Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

    As one of the most important families of porous materials, metal–organic frameworks (MOFs) have well-defined atomic structures. This provides ideal models for investigating and understanding the relationships between structures and catalytic activities at the molecular level. However, the active sites on the edges of two-dimensional (2D) MOFs have rarely been studied, as they are less exposed to the surfaces. Here, for the first time, we synthesized and observed that the 2D layers could align perpendicular to the surface of a 2D zeolitic imidazolate framework L (ZIF-L) with a leaf-like morphology. Owing to this unique orientation, the active sites on the edges of the 2D crystal structure could mostly be exposed to the surfaces. Interestingly, when another layer of ZIF-L-Co was grown heteroepitaxially onto ZIF-L-Zn (ZIF-L-Zn@ZIF-L-Co), the two layers shared a common b axis but rotated by 90° in the ac plane. This demonstrated that we could control exposed facets of the 2D MOFs. The ZIF-L-Co with more exposed edge active sites exhibited high electrocatalytic activity for oxygen reduction reaction. This work provides a new concept of designing unique oriented layers in 2D MOFs to expose more edge-active sites for efficient electrocatalysis.

    Introduction

    Crystalline catalysts have been widely used in catalytic fields for many reactions.16 Especially, when the particle size of these crystalline catalysts was reduced to a nanometer scale, shape, and size, it resulted in the tuning of two important parameters (exposed facets and exposed active sites), thereby enhancing the catalytic properties of the nanocatalysts.712 Controlling the exposed facets in these nanocrystal-based materials could change the distance, the arrangement, and the density of the atoms on the surface, further optimizing the adsorption energy of the intermedia during the reaction.1321 Thus, many studies have shown that controlling surface structures of crystalline catalysts is essential and have reported that the exposed facet in nanocatalysts affected both reactivity and selectivity.2226

    Porous crystalline materials have gained attention due to their high surface areas, large pore volumes, tunable channel connectivities, and modifiable surfaces.2730 Metal–organic frameworks (MOFs), among the most important porous crystalline materials, are built from metal centers and organic linkers.3134 The atoms on the outer surface of MOFs are orderly arranged, making a MOF an ideal system to investigate the relationship between structure and catalytic activity. There are various strategies to control the exposed facet and the arrangement of the atoms on the surface, including the use of modulators to control the growth of MOF crystals, selective etching of as-synthesized MOFs to change the exposed facets, and so on.3539 However, it is difficult to use the above-mentioned strategies to control exposed facets of two-dimensional (2D) MOFs. The common interactions between the layers stacked along the vertical direction in 2D MOFs are π–π stacking, hydrogen bonding, van der Waals forces, and others.34,4046 As observed in other typical 2D materials such as graphene,47,48 layered double hydroxides,49,50 transition-metal dichalcogenides,51,52 black phosphorus,53,54 and MXenes,55,56 the exposed atoms on the surface of the 2D materials are those composed on the layer. On the other hand, the atoms on the layer edges are considered more active for electrocatalytic reactions such as edge-active sites of graphene oxides.5764 To expose more edge sites of 2D materials, investigators have attempted to decrease the size of these 2D materials.62,65,66 However, controlling the exposed facets of 2D MOFs, especially to expose more edges of the layered structure, is still a challenge.

    Here, we synthesized 2D zeolitic imidazolate framework L (ZIF-L) materials with a layer-by-layer stacked structure. By employing a combination of selected area electron diffraction (SAED), bright-field transmission electron microscopy (TEM), and powder X-ray diffraction (PXRD) techniques, we found for the first time that the framework of ZIF-L consisted of layers stacked along the c axis created by hexagons and parallelograms in these fabricated 2D MOFs. Owing to the unique orientation of layers, their edges in the ZIF-L were exposed to the surface; hence, they could be used for electrocatalysis. Another layer of ZIF-L-Co could be grown heteroepitaxially on ZIF-L-Zn, where the two layers shared a common b axis but rotated by 90° in the ac plane. Therefore, controlling the exposed facets of the 2D MOFs was achieved successfully. We further investigated the relationship between the exposed facets of ZIF-L and the electrocatalytic activity for oxygen reduction reaction (ORR) activity. This series of 2D ZIF-L with unique oriented layers provides new opportunities to tailor structures to enhance catalytic activities of 2D MOF materials.

