Supporting Online Material for

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1 Supporting Online Material for Double Perovskites as Anode Materials for Solid-Oxide Fuel Cells Yun-Hui Huang, Ronald I. Dass, Zheng-Liang Xing, John B. Goodenough* *To whom correspondence should be addressed. This PDF file includes: Materials and Methods Figs. S1 to S19 Tables S1 to S3 References Published 14 April 6, Science 312, 254 (6) DOI: /science

2 Supporting Online Material Double Perovskites as Anode Materials for Solid-Oxide Fuel Cells Yun-Hui Huang, Ronald I. Dass, Zheng-Liang Xing, John B. Goodenough* Texas Materials Institute, ETC 9.102, The University of Texas at Austin, Austin, Texas 78712, USA * To whom correspondence should be addressed. jgoodenough@mail.utexas.edu This PDF file includes: Materials synthesis and characterization Fuel-cell assembly and performance Figs. S1 to S19 Table S1 to S3 References 1

3 1. Materials synthesis (1) Anode materials. The anode materials Mg 1-x Mn x MoO 6-δ (SMMO) with x = 0,, 0.5 and 1 were prepared by a sol-gel technique; ethylenediaminetetraacetic acid (EDTA) was the chelating agent and Sr(NO 3 ) 2, Mg(NO 3 ) H 2 O, Mn(NO 3 ) H 2 O, (NH 4 ) 6 Mo 7 O H 2 O were the starting materials. The gel was decomposed at C in air for 24 h and then calcined at 800 C for another 6 h. The calcined powder was pelletized and annealed in a flowing atmosphere of 5% H 2 /Ar for the times and temperatures found in Table S1. X-ray powder diffraction (XRD, Philips X-pert, CuKα radiation) showed all samples were monoclinic (P2 1 /n, b-axis unique) characteristic of a well-ordered double perovskite with lattice parameters shown in Table S1. The lattice parameters as well as the unit-cell volume increase with x. Table S1. Synthesis conditions and room-temperature lattice parameters for MMoO 6-δ (M = Mg 1-x Mn x, Mg 0.9 Cr 0.1 ) and (La 0.75 Sr 5 ) 0.9 Cr 0.5 Mn 0.5 O 3-δ (LSCM) M Mg Mg 0 Mn 0 Mg 0.50 Mn 0.50 Mn Mg 0.90 Cr 0.10 LSCM Synthesis Conditions Space Group 1100 C(24h,2), 1 C(12h,2), 5% H 2 / Ar, cool 180 C/h 1100 C(24h,3), 1 C(12h,2), 5% H 2 / Ar, cool 180 C/h 1100 C(24h,3), 1 C(12h,2), 5% H 2 / Ar, cool 180 C/h 1100 C(24h,2), 1 C(12h,2), 5% H 2 / Ar, cool 180 C/h 1100 C(24h,3), 1 C(12h,2), 5% H 2 / Ar, cool 180 C/h P2 1 /n P2 1 /n P2 1 /n P2 1 /n P2 1 /n a (Å) (2) (3) (4) (7) (2) b (Å) (3) (5) (4) (7) (4) c (Å) (7) (3) (3) (4) (3) β ( ) (Monoclinic) (5) (5) (6) (9) (5) V (Å 3 ) (5) (4) 254(4) (7) (3) 1100 C (24h), 1350 C (72h), 1425 C (84h), O 2, cool 180 C/ h. (2) Electrolyte La Sr Ga 3 Mg 0.17 O (LSGM). LSGM was prepared by conventional solid-state reaction. Stoichiometric La 2 O 3, SrCO 3, MgO, and Ga 2 O 3 were mixed and ground well. La 2 O 3 and MgO were dried in the furnace at 1000 C overnight before using. The mixed powder was pressed into pellets and then calcined at 1000 C for 2

