MetalâOrganic Frameworks as Platform Materials for Solar Fuels

Metal−Organic Frameworks as Platform Materials for Solar Fuels Catalysis Marek B. Majewski,† Aaron W. Peters,†,‡ Michael R. Wasielewski,†,‡ Joseph T. Hupp,*,†,‡ and Omar K. Farha*,†,‡,§ †

Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States ‡ Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States § Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia ABSTRACT: Metal−organic frameworks (MOFs) are promising platform materials for solar fuels catalysis owing, in part, to their synthetic tunability and variability. MOFs can be designed and employed as supports for known molecular and nanoparticulate catalysts and for new atomically precise cluster-based catalysts. Rational modification of the organic linkers that define the framework affords the possibility of incorporating known photosensitizers or functional groups to generate hybrid materials that can serve to absorb light, facilitate substrate transport, and catalyze chemical reactions. In this Review, we highlight the diversity of approaches that have been taken to functionalizing, designing, and modifying MOFs for solar fuels generation where MOFs have typically been used as supports or as scaffolds for known catalysts. We additionally highlight the potential of MOFs as platform materials for new and existing cluster-based heterogeneous catalytic systems where both the active sites and the support can be characterized with near atomic-scale precision.

A

Because of their typically high porosity, enormous internal surface areas, varying topologies, and good chemical stability, metal−organic frameworks (MOFs) have emerged as exceptional materials for many candidate applications ranging from gas storage and/or separation to chemical sensing and catalysis.7−15 Composed of metal ions/clusters (“nodes”) and organic linkers, MOFs may be readily synthetically tuned and custom-built for specific applications.16−20 In the context of solar fuels generation, MOFs with a high degree of stability in aqueous solution and with tolerance to varying pH conditions, ranging from strongly acidic to strongly basic, are particularly desirable.21,22 From among the enormous number of known MOFs,23 a select handful have become favorites for solar fuels applications. Foremost among them are members of the UiO family (UiO = Universitetet i Oslo). Featuring hexa-zirconium(IV)-oxy or occasionally hexa-hafnium(IV)-oxy nodes and ditopic, carboxylate-terminated linkers (see Figure 2), these MOFs show (a) good stability in water (albeit, not in phosphate-containing buffers); (b) excellent thermal stability; (c) ability to accommodate appropriately size-matched, mixedlinker compositions (for example, benzene-1,4-dicarboxylate 1 together with potential metal catalyst chelators such as

s our understanding of the cause and effect of climate change has grown, so too has our understanding of the need for a clean energy power sectora necessity for mitigating the impact of climate change and reducing modern society’s reliance on fossil fuels.1,2 Design of sustainable energy systems will require an interplay of many new and emerging alternative energy technologies.3 A potential capstone technology is the direct conversion of solar energy to chemical fuels (artificial photosynthesis) by way of water oxidation and reduction to yield hydrogen as well as the transformation of carbon dioxide (CO2) and water to hydrocarbons or alcohols.4,5 For this approach to be commercially viable, the exploration and development of new catalysts composed of Earth-abundant elements that are capable of driving these processes under aqueous acid or base conditions, under prolonged solar irradiation, and in ambient conditions are paramount.6

The exploration and development of new catalysts composed of Earth-abundant elements that are capable of driving these processes under aqueous acid or base conditions, under prolonged solar irradiation, and in ambient conditions are paramount. © XXXX American Chemical Society

Received: January 2, 2018 Accepted: February 2, 2018

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DOI: 10.1021/acsenergylett.8b00010 ACS Energy Lett. 2018, 3, 598−611

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Cite This: ACS Energy Lett. 2018, 3, 598−611

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Figure 1. Schematic illustrating the routes to modifying metal−organic frameworks and making them amenable for solar fuels generation.

illustrate the utility of functionalized MOFs for light-harvesting, as efficient external electron-transfer quenching (i.e., quenching at the MOF/solution interface) first requires efficient, longrange energy migration through the framework. In the examples cited, energy transfer is thought to occur mainly through a Dexter (through-bond coupling) mechanism. Designing systems for directional and efficient long-range energy transport is particularly challenging and requires careful attention to (a) MOF and dye (chromophore) symmetry; (b) dye ground- and excited-state electronic properties; and (c) dye spacing and alignment, especially if transport is based on Förster-type energy transfer (fluorescence resonance energy transfer). Toward this end, Dolgopolova et al. postsynthetically immobilized a 4-hydroxybenzylidene imidazolinone energy donor into a purpose-built porphyrin MOF.48,49 Quenching of the fluorescence emission from the donor moiety indicated energy transfer to the porphyrin acceptors and was confirmed by time-resolved fluorescence decay measurements (ΦET = 0.65). While the cited example explored only short-range, single-step energy transfer, this approach (more generally) can overcome the complexity associated with long-range chromophore organization by utilizing the rigid and well-defined structure of a MOF to create a well-ordered chromophore ensemble. So and co-workers employed an automated layer-by-layer (LbL) assembly technique50−52 to generate glass- and electrodesupported chromophore-aligned porphyrinic MOF films.45,53,54 This approach, also termed liquid-phase epitaxy,55 yields films where the porphyrin building blocks can be preferentially aligned, by design, either normal or parallel to the film support. Energy transport through films was monitored first by terminating films of various predetermined thicknesses with a red-emitting squaraine dye. Following subsequent selective excitation of the porphyrin components, fluorescence emission was observed exclusively from the squaraine dye.53,54 In single crystals of an appropriately designed porphyrinic MOF, excitons were discovered to execute ∼2000 linker-to-linker hops within their brief (∼3 ns)