    Experimental Methods

    Methods and materials

    Chemical

    2-Methylimidazole (Hmim, 98%), Zn(NO3)2·6H2O (99%), and Co(NO3)2·6H2O (99%) were purchased from Energy Chemical (Shanghai, China). Carbon nanotubes (CNTs) were purchased from Aladdin (Shanghai, China).

    Synthesis of ZIF-L-Zn@ZIF-L-Co core–shell composite nanomaterials

    We initially synthesized ZIF-L-Zn nanomaterials: 2 mmol of Zn(NO3)2·6H2O and 16 mmol Hmim were dissolved in 80 mL of deionized water, and the mixture was stirred at room temperature for 4 h. The ZIF-L-Zn nanomaterials obtained were centrifuged (10,000 rpm, 5 min), washed five times with methanol, and dried in a vacuum freeze dryer. Then 1.7 mmol of ZIF-L-Zn nanomaterials and 4 mmol of Co(NO3)2·6H2O were dissolved in 10 mL of deionized water and stirred for 30 min at room temperature. Afterward, 10 mL of deionized water containing 8 mmol of Hmim was added to the solution with stirring. The mixture was stirred at room temperature for 24 h. The purple precipitates obtained were collected by centrifugation (10,000 rpm, 5 min), washed five times with methanol, and dried in a vacuum freeze dryer for further application.

    In addition, to replace Zn(NO3)2·6H2O with Co(NO3)2·6H2O, leaf-like ZIF-L-Co nanomaterials were prepared by a method similar to that of ZIF-L-Zn nanosheet.

    Characterization of the ZIF-L-Co materials

    SAED patterns and TEM images were recorded using a JEM-2100Plus (JEOL Ltd., Shanghai, China) at 200 kV with a TVIPS TemCam-XF416 camera (TVIPS, Gauting, Germany). High-angle annular dark-field (HAADF) images and energy-dispersive X-ray spectrometry (EDX) mapping were collected using a GrandARM300F instrument (JEOL Ltd., Shanghai, China) at 300 kV. The morphology of the catalysts was obtained with field-emission scanning electron microscopy (FESEM; Hitachi, SU8220, Xi’an, China). PXRD patterns of the catalysts were measured with an X-ray diffractometer (Bruker, D8 Advance, Cu Kα, λ = 1.5406 Å, 40 kV/40mA, Xi’an, China). The Brunauer–Emmett–Teller specific surface area was tested in Micromeritics ASAP 2020 (Xi’an, China).

    Computational details of the ZIF-L-Co materials

    We used atomic simulation environment (ASE)67 to perform simulations for further structural information of the ZIF-L-Co materials. Electronic structure calculations were performed using Vienna Ab initio Simulation Package68 with the Perdew–Burke–Ernzerhof exchange-correlation functional.69 The plane-wave kinetic energy cutoff was set to 500 eV with an electronic and ionic convergence criterion of an energy difference of 10−5 and 10−2 eV, respectively. The Brillouin zone was described using the Monkhorst-pack scheme 70 with a centered k-point sampling of (1 × 1 × 1) and Gaussian smearing with a width of 0.2 eV.

    Cluster models were constructed by truncating the crystal structure (dashed lines of truncated model structures) in perpendicular and horizontal crystal planes. Clusters were placed in a 30 × 30 × 30 Å3 unit cell to avoid periodic interactions. To maintain the bulk geometry, we constrained all the atoms in the cluster except the active sites and their neighboring nitrogen atoms. The adsorption energies of oxygen intermediates (OOH*, O*, and OH*) were calculated at room temperature by adding the thermal and vibrational corrections to the electronic energies. Adsorption energies were calculated using the computational hydrogen electrode model, which exploited the free energy of coupled proton-electron equal to half of the energy of H2 molecule in the gas phase.71

    Electrochemical studies of the ZIF-L-Co materials

    All electrochemical experiments of ORR performance were carried out on A CHI 760E Electrochemical Analyzer (CH Instruments, Austin, TX, United States) and a Pine Modulated Speed Rotator (Pine Research Instrumentation, Inc., Durham, NC, United States) at ambient temperature. Typically, in a three-electrode configuration, all electrochemical measurements were carried out in 0.1 M KOH solution, using a rotating disk electrode with an area of 0.196 cm2 or a rotating ring-disk electrode (RRDE) with an area of 0.247 cm2) as the working electrode, platinum wire as the counter electrode, and a saturated Ag/AgCl as the reference electrode. All potentials in the figure were converted to a reversible hydrogen electrode (RHE). The conversion formula is as follows: E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + 0.059 × pH. Details of electrodes’ preparation are presented in the Supporting Information.

