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. 2020 Mar 17;12(14):16209–16215. doi: 10.1021/acsami.9b20918

Generating C4 Alkenes in Solid Oxide Fuel Cells via Cofeeding H2 and n-Butane Using a Selective Anode Electrocatalyst

Xiaoyu Yan †, Ying Yang †,§, Yimin Zeng ⊥, Babak Shalchi Amirkhiz ⊥, Jing-Li Luo ∥, Ning Yan †,‡,∥,*
PMCID: PMC7146754  PMID: 32180390

Abstract

graphic file with name am9b20918_0002.jpg

Solid oxide fuel cells (SOFCs) offer opportunities for the application as both power sources and chemical reactors. Yet, it remains a grand challenge to simultaneously achieve high efficiency of transforming higher hydrocarbons to value-added products and of generating electricity. To address it, we here present an ingenious approach of nanoengineering the triple-phase boundary of an SOFC anode, featuring abundant Co7W6@WOx core–shell nanoparticles dispersed on the surface of black La0.4Sr0.6TiO3. We also developed a cofeeding strategy, which is centered on concurrently feeding the SOFC anode with H2 and chemical feedstock. Such combined optimizations enable effective (electro)catalytic dehydrogenation of n-butane to butenes and 1,3-butadiene. The C4 alkene yield is higher than 50% while the peak power density of the SOFC reached 212 mW/cm2 at 650 °C. In addition, coke formation is largely suppressed and little CO/CO2 is produced in this process. While this work shows new possibility of chemical–electricity coupling in SOFCs, it might also open bona fide avenues toward the electrocatalytic synthesis of chemicals at higher temperatures.

Keywords: core−shell catalyst, butane dehydrogenation, SOFC, triple-phase boundary, value-added chemical

1. Introduction

Catalysis and catalytic processes not only have shaped the modern chemical industry but will also significantly contribute to the transformation toward a sustainable society. Solid oxide fuel cells (SOFCs) are such an example: with the assistance of active materials at both electrodes, the electrocatalytic reaction inside offers direct and efficient chemical-to-electricity conversion, featuring high fuel flexibility and power density.1−8 It is estimated that the overall efficiency of SOFC-based power generation can be as twice as that of solely based on the combustion engine.9 When fueled with the readily available light hydrocarbon fuels (e.g., natural gas), the anode catalyst must be capable of catalyzing their complete oxidation into CO2 and H2O, ensuring high efficiency and low emission.2−4

In fact, an SOFC is more than a simple power source: the high operation temperature together with the closed electrode chamber with catalyst makes SOFC an ideal chemical reactor. Various reaction parameters, such as atmosphere, conversion, and selectivity, can be easily tuned by adjusting the potential bias. This renders an attractive advantage compared with the conventional reactor. Over the past decades, great efforts have been devoted to the use of SOFCs in the cogeneration of electrical power and value-added chemicals.10−13 In 1980, Farr and Vayenas first realized this concept by producing NO from NH3 feedstock in an SOFC using a noble-metal catalyst.10 Among other pioneering research studies, Jiang et al. showed the partial oxidation of methane in the Ag-containing solid electrolyte support.13 This industrially important reaction gave 88% selectivity to ethylene when methane conversion is 97% and later inspired the electrocatalytic C–C coupling reaction in SOFCs.14 As we and others have shown recently, SOFCs could also be employed in the methane dry reforming reaction, converting CH4/CO2 into synthesis gas while generating a substantial amount of electricity efficiently.15,16 Recently, solid-state chemicals were also derived from this type of reaction: the carbon nanotubes were made from plastic waste, which further expand the application of the SOFC reactor.17 Despite these advances, the produced chemicals now are often restrained in the C1 and C2 compounds since high-C compounds easily induce coking at SOFC operating conditions.18

Another challenge is to simultaneously achieve high efficiency of transforming feedstock to value-added products and of generating electricity. When the cogeneration mode is applied, the SOFCs often show low electrical efficiency compared with the conventional counterpart.10,14,17 Besides, external heat must be provided to these SOFCs in many cases since the heat released by the reaction is insufficient.15 Intuitively, we are wondering that can we create the “best of both worlds” scenario by simultaneously achieving good power output and high yield of high-C chemicals?