4 20 h. After cooling down, the pellets were reground, re-pelletized, and calcined at 1 C for 20 h in air. The thus-obtained precursor was ball-milled into very fine powder, mixed carefully with 1 wt% polymer binder (PVB), and then pelletized into disks with a diameter of 2 cm. The disks were finally sintered at 1450 C for 20 h to achieve a pure perovskite phase (cubic symmetry with space group of Pm-3m (No. 221)). The sintered ceramic disks were automatically polished by a machine to become thin and smooth. The thickness was controlled to 300 µm. (3) Cathode SrCo Fe O 3-δ (SCF). SCF was also prepared by conventional solid state reaction. Stoichiometric SrCO 3, Co 3 O 4, and Fe 2 O 3 were mixed, ground and pelletized. First, the pellets were calcined at 900 C for 10 h, then at 1000 C for 10 h, and finally sintered at 1250 C for 10 h in air. The XRD pattern shows cubic symmetry with space group of Pm-3m (No. 221). (4) Buffer layer La Ce O 2-δ (LDC). The LDC was obtained by the EDTA complex gel method with La 2 O 3 and CeO 2 as starting chemicals. Final sintering was carried out at 1450 C for 24 h in air. The XRD pattern shows cubic symmetry with space group of Fm-3m (No. 225). 2. Conductivity of the anode materials We measured conductivity of MgMoO 6-δ and MnMoO 6-δ with a standard four-probe method. Pt wire and Pt paste were used to make the four probes. The samples were pressed and polished into rectangular bars and then sintered in 5% H 2 /Ar at 1150 C for 24 h. Before measurement, the sample was again reduced in 5% H 2 /Ar at 800 C for 20 h to ensure formation of oxygen vacancies. The conductivity was measured isothermally at 800 C as a function of oxygen partial pressure po 2. The measurements started from pure H 2 and the sample was slowly allowed to reoxidise with a slow air leak to change and stabilize po 2. The oxygen partial pressure was monitored by CG1000 oxygen analyzer (AMETEK/Thermox); it consists of an electrochemical cell with zirconium oxide as the working element of the gas sensor. The value of po 2 was calculated from the formula: 3

5 E = AT log 09( atm) po 2 ( atm) (AT = 44.0 mv at 615 C) (1) where E is the cell voltage that can be read directly from the display of the analyzer, A is a constant, T is the cell temperature on an absolute scale, and po 2 is the oxygen pressure to be measured. Figs. S1 and S2 show Arrhenius plots of the electronic conductivity σ of MgMoO 6-δ and MnMoO 6-δ from to 800 C in different atmospheres. Both compounds exhibit polaronic conduction. The value of σ depends strongly on the reduced state of the sample, since reduction in the reducing atmosphere greatly enhances the conductivity. After being reduced in 5% H 2 /Ar at 800 C for more than 5 h, the sample exhibits stable conductivity. As confirmed by the thermogravimetric analysis (TGA) result, reduction in 5% H 2 /Ar leads to the formation of oxygen vacancies in the double-perovskite structure. Each oxygen vacancy reduces two Mo(VI) to Mo(V). A mixed-valent Mo(VI)/Mo(V) couple can provide a good electronic conduction. The conductive behaviors of MgMoO 6-δ and MnMoO 6-δ are almost similar. The temperature dependence of the conductivity can be described by a small-polaron hopping mechanism, σ = (A/T)exp(-E a /kt) where E a = H m + H t /2 is the sum of the motional enthalpy H m of the polarons and the enthalpy H t to free a Mo(V) from the oxygen vacancy that creates it. The value of E a can be obtained from the slopes of the Arrhenius plots of ln(σt) vs. 1/T. The average values of E a in the interval 800 C and electronic conductivity σ at 800 C are listed in Table S2. E a is around 0.1 ev in H 2 and H 2 /H 2 S for the two samples. In CH 4, E a becomes even lower. With moisture in H 2 and CH 4, no obvious change was observed in the conductivity and E a. At 800 C, the σ values in H 2 or CH 4 are around 10 S/cm for both MgMoO 6-δ and MnMoO 6-δ. In CH 4, σ of MnMoO 6-δ rapidly increases when the temperature is higher than 750 C. Fig. S3 shows the conductivity measured in H 2 with leaking air at 800 C as a function of oxygen partial pressure po 2. With increasing po 2, the conductivity decreases for both samples, indicative of n-type conductivity as the dominant electronic mechanism. However, σ is more sensitive to po 2 for MnMoO 6-δ than for MgMoO 6-δ. 4