2,3-dihydroxy-1,4-benzenedicarboxylate 3, or 5,5′-dicarboxy2,2′-bipyridine together with [1,1′-biphenyl]-4,4′-dicarboxylate); and (d) despite strong node-linker bonding, an ability to engage in solvent-assisted linker exchange (SALE), thereby permitting complex structures to be framework-incorporated under mild, postsynthetic conditions.24 Related MOFs with tetratopic linkers, for example, tetrakis(4-carboxyphenyl)porphyrin 10, offer slightly higher chemical stability25,26 but appear to lack the ability to engage in synthetically useful linker-exchange. Notably, the porphyrin units can be rendered catalytic via metalation. Depending on MOF topology, these compounds may also present linker-free sites on nodes; these sites can be used to support inorganic catalysts or to graft useful molecular species.27−29 These (and other water-, acid-, and/or base-stable) frameworks have been functionalized de novo, postsynthetically, and in situ to yield MOFs bearing photosensitizers,30−33 heterogenized molecular catalysts,34−36 nanoparticles,37,38 or active sites modeled after those of enzymes (Figure 1).39,40 These hybrid materials have, in turn, been used to study energy and electron transfer as well as electrocatalytic and photocatalytic processes. While many reports have explicated the virtues of MOFs as heterogeneous catalysts,13,41−44 we seek here to highlight MOF-centric approaches to solar fuels catalysis, giving particular attention to an emerging class of atomically precise catalysts supported on frameworks. Metal−Organic Frameworks for Electron and Energy Transfer. The installation of photosensitizers into MOFs to yield light-harvesting frameworks falls outside the scope of our Review;45−47 however, these important efforts mark stepping stones in mimicking the natural photosynthetic process.31 Kent et al. have prepared Zn-based MOFs constructed from wellstudied MII(4,4′-(CO2)2-bpy)2(bpy)2− and MII(4,4′-(CO2)2bpy)2(CN)24− (M = Ru, Os; bpy = 2,2′-bipyridine; 4,4′-(CO2)2bpy = 4,4′-dicarboxylate-2,2′-bipyridine) building blocks that have exhibited antenna behavior, as well as good efficiencies for reductive and oxidative quenching.32,33 Significantly, these findings 599


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used to achieve site isolation, an important consideration for catalysts that may otherwise be rendered inactive because of unproductive catalyst dimerization or because of damaging attack by other catalysts that have mistaken the catalysts for substrates. Conversely, MOFs can serve to closely position pairs of active sites, thereby enabling them to cooperatively catalyze desired reactions. Examples of cooperative catalysis include “preconcentration” of one of a pair of reactants and preferential alignment of pairs of reactants.65−67 MOFs additionally can be used to boost the concentration of a molecular catalyst, for example, at or near an electrode or photoelectrode surface, but at the expense of only miniscule total amounts of catalyst. Concentrations of catalytic nodes or linkers within MOFs generally are in the range of a hundred to several hundred millimolar (i.e., much higher than can typically be achieved for complex molecular catalysts in electrolyte-containing solutions). All else being equal, and under conditions where reactant transport is not rate-limiting, higher catalyst concentrations translate into smaller overpotentials for a given catalytic current, or higher currents at a given overpotential.68 Among the earliest studies of well-defined solar fuels catalysis using MOFs is a report by Wang and co-workers.34 By derivatizing ligands to make them MOF struts, for example, dcbpy (= 5,5′-dicarboxylate-2,2′-bipyridine) as a proxy for [1,1′-biphenyl]4,4′-dicarboxylate in UiO-67, they succeeded in installing (a) two well-studied Ir(III)-based water oxidation catalysts; (b) Re(dcbpy)(CO)3(Cl), a known photocatalyst for CO2 reduction; and (c) two representative photocatalysts for organic transformations. Notably, the immobilized iridium catalysts retained their competency for conversion of H2O to O2 with turnover frequencies (TOFs) ranging from 0.4 to 4.8 h−1, while the frameworks themselves held up to aqueous Ce4+, a potent sacrificial chemical oxidant. Dioxygen is not a fuel, but reductive fuel production requires an oxidation reaction to balance or complete the process. As dioxygen is a benign byproduct, water-to-O2 is an obvious candidate reaction. The MOF-immobilized rhenium(I) complex likewise retained catalytic competency, executing five turnovers for conversion of CO2 to CO over the course of 6 h. It is important to note that CO is a two-electron reduction product, implying that two photons are needed for conversion. Catalysts of this kind are effective, in part, because the one-electron reduction potentials of metal [Re(I)] and ligand (bipyridine) are similar, setting up the photoreduced system for delivery of two reducing equivalents. Additionally, metal reduction converts the catalyst from a stable 18-electron organometallic compound to a less stable, 19-electron species. The instability is expressed as halide-ligand labilization and creation of an open site for CO2 binding and reduction.69 Alternatively, reduction and labilization of the catalyst’s metal center can lead to metal−metal bond formation between pairs of catalysts. Lacking open coordination sites, the dimerized complexes are ineffective as catalysts; further reduction, however, destroys the metal−metal bond and generates catalytically competent monometallic species. For free-molecular, as well as amorphous-polymeric, versions of the catalyst both activation pathways have been observed.70−72 Notably, under electrocatalytic conditions, the dimer-free pathway typically requires less kinetic overpotential and thus is preferable from an energy efficiency perspective. The structural regularity and semirigidity of crystalline MOFs offer a means for blocking catalyst dimerization.35 Another means of diminishing the needed driving force is to replace rhenium(I) with manganese(I). Fei et al. showed that Mn(dcbpy)(CO)3Br 6 can be incorporated in a UiO-67

