Metal–Organic Frameworks for Electrocatalysis: Beyond Their Derivatives

not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/smsc.202100015 This article is protected by copyright. All rights reserved DOI: 10.1002/smsc.202100015 Article type: Feature Review Metal-Organic Frameworks for Electrocatalysis: beyond Their Derivatives Yongchao Yang, Yuwei Yang, Yangyang Liu, Shenlong Zhao* and Zhiyong Tang*

design of MOFs to target the different electrochemical reactions.
In terms of catalysis, MOFs are ideal candidates for the exploration of structureperformance relationships due to their periodic atomic arrangement and well-defined structure. Recently, the lattice-strained NiFe-MOFs were employed as highly active bifunctional electrocatalysts for identifying the active sites and key intermediates during the oxygen redox reaction. [13c] Electrochemical results suggested that the as-  Accepted Article 8). [18a] Accepted Article [18a] Both experimental results and density

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Both experimental results and density calculation revealed that the electron Accepted Article calculation revealed that the electron This article is protected by copyright. All rights reserved 90.57 % was achieved by mixing the doped ZIFs with a conductive filler.
As for the practical application, it is well established that the major limiting factors are the cost and the performances of catalysts or electrode materials. For instance, even though Ru/Ir-based materials possess high catalytic activities for OER, their high cost prevents them from being used in industrial-scale electrolyzers. As  is required, 1 kg terephthalic acid and 3.5 kg zinc acetate dihydrate with 63% yield, and these raw materials cost 414.6, 34.0, and 78.7 USD, respectively. Hence, its total cost is roughly 527 USD kg -1 , which is much lower than that of equivalent Ru or Ir. [26] Armed with superior catalytic performances, MOFs are becoming the favored choice for mass production and practical application to replace the expensive commercial noble metals.

ORR
The cathodic ORR involving a four-electron transfer is the rate-determining step (RDS) in various sustainable and efficient energy conversion techniques including fuel cells and metal-air batteries owing to its sluggish kinetics. [27] Presently, carbonsupported Pt nanoparticles (Pt/C) have been considered as the commonly used ORR catalysts with excellent electrocatalytic performances. [28] Nevertheless, the scarcity two-dimensional (2D) Ni 3 (HITP) 2 MOF (HITP = 2,3,6,7,10,11hexaiminotriphenylene) with decent electrical conductivity of σ = 40 S cm -1 was successfully synthesized and used as the electrocatalyst for ORR (Figure 3). [30] Remarkably, Ni 3 (HITP) 2 MOF presented a positive E onset of 0.82 V and a small Tafel  (Figure 4). [31] A highly crystalline structure and optimized spin state of the cobalt centers in PcCu-O 8 -Co were obtained, where the unpaired electron in the σ* antibonding orbital ( e g = 1) in the cobalt node brought about the remarkable ORR activity of Co-O 4 centers. [16f] Notably, the resultant This article is protected by copyright. All rights reserved during ORR and OER, further revealing a 4eredox reaction mechanism of highperformance NiFe-MOFs.

HOR
Apart from the most commonly used proton-exchange membrane fuel cells catalytic HOR activities when transferred from acidic electrolytes to basic environments. [34] Also, the high price of PGM catalysts is another big concern, with ∼80% cost on the cathode side and the rest on the cathode side. [35] Hence, seeking high performance, low-cost and earth-abundant HOR electrocatalysts for HEMFCs is highly desired to minimize Pt utilization. Making use of the modular nature and decent controllability of MOFs, pristine MOF-based materials stand out from a variety of HOR catalysts. For example, Sun and co-workers creatively reported a Nibased MOF (Ni-BTC, BTC = benzene-1,3,5-tricarboxylic acid) transformed to a costeffective Ni/NiO/C electrocatalyst with abundant Ni/NiO interfacial sites for HOR in alkaline media (Figure 5a, b). [36] The as-fabricated MOF-based material exhibited a much-boosted electrocatalytic HOR performance, which was firmly evidenced in terms of its E onset of nearly 0, superior anti-carbon-monoxide poisoning ability and long-term stability compared to commercial Pt/C, thus manifesting its super-high

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Apart from the most commonly used proton