6 4.0 log(στ ) (Scm -1 K) % H 2 /Ar H 2 97% H 2-3% H 2 O H 2 /5H 2 S CH 4 97% CH 4-3% H 2 O /T (K -1 ) Fig. S1 Temperature dependence of the electronic conductivity of MgMoO 6-δ in 5% H 2 /Ar, H 2, H 2 /5H 2 S and CH 4 after reduction in 5% H 2 /Ar at 800 C for 20 h. log(σt) (Scm -1 K) % H 2 /Ar H 2 97% H 2-3% H 2 O H 2 /H 2 S CH 4 97% CH 4-3% H 2 O /T (K -1 ) Fig. S2 Temperature dependence of the electronic conductivity of MnMoO 6-δ in 5% H 2 /Ar, H 2, H 2 /5H 2 S and CH 4 after reduction in 5% H 2 /Ar at 800 C for 20 h. 5

7 σ (S/cm) MgMoO 6-δ MnMoO 6-δ log po 2 (atm) Fig. S3 Electronic conductivity measured at 800 C as a function of oxygen partial pressure. Table S2. Values of the activation energy E a of electronic conduction for 800 C and the electronic conductivity σ at 800 C in different atmospheres Atmosphere MgMoO 6-δ MnMoO 6-δ E a (ev) σ (S/cm) E a (ev) σ (S/cm) 5% H 2 /Ar ± ± H 2 84 ± ± % H 2 3% H 2 O ± ± H 2 /H 2 S 96 ± ± CH ± ± % CH 4 3% H 2 O ± ± Fabrication and test of single fuel cells Single test cells were fabricated by an electrolyte-supported technique. LSGM with a fixed thickness of 300 µm was used as the electrolyte. The LSGM pellets were polished with a diamond wheel to glazed disks. Mg 1-x Mn x MoO 6-δ (SMMO) was used as anode material, SrCo Fe O 3-δ (SCF) was used as cathode material, and La Ce O 2-δ (LDC) was used as a buffer layer between the anode and the electrolyte. The thin LDC buffer layer has proven effective in numerous studies with 6

8 LSGM electrolyte (1-3); the fluorite structure of the buffer layer prevents interdiffusion of ionic species between perovskite anode and electrolyte. LDC, SMMO and SCF were made into inks with a binder, V-006 (Heraeus). LDC ink was screen printed onto one side of the LSGM disk followed by firing at 1300 C in air for 1 h. SMMO was subsequently screen-printed on the LDC layer and baked at 1275 C in air for 1 h. SCF was finally screen printed on the other side of the LSGM disk and fired at 1100 C in air for 1 h. Micrographs were taken with a scanning electron microscope (SEM, Hitachi: S4500). Fig. S4 shows clear interfaces of LSGM/SCF, LSGM/LDC and LDC/SMMO for the cell after testing in H 2, H 2 /H 2 S and CH 4 at C. From the SEM images, the thicknesses of the LDC, SMMO and SCF layers are estimated to be 30, 100 and 20 µm, respectively; the grain and particle sizes for LDC, SMMO and SCF vary from 1 to 2 µm. Furthermore, the anode SMMO exhibits a fine uniform microstructure with an estimated 30% porosity. a LSGM LDC SMMO b SCF LSGM SCF F E DC B A c d LSGM LDC SMMO Fig. S4 SEM images for the cell after testing in H 2, H 2 /H 2 S and CH 4 at C: (a) cross section of the cell configuration, (b) section between SCF and LSGM, (c) section between LSGM and LDC buffer layer, and (d) the surface of the MgMoO 6-δ anode. 7

9 Fig. S5 shows the fuel cell configuration. The open-circuit voltage (OCV) is given by the Nernst equation: OCV = nrt i 4F ln po2 ( cathode) po ( anode) 2 (2) where n i is the ionic transference number, which is equal to that for pure ionic conduction, R is the gas constant, T is the temperature, F is the Faraday constant, and po 2 is the oxygen partial pressure. The cell terminal voltage is given by E cell = OCV η c η a ir V (3) where η c is the cathode overpotential, η a is the anode overpotential, ir and V are the voltage drops across the electrolyte and that from leaking to the reference electrode, respectively. In most cases, V is negligible. Fig. S5 Schematic diagram of the fuel cell test The working electrode area of the cell was 4 cm 2 ( cm cm). Reference electrodes of the same materials as the working electrodes were used to monitor the overpotentials of the cathode and anode in the cell configuration, as described elsewhere (4). The reference electrode area was 0.1 cm 2 ( cm 5 cm). In order to avoid potential gradients along the electrolyte surface (5), the reference electrode was placed 5 cm away from the working electrode, which is greater than eight electrolyte thicknesses. We used Pt gauze (Alfa Aesar, 52 mesh woven from 0.1 mm diameter wire, 99.9% metal basis) with a small amount of Pt paste (Heraeus) in separate dots as a current collector at both the anode and cathode sides for ensuring contact. The use of Pt paste covering the oxide anode can also give a high lateral conductance and relax the requirement for high electronic conductivity in the oxide anode. In our case, Pt paste was used with several separate dots for contact that did not 8