To achieve this, we herein demonstrate a simple approach of cofeeding conventional fuels and chemical feedstock to SOFC. With the nanoengineered WOx and anode triple-phase boundary (TPB), the oxygen ions coming from the cathode selectively oxidize butane to C4 alkenes without performing deep oxidation. The obtained butenes are of high industrial value as the industrial feedstock of making 1,3-butadiene, the monomer of producing synthetic rubber. Besides, this partial oxidation, combined with H2 oxidation from the cofeed, ensures efficient generation of electricity in the SOFC. This novel yet facile approach may encourage scientists to revisit the possibility of chemical–electricity coupling in SOFCs and open bona fide avenues toward the electrocatalytic synthesis of chemicals at higher temperatures.

2. Results and Discussion

Figure 1a shows the schematic of the cofeeding approach. A mixture of H2 (fuel) and n-butane (feedstock) is fed into the anode chamber of the SOFC reactor simultaneously. This combination well addresses the coking and low-conversion issues when pure chemical feedstock is used in the conventional approach. The H2 fuel guarantees the efficient power generation of SOFCs. Besides, such a mixture effectively facilitates butene desorption and suppresses the carbon deposition, as proven in heterogeneous butane dehydrogenation.19−21 After the catalytic reaction, all C4 products (including the unconverted butane) can be easily separated by liquefaction through cooling or pressurization. Other residuals, containing H2 and C1–C3 compounds, can be recycled and used as the fuel in SOFCs.

Figure 1.

Figure 1

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Schematic of cogenerating C4 alkenes and electricity via the cofeeding of fuel and feedstock strategy. The magnified cartoon shows the hierarchical structure of the anode TPB comprising YSZ, black LST, and Co7W6@WOx core–shell nanoparticles.

To enable this concept, we have nanoengineered the anode TPB with a hierarchical structure (see Figure 1). The yttrium-stabilized ZrO2 (YSZ) scaffold was infiltrated with a layer of black La0.4Sr0.6TiO3 (BLST), which is rich in surface disorder and oxygen vacancies (vide infra). This percolated thin interlayer was catalytically inert yet highly conductive, offering pathways for electrons and oxygen ions. On the surface of BLST, core–shell nanoparticles, comprising a WOx shell and Co7W6 core (Co7W6@WOx), were uniformly deposited to catalyze the dehydrogenation reaction while facilitating coke removal (vide infra). The former was also widely used in the dehydrogenation reaction including that of ethane and propane.22−26

The X-ray diffraction (XRD) patterns of the prepared catalyst is shown in Figure 2a, which reveals the structure evolution from Co7W6@W to Co7W6@WOx upon the sequential heat treatment in the controlled atmospheres. The schematic of this process was illustrated in Figure S1 in the Supporting Information. After infiltrating stoichiometric amounts of W and Co, the consequential calcination at 800 °C transformed them into CoWO4. We then reduced it in hydrogen at 900 °C to yield W-Co7W6, which was confirmed by the XRD pattern in Figure 2a. This also converted LST into BLST. The abundant surface disorder causes the discoloration of LST to become black, as we and others have shown in the previous studies.27−29 Such temperature was sufficiently high to fully reduce CoWO4 as shown by the H2-TPR (temperature-programmed reduction) profile in Figure S2. Then, we carried out the thermogravimetric analysis coupled with differential scanning calorimetry (TGA–DSC) in air. The obtained curve in Figure S3 indicates that W started to oxidize at ca. 300 °C while Co7W6 remained robust till 800 °C, in accordance with the XRD pattern of Co7W6@WOx after calcining the catalyst at 370 °C. While those peaks of metallic W were gone, the XRD peaks of WOx became visible. The formation of WOx was evidenced by the H2-TPR profile in Figure S2. It should be noted that a higher oxidation temperature will cause the degradation of the Co7W6 core.

Figure 2.

Figure 2

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(a) XRD patterns of Co7W6@W and Co7W6@WOx on BLST-YSZ; high-resolution XPS spectra of (b) W 4f, (c) Co 2p, and (d) O 1s core levels.

