (Table 2 and Figure S2). The electronic resistance increases by 38% with growing pH2O, and therefore, it favors the electrolytic boundary extension for the BCZD membrane. At the same time, the increment of total film conductivity amounts 17% compared to the initial level. The latter is realized because of the mentioned deterioration of electronic transport and the increase of protonic transport that follows from eq 13: σionic ≈ σH ∼ (pH2 O)1/2

5. CONCLUSIONS

(17)

This work reports for the first time on a new concept for testing protonic ceramic fuel cells (PCFCs) using a complex approach of traditional volt−ampere measurements with the current interruption method and electromotive force measurements corrected by the polarization resistances of the electrodes. The proton-conducting BaCe0.5Zr0.3Dy0.2O3−δ (BCZD) material is selected as the electrolyte object of PCFC. The obtained results show that the difference between the theoretical achievable and open-circuit voltages comes from the presence of the electronic transport in the BCZD membrane, which significantly increases with temperature growth. The air and hydrogen humidification can be considered as an attractive approach in terms of reducing ohmic resistance and improving ionic transport. However, this inversely affects the electrodes’ electrochemical behavior, leading to an increase in polarization resistance. These fundamental correlations provide the margins for the optimization of PCFC performance and the search of new electrode systems having a positive effect (or as minimal a negative effect as possible) with increasing water vapor concentration in gas atmospheres.

As a result, the corrected value of the average ion transport number is 0.92 at p′H2O = p′′H2O = 0.10 atm against 0.87 obtained at p′H2O = p′′H2O = 0.03 atm. Comparing the ratio of conductivity change with that of the pH2O change, a rough estimation of the contribution of the protonic conductivity can be made. To be precise, the experimental conductivity ratio is 1.171 against the maximum achievable ratio of 1.175 for a purely protonic conductor. This again confirms that proton transport predominates for the BCZD membrane in fuel cell mode under high pH2O values in gases, i.e., t i ≈ tH̅ . Regardless of the positive effect of humidification on the electrolytic properties, the electrode response differs. The polarization resistance of the electrodes grows considerably (by a factor of 2), having a decisive influence on the total cell resistance (increases by 18%; Table 2) and PCFC performance (decreases from 331 to 313 mW cm−2; Figure 5a). The effect of increased electrode overpotential with gas humidification was mentioned recently for PCFCs based on different protonconducting electrolytes.57−59 A complex study of electrode behavior carried out on symmetrical cells with protonic electrolytes60−64 indicates that the presence of water in an oxidant results in a lower number of electrochemical active parts for the oxygen reduction reaction that inhibits the oxygen adsorption and dissociation of surface diffusion (Table S1). At the same time, independence of the polarization resistance on the pH2O variation in hydrogen atmospheres has been demonstrated for fuel electrodes.65 In conclusion, the pH2O increase adversely affects the output characteristics of PCFCs: however, it extends the electrolytic properties of the membrane, leading to higher ionic conductivity, average ion transport numbers, and open-circuit voltage and, therefore, can increase the efficiency of fuel utilization; however, the performance can be reduced due to imperfect electrode properties for both oxygen-reduction and water-generation reactions. It is evident that there is a certain

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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/acsami.7b07472. Concentration of charge carrier defects (holes, oxygen vacancies and protons) depending on water vapor partial pressure (Figure S1), examples of PCFC’s voltage change as a function of resistance of the electronic load (Figure S2), and the pathways of oxygen reduction and water generation reactions realized in the electrolyte/cathode interface of PCFCs depending on the transport behavior of electrode system (Table S1). (PDF) I


