Fast and Anisotropic Proton Conduction in a Crystalline

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Fast and Anisotropic Proton Conduction in a Crystalline Polyphosphate Naoyuki Hatada, Kazuaki Toyoura, Takayuki Onishi, Yoshinobu Adachi, and Tetsuya Uda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5105642 • Publication Date (Web): 05 Dec 2014 Downloaded from http://pubs.acs.org on December 9, 2014

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The Journal of Physical Chemistry

Fast and Anisotropic Proton Conduction in a Crystalline Polyphosphate Naoyuki Hatada,* Kazuaki Toyoura,† Takayuki Onishi, Yoshinobu Adachi, and Tetsuya Uda Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan

1

ACS Paragon Plus Environment


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KEYWORDS intermediate-temperature fuel cells; solid electrolytes; single crystals; nudged elastic band (NEB) method; kinetic Monte Carlo (KMC) simulations

ABSTRACT

Lanthanum polyphosphate (LaP3O9) is an attracting candidate for the electrolytes in fuel cells because of its relatively high proton conductivity. However, the proton conduction mechanism in LaP3O9 (i.e., proton transport pathways and its relationship with the crystal structure) still remains poorly understood, thus there has been no clear strategy for enhancing the conductivity. In this paper, we show that the fast and anisotropic proton conduction pathways exist along the b- and c- axes in the crystal lattice of LaP3O9, and the proton conductivity can be remarkably improved by controlling the microstructure of the electrolyte membranes. The first-principles calculations reveal that protons migrate only along the neighbors of specific oxide ions in the PO4 chains, leading to the conductivity anisotropy, which is readily confirmed using Sr-doped LaP3O9 single crystals. The c-axis oriented, coarse-grained polycrystalline membranes of Srdoped LaP3O9 prepared by solution synthesis techniques exhibit markedly enhanced conductivity compared to randomly oriented polycrystals prepared by solid state reaction, and have direct applicability to fuel cell electrolytes. The discovery of fast proton conduction pathways in LaP3O9 will motivates further development of LaP3O9-based electrolytes, as well as exploration of new proton conducting crystalline polyphosphates with infinite chains of PO4 tetrahedra.

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1. Introduction The development of electrolyte materials with high ionic conductivity is a major challenge for the commercialization of fuel cells. Lanthanum phosphates have attracted attention because of their potential use as solid electrolytes in intermediate temperature fuel cells. Several types of lanthanum phosphates (La7P3O18 1, LaPO4 2,3, LaP3O9 4,5, and LaP5O14 6) have been reported to exhibit proton conductivity of about 10-4 S cm-1 at 600 °C when doped with alkaline earth elements (Ca, Sr, and Ba). Although their conductivity values are two orders of magnitude lower than those of common solid state proton conductors such as perovskite-type oxides

7,8

(e.g.,

BaZrO3 and BaCeO3) and solid acids 9,10 (e.g., CsH2PO4), lanthanum phosphates possess several superior properties including pure proton conductivity at high temperatures (above 600 °C), chemical and thermal stability, and water insolubility. Thus lanthanum phosphates will be more attractive for technological applications if higher proton conductivity is achieved. A particularly interesting lanthanum phosphate is lanthanum polyphosphate (LaP3O9) doped with alkaline earth elements. Its proton conductivity is not only higher than those of other lanthanum phosphates, but also almost independent of water vapor pressure at temperatures below 700 °C 5. This is a distinct advantage, since most of proton conducting oxides and phosphates tend to lose proton conductivity at higher temperatures and lower water vapor pressures due to dehydration. In addition, LaP3O9 is crystallographically unique in that it has infinite helical chains of PO4 tetrahedra running along the c-axis, while many other phosphatebased proton conductors (e.g., LaPO4, CsH2PO4, SnP2O7) consist of isolated monomers or dimers of PO4 tetrahedra. However, these characteristics of LaP3O9 make it difficult to apply general pictures of proton incorporation and conduction in oxides and phosphates, thus there has been no clear strategy for enhancing the conductivity.

