Liam A.
McNeil‡
a,
Greg A.
Mutch‡
a,
Francesco
Iacoviello
b,
Josh J.
Bailey
b,
Georgios
Triantafyllou
a,
Dragos
Neagu
a,
Thomas S.
Miller
b,
Evangelos I.
Papaioannou
a,
Wenting
Hu
a,
Dan J. L.
Brett
b,
Paul R.
Shearing
b and
Ian S.
Metcalfe
*a
aSchool of Engineering, Newcastle University, Merz Court, Newcastle Upon Tyne NE1 7RU, UK. E-mail: ian.metcalfe@newcastle.ac.uk
bElectrochemical Innovation Lab, Department of Chemical Engineering, UCL, London WC1E 7JE, UK
First published on 29th April 2020
Membranes for CO2 capture should offer high permeant fluxes to keep membrane surface area small and material requirements low. Ag-supported, dual-phase, molten-carbonate membranes routinely demonstrate the highest CO2 fluxes in this class of membrane. However, using Ag as a support incurs high cost. Here, the non-equilibrium conditions of permeation were exploited to stimulate the self-assembly of a percolating, dendritic network of Ag from the molten carbonate. Multiple membrane support geometries and Ag incorporation methods were employed, demonstrating the generality of the approach, while X-ray micro-computed tomography confirmed that CO2 and O2 permeation stimulated self-assembly. We report the highest flux of Ag-supported molten-salt membranes to date (1.25 ml min−1 cm−2 at 650 °C) and ultrahigh permeability (9.4 × 10−11 mol m−1 s−1 Pa−1), surpassing the permeability requirement for economically-competitive post-combustion CO2 capture, all whilst reducing the membrane-volume-normalised demand for Ag by one order of magnitude.
Broader contextCO2 separation will likely be required for climate change mitigation and process intensification at an unprecedented scale in the future. Processes for CO2 separation include absorption, adsorption and the use of membranes. Membranes are operated under non-equilibrium conditions, as a driving force for e.g. CO2 transport is applied across the membrane. In gas separation, this driving force typically comes from a partial pressure difference between the feed and permeate sides of the membrane. Here, we exploited such non-equilibrium conditions to stimulate the self-assembly of an Ag dendritic network within a dual-phase, supported, molten-salt membrane. The percolating dendrites endowed an inert and low-cost membrane support with electronic conductivity, in turn, enhancing CO2 permeation to a level that surpassed performance metrics required for economically-competitive CO2 separation. The self-assembly occurred in multiple membrane geometries and using different methods of Ag incorporation, which will allow the development of a range of cost-effective and scaleable membrane fabrication routes. By exploiting the non-equilibrium conditions of permeation, typically regarded as challenging for the stability of membrane materials, we have demonstrated a new and general concept for in situ membrane fabrication. |
Dual-phase membranes can, in general, be categorised into two classes based on the nature of the solid support; ceramic–carbonate,11–23 and metal–carbonate.24–33 More recently, a combination of the two has been explored.34–36 Ceramic–carbonate membranes are most commonly fabricated using ceramics with oxide-ion conductivity. It is proposed in the literature that ceramic–carbonate membranes exploit reversible reaction (1) at the interface between MC, ceramic support and gas phase:
CO2(g) + O2−(s) ⇌ CO32−(l) | (1) |
![]() | (2) |
The first reported metal–carbonate membranes were fabricated from porous stainless-steel infiltrated with MC,30 with CO2 and O2 fluxes stabilising at a level lower than that predicted by reaction (2). The authors suggested that the formation of an electronically-insulating LiFeO2 layer (10−3 S cm−1), through reaction with MC in the presence of O2 at the feed side, lowered the electronic conductivity of the support to a value below the ionic conductivity of MC (100 S cm−1). In this case, and if reaction (2) is sufficient to explain permeation, CO2 and O2 fluxes would be limited by the lower electronic conductivity of the support. To avoid these limitations, Ag-supported, metal–carbonate membranes were later investigated.24,26,29 Ag does not form an oxide layer when in contact with MC at high temperature and offers the highest electronic conductivity of all metals (107 S cm−1). Early Ag-supported dual-phase membranes achieved peak CO2 and O2 fluxes six times higher than stainless-steel membranes, 0.82 and 0.43 ml min−1 cm−2 at 650 °C, respectively, with the 2:
