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Readily processed protonic ceramic fuel cells with high performance at low temperatures

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Science  18 Sep 2015:
Vol. 349, Issue 6254, pp. 1321-1326
DOI: 10.1126/science.aab3987

Cooler ceramic fuel cells

Ceramic ion conductors can be used as electrolytes in fuel cells using natural gas. One drawback of such solid-oxide fuel cells that conduct oxygen ions is their high operating temperatures (at least 600°C). Duan et al. have made a proton-conducting ceramic fuel cell with a modified cathode material that exhibits high performance on methane fuel at 500°C (see the Perspective by Gorte).

Science, this issue p. 1321; see also p. 1290

Abstract

Because of the generally lower activation energy associated with proton conduction in oxides compared to oxygen ion conduction, protonic ceramic fuel cells (PCFCs) should be able to operate at lower temperatures than solid oxide fuel cells (250° to 550°C versus ≥600°C) on hydrogen and hydrocarbon fuels if fabrication challenges and suitable cathodes can be developed. We fabricated the complete sandwich structure of PCFCs directly from raw precursor oxides with only one moderate-temperature processing step through the use of sintering agents such as copper oxide. We also developed a proton-, oxygen-ion–, and electron-hole–conducting PCFC-compatible cathode material, BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1), that greatly improved oxygen reduction reaction kinetics at intermediate to low temperatures. We demonstrated high performance from five different types of PCFC button cells without degradation after 1400 hours. Power densities as high as 455 milliwatts per square centimeter at 500°C on H2 and 142 milliwatts per square centimeter on CH4 were achieved, and operation was possible even at 350°C.

Among the various types of fuel cells, ceramic fuel cells possess several attractive advantages such as fuel flexibility (including the potential to directly use hydrocarbon fuels), high efficiency, and the absence of a requirement for precious-metal catalysts (13), but high operating temperatures [700° to 1000°C for conventional “first generation” yttria-stabilized zirconia (YSZ)–based solid oxide fuel cell (SOFCs)] result in high costs and materials compatibility challenges (4, 5). “Second-generation” SOFCs (5, 6), based on newer oxygen-ion–conducting electrolytes [such as samarium-doped ceria (SDC)], lowered operating temperatures to ~600°C (Fig. 1A). Nanostructured “third generation” SOFCs incorporating rare-earth elements such as Eu or Ru, and ultrathin multilayer electrolytes or core-shell nanofiber composite electrodes, have achieved exceptional performance at 450° to 600°C (7, 8), but performance drops rapidly with decreasing temperature because of the high activation energy (Ea) associated with oxygen-ion conduction (Fig. 1B).

Fig. 1 Comparison of PCFCs and SOFCs.

(A) Performance of current “first-generation” (YSZ-based) SOFCs, “second-generation” [SDC, GDC (gadolinium-doped ceria), and LSGM (strontium- and magnesium-doped lanthanum gallate)-based] SOFCs, and “first generation” PCFCs versus the SSRS-fabricated PCFC with triple-conducting oxide cathode reported here. The new SSRS-based PCFC shows excellent promise in the intermediate- and low-temperature regime (350° to 600°C). (B) Performance of recently reported nanostructured “third generation” SOFCs, which generally incorporate exotic elements such as Eu or Ru, ultrathin multilayer electrolytes, and/or highly nanostructured electrodes versus predicted performance of PCFCs based on the currently achievable area-specific resistance of a 10-μm-thick protonic ceramic electrolyte and assuming electrode resistances identical to those reported in this work. The predicted performance suggests that PCFCs can eventually deliver excellent performance in the IT range (250° to 550°C)—rivaling, if not surpassing, the best third-generation SOFCs. Moreover, the decreased activation energy of PCFC electrolytes compared to SOFC electrolytes suggests that PCFCs can be particularly attractive at lower temperatures. (References used for assembling the data points in Fig. 1, A and B, are provided in the supplementary materials.) (C) Schematic illustration of SOFC and PCFC operation. PCFCs can offer a number of other potential benefits compared to SOFCs, particularly when operating on hydrocarbon fuels. These advantages include higher CH4 conversion because of direct proton (hydrogen) removal from the anode and higher carbon coking resistance due to conditions disfavoring the Bouduard reaction (supplementary text and fig. S1).