singlet lifetimes; furthermore, the hopping was found to be highly directional, effectively mimicking ensembles of pigments in nature’s photosynthetic aparatus.56 The functional importance of this point (directional energy transport) cannot be overemphasized: For a high-symmetry MOF where, for any given step, energy transfer is equally probable in the x, y, and z, and −x, −y, and −z, directions (3D transport; six directions total), the net displacement of the exciton from its point of formation (i.e., the net energy transport distance) increases as N1/6, where N is the number of hops accomplished within the relevant excited-state lifetime (molecular-exciton lifetime). For a lower-symmetry MOF, where hopping is rapid, i.e. likely, only in the x and −x directions (1D transport), the net energy transport distance scales as N1/2 and much larger net distances can be traversed. For the above example where N = 2000, the values of N1/2 and N1/6 are 45 and 3.55, respectively.57 The pillared-paddlewheel arrangement is a good example of a MOF motif that can be used to enforce exciton hopping primarily along one crystallographic axis, albeit in both forward and reverse directions.58,59 More challenging is refining the design to move excitons in a single forward direction, or at least to favor forward over reverse. Nature accomplishes this in photosynthesis by creating an energy cascade comprising antenna chromophores and chlorophyll; excitons move from higher energy to lower energy, but not the reverse. In work that capitalized on the modularity of the LbL approach, Park et al. grew panchromatic MOF films that function as primitive energy cascades and that display antenna-like directional energy (exciton) transport.60 The films contained two kinds of rylenediimides and a free-base porphyrin. LbL was exploited to assemble, as supported thin-films, compositionally varying MOFs containing zones of predetermined optical-density and thicknesses. More generally, the LbL approach provides a means of incorporating MOFs into devices for photochemical energy conversion (where other interesting and useful strategies for synthesizing electrode-supported MOF films include spray-drying, templatedirected growth, and electrophoretic deposition).61−64 Metal−Organic Frameworks as Catalysts for Solar Fuels Generation: Known Molecular Catalysts Installed in Metal−Organic Frameworks. Many efforts have been made to support known molecular solar fuels catalysts in MOFs. Often these materials are prepared through postsynthetic modification techniques or by purposely derivatizing molecular catalysts in such a way as to allow the catalyst to act as a linker in the de novo preparation of the MOF. Typically, the targets are well-studied catalysts whose activity and functionality in homogeneous solutions are understood.

MOF-immobilization of molecular solar fuels catalysts provides a means for examining the catalysts in environments, for example, aqueous solutions, in which their evaluation would otherwise be inhibited by molecular insolubility or aggregation. While perhaps not yet widely appreciated, MOF-immobilization of molecular solar fuels catalysts provides a means for examining the catalysts in environments, for example, aqueous solutions, in which their evaluation would otherwise be inhibited by molecular insolubility or aggregation. MOFs can also be 600


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framework and that the installed species, when doubly reduced, is catalytic for CO2 reduction to formate.35 As the manganese complex does not absorb visible light, the catalysis was facilitated by a solution-phase chromophore, Ru(dmb)32+ (and triethanolamine as a sacrificial redox quencher that yields Ru(dmb)3+ after photoexcitation; dmb = 4,4′-dimethyl-2,2′bipyridine). In this study, the quantum efficiency for formate production was 14%, with a TON of ∼50 based on 4 h, while the selectivity for formate over H2, CO, and other possible products was close to unity. Two reducing equivalents are needed for conversion of CO2 to either CO or formate (vide supra). With ReI(bpy)(CO)3Xtype photocatalysts (X = halide or solvent), the required doubly reduced form is generated via redox disproportionation of pairs of singly reduced catalysts. Thus, appreciable MOF-based photocatalysis should be possible only when catalyst doping is high enough to support redox disproportionation. With the aim of optimizing photocatalyst TONs, Choi and co-workers systematically varied the loading of Re(dcbpy)(CO)3Cl in UiO-67 (via replacement of [1,1′-biphenyl]-4,4′-dicarboxylate).73 Denoting these materials as Ren-MOFs, and varying n from 1 to 24 Re centers per MOF unit cell (where 24 corresponds to one rhenium siting on every linker), they obtained the best performance (highest TON) with three Re moieties per unit cell. At higher loadings, evidence of catalyst crowding appears in the CO portion of the infrared spectrum, while structural models point to congestion in the MOF pores. The empirically determined optimal loading of three molecular photocatalysts per unit cell appears to yield the best balance between (a) fast redox disproportion (favored by a high density of rhenium sites) and (b) fast diffusive delivery of sacrificial molecular redox reagents (favored by a low density of sterically demanding rhenium sites). While further loading could enhance the light-harvesting efficiency of Ren-MOF, the observed drop-off in catalyst TON for n > 3 is sufficiently steep that this would likely be a losing strategy. An intriguing alternative approach to boosting catalyst lightharvesting and, in turn, photocatalytic activity, has been described by the same team. Briefly, Re3-MOF was coated onto Ag nanocubes to yield a hybrid material (Ag⊂Re3-MOF); depending on the MOF layer thickness, these exhibited activity enhancements of up to 7-fold relative to Re3-MOF alone. The enhancements come from optical excitation of a localized surface plasmon resonance (LSPR) associated with the free electrons in the Ag nanocubes. The cubes are small enough that essentially all the free electrons within a given cube can be excited by absorption of a single photon. The resulting collective oscillation of free electrons yields a large electromagnetic field that drops off substantially as one moves nanometers or tens of nanometers away from the cube surface. Notably, cubes display larger LSPRderived fields, and broader plasmon extinction spectra, than do spheres of equivalent volume.74,75 These near-surface fields effectively enhance the absorption cross sections of proximal molecules having electronic transitions at wavelengths coincident with those for LSPR excitation. The weakly absorbing tail of the photocatalytically relevant metal-to-ligand charge transfer (MLCT) overlaps nicely with the LSPR extinction maximum.73 It is worth noting that silver itself is competent as an electrocatalyst for CO2 reduction.76 An intriguing, but not yet explored, variant of the work by Choi et al. would be to replace weakly absorbing, but catalytic, rhenium units with strongly absorbing, but noncatalytic linkers that, when photoexcited or reductively quenched, inject electrons into silver nanoparticles, with the nanoparticles then catalyzing the reduction of CO2.

