Demonstration of efficient electrochemical biogas reforming in a solid oxide electrolyser with titanate cathode

Abstract

Biogas reforming is a renewable and promising way to produce syngas. In this work, we demonstrate a novel strategy to directly and electrochemically convert CH₄–CO₂ into H₂–CO. Electrochemical reforming of dry CH₄–CO₂ (1 : 1) mixture is successfully achieved in a 10 μm-thick titanate cathode with oxygen byproduct generated in anode in an oxide-ion-conducting solid oxide electrolyser under external voltages. In addition, loading iron nanocatalyst in titanate cathode or/and increasing applied voltages has further improved CH₄–CO₂ conversion. The highest methane conversion of approximately 80% is demonstrated for direct electrochemical reforming in cathode in contrast to the low conversion under open circuit condition in oxide-ion-conducting solid oxide electrolyser cathode.

---

Introduction

Significant efforts are being devoted to hydrocarbon reforming for syngas production because of the natural abundance of CH₄–CO₂ and their high-value derivatives.^1–6 Currently, most biogas reforming is preferentially performed using heterogeneous catalysis on transition metal oxides loaded with metal nanocatalysts at intermediate to low temperatures.^7–9 Normally, this catalytic process requires the activation of methane or carbon dioxide through chemical adsorption on surface defect sites or the thermal splitting of methane on a metal catalyst. The stepwise decomposition of CH₄ into CH_x fragments on a metal surface can significantly benefit the catalysis conversion. On the other hand, the chemisorption and dissociation of CO₂ on a transition metal surface is also favorable for an efficient dry CO₂ reforming of CH₄. The recycling of CO₂ in this new way would potentially produce carbon-neutral renewables and a sustainable energy cycle.^10–14

Solid oxide electrolyser is a highly efficient energy conversion device to produce clean fuel via high-temperature electrolysis with favourable kinetics and thermodynamics.^15,16 Oxide-ion-conducting solid oxide electrolysers can directly electrolyze carbon dioxide into carbon monoxide and oxygen using external electricity.^17–20 In this process, CO₂ molecules in the cathode are firstly split into active CO˙ and O˙, which are then electrochemically transformed into CO and O²⁻, respectively. The oxide ion is then simultaneously transported through electrolyte to anode and forms oxygen. We have recently demonstrated the successful electrochemical conversion of CO₂–H₂O into syngas in a solid oxide electrolyser with a configuration (La_0.8Sr_0.2)_0.95MnO_3−δ/YSZ/La_0.2Sr_0.8TiO_3+δ.²¹ The hydrogen species produced from steam splitting can further react with CO₂ or CO to give rise to synthetic fuels in the presence of appropriate metal catalysts. Therefore, this feature of CO₂ electrolysis makes it possible to in situ utilise the O˙ at the three-phase boundary of the cathode to efficiently oxidize methane into syngas in the presence of appropriate metal catalysts before it is transformed to O²⁻ and transported to the anode under sufficient external loads. The thermal splitting of CH₄ into CH_x fragments on thin electrode surface also benefits the catalytic oxidation process.

The conventional Ni–YSZ electrode has exhibited excellent high-temperature electrolysis performance under a reducing atmosphere; however, Ni-cermets are not redox-stable and require a high concentration of reducing gas flowing over the Ni metal to avoid the oxidation of Ni to NiO.^22,23 We have also found that the mixture of CO₂–H₂O can quickly oxidize Ni metal to amorphous phase composed of NiO and Ni(OH)₂, which leads to rapid cathode performance degradations.²⁴ Furthermore, the catalytic activity of Ni metal towards CO₂ splitting is relatively high, and as a consequence carbon deposition most likely occurs and results in cell performance degradation. Some researchers have demonstrated that the carbon deposition is likely caused by reactions that occur over the catalyst and preferentially occur only when CO is present in the chemical reaction system.^25–28 In contrast to Ni–YSZ, perovskite-type La_xSr_1−xTiO_3+δ (LSTO) is an active and redox-stable material with high n-type conductivity upon reduction, which has attracted a great deal of attention in the field of electrode material for solid oxide electrolysers.^29–31 An n-type conductivity of ∼100 S cm⁻¹ has been demonstrated when LSTO is strongly reduced.³² The n-type conduction mechanism would easily fit the strong reducing condition at the cathode and provide excellent electrode performances.

The direct and efficient CH₄ oxidation is demonstrated in a solid oxide fuel cell with titanate anode.³³ Hirata et al. conducted electrochemical reforming of CH₄ with CO₂ in porous cells with Cu in the anode, and they achieved stable formation of H₂ and CO fuel.³⁴ R. J. Kee et al. reported the experimental and modeling investigations of thermal methane reforming chemistry within porous Ni–YSZ anode of solid oxide fuel cell.³⁵ The electrochemical reaction could directly promote the reforming or conversion of methane at the anode side, which requires an anode material with a good catalytic reforming and electrochemical reactivity.³⁶

In this study, we demonstrate in situ electrochemical biogas reforming in an oxide-ion-conducting solid oxide electrolyser with titanate cathode. The electrochemical catalysis process is studied under an applied voltage of 0 to 1.6 V. Active iron nanoparticles are loaded to titanate cathode to further improve biogas reforming performances.

