Energy Storage in Ultrathin Solid Oxide Fuel Cells - Nano Letters

Letter pubs.acs.org/NanoLett

Energy Storage in Ultrathin Solid Oxide Fuel Cells Quentin Van Overmeere,*,† Kian Kerman, and Shriram Ramanathan Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, United States S Supporting Information *

ABSTRACT: The power output of hydrogen fuel cells quickly decreases to zero if the fuel supply is interrupted. We demonstrate thin film solid oxide fuel cells with nanostructured vanadium oxide anodes that generate power for significantly longer time than reference porous platinum anode thin film solid oxide fuel cells when the fuel supply is interrupted. The charge storage mechanism was investigated quantitatively with likely identified contributions from the oxidation of the vanadium oxide anode, its hydrogen storage properties, and different oxygen concentration at the electrodes. Fuel cells capable of storing charge even for short periods of time could contribute to ultraminiaturization of power sources for mobile energy. KEYWORDS: Vanadium oxide, thin film solid oxide fuel cells, energy storage, anode with 150 μm sidelength were fabricated using lithography, reactive ion etching and KOH anisotropic etching.12 The fuel cells were then fabricated by sequentially sputtering the electrolyte 80 to 95 nm thick from a 8 mol % yttria-stabilized zirconia (YSZ) target, dry-etching the silicon nitride from the bottom side, depositing the vanadium oxide (VOx) and/or porous Pt anode on the bottom side and sputtering the porous Pt cathode through a stainless steel shadow mask. RFsputtering of the oxide electrolyte and vanadium oxide anode were performed at 550 °C at pressures of 5 and 20 mTorr Ar and powers of 100 and 150 W, respectively. The composition of the vanadium oxide target was V2O5. DC-sputtering of Pt porous cathode and anode were performed without substrate heating at 75 and 85 mTorr, respectively. The fuel cells were tested in a custom-built environmental station.12 The anodeside was exposed to humidified 5% H2−Ar (specified purity of 99.999%) flowing at 30 mL min−1, while the cathode-side was exposed to ambient stationary air. As shown schematically in Figure 1a, fuel cells were fabricated with anodes consisting either of a porous Pt film, a VOx film, or a bilayer VOx/porous Pt film (further referred to as VOx/Pt). As we show later, the VOx/Pt anode has higher performance than the VOx anode due to lower anodic polarization losses. The cell potential difference was measured during galvanostatic fuel cell operation while switching the fuel supply off. The potential decreased to 0 V in approximately 15 s for the Pt anode cells at all current densities and temperatures. The potential decreased at a slower rate for the VOx and VOx/Pt anode cells, 0 V being reached in 32−210 s depending on the vanadium oxide film thickness and current density. The evolution of the potential difference after interrupting the fuel supply is shown in Figure 1b for a Pt, a VOx, and a VOx/Pt

n recent years, thin film solid oxide fuel cells (TF-SOFC) with electrolyte thicknesses below 1 μm have demonstrated solid oxide fuel cell operation in the 300−600 °C range.1−3 They have been suggested as a future power source for portable electronics owing to high energy density of hydrocarbon fuels.4 The ultralow scale and weight of TF-SOFC power sources may also be interesting for niche applications such as in miniature autonomous systems and military technologies that require operational characteristics for short time periods.5,6 For these systems, the fuel supply may be discontinuous unlike in stationary applications. Because fuel cells are not intrinsically capable of storing energy, when fuel supply is exhausted the energy output is interrupted. To store energy, the fuel cell could be coupled to an external charge storing device such as a battery but this increases the total weight and volume of the system. Storing charge in the fuel cell materials would therefore be a great advantage. To implement this strategy, fuel cell materials that reversibly transfer charges such as in batteries are needed. We fabricated TF-SOFC with vanadium oxide anodes or vanadium oxide/ porous Pt bilayer anodes. Vanadium is known for its propensity to change oxidation state, a property making vanadium oxide a candidate electrode material for different battery technologies.7,8 Moreover, hydrogen can be inserted in VO2 and V2O5 and nonstoichiometric V6O13 and V3O7, with H/V ratios in the range 0.3−1.9 depending on the oxide.9−11 Our vanadium oxide anode TF-SOFCs supply electricity 2−14 times longer than reference porous Pt anode fuel cells when the hydrogen fuel supply is turned off. During regular fuel cell operation at temperatures of 300−360 °C with humidified hydrogen, the open circuit voltages and maximum power densities of the vanadium oxide/porous Pt bilayer anode fuel cells are similar to that of the reference porous Pt anode fuel cells. TF-SOFC were produced starting from double-side polished (100) silicon wafers coated with 200 nm of low-stress silicon nitride. Free-standing silicon nitride square membranes arrays