Cooperativity between immobilized molecular catalysts is another example of desirable behavior that appropriately designed MOFs can facilitate. Cooperativity between immobilized molecular catalysts is another example of desirable behavior that appropriately designed MOFs can facilitate. Ozawa and Sakai have observed (in solutionphase studies) that diplatinum(II) complexes featuring ammonia, pyridine, or other nitrogen ligands are photocatalytically active for water reduction to H2 and considerably more so than monoplatinum(II) complexes.77 The catalysis is believed to proceed via a Pt(II)−Pt(III)-hydride intermediate, where hydride formation is concomitant with platinum−platinum bond formation. Zhou and co-workers proposed that MOF architectures could be used to similarly position pairs of (5,5′-dicarboxylate2,2′-bipyridine)dichloroplatinum(II) 5 complexes and thereby engender heterogeneous catalytic activity for hydrogen evolution.36 Their approach is instructive: As a catalyst scaffold the authors used MOF-253, which has the empirical formula Al(OH)(dcbpy) (Figure 2).78 Al(III)-oxygen(carboxylate) bonds are not only strong, especially by coordination chemistry standards, but also inert to substitution, even in the presence of sizable concentrations of aqueous hydroxide. The scaffold is, in principle, well-suited to the challenging conditions of watersplitting chemistry and presents square channels, with closely spaced dcbpy units defining the channel walls. These units lend themselves to reaction with cis-Pt(DMSO)2Cl2 to form bluelight-absorbing Pt(bpy)(Cl)2 moieties. When paired and illuminated, the MOF-installed platinum units photocatalytically convert H2O to H2,36 presumably via a mechanism akin to that described by Ozawa and Sakai.77 A variety of indirect indicators (including the absence of a catalysis reaction induction time), appear to rule out more prosaic schemes involving photodegradation of single-platinum complexes and formation of catalytic Pt nanoparticles. The preceding examples rely upon functionalized linkers as entry points for catalyst installation. A complementary strategy is to build a molecular-scale catalyst around the node itself, with a redox-active node constituting the catalytic site and the MOF linkers (ligands) serving both to isolate catalytic units and to fix the metal redox potential for catalysis. Perhaps the best examples are cobalt(II)-containing ZIFs (zeolitic imidazolate frameworks, where “zeolitic” signifies that the MOF topology matches that of a known zeolite).7,79 For example, Ru(bpy)32+ sensitization of Co-ZIF-9 in the presence of the sacrificial reagent (TEOA) results in the reduction of CO2 to CO, with high turnover numbers (TON = 90 in a 30 min reaction).80 In Co-ZIF-9, benzimidazolate linkers assemble in tetrahedral fashion around single cobalt(II) ions (Figure 2), with each linker also connecting to a second similarly coordinated cobalt ion. In combination, the nodes and linkers comprise a framework presenting large pores, but small apertures, too small to be permeated by Ru(bpy)3n+, but not so small as to exclude CO2, CO, or water, suggesting catalysis may be driven at near-surface cobalt sites. A similar MOF, Co-ZIF-67, was also shown to photochemically reduce protons to hydrogen with a TOF of 1.1 h−1 (assuming 1 mg of material) by energy transfer from a ruthenium-based dye to the cobalt node.81 That Co-ZIF-9 and Co-ZIF-67 are effective as reduction catalysts points to the dynamic nature of metal-linker bonding 601


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Figure 2. Representations of some MOFs and linkers discussed throughout this Review.

in many, if not most, MOFs. In the absence of linker dissociation, presumably facilitated by metal reduction, individual cobalt centers lack the open coordination site needed to bind and activate carbon dioxide. The network nature of metal− ligand coordination in the ZIF ensures, however, that dissociation is reversible and that photoreduction does not structurally degrade the material. Notably, individual cobalt(I) ions (i.e., photoreduced nodes), in isolation, can deliver only one of the two electrons needed to convert CO2 to CO while the network arrangement of metal ions provides for hopping-based delivery of reducing equivalents from a neighboring cobalt(I). Bioinspired Molecular Catalysts Supported in Metal−Organic Frameworks. Enzymes and enzyme cofactors are one obvious place to turn for inspiration or insight into catalyst and photocatalyst design. For example, sulfur-containing diiron complexes that borrow structural features from the metal-containing site of hydrogenase have been found to function in solution as hydrogen evolution catalysts.82 Typically, however, their stability is insufficient to support more than a few turnovers when driven in a solution-phase ensemble with a molecular photosensitizer.83,84 Marrying bioinspired catalysts with metal−organic frameworks is a promising strategy for isolating and stabilizing such catalysts. One example is the hydrogenase-inspired catalyst [FeFe](dcbdt)(CO)6 (dcbdt = 1,4-dicarboxybenzene-2,3-dithiolate) 4. Devised by Pullen and co-workers and installed under mild postsynthetic conditions in a dcbdt-linker-doped version of UiO-66, this catalyst proved competent for hydrogen evolution (TON = 5, 60 min experiment).39 The required reducing equivalents were provided by excitation and sacrificial redox quenching of Ru(bpy)32+ in solution. As the diameter of Ru(bpy)32+ (∼11 Å)85 exceeds the width of the apertures in UiO-66 (∼6 Å),86 catalysis likely occurs mainly at or near the exterior surface of the MOF (Figure 3). In a similar example, biomimetic complex [(i-SCH2)2NC(O)C5H4N]-[Fe2(CO)6] was directly coupled to a porphyrin-based zirconium MOF

Figure 3. Simplified reaction scheme for the photocatalytic reduction of protons with UiO-66-[FeFe](dcbdt)(CO)6 and schematic representation of the framework components.39 Following sacrificial reduction of photoexcited Ru(bpy)32+, ascorbate chemically decomposes, thereby rendering the redox quenching ascorbate/ ruthenium reaction irreversible. For net energy conversion to be accomplished, these sacrificial processes ultimately must be eliminated, and the desired reductive photochemical reaction must be coupled to a viable oxidative process such as the conversion of water or hydroxide ions to dioxygen. Nevertheless, the use of sacrificial reactions greatly facilitates the isolation and investigation of specific reactions or reaction steps in otherwise complex systems.

also via postsynthetic modification. In the resulting material, the porphyrin acts to absorb incoming photons, obviating the need for an external photosensitizer.40 Photocatalytic H2 yields after 60 min of irradiation approach 3 μmol and coincidentally mirror those reported for UiO-66-[FeFe](dcbdt)(CO)6. 602