1 flux ratio imposed by reaction (2) observed.29
While the initial performance of Ag-supported membranes is very good, it is generally not lasting (e.g. after ∼80 hours of operation at 650 °C, CO2 flux had decreased to ∼0.70 ml min−1 cm−2).29 The high temperature stability issues are thought to be related to poor MC wettability on Ag, and sintering of Ag resulting in a reduction of porosity and ejection of MC.29 Attempts have been made to address stability issues by decreasing the average pore size of the Ag support to increase capillarity,27,33 or by coating the surface of the Ag support with a nano-layer of Al2O3 to improve MC wettability.25,26,28 Alternatively, to limit Ag sintering, a refractory layer of ZrO2 has been deposited.31 It should also be noted that Ag is soluble in MC at the operating temperatures of dual-phase membranes,39,40 through corrosion by MC in the presence of O2,41via reversible reaction (3):
Ag0(s) ⇌ Ag+(1) + e− | (3) |
In this work we show how such an oxygen cycle, i.e. dissolution and precipitation of Ag, can be exploited advantageously to self-assemble a dendritic Ag network within low-cost Al2O3 dual-phase membrane supports. Self-assembly can be stimulated by the non-equilibrium conditions encountered during membrane operation, ultimately endowing inert membrane supports with electronic conductivity, and in turn, high CO2 and O2 fluxes. The generality of this scaleable and materials-efficient approach has been demonstrated by using multiple methods of Ag incorporation and multiple Al2O3 support geometries. High temperature stability relative to wholly Ag-supported membranes is shown to be greatly improved, due to retained contact between the highly wettable Al2O3 support and MC. Ultimately, through permeation experiments and detailed characterisation of membranes using ex situ X-ray micro-computed tomography, we demonstrate how self-assembling Ag dendrites in MC functionalise membranes so that they easily surpass the separation performance requirements for economically competitive CO2 capture. This is achieved whilst lowering the membrane-volume-normalised requirement for Ag by one order of magnitude.
After infiltration with MC at 450 °C, during which both feed and permeate sides of the membrane were fed with a 50 mol% CO2, 25 mol% O2 and 25 mol% N2 mixture to prevent carbonate decomposition, a permeation experiment was conducted at 650 °C, in a custom-made membrane permeation apparatus (Fig. S4, ESI†). The membrane feed-side inlet was kept the same (50 mol% CO2, 25 mol% O2 and 25 mol% N2 mixture) with Ar fed to the permeate-side inlet, to generate a pCO2 difference between feed and permeate sides, i.e. the driving force for CO2 permeation. After 80 hours of operation a stable flux of 5.0 × 10−2 ml min−1 cm−2 was achieved, a five-fold improvement on a membrane without Ag deposition (Fig. S7, ESI†).§ SEM-EDX analysis post experiment revealed that the Ag layer on the feed-side had been corroded to a significant extent, with Ag appearing on the permeate side (Fig. 1b, 2a and Fig. S8, ESI†). SEM image analysis revealed that ∼3% of the 17 mg of Ag initially deposited on the feed side had migrated to the permeate-side surface of the membrane (Fig. S9, ESI†). To determine the location and structure of the remaining ∼97% of the initially-deposited Ag, ex situ X-ray micro-computed tomography (micro-CT) analysis was conducted to visualise the internal structure of the membrane (Fig. 2b and Experimental methods, ESI†). Ag was clearly visible on the permeate-side surface, and within the bulk of the porous pellet as a dendritic structure, confined to the central ∼7 mm ∅. The location of the dendrites strongly suggested that Ag migration was driven by gas permeation, given that MC was infiltrated into the pores of the entire 20 mm ∅ Al2O3 pellet, the plated Ag layer was 14 mm ∅, and the active ∅ for permeation was ∼7 mm due to membrane sealing (Fig. 2a and Experimental methods, ESI†). This dendritic network endowed electronic conductivity to the Al2O3 support, evidenced by four-point DC measurements, elaborated upon below.