3



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In the case of lanthanum orthophosphate (LaPO4) doped with alkaline earth elements, protons are considered to be incorporated into and diffuse through the crystal lattice 11,12,13,14. When La3+ in LaPO4 is partially substituted with M2+ (M = Ca, Sr, and Ba), pyrophosphate ions P2O74- are formed by condensation of isolated orthophosphate ions PO43-, to maintain the charge neutrality;

1 / 2 M2P2O7 → M′La + 1/2 (P2O7 )2PO4 . ••

(1)

Then protons can be incorporated into LaPO4 from water vapor H2O (g), forming hydrogen phosphate

HPO42-;

ions

1/2 (P2O7 )2PO 4 + 1/2 H 2O(g ) → (HPO4 )PO 4 . ••

•

(2)

This situation is similar to that found in many acceptor doped proton conducting oxides, i.e., • (P2 O 7 )•2PO

4

• • and (HPO4 )PO 4 correspond to VO•• and H i , respectively. These defect species in

LaPO4 and their equilibrium relations with water vapor (Equation. (2)) have been confirmed both experimentally

11,12

and computationally

15

. As for proton conduction pathways, which are

generally assumed to be in either crystalline grains or grain boundaries, recently the ones in the crystal lattice of LaPO4 have been clarified theoretically 13,14. The calculated activation energies for proton conduction along the conduction pathways agree well with those experimentally derived, taking into account the association effect between protons and dopants 14. Thus the above picture of proton incorporation and conduction in the crystal lattice of LaPO4 should be feasible, and can be utilized for achieving preferable proton conduction properties. On the other hand, these kinds of defect structures and proton conduction pathways in LaP3O9 have been poorly understood due to its unique proton conduction behavior and crystal structure as mentioned above. It has only recently been reported that the hydration enthalpy for LaP3O9 calculated from first principles is negatively large compared with those for proton conducting oxides

16

. This indicates that protons are confined in the LaP3O9 crystal lattice even under low

4


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water


vapor

pressures,

leading

to

difficulty

in

the

experimental

observation

of

hydration/dehydration behavior at intermediate temperatures 4. However, a fundamental question still remains unanswered: Where can protons diffuse in LaP3O9? In this work, we approached this problem by the combination of theoretical calculations and experiments. First, the low-energy proton conduction pathways in the crystal lattice of LaP3O9 were theoretically explorered using first-principles calculations. Then the temperature dependence of the proton conductivities along each crystallographic direction was estimated by stochastic diffusion simulations based on the kinetic Monte Carlo (KMC) method, taking into account the proton trapping effect by dopants. In addition, the temperature dependence of the proton conductivities along each crystallographic derection was also experimentally evaluated using Sr-doped LaP3O9 single crystals. Both results were compared to verify the proton conduction mechanism derived by the first-principles method. Finally, the enhanced electrochemical performance of the Sr-doped LaP3O9 electrolyte membranes having a preferable microstructure was demonstrated. 2. Computational and experimental methods 2.1. Computational methods All the computational studies were based on first-principles calculations, which were performed using the projector augmented wave (PAW) method implemented in the VASP code 17,18,19

. The generalized gradient approximation (GGA) parameterized by Perdew, Burke and

Ernzerhof

20

was used for the exchange-correlation term. The plane wave cutoff energy was 400

eV. The 5s, 5p, 6s, and 5d orbitals for lanthanum, 3s and 3p for Phosphorus, 2s and 2p for oxygen, and 1s for hydrogen were treated as valence states. A supercell consisting of 1×2×2 unit cell of LaP3O9 including 208 atoms was used with a k-point grid of 2×2×2. The lattice

5



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parameters and atomic positions in the perfect crystal were determined by structural optimization of the unit cell using the experimentally reported crystal structure 21 as the initial structure (a 2×4×4 k-point grid). For finding proton sites, i.e., local energy minima in the crystal, firstly potential energy surface of a proton was calculated with the fixed atomic positions. The grids for the potential energy surfaces of a proton include 30×20×20 = 12,000 points per unitcell. Then, all the atomic positions were fully optimized using the local energy minima in the PES as initial structures until the residual forces became less than 0.02 eV/Å. Four proton sites with potential energy below 1 eV were found around each of the O3 and O4 ions. Proton conduction paths and their energy profiles were evaluated using the nudged elastic band (NEB) method

22

for all possible paths

connecting the proton sites within 3.5 Å distance. Using seventeen conduction paths with calculated potential barriers below 1 eV, the KMC simulations 23 were finally performed to evaluate the proton diffusivity and conductivity. The mean jump frequency of each migration path was estimated using the conventional equation, ν*exp(−∆Emig/kBT), where kB is the Boltzmann constant, T is the temperature, ∆Emig is the potential barrier of the jump, and ν* is the vibrational prefactor set to 1013 Hz referring to previously reported values in proton-conducting phosphates