Proton conduction in oxides generally has a lower Ea compared to oxygen-ion conduction, so protonic ceramic fuel cells (PCFCs) offer intriguing potential for high-performance (913), lower-temperature ceramic fuel cell operation. PCFCs also offer a number of other potential benefits compared to SOFCs, particularly when operating on hydrocarbon fuels. These advantages, illustrated in Fig. 1C, include higher CH4 conversion because of direct proton (hydrogen) removal from the anode and higher carbon coking resistance because of unfavorable Bouduard reaction. However, current PCFC performance lags far behind SOFC performance (Fig. 1A), although in the late 1990s, Kreuer et al. provided an important step toward enabling viable PCFCs with the demonstration of stable yttrium-doped barium zirconate (BZY) proton conductors with high (bulk) proton conductivity (14, 15). Despite this advance, the high grain boundary resistance and fabrication challenges associated with this refractory material system have, until now, constrained its application. Nevertheless, the intrinsic conductivities of currently available protonic ceramic electrolytes suggest that PCFCs can eventually deliver excellent performance between 250° and 550°C. The predicted PCFC performance values based on the limits of current PCFC electrolytes (Fig. 1B) are distributed between 0.2 and 1.6 W cm−2 at 350° to 600°C, based on a 10-μm-thick electrolyte (similar to current third-generation SOFCs) and assuming electrode resistances identical to those reported in this study. Moreover, if epitaxial or “bamboo-structured” PCFC electrolytes can be achieved, thereby mitigating the deleterious effect of blocking grain boundaries, PCFC power densities >2.0 W cm−2 could be reached.

Two major reasons why PCFCs have lagged their more mature SOFC counterparts are a lack of suitable cathodes expressly designed for PCFC operation and fabrication challenges stemming from the refractory nature of most PCFC electrolytes. Here, we introduce advances that address both of these issues, leading to good PCFC performance at temperatures between 350° and 500°C with power densities of 100 to 455 mW cm−2.

The poor performance of most PCFCs is attributed, in part, to their use of cathodes that were developed for SOFCs operating at much higher temperatures (700° to 1000°C) when target PCFC operation temperatures are near 500°C. We have developed a perovskite cathode composition, BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1), that is specifically designed for PCFCs (figs. S2 to S3) (see the supplementary materials). BCFZY0.1 is a Y-doped modification of BaCo0.4Fe0.4Zr0.2O3-δ (BCFZ), which we previously reported as a highly active and chemically compatible cathode material for PCFCs (16, 17). BCFZY0.1 is a transition-metal–doped derivative of the well-known proton-conducting oxide BaZrxY1-xO3-δ (BZY) (18). Although BZY is an excellent proton conductor and also exhibits some oxygen-ion conductivity in dry reducing atmospheres (19), its electronic conductivity is extremely small. By heavily doping the B site of BZY with transition-metal cations (Co and Fe), the electronic percolation threshold is exceeded, thus activating electronic conduction while maintaining ionic conductivity (figs. S4 to S6). The result is a “triple conducting” cathode material (20, 21) that exhibits simultaneous proton, oxygen-ion, and electron-hole conductivity (figs. S7 and S8). As illustrated in fig. S9, the application of conventional SOFC cathodes (which are based on either electron-conducting oxides or mixed oxygen-ion and electron-conducting oxides) to PCFC electrolytes restricts the cathode reaction only to points where the electrolyte and electrode phases meet. In contrast, the triple-conducting BCFZY0.1 cathode eliminates the triple-phase boundary constraints associated with traditional composite cathode architectures: The entire cathode becomes electrochemically active, which offers the chance to lower the viable operating window of PCFC devices to <400°C compared to >700°C today (table S1).

Fabrication complexity has also restrained the commercial development of PCFC technology. The basic structure of a PCFC consists of a fully dense proton-conducting ceramic electrolyte membrane sandwiched between a porous anode and a porous cathode. Traditionally (Fig. 2A), the high-quality componential powders (electrolyte, anode, and cathode) must be synthesized from expensive precursors (e.g., nitrates) by complicated wet-chemistry routes (or by time- and energy-consuming solid-state reaction procedures) followed by multiple drying, grinding, and high-temperature calcination (≥1000°C) steps. The anode support is then prepared and bisque fired, after which the electrolyte layer is deposited and the anode/electrolyte “half-cell” is cofired at temperatures higher than 1600°C to achieve acceptable electrolyte density. The high sintering temperature required to achieve densification of the protonic ceramic electrolyte generally also leads to undesirable coarsening of the anode structure. Finally, a porous cathode layer is deposited and the cell is fired a third time to complete the structure. This separate cathode deposition and firing step frequently leads to interfacial weakness between the cathode and the electrolyte and can constrain the choice of materials options.