The generation of WOx was also confirmed by X-ray photoelectron spectroscopy (XPS). Figure 2b is the W 4f core-level spectrum, showing the characteristic doublets due to the spectrum overlap of W 4f5/2 and W 4f7/2. In the deconvolution, the binding energies of W 4f7/2 at 34.05, 34.75, and 35.72 eV were assigned to W4+, W5+, and W6+, respectively.30 The presence of these W moieties were also verified in the H2-TPR in Figure S2. Co moieties were still detectable on the surface, but the concentration was low. This rendered a W/Co molar ratio close to 10 (vide infra). Both the metallic and oxide signals were detected. However, we argued that the oxides were presumably formed once the sample was exposed to air, which was commonly observed in the literature for Co nanoparticles.31,32 The intensity of the O 1s core-level spectrum was rather strong. The peak at 530.12 eV was assigned to the adsorbed oxygen moieties including H2O and CO2. The stronger peak at 531.38 eV corresponded to lattice oxides, implying that the surface might be predominantly covered by an oxide layer.

We further investigated the nanostructure of the TPB using the microscopy analysis. The catalysts were exposed to 90% H2 + 10% n-butane at 650 °C for 6 h. Figure 3a demonstrates the energy-dispersive X-ray spectroscopy (EDX) elemental mapping of Co7W6@WOx on the surface of BLST-YSZ from the transmission electron microscope (TEM) measurement. The core–shell particles were uniformly distributed on the support, sizing of ∼50 nm. The larger red spots were caused by the particles that are adjacently deposited. Figure 3b shows the high-angle annular dark-field (HAADF) image of a typical singular WOx@Co7W6 particle in which a shell structure with a lighter color is observed. This shell was not resulted from the carbon deposition as few carbon signals were recorded in the EDX. Instead, we can notice that while Co and W are both evenly dispersed in the core part, it seems that W expands to larger areas than Co, implying the presence of a W- and O-rich shell. This is in good agreement with the electron energy loss spectroscopy (EELS) examination of the particle composition. Spot ① labeled in the shell region in Figure 3b is rich in oxygen (cf. EDX mapping). Conversely, no oxygen signal was detected in spot ②. Both EELS and EDX show that the W/Co atomic ratio in the shell is ca. 12 (this was 0.90 for the core), close to that recorded by XPS. This suggests that the WOx shell may contain the Co dopant, which is able to facilitate the electrocatalytic oxidation of hydrogen (vide infra).

Figure 3.

Figure 3

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(a) TEM-EDX elemental mapping of Co7W6@WOx on BLST-YSZ, (b) HAADF image and the corresponding EDX elemental mapping of a Co7W6@WOx nanoparticle; Zr (green), W (red), Co (yellow), O (blue), and C (orange) are displayed. (c) EELS spectra comparison of the two spots selected in the shell and core part in b. (d) HRTEM micrograph and FFT pattern of Co7W6@WOx. The catalysts were exposed to 90% H2 + 10% n-butane at 650 °C for 6 h.

We then obtained a high-resolution TEM (HRTEM) image of the particle, shown in Figure 3d. On the surface of the BLST support, the lattice disorder of BLST is clearly visible (see the white arrows), confirming the formation of the surface disordered LST after the hydrogenation treatment. The deposited nanoparticle has a core–shell structure with a well-crystallized core. The d-spacing of the core is 2.36 Å, matching the (110) orientation of Co7W6.33 This phase is also proven by the fast Fourier transform (FFT) pattern in the inset. On the contrary, the shell is fully amorphous with an average thickness of 2.5 nm. This value is very consistent among of all the particles, thanks to their simple and reproducible formation mechanism. The overall reaction is shown in eq 1 below (see the detailed synthesis steps in the Supporting Information). Note that the molar ratio of W/Co7W6 is always fixed to maintain the mass balance; this corroborates the uniform thickness of WOx shells after oxidation.

2. 1

As the TPB, this core-shell structure is of advantage compared with simply depositing WOx particles on BLST. Since WOx is semi-conductive,34,35 the established electron transfer pathway via the WOx thin shell and the metal-like Co7W6 can minimize the ohmic loss. Besides, the shell also provides a pathway for oxygen ions as well as the active sites for butane dehydrogenation and hydrogen oxidation.