Experimental

All the powders used in this current investigation were of analytical grade and purchased from SINOPHARM Chemical Reagent Co. Ltd (China), unless otherwise specified. The La_0.2Sr_0.8TiO_3+δ (LSTO) powders were synthesized by a combustion method.³⁷ The collected product was fired at 1300 °C with a heating and cooling rate of 5 °C min⁻¹ up to 6 h in air. The (La_0.8Sr_0.2)_0.95MnO_3+δ (LSM) powders were prepared by the solid-state reaction method, as described elsewhere,³⁸ with the final heat treatment temperature at 1100 °C (3 °C min⁻¹) for 3 h in air. A combustion method was employed for the Ce_0.8Sm_0.2O_2−δ (SDC) powders synthesis, using Sm₂O₃ and Ce(NO₃)₄·6H₂O, followed by a heat treatment at 800 °C (3 °C min⁻¹) for 3 h in air. The phase formations of the above prepared powders were analyzed and identified by X-ray diffraction (XRD, CuKα, 2θ = 3° min⁻¹, D/MAX2500V, Rigaku Cooperation, Japan) with 2θ ranging from 10° to 80°. High-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL Ltd, Japan) with selected area diffraction at 200 kV was performed to observe the oxidized and reduced LSTO powders. The valence states of the elements in the oxidized and reduced samples were determined using X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 250 with Al-Kα (1486.6 eV) radiation source. The binding energies were calibrated to the C_1s peak at 285 eV.

About 2.0 g of LSTO powders were pressed into bars at a pressure of 6 MPa, and then sintered at 1400 °C (3 °C min⁻¹) for 5 h in air to obtain samples for the conductivity tests. The bar with a relative density of approximately 85% was reduced at 1400 °C (3 °C min⁻¹) for 10 h in 5% H₂–Ar (99.99% in purity) at a flow rate of 32 mL min⁻¹. The conductivity was tested in 5% H₂–Ar using a DC four-terminal method from room temperature to 800 °C with the conductivity recorded at temperature steps of 0.5 °C using an online multi-meter (Keithley 2000, Keithley Instruments Inc., USA).³⁹ The pO₂ was simultaneously recorded using an online oxygen sensor (Type 1231, ZrO₂-based oxygen sensor, Noveltech, Australia) at 800 °C. The air flow was controlled at 0.5 mL min⁻¹ to change pO₂ at 800 °C. A group of 2 mm-thick YSZ supports were prepared by dry-pressing YSZ powders at a pressure of 8 MPa with a diameter of 20 mm and thickness of 3 mm, and then sintered at 1550 °C (3 °C min⁻¹) for 20 h in air. The prepared LSTO and SDC powders were mixed in alpha-terpineol at a 65 : 35 weight ratio with appropriate amounts of cellulose additive (approximately 10% weight ratio, in contrast to the ceramic powders) to form electrode slurry. The YSZ disc surfaces were then coated with the slurry in an area of 1.0 cm², and then treated at 1000 °C (3 °C min⁻¹) for 3 h in air to assemble symmetrical cells. The electrode based on LSTO–SDC loaded with 2 wt% iron oxide was prepared by infiltrating the required amount of nitrate, followed by heat treatment at 550 °C (5 °C min⁻¹) for 30 min in air. In order to determine the particle size, the impregnation solution was dried and then treated at different temperatures from 400 to 800 °C. Then, XRD was used to test the iron oxide powders, and the particle size was calculated according to the Scherrer equation.^40–43 The size of iron oxide in cathode was found to be around 25 nm; however, it grows to 55 nm at the operation temperature of 800 °C, as shown in Fig. S11.† The current collection layer was constructed by printing silver paste (SS-8060, Xinluyi, Shanghai, China) on both electrode surfaces. An external circuit was constructed using a silver wire (0.4 mm in diameter), and it was connected to current collectors using conductive adhesive (DAD87, Shanghai Research Institute for Synthetic Resins, Shanghai, China), followed by a heat treatment at 550 °C (3 °C min⁻¹) for 30 min in air. Single solid oxide electrolyser with LSM–SDC anode was prepared in the same way, as discussed above. The electrode microstructures were investigated by scanning electron microscopy (SEM, JSM-6490LV, JEOL Ltd, Japan) coupled with energy dispersive spectroscopy (EDS).