10 cover the entire anode surface. Considering the possible extraneous catalytic effect of Pt, all the cells in this work were fabricated and tested under strictly identical conditions. A double-layer sealing design was applied in all single-cell tests. The assembled test cells were placed in the hot zone of a vertical furnace with air directly supplied to the cathode surface and the fuel to the anode surface at a flow rate of 30 ml/min. Before testing, the cells were exposed to 5% H 2 /Ar for 20 h at 800 C to reduce the SMMO and then purged with the fuel gas for 2 h. The performance measurements were typically carried out in the temperature range from 650 to 850 C, and a constant potential was provided by an EG&G potentiostat/galvanostat model 273 running on a homemade LabView program. During a typical measurement, the cell voltage was varied from open-circuit voltage (OCV), which is around V at 800 C, to V and then back to OCV in a total of 30 steps and holding 10 s at each step. 4. Performance of single cells in H 2 and H 2 /H 2 S (H 2 contains 5 ppm H 2 S) (1) MgMoO 6-δ Fig. S6A shows the power density and cell voltage at different temperatures for MgMoO 6-δ in H 2. The maximum power density (P max ) reaches 968 mw/cm 2 at 850 C, 838 mw/cm 2 at 800 C, 450 mw/cm 2 at 700 C, and 243 mw/cm 2 at 650 C. Fig. S6B shows the OCV, cell voltage, anode and cathode overpotentials at 800 C. Fig. S6C and Fig. S6D show the power density and cell voltage for MgMoO 6-δ in H 2 /H 2 S and CH 4, respectively. The decrease in OCV as a function of current density is due to a result of increased fuel utilization as current density increases when a constant fuel flow is used during the measurement. The existence of 5 ppm H 2 S only causes less than 5% loss in power density. In dry methane, the power density of a cell with a MgMoO 6-δ anode was as high as 438 mw/cm 2 (much higher than most of the reported values with catalyst), which indicates that MgMoO 6-δ has great potential to be directly used for methane oxidation without steam. In most SOFC cells with natural gas like CH 4 as a fuel, CH 4 reforms with steam on the anode into synthesis gas CH 4 + H 2 O = CO + 3H 2 (4) 9

11 followed by electrochemical oxidation of hydrogen at the same anode H 2 + O 2- = H 2 O + 2e - (5) For the MgMoO 6-δ anode, a very high power density is obtained in dry CH 4. With steam, the performance becomes lower. Steam is not necessary for this anode. Perhaps CH 4 can be converted by direct electrochemical oxidation on the anode accompanied by oxide ions transferred through the electrolyte CH 4 + 4O 2- = CO 2 + 2H 2 O + 8e - (6) Cell voltage (V) MgMoO 6-δ H O C 800 O C 750 O C 700 O C 650 O C Voltage (V) E cell η c η a OCV MgMoO 6-δ 800 o C, H Fig. S6A Power density and cell voltage at different temperatures for MgMoO 6-δ in H 2. Fig. S6B OCV, cell voltage, anode and cathode overpotentials at 800 C for MgMoO 6-δ in H 2. Cell voltage (V) MgMoO 6-δ 850 O C 800 O C 750 O C H 2 /H 2 S Cell voltage (V) MgMoO 6-δ CH O C 700 O C Fig. S6C Power density and cell voltage at different temperatures for MgMoO 6-δ in H 2 /H 2 S. Fig. S6D Power density and cell voltage at different temperatures for MgMoO 6-δ in dry CH 4. 10