The SOFC button cell was configured as the electrolyte-supported, comprising a 70 μm-thick dense YSZ electrolyte and two porous YSZ layers at the opposite sides as the electrodes. The detailed fabrication procedures are shown in the Supporting Information. The scanning electron microscope (SEM) images of the button cell cross section in Figures S4 and S5 exhibit the anode microstructure. BLST covered all the YSZ surface uniformly, yet the pores for mass transfer were sustained without being blocked.

Figure 4a compared the polarization and power density (PD) curves of the button cell at 650 °C when fueled with pure H2 and 80% H2 + 10% N2 + 10% C4H10. Note that the inert N2 was used as the internal standard in calculating C4 conversion/selectivity. In H2, the peak PD (PPD) reached 245 mW/cm2, comparable with the state-of-the-art YSZ-based SOFC with similar electrolyte thickness. When butane was cofed into the anode chamber, the peak PD dropped slightly to 212 mW/cm2. The polarization profile indicated that such decrease was mainly attributed to the larger overpotential at high current density. This suggests that in addition to the N2-diluting effect, the mass transfer of C4 molecules might increase the concentration polarization. We also performed electrochemical impedance spectroscopy (EIS, see Figure 4b). The charge transfer resistance of the H2-powered SOFC was ca. 0.6 Ω cm2, reflecting the excellent activity in the electrochemical oxidation of H2. When the fuel was switched to the mixture, the spectrum at the high-frequency side stayed essentially the same. In contrast, the low-frequency side showed the increase of charge transfer resistance. Besides, an upgoing tail started to appear at 0.2 Hz, implying that the diffusion effect became prominent in the cofeed SOFC. The performance of the SOFC at different temperatures and feeds was also evaluated (see Figure 4c), and a complete comparison can be found in the Supporting Information. The PPD increased nearly exponentially from 36 mW/cm2 at 550 °C to 456 mW/cm2 at 700 °C in hydrogen. This trend also applied in the cofeed mode. Nonetheless, we did not fuel the SOFC with butane at 700 °C, which was known to cause severe coking and butane cracking.

Figure 4.

Figure 4

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(a) Voltage–current and power density profiles and (b) EIS spectra of the SOFC; (c) PPD comparison of the SOFC at various temperatures; (d) performance of the SOFC in butane dehydrogenation at various temperatures (the cells are biased at 0.5 V in the data recording, and the concentration of all species was the average of three GC measurements with an error limit of ±3%); (e) stability test for the generation of C4 alkenes and electrical power. SOFCs are fed by either H2 or 80% H2 + 10% N2 + 10% C4H10, and the temperature is 650 °C other than specified.

In the determination of butane dehydrogenation performance, the conversion data was recorded when cells were biased at 0.5 V. The conversion of n-butane at 550 °C was up to 55.4% with a high selectivity of 93.7% to C4 alkene products (including butenes and 1,3-butadiene). The selectivity to C1–C3 hydrocarbon products was as low as 4.5%. At higher temperatures, although butane conversion rose, the selectivity of C4 alkenes decreased whereas that of C1–C3 products increased dramatically. Yet, a high yield of C4 alkenes (>50%) was maintained from 550 to 650 °C, which was comparable or even better than that achieved in the conventional flow reactor. Importantly, trace amounts of CO and absolutely no CO2 were detected at all the examined temperatures. This also applied in the control experiment without the presence of hydrogen feed when the SOFC was fueled by 10% C4H10 + 90% N2 at 650 °C (see Tables S1 and S2). Hence, we concluded that the H2 electrocatalytic oxidation was extremely selective, outperforming the oxidative dehydrogenation (ODH) using molecular oxygen and the conventional reactor.36,37 This also suppressed the oxidative butane cracking, rendering a relatively low C1–C3 selectivity. Besides, this process in fuel cells precluded the safety concerns of mixing O2 and butane at high temperatures.38,39 The formation of trace amounts of CO might be ascribed to the reforming reaction since the electrocatalytic oxidation of H2 produced water vapor.