The symmetrical cell was tested under different hydrogen or methane partial pressure at 800 °C using an electrochemical station (IM6, Zahner, Germany) with a frequency range of 4 MHz–0.1 Hz in a two-electrode mode. The electrode polarization resistance was calculated by modeling the spectra using Zview software.³⁸ The hydrogen partial pressure and methane partial pressure were changed by adjusting the gas flow rates of H₂ (99.99%), CH₄ (99.99%) and Ar (99.999%) using a mass flow meter (D08-3F, Sevenstar, China) while the total flow rate was maintained at 20 mL min⁻¹. Then, a single solid oxide electrolyser with LSTO–SDC and iron-loaded LSTO–SDC cathodes was investigated for biogas reforming. The solid oxide electrolysers were sealed to a home-made testing jig by using ceramic paste (JD-767, Jiudian, Dongguan, China) for electrochemical measurements. The AC impedance spectroscopy and current-versus-voltage curve (I–V curve) of the electrolysers were recorded at steps of 0.004 V s⁻¹. The mixed gas of 20% CH₄/20% CO₂/60% Ar was supplied to cathode while the anode was exposed to static air. The gas flow rate was maintained at 20 mL min⁻¹ using mass flow meters. The output gas from cathode was analyzed using an online gas chromatograph (GC9790II, Fuli, Zhejiang, China) to detect the concentration of generated hydrogen and carbon monoxide.

Results and discussion

The XRD Rietveld refinement patterns of the LSTO and iron oxide-loaded LSTO are shown in Fig. 1(a1) and (b1), respectively. Note that Fig. 1(a2) and (b2) represent the reduced samples treated in 5% H₂–Ar for 3 h at 800 °C. The experimental and calculated results indicate that the phase structure of both oxidized and reduced LSTO samples are in Pm3̄m space group.⁴⁴ As shown in Fig. 1(a1) and (a2), the crystal cell volume is 59.794 ų for the oxidized LSTO, which is slightly smaller than that of the reduced sample (59.845 ų). This is probably due to the fact that titanium is predominant in Ti⁴⁺ form in the oxidized LSTO, whereas there is a part of Ti⁴⁺ (0.605 ų) reduced to Ti³⁺ (0.670 ų), which might give rise to cell volume expansion. HR-TEM images of the oxidized and reduced LSTO reveals lattice spacing of 0.274 nm (110) and 0.291 nm (110), are shown in Fig. 2(a) and (b). The increased lattice spacing of reduced LSTO firmly confirms lattice expansion for the reduced sample as determined by the XRD analysis. Fig. 1(b1) shows the XRD pattern of LSTO loaded with iron oxide, which confirms the presence of α-type iron oxide in cathode. Fig. 1(b2) reveals a sharp diffraction peak (2θ = ∼44.6°) of iron crystal plane (110) for the impregnated LSTO after reduction in a reducing atmosphere, which indicates that the iron oxide is transformed into metallic iron metal in 5% H₂–Ar at 800 °C.

Fig. 1 XRD Rietveld refinement patterns of the oxidized (a1) LSTO and (b1) iron-loaded LSTO; the reduced (a2) LSTO and (b2) iron-loaded LSTO.

Fig. 2 TEM graph of the (a) oxidized LSTO; (b) reduced LSTO.

XPS analysis was carried out to establish the chemical states of the oxidized and reduced samples. All spectroscopies were fitted with a Shirley-type background subtraction method.⁴⁵ The background-function for different spectroscopies of elements is fitted by 80% Gaussian and 20% Lorentzian peaks. As shown in Fig. 3(a), only Ti⁴⁺ is observed in the oxidized LSTO sample; however, part of the Ti⁴⁺ is chemically reduced to Ti³⁺ by treating the LSTO sample in a reducing atmosphere. The reduction of Ti⁴⁺ to Ti³⁺ offers free electrons as charge carriers, which is expected to significantly contribute to the electronic conductivity of the reduced ceramic. Similar changes in the chemical state of Ti were also observed in LSTO loaded with iron oxide before and after reduction, as shown in Fig. 3(b1) and (b2). The Fe2p core level XPS spectra are shown in Fig. 3(c1) and (c2). Moreover, as shown in Fig. 3(c1), the Fe³⁺ (2p1/2) and Fe³⁺ (2p3/2) peaks are observed at 724.31 and 710.90 eV,^46,47 respectively. Furthermore, as shown in Fig. 3(c2), the Fe³⁺ (2p3/2) peak is observed at 711.46 eV and Fe³⁺ (2p1/2) is at 724.31 eV, with Fe (2p3/2) occurring at 710.43 eV.^46,47 This further confirms that the iron metal appears after reduction. We believe that the iron metal nanocatalyst is expected to contribute to the improved electrode performance.

Fig. 3 XPS results for Ti (a1) in the oxidized LSTO, Ti (a2) in the reduced LSTO; Ti (b1) and Fe (c1) in the oxidized iron-loaded LSTO; Ti (b2) and Fe (c2) in the reduced iron-loaded LSTO.