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© XXXX American Chemical Society

Received: April 27, 2012 Revised: June 14, 2012

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Figure 1. (a) Cross-sectional diagram of the thin film fuel cells with the three different anodes studied. (b) Evolution of the potential difference during galvanostatic operation at 0.2 mA cm−2 at 300 °C when the hydrogen fuel supply is switched off. The fuel cells with vanadium oxide anodes (blue curve, 25 nm VOx; red curve, 25 nm VOx/porous Pt bilayer) continue producing electricity longer than fuel cells with porous Pt anodes (black curve).

anode fuel cell operated at 300 °C at a constant current density of 0.2 mA cm−2. Vanadium oxide anode TF-SOFC can thus generate electricity in the absence of fuel flow for short time periods. For the experiments in Figure 1b, the charge densities obtained after interrupting the fuel supply until 0 V is reached were 2.5, 26, and 20 mC cm−2, while the energy densities were 1.5, 8.2, and 8.1 mJ cm−2 for the Pt, VOx, and VOx/Pt anode cells, respectively. The charge and energy densities available without fuel supply were found to be dependent on the vanadium oxide film thickness, and the current density at which the cell was operated. These were relatively independent of the temperature within the investigated range from 220 to 360 °C. Lowering the temperature further limits the maximum current density at which the fuel cell can be operated because of the associated decrease of electrolyte conductivity and electrocatalytic activity at the electrode surface. Our results are summarized in Figure 2a−c. As previously mentioned, the potential difference decreased to 0 V in 15 s for the Pt anode cells, independent of the current density and the temperature. Thus, the charge density q = tj increased proportionally to the current density j. Accordingly, the data for the Pt cells in Figure 2b could be fit with q = (14 s)j. For the VOx/Pt anode cells, the charge density followed a linear relationship q = q0 + τj with q0 = 28 and 24 mC cm−2 and τ = 58 and 32 s for the 27 and 17 nm thick VOx/Pt anode respectively. The charge available thus increases with the vanadium oxide film thickness. The average ratio of charge densities for the 27 and 17 nm thick VOx anode at identical temperatures and current densities is equal to 1.5 ± 0.2, which matches with the thickness ratio ∼1.6. The progressive transition from a relatively constant charge density regime to a constant time regime with increasing current density indicates that at least two charge storage processes may occur in parallel, one of them being deactivated at a constant rate. The energy density delivered by the vanadium oxide anode fuel cells after shutting the fuel supply off ranged from 8 to 140 mJ cm−2, as shown in Figure 2c. The highest energy density measured was 140 mJ cm−2 for the 27 nm thick VOx/Pt anode cell at 360 °C at a current density of 7.8 mA cm−2. The energy density increased with the current density up to the current

Figure 2. Current density and temperature dependence of (a) time to 0 V, (b) charge density, and (c) energy available when hydrogen supply switched off. Temperatures are 300 (blue), 320 (green), 340 (orange), and 360 °C (red). Fuel cell anodes consisted of porous Pt (open triangles), 17 nm thick vanadium oxide/porous Pt bilayer (closed squares) and 27 nm thick vanadium oxide/porous Pt bilayer (open circles).

density at which the fuel cell operates at maximum power density. Three mechanisms for power generation in the absence of fuel were considered. The corresponding reactions at the anode have been illustrated in Figure 3. The first hypothesized mechanism is the oxidation of vanadium oxide when the fuel supply is interrupted. To assess this possible origin of the charge transfer in the absence of fuel, high-resolution XPS spectra of the vanadium oxide anode were acquired from three identical fuel cells operated in different conditions. Spectra were acquired with a PHI VersaProbe II instrument using a monochromatic Al K α source. The probe size was 50 μm with a power of 12.5 W. The pass energy was 11.75 eV. The spectra were charge-corrected for the main component of the O1s peak set at 530.0 eV.13 The XPS spectra of the as-deposited vanadium oxide anode is identical to the spectra of the anode after operation in hydrogen at 300 °C, including one discharge cycle without fuel B