Fig. 2 Schematic illustration of the fabrication and structure of PCFC button cells.

(A) traditional approach, (B) composite cathode SSRS approach, and (C) thin-film cathode SSRS approach.

Our PCFC fabrication method enables the full cell (i.e., porous anode, dense electrolyte, and porous cathode) to be created in a single reduced-temperature (1400°C) firing step directly from the raw precursor oxides (Fig. 2B). This approach leverages the recent development of solid-state reactive sintering (SSRS) (10), whereby carefully selected sintering aids can be used to assist the conversion of appropriately mixed raw precursor oxides and carbonates (e.g., BaCO3 + CeO2 + ZrO2 +… etc.) directly into the final phase-pure anode (fig. S10), electrolyte, and cathode perovskite compositions during the single firing step. By using different sintering aids for the electrolyte versus the cathode, the former can be rendered fully dense, whereas the latter can maintain a highly porous and active nanostructure under the same sintering conditions (fig. S11). To further improve cathode performance, a second, optional step (Fig. 2B) subsequently deposits a nanoscale cathode catalyst phase into the porous cathode bone with solution infiltration followed by calcination at moderate temperatures (500° to 900°C). Alternatively, as shown in Fig. 2C, an anode + electrolyte half-cell can be sintered directly from raw precursor oxides in a first, moderate-temperature sintering step (~1400°C) with the subsequent incorporation of a single-phase thin-film cathode via a second lower-temperature (~900°C) sintering step to ensure high cathode surface area and activity.

To illustrate the versatility of this new approach, we used the SSRS method to fabricate five different types of PCFC button cells (Table 1). The button cells feature three different well-known PCFC electrolytes—BaZr0.8Y0.2O3-δ (BZY20), BaCe0.6Zr0.3Y0.1O3-δ (BCZY63), and BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb)—in combination with two different sintering aids (CuO or NiO) and the triple-conducting (electron hole, oxygen ion, and proton) oxide BCFZY0.1 cathode. These varied cell compositions demonstrate the generality and reproducibility of our approach. BZY20, the prototypical PCFC electrolyte material, is notoriously difficult to sinter and densify. It has excellent stability, but high grain boundary resistance. BCZY63 provides improved sinterability and lower grain boundary resistance, but decreased stability compared to BZY20 (22, 23). BCZYYb (24) demonstrates one of the highest conductivities ever reported for a proton-conducting perovskite, but at the cost of further decreased stability, especially in H2O or CO2-containing environments (fig. S12). Nevertheless, successful fabrication of BCZYYb button cells by the single-step SSRS fabrication technique demonstrates that this approach is applicable even to compositionally complex perovskites (e.g., in this case, BCZYYb has five cations). (Experimental details on the preparation and testing of the five different PCFC button cells are provided in the supplementary materials.)

Table 1 Fabrication method, cell composition, and peak power density of cells 1 to 5.

View this table:

Figure 3 summarizes key results from testing of the five PCFC button cells. In Fig. 3A, the current-voltage (I-V) performance of all five cells is compared under H2/air operation at 500°C. The open-circuit voltage (OCV) values for all five cells are higher than 1.05 V, suggesting that both electronic and mechanical leakages are small. Previous detailed studies of SSRS-fabricated BCZYYb–1.0 weight % (wt %) NiO electrolytes in reducing environments have demonstrated that the electronic conductivity of these electrolyte materials remains extremely small (te < 0.01), despite the presence of the NiO sintering aid, over a wide temperature window (100° to 800°C) (25). The reduced sintering temperatures enabled by our SSRS fabrication process (≤1450°C) are sufficient to fully densify the thin electrolyte layers in these cells. Figure S13 shows that dense and defect-free BZY20 electrolytes around 30 μm in thickness can be successfully fabricated with the SSRS method. All cells shown here were fabricated with 20- to 30-μm-thick electrolytes and exhibited good reliability and reproducibility.

Fig. 3 Performance and microstructure of selected cells under H2/air operation.

(A) I-V and power density of cells 1 to 5 under H2/air at 500°C; (B) I-V and power density of cell 2 under H2/air at different temperatures; (C) terminal voltage and power density at a current density of 0.3 A cm−2 at 500°C for cell 2 under H2/air for over 1100 hours; and (D) a cross-sectional view of cell 2 after operation on H2 for over 1100 hours (inset figure is the high-magnification view of BCFZY0.1 cathode after 1100 hours operation).