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Apart from the most commonly used proton Accepted Article [34] Also, the high price of PGM catalysts is another big concern, with Accepted Article Also, the high price of PGM catalysts is another big concern, with 80% cost on the cathod A "volcano" relationship between the binding energy of hydrogen and HOR activity strongly proves that the adsorbed hydrogen is a key HOR intermediate. [37] In consequence, skillfully adjusting hydrogen adsorption will be a powerful strategy to accelerate HOR electrocatalysis of MOFs. In this respect, a highly porous Ni-MOFbased catalytic material (Ni/NiO/NC) composed of Ni 2+ metal sites and 2methylimidazole ligands was developed most recently (in 2021) and then probed for its bifunction for both HOR and ORR catalysis under an alkaline environment (Figure 5c-g). [38] Benefiting from its favorable composition and well-defined heterostructures, this electrocatalyst showed a high mass activity of 9.09 mA mg Ni -1 at an overpotential of 50 mV, outperforming the previously reported Ni-based catalysts for the 2etransfer HOR process. [36,39]

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A "volcano" relationship between the binding energy of hydrogen and HOR activity

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A "volcano" relationship between the binding energy of hydrogen and HOR activity strongly proves that the adsorbed Accepted Article strongly proves that the adsorbed consequence, skillfully adjusting hydrogen adsorption will be a powerful strategy to    Accepted Article g). [38] Accepted Article [38] heterostructures, this electrocatalyst showed a high mass activity of 9.09 mA mg Impressively, the asprepared NENU-500 MOF with sufficient exposed active sites afforded a high catalytic HER activity in acidic media with a small Tafel slope of 96 mV dec -1 , an E onset of 180 mV, and a current density of 10 mA cm -2 at a relatively low overpotential of 237 mV (Figure 6a, b). Moreover, it exhibited not only decent stability in the air but also excellent tolerance in both acidic and basic media. In another study, Hod et al. Accepted Article electrolyzers. [41] Accepted Article [41] Among the numerous works of replacing Pt

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Among

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-500 MOF with sufficient exposed active sites afforded a high Accepted Article 500 MOF with sufficient exposed active sites afforded a high Accepted Article

Figure 6
This article is protected by copyright. All rights reserved overpotential for HER on the NU-1000 electrode was ~210 mV lower than that of pristine electrodeposited Ni-S under identical conditions. The excellent activity was attributed to the facilitated proton transport and the improved immediate chemical environment caused by NU-1000 MOF during the HER electrocatalysis.
Apart from the metal nodes, the organic linkers also exert significant impacts on the Simultaneously, the 3d projected density of states (PDOS) revealed that defect-rich MOF nanosheet arrays nearly displayed a metallic phase property owing to the incorporation of K + , suggesting its better electrical conductivity. The work highlighted that the creation of defects and the improvement of electrical conductivity were vital to tailor the catalytic properties of MOF-based catalysts.

OER
OER at the anode involving a four electrons transfer is the key step that limits the energy conversion efficiency of many clean energy devices, such as water electrolyzers and rechargeable metal-air batteries. [42] Ru-or Ir-based catalysts are regarded as the most efficient OER catalysts so far, however, the prohibitive cost and scarcity severely hinder their widespread implementation. In the past few decades, tremendous efforts have been devoted to the development of various nanomaterials (such as metal oxides, hydroxides and perovskites) to replace noble metal-based ones.
[43] Among them, MOFs are the ideal platforms for constructing these efficient OER electrocatalysts. [44] First, different from traditional doped or compound catalysts, the metal tunability and periodic element arrays of MOFs allow us to optimize the activities of bimetallic or multimetallic catalysts at a precisely controllable atomic level, which is extremely beneficial for promoting the OER catalytic activity via electronic coupling. [45] Meanwhile, the periodic structure of MOFs provides an explicit molecular mode to explore the exact structure-performance relationship at the atomic or molecular level in various electrocatalytic processes. [46] Additionally,

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techniques were employed to probe the structural transformation occurring at metal nodes of MOF-74. Noteworthily, there was a two-phase structural transition found at  Accepted Article novel hierarchical foam-like MOF architecture, which provided an effective platform towards the tunability of MOFs' morphologies and compositions.