12 To test the performance stability of the MgMoO 6-δ anode in the presence of sulfur, we ran the cell in H 2 containing 50 ppm H 2 S with a fixed current density of 920 ma/cm 2. As shown in Fig. S7, the power density dropped from the initial 595 ma/cm 2 to 566 ma/cm 2 after hours, indicative of only 4.8% fade in the power density. 800 Voltage (V) E cell Time (hour) P η a η c 600 Fig. S7 The variation with time of the cell voltage, power density, cathode and anode overpotentials for MgMoO 6-δ in H 2 containing 50 ppm H 2 S at 800 C. Current density was fixed at 920 ma/cm 2. (2) Mg Mn MoO 6-δ The power density, cell voltage, anode and cathode overpotentials for Mg Mn MoO 6-δ in H 2 and H 2 /H 2 S are given in Fig. S8. With the doping of Mn(II) in MgMoO 6-δ, the power density decreases. The maximum output is about 700 mw/cm 2 at 800 C in H 2. In H 2 /H 2 S, it drops to 660 mw/cm 2 at 800 C, which is about a 5 % loss compared with that in pure H 2. The values of anode and cathode overpotentials are almost identical whether in H 2 or in H 2 /H 2 S. The presence of a small amount of sulfur has no obvious influence on the anode. 11

13 Cell voltage (V) Mg Mn MoO 6-δ H O C 750 O C 700 O C 650 O C Voltage (V) Mg Mn MoO 6-δ E cell η c η a OCV 800 O C, H Fig. S8A Power density and cell voltage at different temperatures for Mg Mn MoO 6-δ in H 2. Fig. S8B OCV, cell voltage, anode and cathode overpotentials for Mg Mn MoO 6-δ in H 2. Cell voltage (V) Mg Mn MoO 6-δ H 2 /H 2 S O C 750 O C 700 O C 650 O C 600 Fig. S8C Power density and cell voltage at different temperatures for Mg Mn MoO 6-δ in H 2 /H 2 S. Voltage (V) Mg Mn MoO 6-δ E cell η c η a OCV 800 O C, H 2 /H 2 S Fig. S8D OCV, cell voltage, anode and cathode overpotentials at 800 C for Mg Mn MoO 6-δ in H 2 /H 2 S. (3) Mg 0.5 Mn 0.5 MoO 6-δ The power density, cell voltage, anode and cathode overpotentials for Mg 0.5 Mn 0.5 MoO 6-δ in H 2 and H 2 /H 2 S are given in Fig. S9. With increasing Mn(II) doping in MgMoO 6-δ, the power density further decreases and the anode overpotential increases, especially in H 2 /H 2 S. The maximum output is about 651 mw/cm 2 at 800 C in H 2. In H 2 /H 2 S, it drops to 577 mw/cm 2 at 800 C, which is 12

14 about an 11 % loss compared with that in pure H 2. The anode overpotential in H 2 /H 2 S becomes larger than that in H 2. Cell voltage (V) Mg 0.5 Mn 0.5 MoO 6-δ O C 750 O C 700 O C 650 O C Fig. S9A Power density and cell voltage at different temperatures for Mg 0.5 Mn 0.5 MoO 6-δ in H 2. Voltage (V) Mg 0.5 Mn 0.5 MoO 6-δ E cell η c η a OCV 800 O C, H Fig. S9B OCV, cell voltage, anode and cathode overpotentials for Mg 0.5 Mn 0.5 MoO 6-δ in H 2. Voltage (V) Mg 0.5 Mn 0.5 MoO 6-δ E cell η c η a OCV 800 O C, H 2 /H 2 S Fig. S9C OCV, cell voltage, anode and cathode overpotentials for Mg 0.5 Mn 0.5 MoO 6-δ in H 2 /H 2 S. Voltage (V) Mg 0.5 Mn 0.5 MoO 6-δ E cell η c η a OCV 800 O C, CH Fig. S9D OCV, cell voltage, anode and cathode overpotentials for Mg 0.5 Mn 0.5 MoO 6-δ in dry CH 4. (4) MnMoO 6-δ The power density, cell voltage, anode and cathode overpotentials for MnMoO 6-δ in H 2 are shown in Fig. S10. The maximum output is about 660 mw/cm 2 at 800 C in H 2. In H 2 /H 2 S, it drops to 591 mw/cm 2 at 800 C, which is about a 10.5 % loss compared with that in pure H 2. The anode overpotential remains smaller than the cathode overpotential. 13