Reduced LSTO samples are a typical n-type electronic conductor with electronic conductivity as high as 10–100 S cm⁻¹ in a reducing atmosphere at intermediate temperatures. Fig. S2(a) and (b)† show that the reduced sample displays typical metallic behaviours with negative temperature coefficients in a 5% H₂–Ar atmosphere. Fig. 4 shows the mixed conductivities of the porous LSTO electrode with/without impregnated iron oxide supported on YSZ disks in redox-cycling atmospheres. Two obvious steps are observed in each figure, which are identified as the exposure to different atmospheres. The conductivity pulse appears after flowing 5% H₂–Ar over the electrode to expose the porous electrode in a strong reducing atmosphere. The atmosphere with low oxygen partial pressure leads to the conductivity increase because of the transition of Ti⁴⁺ to Ti³⁺. It is observed that the conductivities of the porous LSTO electrode shift between approximately 0.70 to 1.10 S cm⁻¹ and 0.68 to 1.07 S cm⁻¹ in the redox cycling atmospheres at 800 °C, which further validates the superior redox stability of the LSTO electrode material. In comparison, a similar conductivity change has also been observed for the iron oxide loaded LSTO electrode between 0.58–1.54 S cm⁻¹ and 0.52–1.48 S cm⁻¹ in the redox cycling atmospheres. The loading of iron oxide into the porous LSTO electrode has clearly improved the conductivity, which is accordingly enhanced by 40% in a reducing atmosphere because of the presence of iron metal in the electrode. The iron nanoparticles are expected to strongly increase the catalytic activity in the electrochemical biogas reforming process. The different loading contents of iron catalysts in the electrode have been studied by several researchers.^48–50 They suggested that metal catalyst agglomeration occurs with higher loading contents (wt% > 4%) and it causes electrode performance degradation.^51,52 Normally, the loading content of 0.5–3.5 wt% offers optimum electrode performances.^53–55

Fig. 4 Mixed conductivities of the (a) LSTO electrode and (b) iron-loaded LSTO electrode in redox-cycling atmospheres.

Fig. 5 shows the AC impedance of the symmetric cells tested under a series of hydrogen partial pressure (10, 20, 40, 60, 80 and 100% H₂) at 800 °C. The series resistant of the cell (R_s) corresponds to the first intercept, and the difference between the two intercepts is a measure of the electrode polarization resistance (R_p). The Zview software is employed to calculate the R_s and R_p values, as reported in our previous work.⁵⁶ The ionic resistance of YSZ electrolyte mainly contributes to the R_s, which is generally stable in a wide range of hydrogen partial pressure. However, the R_p of the symmetric cell based on LSTO decreased from about 14.74 to 2.19 Ω cm² with a hydrogen partial pressure ranging from 10 to 100%, as shown in Fig. 5(a1) and (a2), suggesting that the stronger reducing atmosphere is beneficial to improve the electrode polarizations. In contrast, the R_p of the symmetric cell based on LSTO–SDC with iron nanoparticles significantly decreases along with the increasing hydrogen concentration from 4.41 to 1.47 Ω cm², which is probably due to the enhanced electro-catalytic activity under strong reducing atmosphere in the presence of iron nanoparticles. In this case, stronger reducing atmosphere favors the presence of metallic iron on LSTO electrode surface and it consequentially promotes the electro-catalytic activity. On the other hand, a low oxygen partial pressure is also favourable to increase the electrical conductivity of the reduced LSTO, which further improves electrode performances. Similar electrode polarizations have also been observed for the symmetric cells in different methane atmospheres. Fig. 6(b1) and (b2) show the AC impedance of symmetric cells tested at a series of methane partial pressures (5, 20 and 40% CH₄) at 800 °C. The R_p values decrease with increasing methane concentration; however, they are still larger than the polarization values in hydrogen, as shown in Fig. 6(a1) and (a2). This is because of the reducing atmosphere of methane, which is not comparable to hydrogen and is not sufficient to reduce the cathode. In contrast, the R_p based on LSTO with loaded iron nanoparticles significantly decreases along with increasing methane concentrations. Therefore, it can be concluded that the iron electro-catalyst significantly enhances the electro-catalytic activity of the composite electrode, and hence improves the electrode polarizations.

Fig. 5 AC impedance spectroscopy of YSZ-supported symmetrical cells based on (a1) and (a2) LSTO electrodes and (b1) and (b2) iron-loaded LSTO at different hydrogen partial pressure at 800 °C.

Fig. 6 AC impedance spectroscopy of YSZ-supported symmetrical cells based on (a1) and (a2) LSTO electrodes and (b1) and (b2) iron-loaded LSTO at different methane partial pressure at 800 °C.

Fig. 7 presents the microstructures of electrolyte-supported half cells with LSTO and iron-loaded LSTO electrodes. It can be observed that the LSTO and iron-loaded LSTO electrodes are porous and they adhere well to the dense YSZ electrolyte. The electrochemical biogas reforming is then investigated in two types of solid oxide electrolysers with LSTO and iron loaded-LSTO cathodes under a series of applied voltages, ranging from 0 to 1.6 V at 800 °C. Fig. 8(a) shows voltage versus current density curves (V–I curves) of the electrolysers for the electrochemical reforming of dry CH₄–CO₂ mixtures. The maximum current density reaches 145 mA cm⁻² at 1.6 V, based on LSTO cathode at 800 °C. In contrast, the cell based on iron-loaded LSTO cathode is greatly improved and the current density finally reaches 215 mA cm⁻² under the same conditions. As shown in Fig. 8(b), the current densities increase along with applied voltages, and they are quite stable at a fixed applied potential, indicating a stable electrochemical process. The current density with iron-loaded LSTO cathodes reaches 200 mA cm⁻² at 1.6 V, which is higher than 120 mA cm⁻² for the bare LSTO electrode. Both trends further confirm that the loading of iron catalyst can improve electrode performances and are therefore expected to increase syngas production via electrochemical biogas reforming. Electrocatalytic reforming of carbon dioxide by methane in SOFC system was studied by D. J. Moon et al., who found that the reaction of CO₂ and CH₄ under the closed-circuit conditions was more stable than that of the open-circuit.⁵⁷ In our work, quite stable current densities were also obtained to direct electrochemical reforming of methane with carbon dioxide in oxide-ion-conducting solid oxide electrolyser cathode.