CO 2 RR
In recent years, the concentration of CO 2 in the atmosphere has been experiencing an increasingly rapid rise, gradually leading to undesired climate change (e.g., ocean acidification and global warming). [49] Hence, converting carbon dioxide into various high value-added organic fuel molecules (e.g., CO, CH 3 OH, CH 4 , HCOOH, etc.) via effective electrochemical technologies has been considered as a promising strategy to recycle and utilize this abundant and inexpensive carbon resource. [50] Furthermore, the electrocatalytic CO 2 RR that takes place at room temperature and under ambient pressure holds enormous promise from both economic and environmental perspectives. [51] However, due to the thermodynamically stable and kinetically inert nature of CO 2 , as well as the competition with HER in aqueous electrolytes, highly active and selective electrocatalysts are particularly significant in this endeavor. In recent years, some pristine MOF-based catalysts have been extensively investigated to electrochemically catalyze the CO 2 RR process, which can outperform many noble metal-based catalysts. [18a, 19g, 19h] As a typical product of CO 2 conversion, CO can be directly used to create multiple complex carbon-based fuels and feedstocks, such as the methanol generated by hydrogenation through gas-to-liquid conversion reactions and the liquid hydrocarbon fuels produced via the Fischer-Tropsch process. TOF of 4.21 s -1 over 48 h, which rivaled the state-of-the-art catalysts. [53] By combining XAS results and theoretical computation, the structure-activity relationship was unambiguously revealed. The superior CO 2 RR activity of STPyP-Co was attributed to the decreased activation energy of RDS. Furthermore, the molecular orbital analysis suggested that the Co with raised d z 2 orbital could be reduced more easily by filling the d z 2, leading to an excellent electrocatalytic CO 2 RR performance.
This work not only offered a general bottom-up synthetic strategy for creating heterogeneous pristine MOF-based catalysts with the optimized catalytic CO 2 RR activities at the molecular orbital level but also laid the foundation towards the establishment of the structure-activity relationship.
To achieve an effective electroreduction of CO 2 into CO using pristine MOFs, introducing external fields like light irradiation also offers an innovative platform towards optimizing catalytic pathways and properties by altering electrocatalysts' electronic performances including electron transfer, band-bending, charge distribution, Fermi level, and desorption energy of intermediates. [54] Based on this, Wang and coworkers recently mimicked the structure of chlorophyll to construct a series of zirconium porphyrinic MOF hollow nanotubes as photo-coupled electrocatalysts for CO 2 RR. [55] Under visible light input, the as-synthesized pristine MOF-based materials

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RR. [55] Accepted Article [55] Under visible light input, the as

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Under visible light input, the as This article is protected by copyright. All rights reserved and stimulate external electron transfer from the ground state to T1 state. With the help of experimental results and DFT computational analysis, this work successfully verified the high feasibility of "photo-coupled pristine MOF electrocatalysis" and provided a unique route for CO 2 electroreduction at a low overpotential.
In addition to CO products, a series of 2D Cu 2 (CuTCPP) MOFs, porphyrin MOF nanosheets based on copper(II) paddle wheel cluster, were recently constructed and then used for efficient electroreduction of CO 2 to high-value products such as formate and acetate (Figure 11). [19h]  intermediates, thereby significantly facilitating the CO 2 RR process due to the synergistic enhancement of porphyrin-Cu(II) complexes. Significantly, this work proposed a new-found concept 'pre-design' for the intelligible design of highperformance pristine MOF-based electrocatalysts.
catalytically active materials into/onto the pores, matrices, or layers of MOFs, is able to enrich the features of MOFs by taking their essence and discarding their dregs, leading to both their activity enhancement and framework stabilization. [56] In 2014, Allendorf's group reported a 'star' Cu-based MOF, HKUST-1 doped with an organic semiconductor 7,7,8,8-tetracyanoquinodimethane (TCNQ), which was the first MOF possessing tunable electrical conductivity in line with the "Guest@MOF" concept. [57] This work certainly laid a solid foundation towards the following-up development of

Precious-Metal-Based MOF-HGCCs
To surmount both some inherent limitations of pristine MOFs (such as low electrical conductivity and the blockage of catalytically active sites by organic linkers) and the major challenge of scale-up of high-performance precious-metal-based To optimize the electrocatalysis efficiency of this type of catalytic materials for critical reactions, enormous efforts have been devoted to tuning their sizes, morphologies and compositions. As an example of monofunctional MOF-HGCCs containing precious metals for boosting electrochemical CO 2 conversion, the guest Ag 2 O nanoparticles were immobilized into the host layered ZIF-7 to synthesize Ag 2 O@ZIF hybrids via a simple one-pot hydrothermal treatment (Figure 12a). [59] As presented in Figure 12b,  Accepted Article presented in Figure 12 Accepted Article  and ultrathin 2D MOF nanosheets with admirable dispersity and stability (Figure   12d). [61] As suggested in Figure 12e Ni foam (Figure 12h). [64] Benefiting from the quantum size and core-shell structure, whereas the Pt QDs@Fe-MOF electrocatalyst possessed ultralow content of Pt (1.84 μg cm -2 ), the overpotential of 33 mV and 191 mV were realized at 10 and electrodes showed exceptional activity and stability with a current density of 10 mA cm -2 at 1.47 V for overall water splitting of at least 100 h. This work proposed a unique porous core-shell structure to judiciously engineer the MOF-HGCCs for water splitting in industrial practices.