In all of the above cases it is reasonable to assume that the majority of Ag was present as a solid within the membrane at operating conditions, as the Ag loadings far exceeded the solubility limit of Ag in MC at 650 °C.39,40 However, in all cases a small amount of Ag+ was dissolved in MC and therefore, its effect on permeation fluxes needs to be discriminated from Ag in the solid networks. A 1.25 × 10−2 mol% Ag membrane (Ag quantity below the solubility limit of Ag+ in MC) was prepared by dry impregnation of Ag onto the Al2O3 support before MC infiltration (dry impregnation was used to add such a small quantity accurately) (Fig. S11, ESI†). Dissolved Ag enhanced flux, albeit to a lesser degree than when solid Ag was present, with fluxes of 1.90 × 10−2 and 0.92 × 10−2 ml min−1 cm−2 for CO2 and O2, respectively (Fig. 3). Interestingly, all membranes, including the 0 and 1.25 × 10−2 mol% Ag membranes, exhibited a ∼2:
1 CO2
:
O2 flux ratio. This suggested that the permeation mechanism in this class of membrane is more complex than that suggested by reaction (2), where an electronically-conducting support is implied as necessary for permeation to occur. It may be the case, as previously suggested in relation to an Al2O3-supported, molten-carbonate membrane,22 that this was qualitative evidence for electronic conductivity in MC.42 However, considering the present results, it is most likely that the metallic sealants employed for sealing dual-phase membranes provide transmembrane electronic conductivity. For example, visual analysis of a 0 mol% Ag membrane sealed using a silver ink showed the presence of transmembrane silver deposits and associated impact on flux during permeation experiments (Fig. S12, ESI†). Nonetheless, for our present goal, flux enhancement was most significant in the cases where Ag was present as solid dendrites, indicating that producing electronically-conductive networks is the most efficient route to achieving high transmembrane CO2 flux.
To investigate Ag dendrite formation, five membranes each loaded with the 6 mol% Ag carbonate mixture previously determined as an efficient loading to realise high fluxes, were quenched at various times up to and including stable flux, for micro-CT and four-point DC analysis (Fig. 4). A further five membranes, loaded with varying amounts of Ag (0.0125–6 mol% Ag), were quenched after attaining stable flux (Fig. S13, ESI†). Regardless of the quantity of Ag in the membrane, all membranes displayed a period of low flux during the first 10 hours (Fig. 4b inset and Fig. S13, ESI†), after an initial decrease in CO2 and O2 mole fraction (0–5 hours) due to clearing the permeation chamber of the 50 mol% CO2, 25 mol% O2 and 25 mol% N2 mixture fed to the feed- and permeate-side inlets during carbonate infiltration (Fig. 4b inset). Micro-CT was performed on membranes quenched by rapid cooling after 2, 6 and 10 hours of operation to investigate the low flux period, as well as on membranes from 20 and 120 hours, to investigate the regions of flux increase and stability (Fig. 4a and b). First, after 2 hours of permeation, Ag had already migrated from the feed-side surface to the permeate-side surface (Fig. 4a), in agreement with SEM images (Fig. 1b, Fig. S8 and S9, ESI†), and the initial micro-CT investigation (Fig. 2). After 6 hours, Ag dendrites had started to grow in the direction of the feed side from the permeate side, until they spanned the thickness of the membrane at 10 hours (Fig. 4a). Up to this point, no electronically-conductive Ag path connecting feed and permeate sides appeared to have formed, restricting fluxes to the low values observed during the first 10 hours (Fig. 4b inset). From 10 hours, the dendrites continued to grow, in both thickness and number, offering electronic conductivity across the membrane and increased flux of CO2 and O2viareaction (2) (Fig. 4b). After 90 hours, it was likely that the majority of the initially-deposited Ag had been corroded and redistributed as dendrites within the membrane, leading to flux stabilisation (Fig. 4b). Four-point DC analysis demonstrated that membranes operated for less than 10 hours had an electrical resistance three orders of magnitude higher than membranes operated for 10 hours or more (Fig. 4c), with calculations performed on the micro-CT 3D reconstructions indicating that a substantial quantity of Ag was present within the bulk of the membrane, from 10 hours onwards, with increasing feed to permeate-side connectivity (Fig. 4d).