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Nanoparticle Catalysts. Nanoparticles (NPs) comprising precious metals such as platinum or palladium can be highly catalytic for fuel-forming reactions.87 Using NPs is desirable in the context of solar fuels catalysis if the following conditions can be met: (a) heterogenization of particles to prevent physical loss of the catalyst; (b) inhibition of agglomeration, so as to obtain maximum benefit from the catalysts; (c) coupling to light absorbers; (d) exclusion of catalyst poisons; (e) buffering of the NP/liquid interface; and/or (f) facilitation of product selectivity. Encapsulating catalytic NPs within metal−organic frameworks is, in principle, a viable approach to obtaining these outcomes. As an example, purpose-built MOFs based on UiO frameworks bearing Ir(ppy)2(bpy)+-derivatives (ppy = 2-phenylpyridine) 8 as linkers have been exploited by Wang and co-workers as light-absorbing frameworks to drive H2 formation via the catalytic intervention of enshrouded platinum NPs.37 Particle installation was achieved via infiltration of MOF pores with a platinum salt, followed by framework-sensitized reduction to Pt(0), which leads spontaneously to NP formation, a strategy that works well if the NPs do not need to be size-matched to specific pores and complete coating of the NP by the porous MOF is not essential. We note that for applications where these additional requirements come into play, other routes to NP encapsulation by frameworks are available.73,88,89 Wang et al. found that irradiation of iridium-sensitized Pt@MOF material, in the presence of a sacrificial agent, resulted in hundreds of turnovers (624−1620) for Pt-catalyzed hydrogen gas formation.37 Using a different framework (MIL-101; Figure 2) to encapsulate Pt NPs, He and co-workers showed that MOF-embedded CdS nanoparticles could be used in place of chromophoric MOF linkers for photocatalytic formation of H2 (TOF ≈ 300 h−1).90 He and co-workers subsequently showed that the concept could be executed simply by adsorbing the green-light-absorbing dye rhodamine B onto Pt@UiO-66 and using it to photochemically inject electrons into platinum nanoparticles91 (H2 production TOF ≈ 45 h−1).38 Photocatalytic Metal−Organic Frameworks Employing Both Nodes and Linkers as Active Components. In most of the preceding examples of MOFs as enablers of solar fuels formation, organic linkers function as photosensitizers or as immobilizing components of molecular catalysts while the role of nodes is solely a structural one (e.g., organizing linkers, enabling porosity, contributing chemical stability, etc.). Or, if the node is catalytically functional, the linkers play a passive role. Nevertheless, in a few cases chemical or photochemical catalysis entails the active participation (redox participation) of both nodes and linkers. One interesting example is found in the work of Fu et al. where they reported on a primary-amine-functionalized version of Ti8O8(OH)4(bdc)6, MIL-125(Ti) (bdc = benzene-1,4dicarboxylate 1; Figure 2), and its behavior as a photocatalyst for CO2 reduction.92 The −NH2 groups of the derivatized linkers (Figure 2; 2) serve to sensitize the material to blue light (λmax ≈ 400 nm), where the relevant electronic transitions are linker-to-metal charge transfers. This arrangement is reminiscent of a subset of a well-known dye-sensitized photoelectrochemical cells that rely upon direct molecule-to-electrode type chargetransfer transitions to absorb visible-region photons.93−97 It is wellunderstood that the bane of ligand-to-metal charge-transfer transitions (absent spin changes, coordination-number changes, etc.) is fast back electron transfer and concomitant loss of transiently stored solar energy. MIL-125(Ti) circumvents this problem by moving the “injected” electron from the initially photoreduced titanium ion to neighboring titanium ions,98,99

where the metal centers are organized in loops of eight oxobridged metal ions (Figure 2). The surfeit of redox-active titanium centers additionally enables (at least in the presence of sacrificial redox species) accumulation of multiple Ti(III)-stored reducing equivalents, a necessity for reduction of CO2 at energetically accessible potentials. In the presence of dissolved CO2 and under irradiation with visible light, NH2-MIL-125(Ti) was observed to generate ca. 8 μmol of HCOO− (∼0.03 or 0.24 TON per molecular unit if each metal center is catalytically active) over the course of 10 h.100 Building on this success, Logan et al. systematically varied the nature of the amine group on the linker to generate a series of frameworks isoreticular to NH2MIL-125(Ti).101 Modifying the linker with secondary N-alkyl substituents (isopropyl, cyclopentyl, and cyclohexyl) resulted in larger apparent quantum yields of CO2 photoreduction (Φapp = 0.015−0.018) than primary alkyl substituents owing to the longer-lived excited-state lifetimes initiated by the secondary substituents. Additionally, a decrease in the optical band gap through the series was observed, from 2.56 eV in the parent NH2-MIL-125(Ti) to 2.29 eV in the cyclohexyl derivative.101 It has been proposed that similar photochemistry is responsible for catalytic activity of d0 NH2−UiO-66(Zr) and NH2− UiO-66(Hf) frameworks. These materials feature hexa-metal(IV) nodes, so one might imagine that they would be similarly effective at slowing back electron transfer. Nasalevich and coworkers have persuasively shown, however, that the 4d and 5d orbitals of Zr(IV) and Hf(IV), respectively, are far too high in energy to yield visible-region linker-to-metal charge-transfer bands.98 Instead, the yellow color and associated photochemistry for these MOFs is purely linker-based. Consistent with this assertion, the excited-state lifetimes of the UiO-66 derivatives are much shorter than for NH2-MIL-125(Ti), and the reported photocatalytic behavior of the Zr- and Hf-MOFs is arguably more tenuous. These assertions lead one to wonder about taking advantage of the energetically accessible 3d orbitals in Ti(IV) and using them in the node of NH2−UiO-66. Unfortunately, titanium centers are limited to hexa-coordination, so they cannot achieve the higher degree of coordination required of metal ions in UiO-type nodes. Nevertheless, one could envision appending one or more Ti(IV) ions to the exterior of a hexa-zirconium-, hexa-hafnium-, or hexa-cerium node, provided that the node is less than 12-connected to linkers, either by design or on account of missing-linker type defects.102 Electrocatalytic Metal−Organic Framework Materials. Although this Review has focused on the generation of solar fuels via photocatalytic MOFs, or MOF/chromophore assemblies, many of the chemical concepts are equally applicable to MOFs as electrocatalysts. By way of example, Lin and co-workers recently described the electrochemical oxidation of water by modified UiO-67 thin films grown solvothermally on conductive glass supports (fluorine-doped tin oxide).103 The modification, which entailed incorporation of a known water oxidation catalyst, Ru(dcbpy)(tpy)(OH2)2+ 9 (tpy = 2,2′:6,2″-terpyridine) as a linker (together with catalyst-free [1,1′-biphenyl]-4,4′-dicarboxylate), yielded porous, electrochemically active, and electrocatalytically competent MOF films. Specifically, the immobilized complexes proved to be stable for more than 1 h under water oxidation conditions (TOF = 0.2 ± 0.1 s−1 at pH 7 with an applied potential of 1.71 V vs NHE, a kinetic overpotential of about 880 mV). MOFs can also function indirectly by providing, in the immediate vicinity of a catalyst, an environment that facilitates 603