Precious-Metal-Free MOF-HGCCs
In recent years, researchers have been persistently committed to further improving the overall catalytic performances of MOF-HGCCs and minimizing their costs as much as possible. Apart from the above-mentioned good employment of precious-metalbased materials, the incorporation of foreign materials without precious metals into MOFs also provides an alternative direction for the synthesis of MOF-HGCCs and simultaneously lowers the activation energy of catalytic reactions through host-guest cooperation within MOFs. [65] Hence, the optimized precious-metal-free MOF-HGCCs can be practically applied to diverse clean energy devices such as fuel cells, electrolyzers and metal-air batteries.
In 2015, Li and co-workers creatively stabilized ε-MnO 2 nanorods on a Fe-MOF support to prepare a highly porous ε-MnO 2 /MOF(Fe) composite for oxygen reduction in alkaline solutions. [66] The resulting Mn-based MOF-HGCCs yielded a superior activity and stability than those of ε-MnO 2 during the ORR process, even comparable to the commercial 20% Pt/C, in which ORR favored an obvious four-electron transfer pathway. To further enhance the ORR activity of MOF-HGCCs, various amounts of CuS nanoparticles were inserted in 3D Cu-MOFs, [Cu 3 (BTC) 2 ⋅(H 2 O) 3 ] (BTC = 1,3,5benzenetricarboxylate), to construct CuS@Cu-BTC composites with exponentially increased (109-fold) conductivity. [67] In spite of the poor porosity obtained, the resultant material exhibited a boosted ORR activity by a quasi-four-electron pathway Accepted Article

Precious
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CuS nanoparticles were inserted in 3D Cu
This article is protected by copyright. All rights reserved in terms of its E onset of 0.91 V and kinetic current density of 11.3 mA cm -2 at 0.55 V.
Except for the introduction of metal-based materials into MOFs for alkaline ORR,

Sohrabi et al. recently proposed a bio-inspired heme-like MOF-HGCC through an
amazing assembly of pyridine-functionalized graphene (G-py) and a 3D MOF for promoting acidic ORR (Figure 13a). [68] Thanks to the favorable role of a stable porous coordination network (PCN-222) in tuning electronic and geometric structures of active metal centers, the ORR process was markedly accelerated with excellent stability in acidic media. These works opened the door for the rational design of inexpensive and highly stable next-generation ORR electrocatalysts for fuel cells without precious metals and the pyrolysis process.
To achieve highly efficient water splitting, various MOF-HGCCs without precious metals have been developed to facilitate half-reactions, i.e., hydrogen evolution and/or oxygen evolution. As a promising example for HER, a copper(II)phthalocyanineincorporated MOF-HGCC (denoted as CuPc@MOF) was recently synthesized with an exceedingly boosted HER activity compared to the corresponding pristine MOF. [69] And the synergetic effect of the CuPc crystals on the MOF matrix was revealed as the main justification for its performance improvement. Most recently, Do et al. followed host-guest chemistry using a simplistic solvothermal strategy to develop cost-effective HER catalysts by combining amorphous molybdenum sulfide (MoS x ) and Ni-MOF-74 (Figure 13b, c). [70] They also suggested that NiMoS phases can lower the  This article is protected by copyright. All rights reserved basic media, which exhibited better catalytic properties than commercial IrO 2 . [71] In 2021, Yao et al. reported an in situ cathodic electro-transformation method to synthesize a cobalt hydroxide coated Co-MOF (named as Co(OH) 2 /Co-MOF) as the latest precious-metal-free MOF-HGCCs for OER electrocatalysis (Figure 13d-f). [72] Experimental analysis and DFT calculations revealed that the enhanced OER performance of the as-prepared material including long-term stability and high activity with an overpotential of 196 mV at 10 mA cm -2 for 15-hour continuous testing in a 1 M KOH solution, was ascribed to the tailored adsorption free energy of oxygenic intermediates and unique structure with abundant exposed active centers and well-tuned gas release ability. This work laid the foundation for the future design of MOFs with coordinatively saturated metal centers for OER. In another study, the boosted overall water splitting including HER and OER can be attained by a low-cost, highly active and stable Co-based MOF-HGCC (Co 3 O 4 @Co-MOF). [73] The dualfunction MOF-based hybrid was easily produced by confining guest Co 3 O 4 nanocubes on the surface of the Co-MOF nanosheet in the light of a facile one-pot hydrothermal method, which had great potential to be extended to the fabrication of other metal oxides/hydroxides@MOF composite materials.
As one of the high-efficiency heterogeneous electrocatalysts with minimal cost, the precious-metal-free MOF-HGCC has also turned out to be an encouraging candidate for electroreduction of CO 2 . For instance, Kung et al. for the first time immobilized copper(II) clusters into the solvothermally grown thin film of a zirconium MOF (NU-1000) by a solvothermal deposition method to synthesize a single-site Cu-based MOF-HGCC for CO 2 reduction in 2017. [74] In particular, the metallic Cu