Fig. 7 SEM picture of the (a) LSTO–SDC and (b) iron-loaded LSTO–SDC electrodes on the YSZ electrolyte.

Fig. 8 (a) The I–V curves of two electrolyzers at 800 °C in 20% CO₂/20% CH₄/60% Ar; (b) the performances of biogas dry reforming for electrolyzer at 800 °C in 20% CO₂/20% CH₄/60% Ar.

Fig. 9 shows the in situ AC impedance spectroscopy of cells with LSTO and iron-loaded LSTO cathodes in a series of applied voltages ranging from 0 to 1.6 V at 800 °C in 20% CH₄/20% CO₂/60% Ar. As shown in the figures, R_s values are stabilized at approximately 1.5 Ω cm², whereas R_p considerably decreases as the applied voltage increases from 0 to 1.6 V. Increasing the voltage is expected to activate the electrodes to the extent that R_p will remarkably decrease as well as improve the kinetic process of the electrode with accompanying improvement in electrode polarizations. The applied voltage gradually and electrochemically reduces the composite cathode, which improves the mixed conductivity, and hence enhances the electrocatalytic activity of the composite electrode. Two semicircles are observed in the impedance spectra: the high-frequency arcs (R₁) and low-frequency arcs (R₂). At high-frequency, R₁ of the solid oxide electrolysers with cathodes is stabilized approximately at 0.5 Ω cm², or it improves in a narrow range in a wide voltage range. At low frequency, the mass transfer, R₂, dominates the process, which is probably due to the gas conversion, dissociative adsorption and species transfer at TPB. The R₂ remarkably improves from 2.94 to 1.21 Ω cm² for the cell based on the LSTO cathode with an applied voltage ranging from 1.0 to 1.6 V, suggesting the improved kinetics of gas conversion at high voltages. In contrast, the R₂ is significantly reduced to 1.34 Ω cm² at low voltages and further enhanced to 0.75 Ω cm² at high voltages for the cell based on iron-loaded LSTO cathode. Here it may be assumed that the metallic nanoparticles significantly improve the electrocatalytic activity of the composite cathodes, and the passing current further activates the composite electrodes and improves the electrode polarization.

Fig. 9 The AC impendence of the electrolysis cells based on LSTO (a1 and a2) and iron-loaded LSTO (b1 and b2) with the flow of 20% CH₄/20% CO₂/60% Ar at 800 °C.

Fig. 10 shows the short-term performance of electrochemical biogas reforming in solid oxide electrolysers based on LSTO and iron-loaded LSTO cathodes. As shown in Fig. 10(a), the conversion of CO₂ and CH₄ are 7% and 22% for LSTO cathode under an open circuit at 800 °C. Moreover, the conversion of CO₂ and CH₄ reaches 10% and 25% for the cell based on the iron-loaded LSTO cathode without any applied voltage. The conversion difference between CO₂ and CH₄ is also observed for the repeated experiments in Fig. S8.† The reforming of CH₄ with CO₂ is a dominant reaction at 800 °C; however, the simultaneous occurrence of reverse water gas shift would change the carbon oxide conversion (CO₂ + H₂ ↔ CO + H₂O).^58,59 In the meantime, the thermal splitting of CH₄ probably occurs and produces C (CH₄ → C + 2H₂) while the Boudouard reaction probably takes places (2CO ↔ C(s) + CO₂).^60,61 This is expected to further change the conversion of methane and carbon dioxide. However, the Sabatier reaction may be negligible because of the high operation temperatures (CO₂ + 4H₂ ↔ CH₄ + 2H₂O; CO + 3H₂ ↔ CH₄ + H₂O).^62,63 It is observed that the conversion of CO₂ and CH₄ increases along with the applied voltages, indicating that high voltage remarkably facilitates the electrochemical process in cathodes. The conversion of CO₂ and CH₄ are 25% and 35% through the in situ electrochemical reforming in a 10 μm-thick LSTO cathode at 1.6 V at 800 °C, which is larger than the values without applied voltages, as shown in Fig. 10(a). In addition, the conversion is remarkably enhanced by loading iron catalysts in the LSTO cathode. As shown in Fig. 10(b), the remarkable conversion of CO₂ (32%) and CH₄ (78%) is demonstrated when improving the cathode activity by loading catalyst-active iron nanoparticles. The presence of iron catalyst is expected to thermally split CH₄ into CH_x fragments, and therefore favors the in situ biogas reforming in thin-layer cathode.