Ag precipitation in molten salts to form dendritic structures under an electrochemical driving force and when the molten salt is below the Tm of the metal is well-known.43,44 In our case, self-assembly occurred due to the electrochemical gradient established within the membrane as a result of the non-equilibrium conditions of permeation. It is likely that CO32− formation (reaction (2)) consumed electrons produced by Ag dissolution at the high pO2 feed side (reaction (3)). CO32− and Ag+ ions subsequently migrated down their electrochemical potential gradient from feed to permeate side. At the permeate side, CO32− ions decomposed to release electrons, reforming Ag from Ag+, and releasing CO2 and O2 (Fig. 5). Ag first formed at the triple-phase-boundary on the low pO2 permeate side, acting as the nucleation site for continued dendrite formation back towards the feed side. Subsequent electrons released during carbonate decomposition were transported to the tip of the growing dendrite, to recombine with Ag+ from the melt, shortening the diffusional pathway of Ag+, and gradually extending the Ag dendritic network until it spanned the width of the membrane. Such dendritic Ag structures have previously been used to produce electronically-conductive composites, where the unique three-dimensional fractal geometry endowed an ultra-low electrical percolation threshold.45 Our findings demonstrate that the growth of such a dendritic network can also be used to produce self-assembling, electronically-conductive, dual-phase membranes with a substantially lower demand for Ag, elaborated upon below.
![]() | ||
Fig. 5 Ag dendrite self-assembly. Dissolution, migration and precipitation of Ag, driven by the non-equilibrium conditions of permeation. |
The parallel-pore membrane also offered the opportunity to measure permeation in a low-tortuosity and reduced-thickness dendritic Ag membrane. It is known that reducing tortuosity,46 and reducing thickness in dual-phase membranes results in higher fluxes.14 A flux of 1.25 ml min−1 cm−2 at 650 °C was achieved (Fig. 6c), a factor of 13 higher than the pellet membranes and almost two orders of magnitude higher than a parallel-pore membrane control experiment without Ag addition (Fig. S16, ESI†). Peak fluxes for the highest flux Ag-supported membranes in the literature are typically 0.8–0.9 ml min−1 cm−2 at 650 °C,26,29 with the ZrO2-coated Ag membrane achieving 0.9 ml min−1 cm−2, albeit at 850 °C,31 and an Ag membrane with a H2 containing sweep gas reaching 1.3 ml min−1 cm−2 at 700 °C.27|| Ceramic–carbonate membranes have achieved higher still CO2 fluxes employing pre-combustion feed gases (i.e. containing H2 or lacking O2 relative to our experiments) and higher temperatures.20,47
|| We note that, to the best of our knowledge, the CO2 flux reported here at 650 °C is the highest of any ceramic- or metal-carbonate dual-phase membrane reported to date employing a post-combustion feed gas and an Ar sweep gas.31
Permeability, which normalises the effect of membrane thickness and the driving force for permeation (i.e. pCO2 and pO2 difference between feed and permeate sides), of parallel-pore and pellet membranes was 9.4 × 10−11 mol m−1 s−1 Pa−1 and 2.5 × 10−11 mol m−1 s−1 Pa−1, respectively, of the same order of magnitude as Ag membranes in the literature.** However, the quantity of Ag used in our membranes was three orders of magnitude lower (e.g. 4 mg compared with ∼4 g used in Xu et al.),29 so that when permeability was normalised for the quantity of Ag used and the volume of membrane, it was clear that dendritic Ag membranes provided ultrahigh permeability at reduced cost (Fig. 6d). While it is possible that laser-drilled membrane supports will not be an economically-viable option for cost-effective scale-up, the principle of reducing the quantity of Ag is clear. A well-controlled pore forming technique such as phase inversion synthesis could be adopted to fabricate parallel-pore membranes at reduced cost, or modular membrane support geometries such as hollow fibres could be used, as Ag network growth appears to be independent of support geometry.