    Results and Discussion

    2D ZIF-L was synthesized using Co or Zn ions as metal centers and 2-Methylimidazole molecules (Hmim) as ligands (Figure 1). The low-mag TEM images showed that ZIF-L-Co and ZIF-L-Zn were leaf-like nanomaterials with 4.5–5.0 μm length and 200 nm thickness (Figures 1a and 1b). The PXRD patterns indicated that the ZIF-L materials had the same framework structure as those reported ( Supporting Information Figure S1a). Low magnification SEM images obtained confirmed the uniformity of the prepared leaf-like ZIF-L ( Supporting Information Figure S1b). The structure–morphology relationship of these 2D ZIF-L is further investigated using SAED patterns. Electrons interact with matter through electrostatic potential and have much stronger interactions than X-rays, enabling local structural analysis of 2D ZIF-L nanoleaves through electron diffraction and electron microscopy. SAED patterns were collected from ZIF-L-Co and ZIF-L-Zn perpendicular to the leaf plate (Figures 1c and 1d). Both ZIF-L-Co and ZIF-L-Zn gave strong and sharp diffraction spots, indexed to be 0kl reflections based on an orthorhombic crystal system with unit cell parameters a = 24.1 Å, b = 17.1 Å, c = 19.7 Å, and space group Cmca. According to a previous report, the framework of ZIF-L solved by PXRD consisted of 2D nets connected by hexagons and parallelograms.72 The 2D nets further stack along the c axis. In previous reports, the layered structure was thought to be arranged parallel to the surface of ZIF-L-Zn nanoleaf, generally found in many typical 2D materials.73,74 However, based on our electron microscopy studies, we found that the 2D layers synthesized by us stacked along the c axis as a crystalline structure, arranged perpendicular to the surface of the ZIF-L nanoleaves (Figure 1e). To further demonstrate the unique layer orientations in ZIF-L, iron 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (FeTCPP) molecules were introduced into the complex to break hydrogen bonding between the layers. The weak acid nature of FeTCPP protonated the imidazole group in ZIF-L, and the electrostatic repulsive force between the layers due to the positive charges and/or steric effect caused the exfoliation of the layers.75 As shown in Figure 1f, after treatment with FeTCPP solution, the peak positions in PXRD patterns of leaf-like ZIF-L did not change, indicating that the crystalline structure of ZIF-L was retained after mild acid etching. SEM image showed that the leaf-like ZIF-L particles cracked along the direction of the layers and formed tetragonal nanorods. By breaking the interactions between layers, the results further confirmed the unique orientation of the layers in ZIF-L, signifying the first time discovery of such a feature in ZIF-L nanosheets.

    Figure 1

    Figure 1 | Characterization of ZIF-L. (a and b) TEM images and (c and d) SAED patterns of an individual crystal of ZIF-L-Co (a and c) and ZIF-L-Zn (b and d). (e) Schematic and proposed structure of ZIF-L-Zn viewed along the black arrow. (f) Schematic illustration, SEM image, and PXRD pattern of ZIF-L-Co after breaking hydrogen bonds.

