Tao
Li
a,
Xuekun
Lu
b,
Mohamad F.
Rabuni
ac,
Bo
Wang
a,
Nicholas M.
Farandos
d,
Geoff H.
Kelsall
d,
Dan J. L.
Brett
b,
Paul R.
Shearing
b,
Mengzheng
Ouyang
e,
Nigel P.
Brandon
e and
Kang
Li
*a
aBarrer Center, Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK. E-mail: kang.li@imperial.ac.uk
bElectrochemical Innovation Lab, Department of Chemical Engineering, University College London, London WC1E 7JE, UK
cDepartment of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
dDepartment of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
eDepartment of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK
First published on 19th May 2020
Utilization or emission of low calorific value gases (LCVGs) containing <20% CH4 constitute economic and environmental challenges. Ceramic fuel cells offer a possible solution, but their performance is hindered by carbon formation (‘coking’). Herein, we report a novel fuel cell designed to mitigate coking, yielding superior performances but using conventional commercially-available materials. The new micro-monolithic design has an extraordinary geometrical asymmetry that separates the mechanical support and anode current collector from the electrochemically active region and results in significantly facilitated mass transport, yielding power densities of 1.77–2.22 W cm−2 using LCVGs. In addition, the effluent containing only H2, CO and CO2 is of great industrial interest for methanol synthesis, if their ratios are adjusted appropriately. The new fuel cell developed was almost free from coke deactivation and was stable for over 500 h, indicating great promise for both efficient and environmentally benign use of LCVGs.
Broader contextDespite the greater role of methane in the global energy blueprint, many methane sources, both natural and artificial, have been untapped due to their low methane concentrations (typically <20%) and correspondingly low heating values. Generally known as low calorific value gases (LCVGs), they are not useful unless the CH4 concentration can be increased, which requires a significant cost even with state-of-the-art technologies. Therefore, efficient utilization of LCVGs constitute significant economic and environmental challenges. In this study, we demonstrated that a ceramic fuel cell with commercially available materials could yield promising performances (>2 W cm−2) even with 10–20% methane in the fuel. The new ‘micro-monolithic’ conceptual design has displayed not only the advantages in minimizing concentration polarization, but also a unique mechanics in suppressing coking with >500 h stability test conducted. Therefore, with the depletion of existing fossil fuels and the urgency to mitigate climate change, this work demonstrates a promising technology for both efficient and environmentally benign use of LCVGs. |
Despite the great importance of methane in the global energy blueprint, many methane sources have been untapped due to their comparatively low methane concentrations, and correspondingly low heating values, so are known as low calorific value gases (LCVGs). There is no rigid LCVG definition as it was associated initially with the products of coal gasification, with heating values below 7 MJ m−3.3,4 There are other sources of LCVGs, e.g. a significant number of natural gas reservoirs with CO2 contents up to 87% across the world that remain unexplored, despite trillions of cubic metres of recoverable reserves, due to economic concerns.5–8 Such gas is not useful unless the CH4 concentration can be increased, which typically requires a significant cost, even with state-of-the-art technologies. Another important source of LCVGs is from the upgrading procedures necessary to obtain marketable bio-methane from biogas, produced from domestic and agriculture wastes. Though an important source of renewable energy, they could cause tremendous amount of off-gas emissions of 10–15% methane, the balance being CO2.9,10 Methane in these LCVGs is uneconomic to use, and would contribute to climate change, since methane is one of the main greenhouse gases with a global warming potential (GWP) 25–34 times higher than that of CO2.11,12 Presently, these LCVGs commonly are treated by flaring and in some cases, it is necessary to have further addition of fuel to sustain the combustion, which has economic and environmental implications, i.e. greater CO2 emissions. Therefore, with the depletion of existing fossil fuels and the urgency to mitigate climate change, a technology that can achieve clean and efficient use of these low utility LCVGs is in great need.
As examples of a well-studied technology, ceramic fuel cells are promising candidates to utilize LCVGs via internal dry reforming of methane (DRM) reaction (1),12–14 as the high operating temperatures favour high equilibrium conversion (endothermic reaction, ΔH298K = 247 kJ mol−1). DRM reaction converts the two greenhouse gases into a mixture of H2 and CO (syngas), which is the fundamental feedstock for valuable chemicals via processes such as the Fischer–Tropsch synthesis,15–18 making the whole concept promising for CO2 sequestration to mitigate global warming.