13,14

. The diffusion coefficients

along the a-, b- and c-axes were estimated from the mean-square dispracements of 10,000 independent protons after the KMC simupations at each temperature between 500 K and 1500 K. The KMC step was determined at each temperature in the range of 1.5×105 and 3×108 by carefully checking the convergence of the estimated diffusion coefficients. The proton trapping effect by Sr dopants was taken into account as reduced concentration of mobile protons in the crystal, c H + ,mob . The association energy between a proton and a Sr dopant

6


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for each site was here evaluated from the energy difference between the two supercells including both a proton and a strontium ion with the nearest and furthest H+-Sr2+ distances. The relative potential energies of proton sites in the furthest region from the strontium dopant coincide with those in the undoped supercell within 0.02 eV differences, which means the furthest region is out of range affected by the dopant. c H+ ,mob was estimated using the partition functions of mobile and trap sites (Zmob and Ztrap) according to the following equations:

cH+ ,mob = rmob x = (Z mob /(Z mob + Z trap )) x

(3)

, where rmob and x are the ratio of mobile protons and all the proton concentration incorporated in the crystal, respectively. Only the first-nearest-neighbor proton sites from the Sr dopant were treated as the trapped sites, and all the other sites were assumed to be the mobile sites with the same site energies as those in the furthest region. 2.2. Materials preparation and characterization Sr-doped LaP3O9 single crystals and polycrystalline membranes were synthesized in homogeneous phosphoric acid solutions containing La2O3 and SrCO3 as described elsewhere 24. La2O3, SrCO3 and H3PO4 (aq) were mixed in a glassy carbon crucible and kept at 190 °C in the ambient atmosphere to obtain a transparent solution. Then the single crystals and polycrystalline membranes of Sr-doped LaP3O9 were precipitated either at 300 – 360 °C in steam (partial pressure of water, pH2O = 1 atm) or at 190 – 250 °C in humidified air (pH2O = 0.03 atm), respectively. Phase identification was carried out via X-ray diffraction (XRD) analysis on PANalytical X’pert-Pro MPD using Cu Kα radiation at room temperature. The morphology of the samples was investigated using KEYENCE VE-7800 scanning electron microscopy (SEM). Chemical

7



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analysis of the samples was performed by inductively coupled plasma atomic emission spectrometry (ICP-AES) on SII NanoTechnology SPS3500. 2.3. Electrochemical measurements For electrochemical measurements, silver elecrodes was attached to Sr-doped LaP3O9 single crystals using silver paste, while platinum electrodes were deposited on Sr-doped LaP3O9 polycrystalline membranes by sputtering. The electrical conductivity of Sr-doped LaP3O9 single crystals was evaluated by electrochemical impedance spectroscopy (EIS) using a frequency response analyser Solartron 1260 in the frequency range 10 – 107 Hz under a humidified gas mixture of hydrogen and argon. The electrical conductivity of Sr-doped LaP3O9 polycrystalline membranes was evaluated in the same manner in the frequency range 0.1 – 107 Hz. The performance of the fuel cell with Sr-doped LaP3O9 polycrystalline membrane electrolyte was evaluated using humidified hydrogen and oxygen (pH2O = 0.03 atm) as the fuel and oxidant, respectively. 3. Results and discussion 3.1 First-principles study of the proton conduction pathways in the crystal lattice of LaP3O9 In general, protons are considered to reside around oxide ions in the crystal lattice of proton conducting oxides, phosphates, sulphates, and other compounds containing oxide ions, and to migrate by repetition of rotations around the oxide ions and hoppings between the rotational orbits. Thus, configuration of oxide ions in crystals is important in the proton conduction. In LaP3O9, oxide ions are located at the corners of PO4 tetrahedra, crystallographically classified into five, O1 – O5. The O1 and O5 ions are corner-shared by two PO4 tetrahedra, to form infinite

8


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helical PO4 chains along c-axis (Figure 1(a)). One, therefore, pictures that the dominant proton conduction in LaP3O9 could be along c-axis from the viewpoint of the crystal structure.