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Figure 4. (a) Schematic for electrodeposition of NiSx with a film of NU-1000 and (b) SEM image of an NU-1000_NiS film showing hexagonal rod-shaped crystals of NU-1000 on top of the FTO substrate (1 μm) with electrodeposited NiSx within the porous base of the MOF.104

eliminated if the system could be re-engineered to transport charges even a few to several times faster. The principles relevant to accomplishing efficient and effective MOF-based electrocatalysis should extend to photoelectrochemical catalysis with MOFs. For MOF-based photochemical catalysis not involving electrochemistry, the requirements (particularly for charge and exciton transport) may be less stringent than for photoelectrochemical catalysis, as excitons and/or charges need not move all the way to an electrochemical inter-

MOFs can also function indirectly by providing, in the immediate vicinity of a catalyst, an environment that facilitates proton transfer, provides local pH buffering, regulates ion transport, or exerts other favorable influences.

For MOF-based photochemical catalysis not involving electrochemistry, the requirements (particularly for charge and exciton transport) may be less stringent than for photoelectrochemical catalysis, as excitons and/or charges need not move all the way to an electrochemical interface.

proton transfer, provides local pH buffering, regulates ion transport, or exerts other favorable influences. An example from Hod and co-workers is illustrated in Figure 4.104 Electrodeposited nickel sulfide (Ni3S2) is moderately electrocatalytic for hydrogen evolution from aqueous HCl (pH 1). When integrated with the MOF NU-1000, however, its activity is substantially enhanced, with the kinetic overpotential for hydrogen evolution at a benchmark current density of 10 mA cm−2 decreasing by ∼240 mV. The precise mechanism for the observed cocatalytic behavior is unclear; notably, NU-1000 is redox inert over the range of potentials studied for hydrogen evolution. The MOF does, however, present weak acids (aqua and hydroxo ligands) in high density, and these presumably facilitate catalysis via one of the effects mentioned above. Another instructive case is the electrochemical conversion of CO2 to CO, via the catalytic participation of porous, electrodesurface-supported, MOF films featuring porphyrinic iron (or cobalt) units as linkers.68,105,106 Catalysis entails redox cycling between iron oxidation states II, I, and 0. Reducing equivalents are delivered to the catalytic sites via redox hopping between the metalloporphyrins themselves. In this scheme, under catalystreaction-rate-limited conditions (as opposed to transport-limited conditions) the per-metalloporphyrin turnover frequency in the MOF film is identical to that for the solution-phase, molecular version of the electrocatalyst. This finding is consistent with the ability of the MOF (specifically, Fe-MOF-525) to isolate individual porphyrins yet present them in high density. Hod et al. reported that the electrophoretically deposited MOF films present the equivalent of ∼900 metalloporphyrin monolayers each, but with high porosity.68 The overpotential, at modest current densities, for electrocatalytic reduction of CO2 to CO is a few hundred millivolts lower for the MOF than for the molecular (solution phase) catalyst. Given identical TOFs, the decreased overpotential is directly attributable to the much higher concentration of catalyst in the MOF (versus solution). At high overpotentials, the molecular catalyst outperforms the MOFimmobilized catalyst, supporting about three times as much catalytic current. This difference is a manifestation of comparatively slow charge transport through the MOF (i.e., an apparent charge diffusion coefficient of around 10−13 cm2 s−1). These results also imply that the charge-transport bottleneck could be

face. Instead, they need only move from a photoexcited linker (or node) to a catalytic site. Few-Atom Clusters for Solar Fuels Catalysis. A variety of enzymes for catalytic reduction reactions make use of metalcluster or metal-dimer containing cofactors, most notably nitrogenase (N2 reduction to ammonia) and various hydrogenases (H+ reduction to molecular hydrogen). Also worth noting is the cubane-like manganese and calcium oxo cluster employed by Photosystem II for catalytic oxidation of water to molecular oxygen.107 The presence of multiple redox-active metal ions in a single cluster provides a basis for storage and release of multiple redox equivalents at nearly identical formal potentials; in favorable cases, they may also provide a basis for uptake and release of multiple protons, where the change of redox state of the cluster provides the thermodynamic driving force. These properties, of course, are potentially of great value if clusters are employed as catalysts for solar fuels forming reactions (a subset of reactions involving transfer of multiple protons and multiple redox equivalents). Inorganic clusters occupy a regime of size and complexity beyond mononuclear metal complexes, but short of nanoparticles. Often, they can be obtained in monodisperse and structurally well-defined form, features that greatly facilitate computational modeling and hypothesis-driven understanding and refinement of the properties of clusters, including properties relevant to their use as catalysts. A small, but growing literature exists on the integration of solar fuels relevant catalytic clusters with metal−organic frameworks. One attractive approach is to start with preformed clusters, typically polyoxometalates (POMs), and install them within the pores of MOFs, either during MOF synthesis or in a 604