Research Article


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Cell with Excellent Performance and Fuel Compatibility. Adv. Mater. 2016, 28, 8922−8926. (14) Gao, Z.; Mogni, L. V.; Miller, E. C.; Railsback, J. G.; Barnett, S. A. A Perspective on Low-Temperature Solid Oxide Fuel Cells. Energy Environ. Sci. 2016, 9, 1602−1644. (15) Zhang, Y.; Knibbe, R.; Sunarso, J.; Zhong, Y.; Zhou, W.; Shao, Z.; Zhu, Z. Recent Progress on Advanced Materials for Solid-Oxide Fuel Cells Operating Below 500 °C. Adv. Mater. 2017, 1700132. (16) Medvedev, D.; Murashkina, A.; Pikalova, E.; Demin, A.; Podias, A.; Tsiakaras, P. BaCeO3: Materials Development, Properties and Application. Prog. Mater. Sci. 2014, 60, 72−129. (17) Kochetova, N.; Animitsa, I.; Medvedev, D.; Demin, A.; Tsiakaras, P. Recent Activity in the Development of ProtonConducting Oxides for High-Temperature Applications. RSC Adv. 2016, 6, 73222−73268. (18) Dippon, M.; Babiniec, S. M.; Ding, H.; Ricote, S.; Sullivan, N. P. Exploring Electronic Conduction Through BaCexZr0.9−xY0.1O3−δ Proton-Conducting Ceramics. Solid State Ionics 2016, 286, 117−121. (19) Sun, W.; Wang, Y.; Fang, S.; Zhu, Z.; Yan, L.; Liu, W. Evaluation of BaZr0.1Ce0.7Y0.2O3−δ-Based Proton-Conducting Solid Oxide Fuel Cells Fabricated by a One-Step Co-Firing Process. Electrochim. Acta 2011, 56, 1447−1454. (20) Mori, M.; Liu, Y.; Hashimoto, S.-I.; Takei, K. Investigation of Current Leakage of Micro-tubular SOFCs with a Ceria Membrane for Low-intermediate Temperature Power-generation Applications. Electrochemistry 2009, 77, 178−183. (21) Lo Faro, M.; Aricò, A. S. Electrochemical Behaviour of an AllPerovskite-Based Intermediate Temperature Solid Oxide Fuel Cell. Int. J. Hydrogen Energy 2013, 38, 14773−14778. (22) Li, X.; Ihara, M. Ionic Conduction and Power Generation Characteristics of Pr-Doped Ba2In2O5 for Proton-Conducting SOFCs. J. Electrochem. Soc. 2015, 162, F927−F938. (23) Magrasó, A.; Fontaine, M.-L.; Bredesen, R.; Haugsrud, R.; Norby, T. Cathode Compatibility, Operation, and Stability of LaNbO4-Based Proton Conducting Fuel Cells. Solid State Ionics 2014, 262, 382−387. (24) Zhu, Z.; Sun, W.; Shi, Z.; Liu, W. Proton-Conducting Solid Oxide Fuel Cells with Yttrium-Doped Barium Zirconate Electrolyte Films Sintered at Reduced Temperatures. J. Alloys Compd. 2016, 658, 716−720. (25) Okuyama, Y.; Kozai, T.; Ikeda, S.; Matsuka, M.; Sakai, T.; Matsumoto, H. Incorporation and Conduction of Proton in Sr-Doped LaMO3 (M = Al, Sc, In, Yb, Y). Electrochim. Acta 2014, 125, 443−449. (26) Danilov, N.; Vdovin, G.; Reznitskikh, O.; Medvedev, D.; Demin, A.; Tsiakaras, P. Physico-Chemical Characterization and Transport Features of Proton-Conducting Sr-Doped LaYO3 Electrolyte Ceramics. J. Eur. Ceram. Soc. 2016, 36, 2795−2800. (27) Gorelov, V. P.; Balakireva, V. B.; Kuz’min, A. V. Partial Conductivities in Perovskites CaZr1−xScxO3−α (x = 0.03−0.20) in an Oxidation Atmosphere. Phys. Solid State 2016, 58, 12−18. (28) Gorelov, V. P.; Balakireva, V. B.; Kuz’min, A. V.; Plaksin, S. V. Electrical Conductivity of CaZr1−x (x = 0.01−0.20) in Dry and Humid Air. Inorg. Mater. 2014, 50, 495−502. (29) Sun, W.; Shi, Z.; Wang, Z.; Liu, W. Bilayered BaZr0.1Ce0.7Y0.2O3−δ/Ce0.8Sm0.2O2−δ Electrolyte Membranes for Solid Oxide Fuel Cells with High Open Circuit Voltages. J. Membr. Sci. 2015, 476, 394−398. (30) Sumi, H.; Suda, E.; Mori, M. Blocking Layer for Prevention of Current Leakage for Reversible Solid Oxide Fuel Cells and Electrolysis Cells with Ceria-Based Electrolyte. Int. J. Hydrogen Energy 2017, 42, 4449−4455. (31) Medvedev, D.; Pikalova, E.; Demin, A.; Podias, A.; Korzun, I.; Antonov, B.; Tsiakaras, P. Structural, Thermomechanical and Electrical Properties of New (1−x)Ce 0.8 Nd 0.2 O2−δ − xBaCe 0.8 Nd0.2 O3−δ Composites. J. Power Sources 2014, 267, 269−279. (32) Cao, J.; Gong, Z.; Fan, C.; Ji, Y.; Liu, W. The Improvement of Barium-Containing Anode for Ceria-Based Electrolyte with ElectronBlocking Layer. J. Alloys Compd. 2017, 693, 1068−1075.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dmitry Medvedev: 0000-0003-1660-6712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Russian Science Foundation (grant no. 16-19-00104). D.M. thanks the Scholarship of the President of the Russian Federation (grant no. CII1885.2015.1). The characterization of powder and ceramic materials was carried out at the Shared Access Center “Composition of Compounds” of the Institute of HighTemperature Electrochemistry.

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