Fig. 10 The conversion of CO₂ and CH₄: (a) based on LSTO and (b2) based on iron-loaded LSTO in the flow of 20% CH₄/20% CO₂/60% Ar at 800 °C.

Fig. 11 shows the production of CO and H₂ from biogas reforming in the cell based with LSTO and iron-loaded LSTO cathodes under a series of applied voltages, ranging from 0 to 1.6 V at 800 °C. The iron nanoparticles can significantly improve the electrocatalytic activity. It is found that composite electrode loaded with iron catalyst demonstrates remarkably high performance for steam electrolysis or carbon dioxide electrolysis when voltage is applied.^50,64–66 On the other hand, applied voltages can electrochemically activate the electrode, as reported by several researchers.^67,68 For this reason, the iron-loaded LSTO demonstrates a superior behavior even under close circuit conditions, in contrast to the bare electrodes. At applied voltages, efficient dry reforming of CH₄–CO₂ mixtures is therefore achieved. Note that the yields of CO and H₂ accordingly increase when applied potential increases from 0 to 1.6 V. Higher applied potentials produce stronger reducing conditions in the cathode and further activate the electrode for electrochemical splitting of CO₂ to oxide radicals, which is beneficial to the in situ oxidation of hydrocarbons. As shown in Fig. 11(a), H₂ is produced at the rate of 3.17 × 10⁻³ mol s⁻¹ with bare LSTO at 1.6 V, whereas the H₂ yield reaches approximately 5.64 × 10⁻³ mol s⁻¹ based on the iron-loaded LSTO cathode. The CO is produced at a rate of 3.37 × 10⁻³ and 5.97 × 10⁻³ mol s⁻¹ at 1.6 V for the cells with LSTO and iron-loaded LSTO cathodes respectively, as shown in Fig. 11(b). Both trends further confirm that loading iron catalyst is able to significantly improve cathode performances, and hence the production of CO and H₂. However, it should be noted that only small amounts of steam are detected at low voltages and the steam is even absent at higher voltages. The steam is a product of biogas reforming, but it further splits to oxidize methane or to produce hydrogen in cathode.

Fig. 11 (a) The production of H₂; (b) the production of CO with LSTO and iron-loaded LSTO in the flow of 20% CH₄/20% CO₂/60% Ar at 800 °C.

Dry CO₂ reforming of methane is an extremely endothermic reaction [CH₄ + CO₂ → 2CO + 2H₂],^69,70 which shows that CO₂ reforming of methane produces a syngas with a H₂–CO ratio close to 1 : 1. Simultaneously, water gas shift reaction occurs [CO₂ + H₂ ↔ CO + H₂O],^58,59 as well as CH₄ decomposition [CH₄ → C(s) + 2H₂],⁶⁰ the Boudouard reaction [2CO → C(s) + CO₂]⁶¹ and the Sabatier reaction [CO₂ + 4H₂ ↔ CH₄ + 2H₂O, CO + 3H₂ ↔ CH₄ + H₂O] occurs.^62,63 All these side reactions sometimes influence the final conversion of methane and carbon dioxide. Electrochemical biogas reforming not only utilizes the above reactions, but also uses the applied voltages to achieve in situ electrochemical reforming in cathode. The carbon dioxide can be electrochemically split into oxide ion and carbon monoxide in cathode. Then, a part of the oxide ion can be in situ utilized to electrochemically oxide methane and generate syngas [CO₂ + 2e⁻ → CO + O²⁻, CH₄ + O²⁻ → CO + 2H₂ + 2e⁻]. This electrochemical process in the cathode is expected to significantly enhance the conversion of carbon dioxide and methane, and it therefore offers a novel and promising alternative for carbon dioxide reforming of methane.

Conclusions

In this work, direct electrochemical biogas reforming is demonstrated in an oxide-ion-conducting solid oxide electrolyser with titanate cathode. Efficient dry reforming of CH₄–CO₂ mixtures is successfully achieved in a thin-layer cathode under external electrical voltages. High conversion of CO₂ (32%) and CH₄ (78%) is obtained through in situ electrochemical reforming in the cathode. The presence of catalytic-active iron nanoparticles leads to enhanced CO₂–CH₄ conversion and CO–H₂ production from the in situ electrochemical biogas reforming. The present results offer an alternative way to efficiently convert hydrocarbons into syngas.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (NSFC), no. 21303037, the China Postdoctoral Science Foundation, no. 2013M53150, the Ministry of Education of Overseas Returnees Fund, no. 20131792, and the Fundamental Research Funds for the Central Universities, no. 2012HGZY0001.