    To demonstrate the advantages of the unique orientation of ZIF-L layers, the materials with the same layered structure but different orientations were prepared as references (Figure 2a). Interestingly, after heteroepitaxial growth of another layer of ZIF-L-Co on ZIF-L-Zn (denoted as ZIF-L-Zn@ZIF-L-Co), additional diffraction spots were observed in the SAED pattern (Figure 2). This observation indicated that ZIF-L-Co grew along a different direction as ZIF-L-Zn. Additional diffraction spots in the SAED pattern were indexed by hk0 reflections of ZIF-L-Co, indicating an intergrowth of bc (ZIF-L-Zn) and ab (ZIF-L-Co) planes (Figure 2b). To reveal the intergrowth behavior of these two structures, the SAED pattern of ZIF-L-Zn@ZIF-L-Co was also collected along the same orientation. The spots marked by yellow arrows belong to the ZIF-L-Co, and most other strong spots belong to the substrate ZIF-L-Zn (Figure 2c). The observation of this intergrowth was very common in the newly synthesized materials ( Supporting Information Figure S2). This unexpected orientation of intergrowth might have led to the relatively rough surface of ZIF-L-Zn@ZIF-L-Co. The shell could not grow into large domains outside the core due to lattice mismatch (a = 24.1 Å and c = 19.7 Å). Occasionally, the overgrown of thin slabs on the substrate (ZIF-L-Zn) was observed ( Supporting Information Figure S2b), which further indicated that the lattice mismatch might have limited the growth of ZIF-L-Co along the [100] direction. Therefore, ZIF-L-Co grew heteroepitaxially on ZIF-L-Zn by sharing a common b axis but rotated by 90° in the ac plane, which was rarely observed in previously reported 2D MOFs (Figures 2d and 2e).

    Figure 2

    Figure 2 | (a) Schematic of heteroepitaxially grown ZIF-L-Zn@ZIF-L-Co. (b) TEM images and (c) SAED patterns of an individual crystal of ZIF-L-Zn@ZIF-L-Co. Reflections marked with yellow arrows belong to the ZIF-L-Co. (d) Proposed structure of ZIF-L-Zn@ZIF-L-Co viewed along the black arrow in (a). (e) Schematic of the relationship between the exposed facets of ZIF-L and ZIF-L-Zn@ZIF-L-Co. The frameworks of 2D MOFs were represented by the network of metal atoms (green for ZIF-L-Zn and purple for ZIF-L-Co), where all of C, N, and H atoms were omitted for clarity.

    ZIF-L-Zn@ZIF-L-Co had similar morphology to ZIF-L-Zn ( Supporting Information Figure S3). The PXRD pattern of ZIF-L-Zn@ZIF-L-Co also confirmed the framework of ZIL-L with high crystallinity ( Supporting Information Figure S1a). In addition, the PXRD pattern of ZIF-L-Zn@ZIF-L-Co also showed the diffraction peak assigned to ZIF-67. This result implied that no ZIF-67 crystal was constructed with the same linker and Co ions as ZIF-L-Co. The EDX mapping of ZIF-L-Zn@ZIF-L-Co showed that the signals from Zn were located in the center of the nanoleaf, while Co was found on the exteriors, demonstrating the formation of the heteroepitaxially grown structure ( Supporting Information Figure S4). N2 adsorption/desorption isotherms of ZIF-L-Zn, ZIF-L-Co, and ZIF-L-Zn@ZIF-L-Co indicated that all of these materials were nonporous ( Supporting Information Figure S5). Thus, confirming a successful preparation of heteroepitaxially grown structure of the ZIF-L-Zn@ZIF-L-Co.

    A 2D network was stabilized by the interaction between the terminal Hmim and the “free” Hmim through hydrogen bonding. There are some reported MOFs where the epitaxial growth caused a change in orientation of the MOF crystals. The main reason for the change of orientation could be attributed to matched lattices of two materials at the interface.7679 In our system, the reason that the structure of the ZIF-L-Co shell changed the direction of growth is due to the hydrogen bonding between Hmim ligands on the surface of the ZIF-L-Zn core and Hmim from the ZIF-L-Co shell ( Supporting Information Figure S6). Thus, we achieved controlled exposed facets of 2D ZIF-L successfully. Then we proposed a structural model to illustrate the intergrowth behavior (Figure 2d and Supporting Information Figure S7).