Recently, there has been an increasing recognition of membrane performance targets for post-combustion CO2 capture.48 Depending on specific mode of operation, CO2/N2 selectivity is targeted at 20–150 for pressure-driven operation (pressure ratio 5–15), or >140 when employing a sweep gas.49 This selectivity must be present in membranes with CO2 permeabilities of 10−13–10−12 mol m−1 s−1 Pa−1 to be economically-competitive with existing approaches.49,50 However, membranes generally exhibit a permeability-selectivity trade-off; for polymeric membranes, this is known as the Robeson upper bound,9,51 and as such few membranes have surpassed these targets (Fig. 7). Dual-phase membranes offer outstanding CO2/N2 selectivity (>1000) as the solubility of CO2 in molten carbonate is ∼104 higher than N2.38,52 As N2 is only detected on the permeate side of a dual-phase membrane as the result of a leak, selectivity is routinely calculated based on solubilities alone. Here, we have adopted a more conservative approach and calculated a minimum selectivity based on the limit of detection of our analytical apparatus (Experimental methods text, ESI†). Across all our permeation experiments, we thus determine a minimum CO2/N2 selectivity of ∼100, rising to ∼370 (although we note that this could be as high as >1000) (Fig. 7). Thus, our results surpass the two most recent CO2/N2 Robeson upper bounds,9,51 and a range of state-of-the-art graphene oxide,53,54 mixed-matrix,55–57 SPONG48 and polymer membranes,.9,51,58 Furthermore, dual-phase membranes are amongst a very limited range of membrane materials that can operate at >400 °C, offering exciting new separation opportunities, in e.g. hot flue gases without the need for gas cooling. It is also important to note that dual-phase membranes have recently been shown to self-heal autonomously at high temperature, providing confidence in their durability during operation.59 Finally, knowledge of permeability allows one to define a membrane thickness required to achieve a target permeance (10−7 mol m−2 s−1 Pa−1 for post-combustion CO2 capture).60 The range of state-of-the-art membranes we have selected for comparison require thicknesses of 10−5–10−7 m, whereas dual-phase membranes can achieve the target permeance at 10−4 m thickness. For this reason, the membranes we report here (∼500 μm) are approaching the permeance required for economically-competitive post-combustion CO2 capture, without the need for extreme membrane thinning and the associated costs.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ee03497h |
‡ These authors contributed equally to this work. |
§ Throughout, we define stable flux as a minimum of 4 hours where the mean flux varied less than ±3%. |
¶ In this case, the apparent activation energy corresponds to all processes that result in a transmembrane flux of CO2 or O2, i.e. surface reactions and bulk transport processes. Although technically incorrect, as the reported activation energy represents multiple processes, the values are useful for comparing our results with other membranes in the literature. |
|| We note that reported fluxes are often the highest flux achieved during the first ∼10–20 hours of a permeation experiment, with stable fluxes at ∼80 hours some ∼10% lower than the values highlighted here. |
** Note that we do not account for the driving force of O2 in our calculation of permeability (and therefore permeance) as this is the procedure widely adopted with Ag-supported membranes, thus allowing us to compare. |
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