Notes and references

1. X. S. Wu and S. Kawi, Energy Environ. Sci., 2010, 3, 334–342.
2. P. Tang, Q. J. Zhu, Z. X. Wu and D. Ma, Energy Environ. Sci., 2014, 7, 2580–2591.
3. T. Shishido, M. Sukenobu, H. Morioka, R. Furukawa, H. Shirahase and K. Takehira, Catal. Lett., 2001, 73, 21–26.
4. E. B. Pereira, P. Ramírez de la Piscina, S. Martí and N. Homs, Energy Environ. Sci., 2010, 3, 487–493.
5. E. P. Murray, T. Tsai and S. A. Barnett, Nature, 1999, 400, 649–651.
6. C. H. Yang, Z. B. Yang, C. Jin, G. L. Xiao, F. L. Chen and M. F. Han, Adv. Mater., 2012, 24, 1439–1443.
7. C. Jin, C. H. Yang, F. Zhao, A. Coffin and F. L. Chen, Electrochem. Commun., 2010, 12, 1450–1452.
8. K. Tomishige, O. Yamazaki, Y. G. Chen, K. Yokoyama, X. H. Li and K. Fujimoto, Catal. Today, 1998, 45, 35–39.
9. J. Wei and E. Iglesia, J. Catal., 2004, 224, 370–383.
10. M. C. J. Bradford and M. A. Vannice, Catal. Rev.: Sci. Eng., 1999, 41, 1–42.
11. G. Kim, G. Corre, J. T. S. Irvine, J. M. Vohs and R. J. Gorte, Electrochem. Solid-State Lett., 2008, 11, B16–B19.
12. J. Sfeir, P. A. Buffat, P. Möckli, N. Xanthopoulos, R. Vasquez, H. J. Mathieu, J. V. Herle and K. R. Thampi, J. Catal., 2001, 202, 229–244.
13. R. J. Gorte, H. Kim and J. M. Vohs, J. Power Sources, 2002, 106, 10–15.
14. N. Laosiripojana and S. Assabumrungrat, J. Power Sources, 2007, 163, 943–951.
15. G. Tsekouras, D. Neagu and J. T. S. Irvine, Energy Environ. Sci., 2013, 6, 256–266.
16. M. Ni, M. K. H. Leung and D. Y. C. Leung, Int. J. Hydrogen Energy, 2008, 33, 2337–2354.
17. K. Xie, Y. Q. Zhang, G. Y. Meng and J. T. S. Irvine, J. Mater. Chem., 2011, 21, 195–198.
18. W. T. Qi, Y. Gan, D. Yin, Z. Y. Li, G. J. Wu, K. Xie and Y. C. Wu, J. Mater. Chem. A, 2014, 2, 6904–6915.
19. K. R. Sridhar and B. T. Vaniman, Solid State Ionics, 1997, 93, 321–328.
20. Y. X. Li, Y. Gan, S. S. Li, Y. Wang, H. F. Xiang and K. Xie, Phys. Chem. Chem. Phys., 2012, 14, 15547–15553.
21. K. Xie, Y. Q. Zhang, G. Y. Meng and J. T. S. Irvine, Energy Environ. Sci., 2011, 4, 2218–2222.
22. D. Neagu and J. T. S. Irvine, Chem. Mater., 2010, 22, 5042–5053.
23. A. Ghosh, A. K. Azad and J. T. S. Irvine, ECS Trans., 2011, 35, 1337–1343.
24. G. J. Wu, K. Xie, Y. C. Wu, W. T. Yao and J. Zhou, J. Power Sources, 2013, 232, 187–192.
25. S. D. Ebbesen and M. Mogensen, J. Power Sources, 2009, 193, 349–358.
26. X. C. Lu and J.-H. Zhu, Solid State Ionics, 2007, 178, 1467–1475.
27. D. Neagu, G. Tsekouras, D. N. Miller, H. Ménard and J. T. S. Irvine, Nat. Chem., 2013, 5, 916–923.
28. X. D. Yang and J. T. S. Irvine, J. Mater. Chem., 2008, 18, 2349–2354.
29. A. D. Aljaberi and J. T. S. Irvine, J. Mater. Chem. A, 2013, 19, 5868–5874.
30. X. Li, H. L. Zhao, X. Zhou, N. S. Xu, X. Zhou, C. J. Zhang and N. Chen, Int. J. Hydrogen Energy, 2009, 34, 6407–6414.
31. D. N. Miller and J. T. S. Irvine, J. Power Sources, 2011, 196, 7323–7327.
32. Y. Gan, Q. Q. Qin, S. G. Chen, Y. Wang, D. H. Dong, K. Xie and Y. C. Wu, J. Power Sources, 2014, 245, 245–255.
33. J. C. Ruiz-Morales, J. Canales-Vázquez, C. Savaniu, D. MarreroLópez, W. Zhou and J. T. S. Irvine, Nature, 2006, 439, 568–571.
34. Y. Suga, R. Yoshinaga, N. Matsunaga, Y. Hirata and S. Sameshima, Ceram. Int., 2012, 38, 6713–6721.
35. E. S. Hecht, G. K. Gupta, H. Y. Zhu, A. M. Dean, R. J. Kee, L. Maier and O. Deutschmann, Appl. Catal., A, 2005, 295, 40–51.
36. S. Matayoshi, Y. Hirata, S. Sameshima, N. Matsunaga and Y. Terasawa, J. Ceram. Soc. Jpn., 2009, 117, 1147–1152.