15 Cell voltage (V) MnMoO 6-δ H O C 750 O C 700 O C 650 O C Voltage (V) 800 O C, H 2 MnMoO 6-δ OCV E cell η c η a Fig. S10A Power density and cell voltage at different temperatures for MnMoO 6-δ in H 2. Fig. S10B OCV, cell voltage, anode and cathode overpotentials at 800 C for MnMoO 6-δ in H 2. (5) Mg 0.9 Cr 0.1 MoO 6-δ In H 2, the power density of Mg 0.9 Cr 0.1 MoO 6 is close to that of MgMoO 6-δ. The maximum power density reaches 790 mw/cm 2 at 800 C. But in H 2 /H 2 S, it drops to 610 mw/cm 2 at 800 C with about a 23 % loss. The doping of Cr 3+ reduces the tolerance to sulfur. As shown in Fig. S11, the anode overpotential becomes much larger than the cathode overpotential in H 2 /H 2 S. Short-term testing in H 2 with a fixed current density of 1360 ma/cm 2 shows a stable performance (Fig. S12). Voltage (V) Mg 0.9 Cr 0.1 MoO 6-δ E cell η c η a OCV 800 O C, H Voltage (V) Mg 0.9 Cr 0.1 MoO 6-δ E cell η c η a OCV 800 O C, H 2 /H 2 S Fig. S11A OCV, cell voltage, anode and cathode Fig. S11B OCV, cell voltage, anode and overpotentials at 800 C for Mg 0.9 Cr 0.1 MoO 6-δ in H 2. cathode overpotentials overpotentials at 800 C for Mg 0.9 Cr 0.1 MoO 6-δ in H 2 /H 2 S. 14

16 Cell voltage (V) Time (h) Fig. S12 The variation with time of the cell voltage and power density for Mg 0.9 Cr 0.1 MoO 6-δ in H 2 at 800 C. The current density was fixed at 1360 ma/cm Investigation of the effect of platinum and La-doped ceria In order to make clear the contribution from Pt paste and La-doped ceria (LDC) on the cell performance, we designed and tested several cells that are described below in detail. (1) Pt mesh-pt paste/ldc/lsgm/scf In order to check the effect of the LDC layer, we used LDC directly as the anode and Pt mesh and Pt paste as current collector. The power density and cell voltage at 800 C in different fuels are shown in Fig. S13. The values of P max are 528 and 486 mw/cm 2 in dry and wet H 2, respectively, much lower than those of cells with MMoO 6-δ as the anode. Furthermore, in methane, the output becomes even lower: 27 mw/cm 2 in dry CH 4 and 52 mw/cm 2 in wet CH 4. Ceria exhibits some catalytic effect on methane reforming with the existence of steam, but it works poorly. For direct oxidation of dry methane, the performance is even poorer. Our MgMoO 6-δ anode shows a P max of 438 mw/cm 2 in dry methane without any steam, which indicates that MgMoO 6-δ plays an important role for direct oxidation of CH 4. 15

17 Cell voltage (V) H 2 97% H 2-3% H 2 O CH 4 97% CH 4-3% H 2 O Fig. S13 Power density and cell voltage at 800 C for LDC/LSGM/SCF cell without doubleperovskite MMoO 6-δ in different fuels. (2) Pt mesh- MMoO 6-δ /LDC/LSGM/SCF In order to check the catalytic effect of Pt-paste, we buried a Pt-mesh current collector into the MMoO 6-δ (M = Mg, Mn) anode before annealing at 1100 C for 1 h. No Pt paste was used. The performance is displayed in Fig. S14. For MgMoO 6-δ, the P max values are 920 and 707 mw/cm 2 in H 2 at 850 and 800 C, respectively, about 10% lower than those of the cell with Pt paste. In dry methane, P max also reaches as high as 340 mw/cm 2 at 800 C. In wet methane, P max is 280 mw/cm 2 at 800 C. For MnMoO 6-δ, the P max values at 800 C are 520 and 175 mw/cm 2 in H 2 and CH 4, respectively, which are comparable to those of the cell with Pt paste. The conductivity of our materials is not poor. Pt paste can enhance the conductivity of the anode, however, the cells with SMMO as anodes still work well without Pt paste. Therefore, the effect of Pt paste on the cell performance in our case is much less than that previously reported (6). 16