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Figure 5. Summary of the structure of NENU-500: (a) structural components PMoV8MoVI4O36(OH)4Zn4 (an ε-Zn-Keggin POM) and the acid form of the linker, benzene tribenzoate, and (b) 3D (3,4)-connected framework.110

postsynthetic step.108,109 Alternatively, POMs themselves can be used as building blocks (nodes) as illustrated by the MOF NENU500, which is composed of PMoV8MoVI4O36(OH)4Zn4 polyoxometalate nodes and benzene tribenzoate linkers (Figure 5).110 Composite films of NENU-500 and carbon black on glassy carbon electrodes are electrocatalytically competent for hydrogen evolution from aqueous acid, displaying a kinetic overpotential of just 240 mV at a benchmark current density of 10 mA/cm2. Consistent with the comparatively low overpotential, DFT calculations indicate a nearly ideal free energy (ΔGH°) of adsorption for atomic hydrogen on the POM’s bridging oxo ligands, where atomic hydrogen can be obtained from solution hydronium ions in combination with the electrochemically reduced POM (i.e., molybdenum-reduced POM). A recurring challenge with nonstructural polyoxometalates in MOFs is to retain MOF porosity after POM incorporation. While not yet widely explored, an initially promising strategy is to use MOFs that are characterized by hierarchical porosity, with one pore type providing POM siting and other pore types providing porosity.111 Alternatively, one may envision a scenario where pore/POM size mismatches are great enough to leave POMs accessible to candidate reactants, but with small enough pore apertures to inhibit POM escape from the MOF (or with complementary charges for guests and pore walls). One example of this methodology is the encapsulation of P2W18O626−, a Wells−Dawson POM, by a UiO-type MOF featuring tetra-aryl linkers, 4,4′-(phenyl-carboxylate)-2,2′-bipyridine, where the linkers double as chelating ligands for Ru(bpy)32+. P2W18O626− is a known electrocatalyst for hydrogen evolution from aqueous acid; visible light excitation of the POM@UiO material in the presence of a sacrificial regenerator engenders sustained evolution of molecular hydrogen (TONs reaching 540 over the course of 36 h of photolysis).112 Qualitatively similar results (1400 turnovers in 72 h) were obtained with Ni4(H2O)2(PW9)34)210− as the POM component and IrIII(2-phenylpyridine)2(bpy)+ 8 as the linker-immobilized light-absorbing component.113 In principle, POM@MOF materials for photocatalytic oxygen evolution should be experimentally accessible as well. Additionally, as purely inorganic constructs, appropriately chosen POMs should be highly resistant to oxidative degradation under catalytic conditions. An intriguing alternative approach takes advantage of accessible aqua and hydroxo ligands (nonstructural ligands) on the chemically and thermally robust nodes of mesoporous MOFs such as NU-1000 and exposes these ligands to volatile and

highly reactive molecular organometallic compounds either in the vapor phase or in an inert solvent such as heptane. Exposure leads to (a) attachment of the molecular compound to the node via shared oxo or hydroxo ligands and (b) protonation and volatile release of a fraction of the molecular compound’s original ligands. The organometallic species can be further reacted with steam or H2S to yield node-supported, metal-oxy or metal-sulfide-oxy species. Depending on the chemical identity of the organometallic precursor molecule, four, six, or eight metal ions are typically installed per node, with the ions selforganizing as clusters, usually at a density of one cluster per node (Figure 6). This two-step installation process is reminiscent of many atomic layer deposition (ALD) schemes. ALD is a self-limiting, conformal, materials synthesis technique that is especially wellsuited to the task of growing ultrathin (atomically thin) metaloxide, -nitride, -sulfide, and/or -carbide films on surfaces, including complex and/or high-area surfaces.114 Notably, the cluster synthesis chemistry is similarly self-limiting and conformal, i.e., clusters are of fixed size, and can be formed on nodes throughout the high-area interior of the MOF. Just as thicker films can be grown on supporting surfaces by repeating the two-step ALD process, larger clusters, including mixed-metal clusters, can be grown on MOF nodes by repeating the corresponding two-step synthesis cycle. Because the clusters are nonstructural components of the MOF, changes in cluster or metal-ion oxidation state, coordination number, or degree of protonation (changes that typically accompany catalytic cycling) do not adversely affect MOF stability. On-node synthesis of perhaps two dozen chemically distinct kinds of clusters via ALD-like methods has now been demonstrated, including oxy and/or oxy, sulfide clusters, most firstrow transition metals, and some main-group metals.115−120 We postulate that this same synthetic methodology may be applied

On-node synthesis of perhaps two dozen chemically distinct kinds of clusters via ALD-like methods has now been demonstrated, including oxy and/or oxy, sulfide clusters, most first-row transition metals, and some main-group metals. 605


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Figure 6. Schematic representation of the automated synthesis of MOF-supported arrays of few-atom clusters comprising metal-oxo, hydroxo or metal-sulfide, oxo species. The clusters are grown in from vapor- or solution-phase reactants via ALD-like chemistry. The self-limiting nature of the two-step (A + B) synthesis cycle fixes the sizes of the clusters. Larger clusters of similarly uniform size can be obtained by running additional AB cycles.

tasks, MOF-specific design principles are in hand; these should accelerate the pace of discovery and facilitate hypothesis-driven research on solar fuels catalysis. The obvious strengths of MOFs as a family of porous functional materials are their enormous chemical diversity, their well-defined structures (an essential feature for efficient exploitation of computational modeling as a design and discovery tool), and their compatibility with modular elaboration. Once developed, a well-chosen mesoporous MOF can be used as a broad platform for installation and presentation of dyes, molecular catalysts, cluster-based catalysts, nanoparticulate catalysts, redox shuttles, and/or other functional entities. Notably, the installed moieties can be precisely sited and can be protected from agglomeration with similar moieties or interference by complementary moieties. The achievable complexity and tunability of the resulting photocatalytic entities, even at this early stage, arguably would be difficult to emulate via conventional molecular or supramolecular synthetic chemistry.