Figure 1. First-principles investigation of the proton conduction in LaP3O9. (a) The crystal structure of LaP3O9. The oxide ions are classified into O1–O5, which are shown in different colour (O1: blue, O2: pink, O3: black, O4: red, O5: water). The O1 and O5 ions are cornershared by two tetrahedra, to form infinite PO4 helical chains along the c-axis. Four PO4 chains are shown in the figure. (b,c) The proton conduction pathway along (b) the c-axis and (c) the baxis in LaP3O9. The black, red, and white small spheres denote the trajectories of rotational orbits around O3 and O4 ions and hopping paths between them, respectively.

Our computational studies using first-principles calculations partially support the picture from the structural viewpoint. The red and white small spheres in Figure 1(b) denote a proton conduction pathway along the c-axis, which is entangled with the single PO4 chain running

9



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through the inside of the infinite helix. The pathway consists of partial rotations around O4 ions and hoppings between the rotational orbits. The calculated potential barrier is 0.32 eV, which is the lowest among all the conduction pathways in LaP3O9. In addition, another fast conduction pathway was also found along the b-axis beyond the picture from the structural viewpoint. Figure 1(c) shows the pathway along the b-axis, which consists of partial rotations around O3 and O4 ions and hoppings between them. The calculated potential barrier is 0.37 eV, comparable to that along the c-axis. By contrast, even the fastest pathway has 1.0 eV potential barrier along the a-axis, i.e., there is no fast conduction pathway in this direction.

Figure 2. Proton sites (white spheres) and rotational orbits around O3 and O4 ions (black and red spheres, respectively) found by first-principles calculations. Eight proton sites with low potential energies below 1 eV were found only around O3 and O4 in spite of five crystallographic oxygen sites (O1-O5).

The anisotropic proton conductivity is strongly related to not only the existence of the infinite helical PO4 chains but also selective preference of protons for oxide ions. It is interesting that protons in LaP3O9 prefer only the O3 and O4 ions (See Figure 2), so that the proton conduction pathways consist of the rotational orbits around the O3 and O4 ions and hoppings between them. The other oxide ions (O1, O2, and O5 ions) are not directly involved in the proton conduction, which is notable in picturing proton conducting network from the crystal structure. The selective

10


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preference of protons display a tendency, that is, protons tend to prefer “unshared” oxide ions to “corner-shared” ones. Therefore, rotational orbits around unshared oxide ions and their configuration could be a useful clue to imagine proton conduction behaviour in phosphates. 3.2. Stochastic proton diffusion simulations and proton trapping by dopants The temperature dependence of the proton conductivity was investigated by stochastic diffusion simulations based on the KMC simulations, and the effect of proton trapping by dopants was also considered by estimating the trapped proton concentration from the association energy between protons and dopants.

11



T ' / ○C 1200 800 600

a 2

log (σ T / S cm -1 K)

400

Without trapping effect With trapping effect b-axis

0 −2

0 c-axis −2

b-axis

−4

c-axis a-axis

−4

log (σ / S cm-1)

4

−6

−6 0.6 0.8

1

1.2 1.4 1.6 1.8 1000 T -1 / K-1

−8 2

T ' / ○C 1200 800 600 1

rmob, rtrap

400

b

0.8

rmob rtrap

0.6 0.4 0.2 0 0.6 0.8

Mobile proton concentration, rmob x (%)

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0.6 0.4 0.2 0 0.2

1

1.2 1.4 1.6 1.8 1000 T -1 / K-1

2

c 800 K (Calc. assoc. energy) − 0.2 eV

0.1 (Calc. assoc. energy) − 0.1 eV 0 0.04 0.02 0 0

(Calc. assoc. energy) − 0.0 eV 1

2 3 4 5 6 Sr doping level, x (%)

7

8

Figure 3. Theoretically derived proton conductivity and mobile proton concentration. (a) The calculated pProton conductivities along the a-, b-, and c-axes as a function of inverse temperature along the a-, b-, and c-axes. The proton concentration is set to be 2 % per La site in the crystal of LaP3O9. The blue doppted dotted lines with solid circles are the proton conductivities under assumption of the constant mobile proton concentration without the proton trapping effects. The

12


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red solid lines with open circles include the trapping effects, decreasing the concentration of mobile protons with decreasing temperature. (b) The estimated rRatios of mobile and trapped protons in 2 % Sr-doped LaP3O9 (denoted by rmob and rtrap, respectively) as a function of inverse temperature. (c) Mobile proton concentration at 800 K as a function of Sr doping level under the proton trapping effect. Three cases with different association energies are simulated (one used for Figure 3(a) and (b), and the others with 0.1 or 0.2 eV smaller association energies). The total (mobile and trapped) proton concentration is set to be equal to Sr doping level. Arrows in the figure indicate the maxima of mobile proton concentration.