18 Cell voltage (V) O C,H O C,H O C,H C,CH Cell voltage (V) 800 O C,H O C,H O C,H O C,CH Fig. S14A Power density and cell voltage for MgMoO 6-δ /LDC/LSGM/SCF cell with buried Pt mesh as current collector. Fig. S14B Power density and cell voltage for MnMoO 6-δ /LDC/LSGM/SCF cell with buried Pt mesh as current collector. (3) Au mesh-au paste / MMoO 6-δ /LDC/LSGM/SCF Figs. S15 and S16 show the power density for MMoO 6-δ /LDC/LSGM/SCF (M = Mg, Mn) cells with Au mesh and Au paste as current collector. P max in H 2 at 800 C is approximately mw/cm 2 for MgMoO 6-δ and 320 mw/cm 2 for MnMoO 6-δ, much lower than the corresponding P max with Pt as current collector. The decrease in P max is more remarkable for MgMoO 6-δ. As observed with an optical microscope, even with several separate dots of Au paste for contact, the Au melted and spread over the entire anode surface after annealing at 800 C, which considerably reduced the effective area of the working electrode. Since in our case the area of the working electrode is only 4 cm 2, which is much smaller than that commonly used (1 cm 2 ), the loss of effective area caused by the melted Au cannot be ignored. We can clearly see from Fig. S15B that P max gradually increases with cycle number, demonstrating that a long time is required for the fuel gas to penetrate through the melted area during the electrochemical process. However, after the 30 th cycle number, P max for MnMoO 6-δ almost reaches 600 mw/cm 2, which is comparable to that with Pt as the current collector. Au readily forms alloys with some metals, which on one hand decreases the melting point of Au and on the other hand destroys the structure of the anode material. In our case, it is easier for 17

19 Au to form an alloy with Mg than with Mn. For this reason, we believe, MgMoO 6-δ exhibits poorer performance compared to MnMoO 6-δ with Au as current collector. Moreover, P max for the MgMoO 6-δ anode with Au as current collector was less than 10 mw/cm 2 in CH 4, which means that the MgMoO 6-δ anode was poisoned by the formation of an Au-Mg alloy Cycle number P max (mw/cm 2 ) Cycle number Fig. S15A Power density in H 2 at 800 C for a MgMoO 6-δ /LDC/LSGM/SCF cell with Au mesh and Au paste as current collector. Fig. S15B P max in H 2 at 800 C vs cycle number for a MgMoO 6-δ /LDC/LSGM/SCF cell with Au mesh and Au paste as current collector O C 700 O C 750 O C 800 O C P max (mw/cm 2 ) Cycle number Fig. S16A Power density in H 2 for a MnMoO 6-δ /LDC/LSGM/SCF cell with Au mesh and Au paste as current collector at different temperatures. Fig. S16B The maximum power density in H 2 at 800 C vs cycle number for a MnMoO 6-δ /LDC/LSGM/SCF cell with Au mesh and Au paste as current collector. 18

20 (4) Au mesh- MgMoO 6-δ /LDC/LSGM/SCF We further tested MgMoO 6-δ /LDC/LSGM/SCF cell with Au mesh buried in the MgMoO 6- δ layer. The Au-mesh-buried MgMoO 6-δ layer was annealed at 1000 C in air for 1 h before cell fabrication. No Au paste was used so that we expected that the effect of melted Au could be suppressed. However, no improvement in performance was attained (see Fig. S17). As reported in our previous paper (3), a small amount of Pt sputtered on the anode surface greatly enhanced the power density of the cell with Pt current collector. We sputtered Pt with the same technique and condition as our previous paper (3) on the Au-mesh-buried MgMoO 6-δ surface, but we did not get any obvious improvement in performance in both H 2 and CH 4. This observation indicates that annealing at 1000 C still caused the Au mesh to form an Au-Mg alloy so that Pt made little enhancement in the cell performance. It can therefore be concluded that the good performance of our cells reported in this study is predominantly ascribed to the contribution from the double-perovskite anode materials. Cell voltage (V) Au mesh Au mesh + Pt sputter Fig. S17 Power density and cell voltage in H 2 at 800 C for MgMoO 6-δ /LDC/LSGM/SCF cell with buried Au mesh in anode as current collector. 6. Composition analysis of the cell after operation (1) Detection of sulfur and carbon after operating in H 2, H 2 /H 2 S and CH 4 19