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at least up to 400 °C but the stability of the other oxyhydrides has not been documented.10 The relatively small solubility of hydrogen in platinum would explain the small charge delivered when the fuel supply is switched off for the Pt anode fuel cells.17 The maximum charge density available in the most favorable case, assuming a hydride composition of H3.8V2O5 would however still be relatively small compared to our measured value. For the 27 nm film, the calculated charge density available by hydrogen storage is equal to 29 mC cm−2 (reaction (2) in Figure 3). A third mechanism that we therefore do not exclude in our case is that when the fuel supply is switched off, the vanadium oxide anode cells supply power due to the difference in oxygen partial pressure at the cathode and anode surfaces (reaction (3) in Figure 3). Note that the driving force for the first and third mechanisms is similar, being the difference in oxygen activity at the cathode and anode. For the first mechanism, however, vanadium cations are oxidized, while for the third mechanism it is oxygen anions that are oxidized. Open circuit potentials up to 400 mV and power densities of 40 nW cm−2 have been reported for concentration cells with nanoscale oxide electrolytes at room temperature.18 Considering the temperature dependence of the Nernst equation, we calculate that for concentration cells with the same activity ratio required at the electrodes to obtain 400 mV at 25 °C, the open circuit potential at 350 °C would be equal to 835 mV. This value is relatively close to the open-circuit potential values of our fuel cells with hydrogen supplied. However, the latter mechanism would be expected to apply to the porous Pt anodes as well and does not account for the increase of the charge density with the vanadium oxide film thickness. The precise origin of the observed charge density delivered in the absence of fuel therefore could arise from more than one mechanism. Regarding the aforementioned deactivation of the energy storage capacity of the vanadium oxide electrode, plausible pathways include oxidation of the vanadium oxide by water or oxygen present on the anode surface, and/or (chemical) hydrogen desorption from the vanadium oxyhydride for the first and second mechanisms, respectively. The performance of fuel cells with a 90 nm thick YSZ electrolyte and a porous Pt or a 28 nm thick VOx or a 28 nm thick VOx/Pt anode operating at 320 °C is compared in Figure 5a. The measured potential difference E is related to the current density j by E = EOC − ρj − η with EOC as the open-circuit potential difference, ρ is the area-specific resistance, and η is the polarization losses. Therefore, the shift in potential of the E versus j curves for the VOx and VOx/Pt anodes indicates that the anodic polarization losses are about 300 mV higher for vanadium oxide compared to platinum at 320 °C. The greater decrease of the potential difference with current density in the ohmic loss regime at intermediate current densities indicates that the conductivity of vanadium oxide may be a limiting factor for the performance. The in-plane conductivity of vanadium oxide films was measured from 25 to 550 °C. In reducing environments, the conductivity was in the range 150 to 300 S cm−1 when the temperature was higher than 68 °C, the metal− insulator transition temperature of VO2, as illustrated in Figure 6. These large electronic conductivity values may indicate that the ionic conductivity of vanadium oxide is a performance limiting factor compared to the Pt anode fuel cells. In Figure 5b,c, the best EOC and maximum power density (Pmax) are represented as a function of temperature for all the fuel cells tested in this work with batch-to-batch variations in electrolyte

Figure 3. Schematic illustration of the reactions at the fuel cell electrodes when the fuel supply is interrupted. Mechanisms (1), (2), and (3) refer to the three hypothesized mechanisms: vanadium oxidation, hydrogen desorption and oxidation, and oxygen concentration cell, respectively.

(Figure 4). Deconvolution of the V2p3/2 spectra using literature peak positions and full width at half-maximum

Figure 4. V2p and O1s XPS spectra of the vanadium oxide anode after deposition, after fuel cell operation in hydrogen at 300 °C, and after operation with the hydrogen supply off at open circuit potential.

values13−15 indicates that the vanadium oxide consists of a small fraction of V2O5 and majority mixed-phases (see Supporting Information for a detailed discussion of the spectroscopy data). Since vanadium oxide is known to be prone to surface oxidation of its outermost surface,13 the small V5+ contribution is possibly unrelated to fuel cell operation. After operation without fuel, the oxide is composed of V5+ and V4+ with 80 and 20 mol % ratios, respectively. Assuming to a first approximation that the charge provided in the absence of fuel results from the change of vanadium oxidation state from 4+ to 5+ (reaction (1) in Figure 3), the charge density in a 27 nm film would be equal to 15 mC cm−2. This is lower than the measured charge densities (see Figure 2b). Another plausible mechanism is that switching the fuel supply off triggers the progressive desorption of hydrogen from the vanadium oxide and its subsequent oxidation at the anode/ gas interface. Hydrogen storage in vanadium oxide has been reported with hydride compositions of HxV2O5 (3 < x < 3.8),16 HxV3O7 (x < 1.51) and HxV6O13 (x < 5.82)11 and HxVO2 (x < 0.37).9 Regarding high-temperature stability, HxV2O5 is stable C