CH4(g) + CO2(g) → 2CO(g) + 2H2(g) | (1) |
One major challenge for hydrocarbon-fuelled ceramic fuel cell is carbon formation or coking, the mechanism of which depends on both thermodynamic and kinetic considerations. The carbon formed mainly via reactions (2)–(4), not only result in the de-activation of the anode by occupying active sites, but also lead to physical blockage of percolated pore structures, impeding gas transport:
Gas phase pyrolysis: CH4(g) + C(s) → 2H2(g) | (2) |
Boudouard reaction: 2CO(g) → + CO2(g) + C(s) | (3) |
Carbon gasification: CO(g) + H2(g) → H2O(g) + C(s) | (4) |
Despite the system complexity due to the multiple reactions involved, the thermochemistry of the gas phase equilibria, depicted by a C–H–O ternary diagram, is well established and accepted as a useful indication of the thermodynamic driving force for coking.12,19 As for coking mitigation, one major strategy is to replace conventional Ni-cermet with alternative materials that do not catalyze carbon formation, e.g. Cu-based electrode and some perovskites.20 The other main approach involves operating Ni-based electrode under well-controlled conditions by addition of reforming agents or even oxygen into hydrocarbon fuels to adjust the C:H:O ratio and so suppress coking.21 Despite much research focused on methane conversion using solid oxide fuel cells,12 state-of-the-art ceramic fuel cells cannot be used for efficient LCVG recovery, because of their cell designs, operating costs, lack of coking resistance and electrochemical performance.
Hence, a new strategy was developed to achieve high utilization efficiency of LCVG methane, with improved coking resistance and long-term stability. Fabricated from commercially available materials, the ceramic fuel cell had an evolved ‘micro-monolithic’ geometry with droplet-shaped channels. These exhibited a new gas transport configuration than conventional designs by offering a unique feature of geometrical ‘asymmetry’, i.e. the electrochemically active region (EAR) was deliberately separated spatially from the electronically conducting mechanical support, dramatically shortening the diffusional length for gaseous fuels to reach the EAR and enabling enhanced control over location where carbon deposition is favourable. This design helps to deliver high electrochemical performance operating with methane concentrations as low as 10%, by minimizing concentration polarization and creating an area-specific atmosphere that suppressed coking in the EAR.
X-ray computed tomography (X-ray CT) was used to reconstruct the three-dimensional structure, the results of which were used for CFD modelling to confirm the atmosphere variation across the cell. It was established that coking occurs only on the inert current collector/mechanical support at the fuel inlet (upstream) region, which had negligible influence on the structural integrity and electrochemical performances. Even with a 1:1 CH4–CO2 feed ratio, predicted thermodynamically to favour coking at <800 °C, the fuel cell operated with negligible performance degradation for >500 h.
After the preparation of the anode substrate, both yttria-stabilized zirconia (YSZ) electrolyte and gadolinium-doped ceria (GDC) were deposited by dip-coating (Fig. 1e), each having a thickness of 4.5 (±0.5) μm; the YSZ electrolyte was highly dense after the co-sintering at 1400 °C (Fig. S3, ESI†). The porous GDC-LSCF cathode was dip-coated to complete a single cell and had a thickness of 25–30 μm after sintering at 1000 °C for 2 hours. Both electrodes were wrapped with silver wires (0.2 mm diameter) as current collector.
Maximum power densities of 0.93, 1.75 and 2.27 W cm−2 were measured at 600, 650 and 700 °C, respectively. To the best of our knowledge, 2.27 W cm−2 at 700 °C using hydrogen fuel is the highest power density reported in the literature for any SOFC design, as shown in Table 1. In addition, the anode's geometrical asymmetry and deliberate separation of mechanical support from the EAR demonstrated effectiveness in facilitating gas transport, as illustrated by electrochemical impedance spectroscopy (EIS) in Fig. 3b. The Nyquist curves were fitted using a classical equivalent circuit LRs(R1||CPE1)(R2||CPE2), where L stands for an inductor, R for a resistor and CPE for constant phase element. The fitted impedance spectra are shown in Fig. S4 (ESI†) and the fitting results are summarized in Table S2 (ESI†). The low-frequency (1–100 Hz) arc of the impedance spectrum, implied an area-specific diffusion resistance of approximately 0.16 Ω cm2, which is ca. 1/3 the values for other micro-monolithic counterparts reported previously,26 suggesting the diffusional resistance inside the porous anode had been decreased significantly by this design's tailored morphology.