Figure 3A reveals that cells 1 and 2, which are based on the BCZYYb electrolyte, yield the best performance, with peak power densities of 455 and 405 mW cm−2, respectively, at 500°C. Previous reported power densities for PCFCs at this temperature are typically 50 to 280 mW cm−2 (Fig. 1A). These two BCZYYb cells differ primarily in the route used to prepare their cathodes. The highest-performing cell (cell 1) was fabricated with the route shown in Fig. 2B, whereas the other BCZYYb cell (cell 2) was fabricated with the route shown in Fig. 2C. The route 2B fabrication process produces a composite two-phase cathode with a highly porous, proton-conducting BCZY63 cathode “backbone” decorated by a nanoparticulate BCFZY0.1 catalyst phase created via a secondary infiltration process [see representative scanning electron micrograph (SEM) images in fig. S14]. In contrast, the route 2C fabrication process uses a single-phase thin-film cathode composed entirely of the BCFZY0.1 catalyst phase without a secondary proton-conducting backbone phase (see representative SEM image for this cell in Fig. 3D; additional images are provided in fig. S15). The thin-film single-phase cathode performs nearly as well as the composite cathode, which substantiates the mixed proton and electronic conduction properties of the BCFZY0.1 cathode material. BCFZY0.1 alleviates the constraints associated with traditional triple-phase boundary composite cathode architectures and enables cells to be produced by the arguably simpler route 2C fabrication process without substantial loss in performance.

Because of the fabrication advantages afforded by the simpler single-phase thin-film cathode design, cells 3, 4, and 5 were also prepared by the route 2C fabrication process. Cells 3 and 4 incorporated a BZY20 electrolyte, whereas cell 5 incorporated a BCZY63 electrolyte. Cell 3 used 1.0 wt % NiO as a sintering aid, which was mixed with the electrolyte precursors to assist in the phase-formation and densification process, whereas cells 4 and 5 used 1.4 and 1.3 wt % CuO, respectively, as a sintering aid for the same purpose. We have previously shown (26) that both NiO and CuO are excellent sintering aids for BZY20 and BCZY63. The BZY20 and BCZY63 cells showed modestly decreased performance compared to the BCZYYb cells, which was expected given the lower conductivity of these electrolytes. Although the electrolyte thickness and overall microstructures of cells 3 to 5 are similar (see figs. S16 to S18), the cell prepared with NiO as a sintering aid (cell 3) showed somewhat better performance. The I-V curves in Fig. 3A show that cell 3, with 1.0 wt % NiO as a sintering aid, has a higher OCV, which we speculate arose from a lower electronic leak compared with cell 4, which used 1.4 wt % CuO as the sintering aid.

Figure 3B provides further details on the performance of cell 2 as an example. The I-V performance of cell 2 as a function of temperature (Fig. 3B) shows that viable power densities (~100 mW cm−2) can still be produced at temperatures as low as 350°C. Indeed, all five cells produced measurable power at 350°C (the I-V curves of the other four cells as a function of temperature under the same conditions are shown in figs. S16 to S19). Exemplary impedance spectroscopy plots of cells 1 and 3 are shown in fig. S20, while the electrolyte and electrode area-specific resistances extracted from these impedance measurements are provided in fig. S21.

Figure 3C demonstrates the stability of the operating voltage and power density during long-term testing of cell 2 under H2/air operation at a constant current density of 300 mA cm–2 at 500°C. Both cell voltage and power density actually increased slightly during the course of the 1100-hour test, which we attribute to the continued reduction of the anode during the first 600 hours of operation. The cell was still fully viable after 1100 hours, and its microstructure (Fig. 3D) was virtually identical to that of an untested cell. The cathode/electrolyte and anode/electrolyte interfaces showed no signs of delamination, and the well-connected interfacial character was preserved without any visible cracking or pore formation, suggesting good thermal expansion compatibility and stability of the electrodes with the electrolyte. Furthermore, the high-magnification image of the cathode in the inset of Fig. 3D shows that even after long-term testing, the cathode maintained its fine nanostructure.