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Figure 6. (a) DC conductivity of a 100 nm thick vanadium oxide film deposited on 100 nm thick polycrystalline yttria-stabilized zirconia. Temperature ramping rate of 10 °C min−1 and hold at 550 °C during 150 min. Electrical contacts consisted of two 200 nm thick dense platinum electrodes deposited 800 μm apart by sputtering through a shadow mask. Morphology of the VO2 film before (b) and after (c) the conductivity measurement. Scale bar applies to both (b) and (c).

either be due to the inhibition of the morphological evolution of the VOx film by the Pt film or to the porous Pt film still providing electronic conduction once the electronic conductivity of the VOx film decreases. These morphological changes and the associated performance degradation at temperatures higher than 340 °C are probably the reason why the use of vanadium oxide for solid oxide fuel cell anodes has only been reported so far for mixed oxides to the best of our knowledge.19 The energy storage capability of the VOx/Pt bilayer anode are potentially encouraging for their integration into high performance thin film solid oxide fuel cells operating in the sub-400 °C range. To summarize, we have fabricated vanadium oxide anode thin film solid oxide fuel cells, which continue to supply electricity for short periods of time when the hydrogen fuel supply is interrupted. A portion of the charge delivered was attributed to the reversible oxidation of the anode to V2O5 and hydrogen storage in the electrode. Vanadium oxide anode fuel cells had performances similar to reference platinum anode fuel cells, with open circuit potential differences of 1.0 V and a maximum power density of 6.6 mW cm−2 at 360 °C. Our results show that materials that can be reversibly reduced and oxidized or store hydrogen in fuel cell operating conditions allow storage of energy that can be delivered when the fuel supply is depleted. This strategy may allow combining characteristics of fuel cells and batteries into single devices for unique applications.

Figure 5. Performance of the porous Pt, vanadium oxide, and vanadium oxide/porous Pt bilayer anode thin film solid oxide fuel cells with humidified 5% H2/Ar fuel. (a) Current density dependence of the potential difference and power density at 320 °C for three fuel cells with identical electrolytes and cathodes, deposited during the same batch. (b) Best open-circuit potential difference and (c) maximum power densities among all fuel cells used in this study with electrolytes and cathodes deposited in different batches.

and anode thicknesses. The VOx anode fuel cells have lower EOC and Pmax because of the higher polarization loss of vanadium oxide for hydrogen oxidation compared to platinum. However, VOx/Pt bilayer anode fuel cells have EOC and Pmax very similar to Pt anode fuel cells. The EOC are identical, while the Pmax of the Pt anode cells are higher by a factor of 1.5, due to the low ionic conductivity of vanadium oxide. For temperatures smaller than 340 °C, the Pmax of the VOx anode fuel cells are proportional to that of the Pt and VOx/Pt anode cells. At temperatures higher than 340 °C, Pmax decreases when the temperature increases. This was found to be a consequence of morphological changes in the vanadium oxide film with a simultaneous degradation of the electronic conductivity, as illustrated on Figure 6. These morphological changes are attributed to the low melting temperature of V2O5, present in the anode during operation without fuel as indicated by the XPS analysis. The melting temperature of V2O5 is 690 °C, the mobility of vanadium and/or oxygen at 340 °C may be high enough for grain growth or diffusion to occur. The morphological changes of the vanadium oxide film resulted in irreversible degradation of the performance of the VOx anode fuel cells. The performance of the VOx/Pt cells does not seem to be affected by temperatures higher than 340 °C. This could

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on the XPS sample characterization. This material is available free of charge via the Internet at http:// pubs.acs.org. D

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 617 495 4469. Address: 29 Oxford Street, Pierce Hall, Cambridge, MA 02138. Notes

The authors declare no competing financial interest. † On leave from the Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium.

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ACKNOWLEDGMENTS The work was funded in part by NSF Grant CCF-0926148. Q.V.O. acknowledges financial support from the F.R.S.-FNRS through a postdoctoral scholarship. K.K. was supported by the Department of Defense through the National Defense Science and Engineering Graduate Fellowship (NDSEG) Program. This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS) a member of the National Nanotechnology Infrastructure Network (NSF Award No. ECS-0335765) and at the MRSEC Shared Experimental Facilities at MIT (NSF Award No. DMR-08-19762).

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