copper(II) clusters into the solvoth
This article is protected by copyright. All rights reserved performance towards CO 2 conversion with formate as the major product, in which the crystallinity and morphology of this MOF-HGCC remained unchanged after the electrocatalysis. Further, they found that the size of the MOF channels exerted decisive impacts on the size of copper nanoparticles acquired using this method. More recently, Qiu and co-workers reported another Cu-based MOF-HGCC towards efficient CO 2 conversion by a time-resolved controllable restructuration from Cu 2 O to Cu 2 O@Cu-MOF in the presence of the 1,3,5-tricarboxylic acid (H 3 BTC) ligand. [75] Armed with ultrahigh specific surface area for enhanced CO 2 adsorption capacity and the boosted charge transfer bought by sufficient Cu 2 O active sites, the as-prepared

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applying them as a robust platform to confine active sites into organic mesh ce Accepted Article applying them as a robust platform to confine active sites into organic mesh ce This article is protected by copyright. All rights reserved

Conclusions and Perspectives
As an emerging class of electrocatalysts, MOFs have great potential to replace or complement current noble metal-based and carbon-based catalysts due to their ultrahigh specific surface area, interconnected porosity and accessible metal sites, as well as high controllability in both components and structures. The development of highly efficient and inexpensive pristine MOFs and MOF-HGCCs for electrocatalysis involved in the key reactions (i.e., ORR, HOR, HER, OER, and CO 2 RR) of renewable energy conversion and storage has become an emerging research field in the past few years. In this article, we summarize the unique merits of MOF-based electrocatalysts associated with their material design principles and mechanism discussion. And a comprehensive but critical review of recent advances in MOF electrocatalysis including oxygen reduction, hydrogen oxidation, water splitting, and CO 2 reduction is provided.
Although tremendous progress has been made, the study on MOF electrocatalysis is still in its infancy, and many challenges (such as low electrical conductivity, bad contact efficiency and poor stability, etc.) are becoming the major limitations for the performance enhancement of MOF-based materials and awaiting to be overcome urgently. Herein, a comparison of the best catalytic performances of pristine MOFs and other materials is presented in

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provided.
Although tremendous progress has been made, the study on MOF electrocatalysis is

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Although tremendous progress has been made, the study on MOF electrocatalysis is still in its infancy, and many challenges (such as low electrical conductivity, bad

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. [24a] Accepted Article [24a] More importantly, the related formation mechanism of MOFs needs to

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More importantly, the related formation mechanism of MOFs needs to be elucidated, which will lay a foundation for the functionalization of MOFs.  Especially, XAS analysis has been extensively applied to determine the oxidation state of metal centers, bond length, short-range disorder, coordination number and local geometry. Sixth, the computational simulations are exceedingly useful to understand the mechanisms related to electrocatalytic activities and stability. Armed with such novel knowledge acquired from experiments and computational calculations, ones will be able to easily functionalize MOFs at the molecular/atomic level and implement the advanced catalytic reactions with high catalytic activities, excellent selectivity and ultra-long-term stability.
In summary, further in-depth research efforts are needed to surmount the challenges discussed above to obtain low-cost, high-performance pristine MOF-based electrocatalysts, and even MOF-based composites, ultimately achieving the benchmarking electrocatalytic performances in the practical application of renewable energy conversion and storage.