We investigated whether direct methane operation of several SSRS-fabricated fuel cells could be maintained in the intermediate-temperature operating regime. As shown in Fig. 4A, a CuO-sintered BZY20-based cell operating on direct methane fuel attains a peak power density of 240 mW cm–2 at 600°C [versus, e.g., 24 mW cm−2 at 750°C (27) for previous direct-methane PCFCs]. Furthermore, the cell achieved stable operation even at 500°C. The cells also maintained excellent stability. The OCV, terminal voltage, and power density for methane-fueled BZY20 fuel cells operating at 550° and 500°C remained highly stable during 500 and 1400 hours testing periods, respectively (Fig. 4, B and C). In both cases, the cells were still fully viable when testing was halted. The microstructure of the BZY20 cell after 1400 hours operation on methane at 500°C (Fig. 4D) revealed no detectable changes in cell morphology, cracking, or delamination, and no evidence of carbon deposition (fig. S22). Long-term stability under OCV conditions at 600°C (>400 hours) on methane operation was also measured for a BZY20-based fuel cell sintered with CuO (fig. S23). Based on the higher performance of the NiO-sintered BZY20 cell, its performance on methane was also tested with a H2O/CH4 ratio of 2.5 without fuel dilution by an inert carrier gas. The cell attained peak power densities of 290, 215, and 142 mW cm−2 at 600°, 550°, and 500°C, respectively (Fig. 4E). Figure 4F confirms the stability of the cell over 200 hours of testing. These direct methane PCFC single cells achieve unprecedented performance compared with previous results reported in the literature (table S2). Although BCZYYb-based cells showed better performance on hydrogen, the instability under methane operation was observed (figs. S12 and S24).

Fig. 4 Performance and microstructure of selected cells under CH4/air operation.

(A) I-V and power density for cell 4 under 20 vol. % CH4 + 50 vol.% H2O + 30 vol. % Ar/air at 500°, 550°, and 600°C; (B) terminal voltage, OCV, and power density at a current density of 155 mA cm−2 at 550°C for cell 4 under 20 vol. % CH4 + 50 vol. % H2O + 30 vol. % Ar/air for over 500 hours; (C) terminal voltage, OCV, and power density at a current density of 80 mA cm-2 at 500°C for cell 4 under 20 vol. % CH4 + 50 vol. % H2O + 30 vol. % Ar/air for over 1400 hours; (D) cross-sectional view of cell 4 after operation under 20 vol. % CH4 + 50 vol. % H2O + 30 vol. % Ar/air for over 1400 hours; (E) I-V and power density of cell 3 under 28.6 vol. % CH4 + 71.4 vol. % H2O/air at 500°, 550°, and 600°C; (F) terminal voltage, OCV and power density of cell 3 at a current density of 150 mA cm−2 at 500°C under 28.6 vol. % CH4 + 71.4 vol. % H2O/air for over 200 hours.

By using a densification-aiding sintering additive in the electrolyte layer, a porosity-stabilizing additive in the cathode bone, and a pore-former in the anode, solid-state reactive sintering can be used to produce a complete PCFC single cell directly from raw binary oxides with just one or two combined phase-formation and sintering steps. Low-temperature PCFC performance is further enabled by a new, triple-conducting BCFZY0.1 cathode material. The SSRS-fabricated PCFCs attain high power densities at intermediate temperature (as high as 455 mW cm–2 at 500°C) with viable power density produced at temperatures as low as 350°C and long-term durability of >1000 hours without loss in performance. Furthermore, SSRS-fabricated PCFCs using BZY20 electrolyte demonstrate very good intermediate-temperature performance and stability under CH4/air testing for over 1400 hours, underscoring the promise of intermediate-temperature PCFCs for direct hydrocarbon operation. These results highlight the potential of the SSRS process to provide a commercially practical, simple, and low-cost approach to scalable solid-state ceramic devices.

Supplementary Materials

www.sciencemag.org/content/349/6254/1321/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S24

Tables S1 and S2

References for Fig. 1, A and B

References (2891)

References and Notes

  1. Acknowledgments: This work was supported by Advanced Research Projects Agency–Energy (ARPA-E) for funding under the REBELS program (award DE-AR0000493), the National Science Foundation Materials Research Science and Engineering Centers program under grant DMR-0820518, and the Petroleum Institute in Abu Dhabi, United Arab Emirates. This work is related to U.S. Patent application 62/101,285 (2015) and U.S. Patent application 14/621,091 (2015) filed by J. Tong et al. J.T. and R.O’H. developed the intellectual concept, designed all the experiments, and supervised this research. C.D. performed the fabrication and testing experiments of PCFC single cells. M.Sh. synthesized and tested the cathode materials. S.N. identified the copper oxide as an effective sintering aid for solid-state reactive sintering. M.Sa. measured proton concentration in cathode material. S.R. contributed to the preparation of the pastes for the electrolytes and cathode bones. A.A. participated in discussion and analysis of the methane-fueled cell testing. J.T., R.O’H., and C.D. analyzed all experimental data and wrote the paper.
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