Based on the aforementioned materials characterization results and the recent understandings of the WOx-based dehydrogenation catalyst, we hypothesized and depicted the butane dehydrogenation mechanism in Figure 5. Note that the infiltrated Pt indeed boosted the SOFC performance, yet the promotion effect was not prominent regarding butane dehydrogenation (see Table S2). We thus conclude that the tungsten phase contributes more to the butane activation. The oxidative dehydrogenation might follow the similar pathway as the well-known Mars van Krevelen mechanism.40,41 Butane initially underwent the chemisorption primarily on the catalyst surface. Sequentially, the WOx redox couple in the amorphous shell (cf. XPS spectrum showing W6+, W5+, and W4+) oxidized the readily formed H intermediate followed by the release of electrons passing via the highly conductive Co7W6 core to BLST. In the meantime, since defect-rich WOx was also ionically conductive,42 the high-valence W was regenerated by accepting the oxygen ions via BLST to close the loop. The other redox couple, CoOx, with much lower surface content, in the amorphous shell might also help via the same mechanism. Besides, together with Pt, CoOx might contribute more to the electrooxidation of the H2 fuel, guaranteeing high power density of the SOFC.

Figure 5.

Figure 5

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Schematics showing the possible electrocatalytic butane dehydrogenation pathways at the TPB of Co7W6@WOx–BLST.

In addition to the excellent SOFC performance and efficient butane dehydrogenation, the cofeed mode and the new catalyst also enabled high stability during the longevity test at 650 °C. After the initial PD drop at 0.6 V, which might be attributable to the electrode structural reconstruction, both C4 yield and SOFC performance were stabilized, showing little degradation after the 7 h test. The 72 h cyclic stability test in Figure S6 also demonstrates little degradation. In fact, coke formation is a common challenge in the heterogeneous dehydrogenation of butane. Our catalyst can form hydrogen tungsten bronze intermediate during the reaction (vide supra), a proven powerful decoking agent.43 Thus, carbon deposition was largely suppressed, rendering the robust SOFC anode under operation.

3. Conclusions

In this work, we showed that efficient dehydrogenation of n-butane in SOFCs can be simultaneously achieved with the excellent fuel cell performance. Through applying the cofeeding strategy and designing the hierarchical structure of the anode TPB containing Co7W6@WOx nanoparticles, the yield of C4 alkenes was above 51% while the SOFC power density reached 131 mW/cm2 in the longevity test. Such performance was significantly better than the conventional ODH of butanes. In the future, we anticipate that the use of more conductive or thinner electrolytes might further boost the SOFC performance. This balanced performance of SOFCs during electricity–chemical generation via cofeeding together with the TPB design might encourage scientists in the exploration of alternative uses of electrochemical cells.

4. Experimental Section

The detailed fabrication procedure, particularly the sequence of infiltration and heat treatment, of the SOFC button cell can be found in the Supporting Information. Briefly, the configuration of the cell comprised a dense YSZ electrolyte layer (∼70 μm-thick) with two porous YSZ scaffolds (∼45 μm-thick) at the opposite sides. LST and La0.6Sr0.4Fe0O3 (LSF) were the anode and cathode backbone materials, respectively. Stoichiometric amounts of cobalt nitrate and ammonium paratungstate were infiltrated to the anode, calcined, and reduced in H2 at 900 °C for 4 h to obtain Co7W6@W. Besides, ∼0.5 wt % Pt was infiltrated into both electrodes to enhance the electrocatalytic performance, particularly the oxygen activation, below 650 °C.

The gaseous products were periodically sampled and analyzed with a gas chromatograph (GC, Agilent 6890) equipped with both FID and TCD detectors. Conversion of n-butane and selectivity for the dehydrogenation products including butenes and 1,3-butadiene were calculated on the basis of carbon balance. The yield of all products was calculated by multiplying conversion of n-butane and selectivity of the specific product. The electrochemical analyses of the cells were performed using both the Gamry Reference 600 potentiostat and the Solartron SI 1287 electrochemical interface equipped with an SI 1260 impedance/gain-phase analyzer.

Acknowledgments

We thank Prof. Raymond Gorte from University of Pennsylvania, USA, for providing the YSZ green tape and the funding through Jiangsu Provincial Department of Science and Technology (BK20190216), and N.Y. also thanks the financial support from the Netherlands Organization for Scientific Research (NWO) NWO-GDST Advanced Materials program (no. 729.001.022).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.9b20918.

  • Experimental details, additional tables, TPR, TGA, and SEM data (PDF)

Author Contributions

# X.Y. and Y.Y. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

am9b20918_si_001.pdf (719.4KB, pdf)

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