Type of fuel | Temperature/°C | Max. power density/W cm−2 | Ref. |
---|---|---|---|
Hydrogen | 700 | 2.27 | This work |
850 | 1.70 | 29 | |
800 | 0.98 | 25 | |
900 | 1.32 | 30 | |
600 | 0.93 | This work | |
600 | 0.62 | 31 | |
600 | 0.39 | 32 | |
600 | 0.431 | 33 | |
600 | 0.65 | 21 | |
600 | 0.8 | 34 | |
LCVG | 750 | 2.2 | This work |
LCVG | 750 | 0.14 | 28 |
Biogas | 800 | 0.19 | 35 |
Biogas | 875 | 0.07 | 36 |
Biogas | 800 | 0.2 | 37 |
Propane | 850 | 1.3 | 29 |
Due to the extraordinary geometrical asymmetry involved in the complex micro-monolithic electrode design, integrated computed fluid dynamics (I-CFD) modelling was undertaken on the cell's real 3D structure to further illustrate the influence of the design on facilitating gas transport. As shown in Fig. 2, the 3D reconstruction of the electrode obtained from a custom-developed multi-length scale X-ray CT technique, which includes the micro-CT for the extraction of macro-structural features, as well as the nano-CT for determining micro-structural parameters.25,27 Integrated computed fluid dynamics (I-CFD), which was developed based on such multi-length scale X-CT technique, has been demonstrated to be capable of quantifying effective mass transport parameters on the hierarchically-structured porous anode, with the pore size spanning over two orders of magnitude. This is often challenging due to the balance between imaging resolution and field of view.25,27 This is the first time that such an I-CFD technique was employed to investigate the new micro-monolithic electrode, providing a more vivid and straightforward insight into gas transport in such a complex, hierarchical micro-structure. As shown in Fig. S5a–c (ESI†), when the gas feed was pure hydrogen, the hydrogen molar fraction in the EAR (Fig. 1b) was very close to the bulk concentration under typical operating current densities of 1 and 2 A cm−2. Even at 4 A cm−2, the hydrogen molar fraction decreased from 100% to only ca. 80%, due to hydrogen depletion by its electrochemical oxidation producing steam, suggesting well-optimized fuel transport. Such observations agree well with the U–j curves shown in Fig. 3a, in which the concentration polarization started to dominate at current densities up to 5.5–6 A cm−2.
The superior power densities can be attributed to the well-designed cell structure that optimizes fuel transport inside the EAR, eliminated the additional transport resistance from the support. It is understood that generally fuel conversion/utilization for typical SOFCs should be limited to 70% to minimize concentration polarization losses and avoid Ni oxidation at low H2:H2O ratios, though such conversions may vary depending on specific geometries and operating conditions.
For simulated LCVGs with low fuel contents, concentration polarization becomes a dominant factor limiting cell performance. Therefore, reducing the diffusive path length from several hundred micrometres to <50 μm, together with the micro-structure tailoring during fabrication, proved essential to achieving efficient conversion of simulated LCVGs. Such explanations could be confirmed further by the I-CFD modelling, results for which are shown in Fig. S5d and e (ESI†). When the inlet gas contains high percentages of inert diluent (set at 80%), the driving force for fuel diffusion decreased accordingly. It is noteworthy that although CO is also a gaseous fuel, H2 had a slightly lower equilibrium potential difference (0.9839 V) than that of CO (0.9907 V) relative to oxygen reduction at 1027 K,38 and a much greater self-diffusion coefficient (11.3 vs. 1.7 cm2 s−1 at 1027 K).39 Under the operating conditions used, hydrogen was expected to be oxidized preferentially, so CO was assumed to be ‘sluggish or inert’ during CFD modelling. Hydrogen concentrations decreased radially from 20% to approximately 15% and 8% under operating current densities of 1 and 2 A cm−2, respectively; fuel starvation was predicted at 4 A cm−2. The permeation flux density (J, mol m−2 s−1) driven by a concentration gradient, can be derived by a modified Fick's first law shown in eqn (S1) (ESI†). As the hydrogen concentration in the initial feed was decreased, micro-structure tailoring, as discussed in the Morphology section, becomes critical to facilitate gas transport, such as by decreasing electrode thickness, increasing the porosity or decreasing the tortuosity. As we reported previously, introducing micro-channels could decrease the tortuosity effectively by several times.25 The structural tailoring, together with the thickness control in the EAR, significantly decreased concentration polarization and contributed to achieving increased electrochemical performances when simulated LCVGs were used as fuel.
As discussed in the Introduction, an important benefit of utilizing LCVGs as fuels in SOFCs is their relatively high operating temperatures (>600 °C) that favour the DRM reaction, according to thermodynamic predictions (Fig. S6 ESI,† endothermic reaction, ΔH298K = 247 kJ mol−1). Based on results of our calculations of thermodynamic equilibria, shown in Fig. S8 (ESI†), complete conversion (conversion rate > 99.9%) of methane could be achieved theoretically at temperatures of 660, 720 and 950 °C with 10%, 20% and 50% initial methane contents, respectively. However, due to the miniaturization of the dimensions of the single cell, the residence time for the DRM reaction was constrained to approximately 0.1 s. Thus, the actual methane conversion can be quite far from thermodynamic equilibrium conditions if the cell performs solely as a catalytic reactor without any current loads, as shown in Fig. S9 (ESI†). On the other hand, once the DRM reaction was coupled with electrochemical reactions, during which both hydrogen and carbon monoxide were formed by reaction (1) then consumed continuously by electrochemical oxidation, the methane conversion could be increased dramatically.