Figure 3(a) shows the calculated proton conductivity as a function of inverse temperature when the proton concentration is 2 % per La site. The three blue dotted lines with solid circles show the conductivities in the a-, b-, and c-directions, respectively, without the proton trapping effect. The conductivities along the b- and c-axes are comparable, while that along the a-axis is much lower reflecting the high potential barrier. The apparent activation energies derived from the slopes of the calculated conductivity curves are 0.34 eV along the b- and c-axes vs. 0.97 eV along the a-axis, approximately equivalent to the corresponding potential barriers. Here, note that all the protons in the crystal cannot contribute to the conductivity due to proton trapping by dopant-proton association. The association energies between protons and Sr dopants are calculated to be in the range of 0.4 – 0.6 eV depending on the proton sites. Figure 3(b) shows the temperature dependence of the ratios of mobile and trapped protons in the crystal estimated from the calculated association energies according to Equation (3). The slope of the mobile proton ratio becomes steeper as decreasing temperature, e.g., the apparent activation energies are 0.30 eV at 1500 K and 0.58 eV at 500 K. Note that a large part of protons are trapped by Sr dopants

13



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and only the other small part contributes to the proton conductivity at intermediate temperatures. The red solid lines with open circles in Figure 3(a) are the estimated proton conductivities taking trapped protons by Sr dopants into consideration. The slopes of the conductivity curves become steeper than those without the trapping effect, yielding an apparent activation energy of 0.92 eV along the b- and c-axes at 573–773 K. The difference between the conductivities with and without the trapping effect reaches several orders of magnitude at low temperatures, e.g., two orders of magnitude lower at 773 K. Figure 3(c) shows the mobile proton concentration as a function of Sr doping level (both expressed as a percentage of La site) under the proton trapping effect. The total (mobile and trapped) proton concentration is assumed equal to Sr doping level. Even though the total proton concentration monotonically increases with Sr doping level, the mobile proton concentration begins to decrease at a certain Sr doping level because of the proton trapping effect. 3.3. Proton conduction properties of Sr-doped LaP3O9 single crystals We aimed to verify experimentally the proton conduction mechanism suggested by the theoretical calculations. Previous experimental studies of the proton conduction properties of LaP3O9 in the literature were conducted using polycrystalline samples or amorphous samples 4,25. Polycrystalline samples generally contain randomly oriented crystalline grains and grain boundaries, hence it is difficult to extract detailed information on the crystalline phase. We have recently developed a synthesis method of Sr-doped LaP3O9 single crystals in condensed phosphoric acid solutions

24

, which opened the way for directly characterizing the proton

conduction properties of the crystalline phase. Figure 4(a) shows the column-shaped crystals of Sr-doped LaP3O9 with length up to 2 mm. The c-axis was always directed parallel to the longer direction of the crystals, as a consequence

14


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of preferred growth along the c-axis 24. As shown in Figure 4 (b), two platinum wires were attached to the both ends of a crystal using silver paste for electrical conductivity measurements.

Figure 4. Experimental characterization of single crystalline Sr-doped LaP3O9. (a) Photograph of Sr-doped LaP3O9 single crystals. (b) Typical SEM (scanning electron microscope) image of a Srdoped LaP3O9 single crystal with platinum wires attached to its ends using silver paste for conductivity measurements. (c) Electrical conductivity of 2.3 mol% Sr-doped LaP3O9 measured along different crystallographic directions. The electrical conductivity of polycrystalline 3% Srdoped LaP3O9 is from Ref. 26. (d) Sr doping level dependence of the electrical conductivity of LaP3O9 along the c-axis at 495±5 °C.