    Through cyclic voltammetry (CV), linear sweep voltammetry (LSV), Tafel analysis, and RRDE electrochemical testing methods, we studied electrocatalytic oxygen reduction performance using ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co as catalysts. Compared with ZIF-L-Zn@ZIF-L-Co, ZIF-L-Co showed a superior ORR activity. In an O2 saturated 0.1 M KOH solution, ZIF-L-Co exhibited an apparent reduction peak at ∼0.73 V (Figure 3a). LSV curves obtained further demonstrated its catalytic activity. ZIF-L-Co showed a peak onset potential of 0.88 V (vs RHE) and a half-wave potential of 0.80 V, which was much higher than those of ZIF-L-Zn@ZIF-L-Co (onset potential of 0.81 V and half-wave potential of 0.70 V) and pure CNT (onset potential of 0.76 V and half-wave potential of 0.645 V) ( Supporting Information Figure S8). Half-wave potential of ZIF-L-Co is only 59 mV lower than that of commercial Pt/C (vs RHE) (Figure 3b). Compared with ZIF-L-Co, incorporating Zn sites into ZIF-L-Zn@ZIF-L-Co would inevitably reduce the content of Co sites. ZIF-67 had a similar coordination form and Co content as ZIF-L-Co was used as a reference ( Supporting Information Figure S9). The E1/2 of ZIF-67 was 0.74 V, while the limiting current density was 3.7 mA cm−2. This result confirmed that the difference in ORR performance between ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co was not caused by Co contents. We also characterized the ORR activity of the ZIF catalyst using Zn ions as the metal centers (ZIF-8 and ZIF-L-Zn; Supporting Information Figure S9) with similar structures as ZIF-67 and ZIF-L-Co, respectively. The activities of ZIF-8 and ZIF-L-Zn were much lower than that of ZIF-L-Co ( Supporting Information Figure S10). Additionally, such high electrocatalytic ORR activity surpassed most previously reported MOF-based catalysts ( Supporting Information Table S1). The Tafel slope value indicated a catalytic ORR kinetics: The Tafel slope of ZIF-L-Co was 86.1 dec−1 and it scored the lowest value compared with Pt/C, ZIF-L-Zn@ZIF-L-Co, and ZIF-67, which exhibited Tafel slope of 97.1, 125.8, and 109.1 dec−1, respectively (Figure 3c). Moreover, we used the K–L plots and RRDE methods to test the number of electron transfer (n) in the ZIF-L-Co electrocatalytic ORR. The calculated value showed that n was ∼3.3, which was more inclined to the four-electron pathway (Figures 3d and 3e and Supporting Information Figure S11). After the j-t chronoamperometry response evaluation, ZIF-L-Co showed high durability. After 10 h of durability test, the current consumption was only 11.5%, which was compatible with the commercial Pt/C. X-ray photoelectron spectroscopy (XPS) spectra of Co 2p and N 1s also showed that the electronic structure of the active center did not change significantly after the catalytic reaction, confirming that excellent stability was maintained during the reaction with ZIF-L-Co (Figure 3f and Supporting Information Figure S12).

    Figure 3

    Figure 3 | (a) CV curves of the ZIF-L-Co electrodes in O2 and Ar saturated 0.1 M KOH. (b) LSV curves and (c) Tafel plots of the ZIF-L-Co, ZIF-L-Zn@ZIF-L-Co, ZIF-67, and Pt/C. (d) LSV curves at different rotation speeds of ZIF-L-Co. (e) K–L plots of rotation speed corresponding to 1/j at different potentials. (f) chronoamperometric responses at 0.664 V.

    To understand the reason behind different ORR activities in ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co, we used density functional theory (DFT) calculations. Based on TEM results (Figure 2), we hypothesized that the heteroepitaxially grown ZIF-L-Zn@ZIF-L-Co had a different exposed facet from ZIF-L-Co. Prior to the heteroepitaxially grown process, the perpendicular crystal plane was the dominant exposed facet in ZIF-L-Co, which changed to a horizontal crystal plane in ZIF-L-Zn@ZIF-L-Co. To test this hypothesis, we made truncated cluster models for perpendicular (representing ZIF-L-Co, Figure 4a, green dashed line) and horizontal (representing ZIF-L-Zn@ZIF-L-Co, Figure 4a, purple dashed line) crystal planes and modeled the ORR reaction. To maintain the crystal structure of the MOF, the tails of the cluster models were fixed in the ZIF-L bulk positions. In both models, Co atoms were connected through Hmim, but the perpendicular model had geometrically more accessible active sites. To model the ORR reaction, we assumed an associative mechanism (see details in the Computation details of the ZIF-L-Co materials). Figure 4b shows the calculated free energy diagram (FED) for both cluster models and the ideal catalyst at U = 0.0 V versus RHE. The calculated limiting potential (UL, i.e., the maximum potential at which all steps are downhill in free energy) was used as a metric to evaluate the activity of the two cluster models of ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co. The inset of Figure 4b shows the calculated limiting potentials, which can be directly compared with our experimental onset potential. Based on these calculations, the last step in the FED, which was the reduction of OH* to H2O, was a limiting step to the ORR activity on the horizontal crystal plane, exposed in ZIF-L-Zn@ZIF-L-Co while reduction of O2 to OOH* was limiting to the perpendicular crystal plane, exposed in ZIF-L-Co. The horizontal crystal plane yielded a limiting potential of 0.60 V, indicative of lower ORR activity. On the other hand, the free energy profile of the perpendicular crystal plane, exposed in ZIF-L-Co was closer to an ideal catalyst, such that it showed a calculated rate-limiting potential of 0.84 V and higher ORR activity. These results are in close agreement with our experimentally measured onset potentials for ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co (Figure 3b and Figure 4b inset). Based on these results, we inferred that the perpendicular crystal plane in ZIF-L-Co was more active toward ORR due to its geometrically abundant active edge sites exposed and accessible to react and drive the ORR electrocatalytic activity high.