As mentioned previously, gas flaring, which is the conventional route to dispose of natural gas via controlled combustion, is the most common approach for LCVG abatement in order to prevent direct emission of methane as a major greenhouse gas. However, presently it is also one of the most detrimental for energy wastage and climate change. For LCVG flaring, the high CO2 concentration frequently necessitates addition of extra fuel to reach the ignition limit and maintain the flame stability.3,4 Therefore, the disposal of LCVGs via flaring is not only a waste of primary energy, but also a significant contributor to global climate change, as shown in Fig. S10 (ESI†). By comparison, operating ceramic fuel cells with LCVGs could result in highly efficient recovery of their calorific values by conversion to electrical energy, with greatly decreased environmental consequences. As predicted by the techno-economic analysis (Fig. S10, ESI†), a minimum electricity price of 6.5 ¢ kW−1 h−1 could generate revenue to off-set capital expenditure, maintenance and operation cost by selling generated electricity. As provide by U.S. Energy Information Administration (EIA), the latest average national electricity price is approximately 13 ¢ kW−1 h−1.40 Therefore, the utilization of LCVGs using ceramic fuel cells could well be profitable.
The effluent gas mixtures were characterized using gas chromatography (GC) to quantify mole fractions of the components, as exemplified in Fig. 5 for 650 °C under open-circuit conditions. Due to the relatively low methane content, the conversion of methane at 650–750 °C for all three LCVG mixtures was close to their thermodynamic equilibrium composition (Fig. S7, ESI†), confirmed by the negligible methane content measured downstream of the reactor. The effluents contained approximately 45 to 61% syngas (CO + H2), the balance being CO2. The H2/CO ratio was slightly less than 1 (stoichiometric ratio in DRM reaction (1)) due to the reverse water–gas shift reaction. With applied current, the effluent compositions changed according to the additional oxygen content. In addition to efficient electricity generation, the effluents of LCVG utilization are still of considerable industrial interest, e.g. for methanol production via catalytic CO2 hydrogenation.41,42 As demonstrated in several industrial plants, methanol manufactured from these H2–CO–CO2 mixtures potentially can be used as a fuel or adopted as feedstock to produce further value-added chemicals. The highly efficient energy conversion resulting from the well-designed cell structure, together with the significant industrial importance of the downstream effluents, provides a new route to utilize LCVGs and to decrease their carbon footprints effectively.
Fig. 5 Concentrations of different compositions in the effluents of LCVG mixtures (10–20% CH4) at 650 °C under open-circuit condition (residual CH4 mole fractions not shown as values were <0.1%). |
Fig. 6 shows the time dependence of current densities of the novel micro-monolithic ceramic fuel cell operating at 0.7 V and 700 °C with 50 cm3 min−1 of CH4–CO2 mixture as the fuel (1:1 ratio), exhibiting a degradation rate over 500 hours of <2% (0.02 A cm−2 kh−1). Fig. 7 shows SEM images around the inner contour after operation. As shown in Fig. 3a, the bulk composition of gaseous fuel varies axially as a result of H2/CO oxidative depletion, with a corresponding increase in steam concentrations. Therefore, the surfaces of the support region (Fig. 7c, region A1) and active region (Fig. 7c, region A2) at both upstream and downstream of the cell were examined. The EAR (A2) along the whole axial direction was free from coking, due to the relatively high steam concentration generated from hydrogen oxidation (Fig. 4a) that maintained the local C:H:O ratio below the coking threshold (Fig. 7a). In contrast, the steam content at fuel inlet was almost negligible and the feed composition (CH4:CO2 = 1:1) was well within the coking region. Hence, carbon filaments were observed on the support (A1) at the upstream region (Fig. 7e and f), in agreement with previous reports,17 whereas the downstream side was free of coking. However, the influences of such coking on current densities at constant potential difference and on structural integrity were almost negligible. Thus, it can be concluded that even with a conventional Ni-based electrode, which is well known to be prone to coking, relatively stable operation could be achieved as result of controlling area-specific C:H:O ratios via morphology design and optimization. Potentially, this provides a new and more cost-effective approach to suppress the long-standing coking problem and enable efficient utilization of LCVGs as an overlooked energy source.
Fig. 6 Effect of time on current densities of a single cell at a constant cell potential difference of 0.7 V fuelled by CH4–CO2 mixture (1:1 ratio) and ambient air as the oxidant. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee00070a |
This journal is © The Royal Society of Chemistry 2020 |