The electrical conductivities of 2.3% Sr-doped LaP3O9 single crystals measured along the caxis and two other directions in the ab-plane are presented in Figure 4 (c). As Sr-doped LaP3O9 is almost pure proton conductor 4, the measured electrical conductivity values can be regarded as proton conductivity values. The anisotropy of the conductivity was clearly observed, i.e., higher conductivity along the c-axis than those in the ab-plane. In addition, compared with the electrical conductivity of polycrystalline 3% Sr-doped LaP3O9 26, that of single crystal along the c-axis was

15



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5 times larger and reached 4.5 × 10-4 S cm-1 at 500 °C. These results provide an experimental evidence for the existence of fast proton conduction pathways along the c-axis in the crystal lattice of Sr-doped LaP3O9. The fivefold increase of conductivity may be due not only to the crystallographic orientation effect, but also to the absence of possibly resistive grain boundaries in the single crystals. The temperature dependence of the measured conductivity along the c-axis yields an apparent activation energy of 0.78±0.04 eV at 300–500 °C, which is slightly smaller than, but in good agreement with our theoretical value (0.92 eV) under the proton trapping effect. It implies that mobile proton concentration is actually limited by the proton trapping effect. Figure 4 (d) shows the Sr doping level dependence of the electrical conductivity of LaP3O9 along the c-axis. The electrical conductivity showed an increasing trend with Sr doping level. This agrees with the trend observed with polycrystalline samples 4, and proves that protons are incorporated into the crystal lattice as a consequence of substitution of Sr2+ for La3+. 3.4. Electrochemical performance of the uniaxially oriented Sr-doped LaP3O9 electrolyte membranes The above understanding of the proton conduction in LaP3O9 (i.e., the existence of the fast and anisotropic proton conduction pathways in the crystal lattice of Sr-doped LaP3O9) provides a guideline for improving the electrolyte performance. It is preferable that polycrystalline electrolytes have less grain boundaries and larger crystalline grains with b- or c-axis orientation. In this regard, we have recently found that this kind of Sr-doped LaP3O9 electrolytes can be prepared via precipitation in condensed phosphoric acid solutions 24. Figure 6(a)(b) shows the image of a polycrystalline membrane of 7.9 mol% Sr-doped LaP3O9 which consists of columnar c-axis-oriented grains. The c-axis orientation normal to the surface of membrane was confirmed by the X-ray diffraction pattern shown in Figure 6 (c). It should also be noted that the Sr doping

16


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level of 7.9% is higher than the previously reported highest doping level, ~3%, achieved by solid state reaction method. (The advantage of solution synthesis in obtaining higher doping levels has been previously demonstrated on Sr-doped LaPO4 12.) Figure 6 shows the electrical conductivity of the membrane as a function of inverse temperature. The overall conductivity of the membrane (2.1 × 10-4 S cm-1 at 400 °C) was 13 times higher than the reported conductivity value for polycrystalline 3% Sr-doped LaP3O9, probably due to the orientation effect, lower grain boundary density, and higher Sr doping level. It is noteworthy that the conductivity of the membrane was comparable to that of 10% Y-doped SrZrO3 (SrZr0.9Y0.1O2.95) 27, a well-known proton conducting perovskite-type oxide. Figure 7 shows the power generating characteristics of 7.9 mol% Sr-doped LaP3O9 membrane in the fuel cell configuration at 400–549 °C. Though the maximum power density (9.3 mW cm-2 at 549 °C) was smaller than those of state-of-art fuel cells, the open circuit voltage value of 1.11 V was confirmed at 549 °C, which was close to the theoretical (Nernst) value of 1.17 V. This demonstrated the direct applicability of Sr-doped LaP3O9 membranes precipitated in condensed phosphoric acid solutions to fuel cell electrolytes.

17



Figure 5. Properties of 7.9% Sr-doped LaP3O9 polycrystalline membranes. (a) Photograph of 7.9% Sr-doped LaP3O9 membrane. (b) Scanning electron microscope image of the cross section of 7.9% Sr-doped LaP3O9 membrane. (c) X-ray diffraction pattern obtained from the upper surface of 7.9% Sr-doped LaP3O9 membrane.

T ' / ○C 700 600 500 1

0

400

300

7.9% Sr-doped LaP3O9 −2 (Precipitated) Ar - 0.1 atm H2 - 0.03 atm H2O SrZr0.9Y0.1O2.95

−3

−1 −4

log (σ / S cm-1)

log (σ T / S cm-1 K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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−2 −5

3% Sr-doped LaP3O9 (Polycrystal) −3 1

1.2

1.4 1.6 1000 T -1 / K-1

1.8

2

Figure 6. Electrical conductivity of 7.9% Sr-doped LaP3O9 membrane. The electrical conductivity of polycrystalline 3% Sr-doped LaP3O9 is taken from Ref. 26 and that of SrZr0.9Y0.1O0.95 is taken from Ref. 27.