    Figure 4

    Figure 4 | (a) Truncated model structures for heteroepitaxially grown ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co. Dashed lines indicate the horizontal and perpendicular crystal planes in ZIF-L. (b) FED for the ORR on truncated model structures in (a) as well as the ideal catalyst (blue). The inset shows the calculated limiting potentials (UL) for the models. Dashed lines represent the experimental onset potentials for the ZIF-L-Co and ZIF-L-Zn@ZIF-L-Co, respectively.

    Conclusion

    We prepared a 2D ZIF-L with a unique oriented layered structure to provide more exposed edge active sites in 2D ZIF-L-Co, beneficial for electrocatalysis. To prove this concept, ZIF-L-Zn@ZIF-L-Co, with different exposed facets, sharing a common b axis but rotated by 90° in the ac plane, was prepared as a reference. As electrocatalysts for ORR, ZIF-L-Co exhibited higher ORR activity than ZIF-L-Zn@ZIF-L-Co by exposing more edge active sites, which showed higher intrinsic activity, demonstrated by theoretical DFT calculation. This work provides a concept of designing a 2D MOF with a unique oriented layered structure to provide more exposed edge active sites for efficient electrocatalysis, thereby advancing the development of devices with high-energy conversion and storage capabilities.

    Supporting Information

    Supporting Information is available and includes details of the catalytic measurements, PXRD, SEM, and TEM images, N2 adsorption/desorption, XPS, proposed structure, and the evaluation of catalytic performances.

    Author Contributions

    H.Z. guided all aspects of this work. Y.W. designed and synthesized the MOF performed SEM, PXRD, and electrochemical measurements. A.H.B.M. and T.J.G. carried out the computational DFT calculations under the supervision of S.S. T.S. performed TEM, ED, EDX mapping, and construction of a structural model of MOF under the supervision of Y.M, Z.L., and Y.Z. carried out the N2 adsorption–desorption isotherm and revised the manuscript. Z.H. contributed to building the structural model of MOF and assisted in characterizing the structure. W.Z. and R.C. contributed to XPS analysis and revised the manuscript. Y.W., T.S., S.S., and H.Z. prepared this manuscript.

    Conflict of Interest

    There are no conflicts of interest to declare.

    Acknowledgments

    The authors are grateful for the support from the National Natural Science Foundation of China (grant nos. 21975148, 21875149, 21835002, 21875140, and 21773146), the Fundamental Research Funds for the Central Universities, the Research Funds of Shaanxi Normal University, Shanghai Natural Science Fund (no. 17ZR1418600) and CℏEM, SPST of ShanghaiTech University (no. EM02161943), and the Swedish Research Council Formas (no. 2020-00831, Z.H.). S.S., A.H.B.M., and T.J.G. gratefully acknowledge the support from the University of Calgary’s Canada First Research Excellence Fund Program, the Global Research Initiative in Sustainable Low Carbon Unconventional Resources. This research was enabled in part by support provided by computational resources at the University of Calgary ( www.rcs.ucalgary.ca) and Compute Canada ( www.computecanada.ca).

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