21 The phase of the anode film was checked by XRD after testing in H 2, H 2 /H 2 S and CH 4 from 650 to 800 C for a total of 1 day; no phase change or impurity caused by sulfur and carbon was observed. Energy dispersive spectrometer (EDS) analysis of the anode surface also confirmed the stability of the SMMO phase and no sulfur species was detected. Fig. S18 shows the EDS result for the MgMoO 6- δ anode after operating in H 2, H 2 /H 2 S and CH 4. Only Sr, Mg, Mo and O were observed. The atom ratio of Sr, Mg and Mo obtained from EDS is 2.19:1:0.95, which is close to 2:1: Sr Counts (cps) Mg Mo O Sr Mo Mo Energy (kev) Sr Fig. S18 EDS for the MgMoO 6-δ anode taken from the surface after operating in dry H 2, H 2 /H 2 S and CH 4 at C for a total of 1 day. (2) Elemental analysis of cross-sectional layers of MgMoO 6-δ /LDC/LSGM/SCF cell We further examined with EDS the elemental distribution along the cross-sectional layers of the MgMoO 6-δ /LDC/LSGM/SCF cell taken after cycling in dry and wet H 2, H 2 /H 2 S and CH 4 at 800 C for a total of 10 days. Six typical areas with a grid size of 1 µm 1 µm were chosen for EDS analysis, which are sketched as labels of A F in Fig. S4A. A was located in the SMMO layer close to the anode surface, B in the SMMO layer close to LDC, C in the LDC layer close to SMMO, D in the LDC layer close to LSGM, E in the LSGM layer close to LDC, and F in the deep LSGM layer. EDS spectra for A F are shown in Fig. S19, and the atom percentages of all the detected elements are 20

22 listed in Table S3. The amount of O was much higher than that expected due to a large amount of O 2- that reacted in these areas. No carbon was detected. After the testing, we did not find any observable carbon deposited on the cell and in the testing setup. Only tiny amounts of S and Pt were found in area A that was close to the anode surface. We should emphasize that no Pt appeared anywhere in the LDC layer and no Pt was found in the deep SMMO layer. Therefore, it is impossible that Pt and LDC catalyze the oxidation of CH 4 together. On the other hand, it can be seen that both C and D areas contain some SMMO and LSGM, and E contains some LDC. This illustrates that SMMO and LSGM phases interdiffuse in the LDC layer. However, no LDC was observed in area B, indicating that the LDC layer protected well the SMMO layer. The main purpose of the LDC layer is to prevent interdiffusion of SMMO and LSGM. Sr O Mg Mo Sr A B Counts (a.u.) La Ga C D OGa Ce Sr La Ce La CeLa Ce Ga Ga Sr E Ga Mg Sr La La La Ga Ga Energy (kev) Sr F Fig. S19 EDS for the MgMoO 6-δ anode taken after cycling in dry and wet H 2, H 2 /H 2 S and CH 4 at 800 C for a total of 10 days. 21

23 Table S3. Elemental analysis by EDS for the MMoO 6-δ /LDC/LSGM/SCF cell with Pt mesh and Pt paste as current collector after operating in dry and wet H 2, H 2 /H 2 S and CH 4 at 800 C for a total of 10 days Atom % A B C D E F O Sr Mg / 1.89 Mo / / La / / Ce / / / Ga / / S 0.36 / / / / / Pt 9 / / / / / References 1. K.Q. Huang, J.B. Goodenough, J. Alloys Compds , 454 (0). 2. K.Q. Huang, J.H. Wan, J.B. Goodenough, J. Electrochem. Soc. 148, A788 (1). 3. J.H. Wan, J.Q. Yan, J.B. Goodenough, J. Electrochem. Soc. 152, A1511 (5). 4. M. Feng, J.B. Goodenough, K. Huang, C. Milliken, J. Power Sources 63, 47 (1996). 5. S.B. Adler, B.T. Henderson, M.A. Wilson, D.M. Taylor, R.E. Richards, Solid State Ionics 134, 35 (0). 6. S. McIntosh, J.M. Vohs, R.J. Gorte, Electrochem. Solid-State Lett. 6, A240 (3). 22