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H2, Pt | 7.9% Sr-doped LaP3O9 | Pt, O2 0.03 atm H2O 1.2

a

400 ○C (VOC: 1.03V) 452 ○C (VOC: 1.08V) 501 ○C (VOC: 1.10V) 549 ○C (VOC: 1.11V)

Voltage, V / V

1 0.8 0.6 0.4 0.2 0

0

10 20 30 Current density, i / mA cm-2

H2, Pt | 7.9% Sr-doped LaP3O9 | Pt, O2 0.03 atm H2O 12 Power density, P / mW cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

10

400 ○C 452 ○C

501 ○C 549 ○C

9.3

8

7.4

6

4.9

4 2.5 2 0

0

10 20 30 Current density, i / mA cm-2

Figure 7. (a) Cell voltage and (b) power density of the fuel cell using 7.9% Sr-doped LaP3O9 membrane as the electrolyte at 400–549 °C. VOC denotes the open circuit voltage. The numerical values in (b) indicate the maximum power densities. 4. Conclusions In this work, we theoretically revealed the existence of fast and anisotropic proton conduction pathways along the b- and c- axes in the crystal lattice of LaP3O9 by first-principles calculations,

Formatted: Not Superscript/ Subscript Formatted: Not Superscript/ Subscript

and provided its experimental evidences by conductivity measurements using Sr-doped LaP3O9 single crystals. As for the theoretical calculations, the low-energy proton conduction pathways along the b- and c-axes in the crystal structure of LaP3O9 with potential barriers of 0.37 eV and 0.32 eV, respectively, were discovered by the NEB method. The temperature dependence of the

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Formatted: Not Superscript/ Subscript


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proton conductivity values was estimated on the basis of the KMC method. When the proton trapping effect by dopants was taken into account, thethe apparent activation energy for the proton conductivity along the b- and c-axes increased to of 0.92 eV was derived for the proton conductivity along the b- and c-axes. As for the experiments, we have measured the proton conductivity of 2.3% Sr-doped LaP3O9 single crystals, to eliminate the effect of grain boundaries and to assess the conductivity anisotropy. The anisotropy of the conductivity was clearly observed, i.e., higher conductivity along the c-axis than those in the ab-plane. The conductivity along the c-axis reached 4.5 × 10-4 S cm-1 at 500 °C, 5 times larger than the reported conductivity value of polycrystalline 3% Sr-doped LaP3O9. These results confirms the existence of fast proton conduction pathways along the c-axis in the crystal lattice of Sr-doped LaP3O9. In addition, it was affirmed that mobile proton concentration is limited by the proton trapping effect by dopants, asthe close agreement of the experimentally derived activation energy for the proton conduction along the c-axis (0.78±0.04 eV at 300–500 °C) is in good agreement with the theoretical value under the proton trapping effect (0.92 eV) affirms the validity of the microscopic proton conduction mechanism of LaP3O9 revealed in this study. Based on the above understanding of the proton conduction in LaP3O9, the proton conductivity of polycrystalline LaP3O9 electrolytes can be enhanced in principle by controlling the crystallographic orientation and reducing the grain boundary density. The polycrystalline membrane of 7.9% Sr-doped LaP3O9 which consists of columnar c-axis-oriented grains showed enhanced conductivity and direct applicability to fuel cell electrolytes, although the performance of the fuel cell still needs to be improved for practical application. The possible future strategies may include the optimization of synthesis procedures for thinner LaP3O9 electrolytes with b- or c-axis oriented grains and less grain boundaries, as well as

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searching for other dopants which suppress the proton trapping effect. Last of all, since our demonstration of the proton conduction mechanism by both computational and experimental approaches is the first of this kind for any polyphosphates with infinite chains of PO4 tetrahedra, exploring new proton conducting crystalline polyphosphates will be very interesting.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses †Department of Materials Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan.

ACKNOWLEDGMENT This work was financially supported by MEXT (Ministry of Education, Culture, Sports, Science and Technology) Elements Science and Technology Project and a Grant-in-Aid for JSPS (Japan Society for the Promotion of Science) Fellows. The authors would like to thank Y. Nose for helpful discussions and A. Kuramitsu for her technical assistance.

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Amezawa, K.; Tomii, Y.; Yamamoto, N. High temperature protonic conduction in LaPO4

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Table of Contents Graphic

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Fast and Anisotropic Proton Conduction in a Crystalline

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