to clusters relevant to catalytic formation of solar fuels, and to date, only a small number of examples have been described, with all of them featuring NU-1000 as the MOF scaffold. Electrocatalytic oxidation of aqueous hydroxide to dioxygen has been demonstrated with node-supported tetra-cobalt(II)oxo, hydroxo clusters.121 Photocatalytic reduction of water to H2 has been demonstrated with tetra-nickel(II) clusters featuring mixed ligation (oxo and sulfide ligands).119 Photocatalysis was initiated by near-UV irradiation of the 1,3,6,8-tetrakis(p-benzoic acid)pyrene 11 linkers of NU-1000, which are potent reductants and rapidly undergo electron transfer to the Ni(II) cluster. A sacrificial reagent then restores the linker to its original form. Notably, at neutral pH the cluster-functionalized framework carries a net negative charge affording the ability to exchange cationic dye molecules (Rose Bengal) into the MOF, where they can then function as auxiliary photosensitizers, expanding the material’s spectral coverage and boosting its quantum efficiency for H2 formation under white light illumination. Finally, electrocatalytic reduction of CO2 in water to CO has been demonstrated with MOF-immobilized, copper nanoparticles.122 Oxide clusters of easily reducible metal ions, such as Cu(II) and Pt(IV), have proven amenable to conversion to metallic clusters and nanoparticles via exposure to H2.89,123 Intuitively, the sizes of the metal(0) clusters and nanoparticles are defined and constrained by the widths of the MOF pores. As numerous other cluster@MOF compositions have been evaluated as atomically precise heterogeneous catalysts for gas-phase reactions, and because innumerable mixed-metal compositions are, in principle, synthetically accessible, considerable further exploration of ALDlike synthetic MOF derivatizations for potential solar fuels applications seems likely. Summary and Future Outlook. Among the many tasks that may need to be accomplished for catalytic production of solar fuels by a given artificial system are (a) broad-spectrum light harvesting, (b) excitonic energy transport, (c) charge transport, (d) reductive catalysis (especially water reduction to H2 and CO2 conversion to hydrocarbons), (e) water oxidation to O2, (f) exciton splitting and electron or hole transfer to catalysts, (g) accumulation of multiple redox equivalents by catalysts (to enable two-, four-, or six-electron chemical transformations), and (h) integration of catalysts and light harvesters with electrodes. At least one example of MOF-based execution exists for each of these candidate tasks. Additionally, for many of the

For many of the tasks, MOF-specific design principles are in hand; these should accelerate the pace of discovery and facilitate hypothesis-driven research on solar fuels catalysis. Clearly there is still much work to be done in devising MOFbased photocatalytic systems that completely eschew sacrificial reagents and that bring together all of the functions needed for stand-alone photocatalytic production of solar fuels, especially if the goal is to obtain these fuels in a reasonably efficient fashion. Narrower challenges are to impart to a broader selection of MOFs good chemical stability in highly basic solutions (a common milieu for catalytic water oxidation) and in phosphate-based aqueous buffers (often used for catalytic CO2 reduction) and to

An enormous variety of new MOF-based or MOF-supported, solar-fuels-relevant photocatalytic systems can be envisioned. 606


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develop broadly applicable ways of imparting significant electronic conductivity to MOFs, but without sacrificing chemical permeability and molecular-scale porosity.124−130 An enormous variety of new MOF-based or MOF-supported, solar-fuels-relevant photocatalytic systems can be envisioned, with perhaps most of them no better or no worse than the few dozen proof-of-concept systems already in the literature. The challenge then is not to expand the number of examples but to deploy MOF-centric chemistry for evaluation of ideas and development of attractive new catalysts or light-absorbers that may be difficult or impossible to evaluate or develop by other means. We suggest that (a) catalysts based on well-defined, fewatom clusters and (b) panchromatic light-absorbers comprising MOF-organized ensembles of chemically simple, narrow-band chromophores are specific topical areas where a MOF-centric approach will prove especially fruitful.

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Omar K. Farha is an Associate Professor of Chemistry at Northwestern University, Distinguished Adjunct Professor at King Abdulaziz University, Chief Scientific Officer of NuMat Technologies, and associate editor for ACS Applied Materials and Interfaces. His current research spans diverse areas of chemistry and materials science ranging from energy to defense-related challenges. Specifically, his research focuses on the rational design of metal−organic frameworks (MOFs) and porous-organic polymers for sensing, catalysis, storage, separations, and light harvesting. Omar was named one of the “Highly Cited Researchers” in 2014, 2015, 2016 and 2017.

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ACKNOWLEDGMENTS Work done at Northwestern was supported by the ArgonneNorthwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under award number DE-SC0001059.

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AUTHOR INFORMATION

Corresponding Authors

DEDICATION Dedicated to Prof. Susumu Kitagawa in honor of his 65th birthday and in recognition of his seminal contributions to metal−organic framework chemistry.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

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Marek B. Majewski: 0000-0002-5190-7193 Michael R. Wasielewski: 0000-0003-2920-5440 Joseph T. Hupp: 0000-0003-3982-9812 Omar K. Farha: 0000-0002-9904-9845

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Notes

The authors declare no competing financial interest. Biographies Marek B. Majewski is an Assistant Professor of Chemistry at Concordia University (Montréal). As a postdoctoral research fellow at the Argonne-Northwestern Solar Energy Research (ANSER) Center and Northwestern University he worked together with Prof. Wasielewski, Prof. Hupp, and Prof. Farha on photoinitiated charge separation and transport to drive molecular solar fuels catalysts and to design functional inorganic materials. He completed his B.Sc. at the University of Saskatchewan followed by his Ph.D. degree under the mentorship Prof. Michael Wolf at the University of British Columbia. Aaron W. Peters is a Chemistry Ph.D. candidate in Professor Joseph Hupp’s group at Northwestern University. He received his B.S. degree in Chemistry in 2013 at Ursinus College. His research is focused on studying structure−function relationships using porous materials in the context of energy-related catalysis. Michael R. Wasielewski is the Clare Hamilton Hall Professor of Chemistry at Northwestern University; Executive Director of the Institute for Sustainability and Energy at Northwestern (ISEN); and Director of the Argonne-Northwestern Solar Energy Research (ANSER) Center, a DOE Energy Frontier Research Center. He received his Ph.D. from the University of Chicago and was a postdoctoral fellow at Columbia University and Argonne National Laboratory. His research focuses on light-driven processes in molecules and materials, artificial photosynthesis, molecular electronics, and molecular spintronics. Joseph T. Hupp is a Morrison Professor of Chemistry at Northwestern University. He was a student of the late Mike Weaver at Michigan State University and Purdue University, completing a Ph.D. degree in 1983. He was a postdoc at the University of North Carolina. His research centers on energy- and defense-relevant materials chemistry, including chemical storage and separations, catalysis, light-to-electrical energy conversion, and catalytic water oxidation. His research findings place him among the world’s most highly cited chemists as assessed by Thomson-Reuters. 607


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ACS Energy Letters

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