37. S. S. Xu, D. H. Dong, Y. Wang, W. Doherty, K. Xie and Y. C. Wu, J. Power Sources, 2014, 246, 346–355.
38. S. S. Li, Y. X. Li, Y. Gan, K. Xie and G. Y. Meng, J. Power Sources, 2012, 218, 244–249.
39. M. Vracar, A. Kuzmin, R. Merkle, J. Purans, E. A. Kotomin, J. Maier and O. Mathon, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 6, 174107.
40. A. Monshi, M. R. Foroughi and M. R. Monshi, World J. Nano Sci. Eng., 2012, 2, 154–160.
41. C. Solliard, Surf. Sci., 1981, 106, 58–63.
42. J. Langford and A. Wilson, J. Appl. Crystallogr., 1978, 11, 102–103.
43. A. W. Burton, K. Ong, T. Rea and I. Y. Chan, Microporous Mesoporous Mater., 2009, 117, 75–90.
44. J. Canales-Vázquez, S. W. Tao and J. T. S. Irvine, Solid State Ionics, 2003, 159, 159–165.
45. Y. X. Li, Y. Wang, W. Doherty, K. Xie and Y. C. Wu, ACS Appl. Mater. Interfaces, 2013, 5, 8553–8562.
46. M. Vracar, A. Kuzmin, R. Merkle, J. Purans, E. A. Kotomin, J. Maier and O. Mathon, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 174107.
47. P. Jiang, L. Bi, X. Y. Sun, D. H. Kim, D. Jiang, G. H. Wu, G. F. Dionne and C. A. Ross, Inorg. Chem., 2012, 51, 13245–13253.
48. X. Y. Xu, C. R. Xia, G. L. Xiao and D. K. Peng, Solid State Ionics, 2005, 176, 1513–1520.
49. C. J. Fu, Q. L. Liu, S. H. Chan, X. M. Ge and G. Pasciak, Int. J. Hydrogen Energy, 2010, 35, 11200–11207.
50. C. Ruan, K. Xie, L. M. Yang, B. Ding and Y. C. Wu, Int. J. Hydrogen Energy, 2014, 39, 10338–10348.
51. Y. X. Li, G. J. Wu, C. Ruan, Q. Zhou, Y. Wang, W. Doherty, K. Xie and Y. C. Wu, J. Power Sources, 2014, 253, 349–359.
52. M. Lomberg, E. Ruiz-Trejo, G. Offer and N. P. Brandon, J. Electrochem. Soc., 2014, 161, F899–F905.
53. P. Kim-Lohsoontorn, Y.-M. Kim, N. Laosiripojana and J. Bae, Int. J. Hydrogen Energy, 2011, 36, 9420–9427.
54. Z. B. Liu, B. B. Liu, D. Ding, Z. Y. Jiang and C. R. Xia, Int. J. Hydrogen Energy, 2012, 37, 4401–4405.
55. C. D. Savaniu and J. T. S. Irvine, Solid State Ionics, 2011, 192, 491–493.
56. Y. X. Li, Y. Gan, Y. Wang, K. Xie and Y. C. Wu, Int. J. Hydrogen Energy, 2013, 38, 10196–10207.
57. D. J. Moon and J. W. Ryu, Catal. Today, 2003, 87, 255–264.
58. M. Wisniewski, A. Boréave and P. Gélin, Catal. Commun., 2005, 6, 596–600.
59. H. Timmermann, D. Fouquet, A. Weber, E. Ivers-Tiffée, U. Hennings and R. Reimert, Fuel Cells, 2006, 6, 307–313.
60. M. Ando, Y. Hirata, S. Sameshima and N. Matsunaga, J. Ceram. Soc. Jpn., 2011, 119, 794–800.
61. S. Assabumrungrat, N. Laosiripojana and P. Piroonlerkgul, J. Power Sources, 2006, 159, 1274–1282.
62. S. K. Hoekman, A. Broch, C. Robbins and R. Purcell, Int. J. Greenhouse Gas Control, 2010, 4, 44–50.
63. K. R. Thampi, J. Kiwi and M. Grätzel, Nature, 1987, 327, 506–508.
64. G. Kim, S. Lee, J. Y. Shin, G. Corre, J. T. S. Irvine, J. M. Vohs and R. J. Gorte, Electrochem. Solid-State Lett., 2009, 12, B48–B52.
65. H. Lv, H.-Y. Tu, B.-Y. Zhao, Y.-J. Wu and K.-A. Hu, Solid State Ionics, 2007, 177, 3467–3472.
66. B. Huang, S. R. Wang, R. Z. Liu and T. L. Wen, J. Power Sources, 2007, 167, 288–294.
67. K. Xie, Y. Q. Zhang, G. Y. Meng and J. T. S. Irvine, J. Mater. Chem., 2011, 21, 195–198.
68. G. Schiller, A. Ansar, M. Lang and O. Patz, J. Appl. Electrochem., 2009, 39, 293–301.
69. S. Wang and G. Q. (Max) Lu, Energy Fuels, 1996, 10, 896–904.
70. A. P. E. York, J. B. Claridge, A. J. Brungs, S. C. Tsang and M. L. H. Green, Chem. Commun., 1997, 39.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05587j

---