Advances in metal–organic frameworks and their derivatives for diverse electrocatalyticapplications

Pan, Yangdan; Abazari, Reza; Wu, Yuhang; Gao, Junkuo; Zhang, Qichun

Published in:Electrochemistry Communications

Published: 01/05/2021

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Published version (DOI):10.1016/j.elecom.2021.107024

Publication details:Pan, Y., Abazari, R., Wu, Y., Gao, J., & Zhang, Q. (2021). Advances in metal–organic frameworks and theirderivatives for diverse electrocatalytic applications. Electrochemistry Communications, 126, [107024].https://doi.org/10.1016/j.elecom.2021.107024

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Electrochemistry Communications 126 (2021) 107024

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Mini Review

Advances in metal–organic frameworks and their derivatives for diverse electrocatalytic applications

Yangdan Pan a, Reza Abazari a, Yuhang Wu a, Junkuo Gao a,*, Qichun Zhang b,*

a Institute of Functional Porous Materials, School of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China b Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China

A R T I C L E I N F O

Keywords: Metal-organic framework Porous material Modified electrode Electrocatalysis Advanced oxidation/reduction

A B S T R A C T

Oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), ni-trogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2RR) are five major reactions in the progress, conversion, and storage of clean energy. Efficient electrocatalysts can be used to decrease the over-potential of these reactions and elevate practical applications of energy-related systems. Metal–organic frame-works (MOFs), via the modification of electrodes, provide new opportunities to develop green and sustainable energy systems and conduct fundamental research on electrocatalytic applications. Benefitting from the versa-tility of engineering and construction strategies, compositional/structural modification, high electrical conduc-tivity, various high-density active sites, and the capability as precursors, MOFs and the derived materials are considered as efficient electrocatalysts. Based on those merits, the electrochemical application of MOFs and their derived materials for the five mentioned reactions are briefly discussed in this mini review. Furthermore, the challenges and perspectives for engineering MOF-based electrocatalysts are also provided.

1. Introduction

The ever-growing request for energy, the limited fossil fuel resources and the growing environmental concerns have convinced researchers to find new sustainable energy conversion and storage technologies with higher efficiency, lower cost, and less greenhouse gas emission [1,2]. Some clean technologies to store and convert energies, such as fuel cells, water splitting, and rechargeable batteries, have drawn huge attention [3–5]. These technologies involve some basic electrocatalytic reactions such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2RR). In the case of these electrocatalysts applications, noble metal catalysts (Pt/C, IrO2, RuO2) have been demonstrated to display excellent electrocatalytic ac-tivities with lower overpotential. However, their high price and limited resources impede their wide applications [6,7]. Therefore, the devel-opment of efficient non-precious metal electrocatalysts with higher ac-tivity and practical durability has drawn a great deal of attention.

Metal-organic frameworks (MOFs) are formed by self-assembly of organic ligands with metal ions or clusters through coordination inter-action, which have drawn growing attention as various electrocatalytic

applications due to their great specific surface area, tunable porous structure, and active metal sites such as HER, OER, ORR, CO2RR, and NRR [8,9]. Since the first MOF-derived material by pyrolysis as an electrocatalyst was reported in 2008 [10], MOF-derived materials with different compositions and morphologies have been designed and applied in the electrocatalytic field [11,12]. Because MOFs can be pre-pared on a large scale through the simple synthesis process [13,14], they have been widely used as electrocatalysts directly or indirectly by electrochemical conversion or high-temperature calcination. However, there are still some challenges and problems. On the one hand, MOFs are used directly as electrocatalysts by electrochemical conversion, which are easily converted into oxides or nitrides at high current or voltage. The electrocatalytic mechanism and active sites are unclear due to chemical and structural transformation. On the other hand, MOFs can be converted into metal/carbon materials by high-temperature carboniza-tion as electrocatalysts with large specific areas, enhanced electrical conductivity and stability, and increased dispersion [15–17]. Never-theless, the porous structure of MOFs is prone to collapse and metal ions are lent to agglomerate in annealing progress. Overall, MOFs and derived materials as electrocatalysts are a double-edged sword, which requires us to adopt good factors and avoid shortcomings to develop

* Corresponding authors. E-mail addresses: jkgao@zstu.edu.cn (J. Gao), qiczhang@cityu.edu.hk (Q. Zhang).

Contents lists available at ScienceDirect

Electrochemistry Communications

journal homepage: www.elsevier.com/locate/elecom

https://doi.org/10.1016/j.elecom.2021.107024 Received 9 February 2021; Received in revised form 2 March 2021; Accepted 12 March 2021

Electrochemistry Communications 126 (2021) 107024

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novel effective and multifunctional electrocatalysts. In this mini-review, the usage of particular MOF-based materials for

a particular type of electrocatalytic reaction is briefly discussed (Scheme 1). Then, some challenges of MOFs and derived materials for electro-catalysis are also reviewed.

2. Recent progress on electrochemistry

Thousands of articles on electrocatalysis have been published over the past decades. According to these findings, it is essential to develop a highly active and stable electrocatalyst to speed up the reaction rate and reduce the reaction energy barrier. Among all electrocatalytic materials with improved energy conversion efficiency, MOFs and their derived materials have attracted extensive interest by considering their unique advantages such as high surface-to-volume ratio, intrinsically uniform metal distribution, and structural tailorability. In recent years, MOFs and their derived materials have been widely applied in electrocatalysis including HER [18–22], OER [23,24], ORR [25,26], CO2RR [27,28], and NRR [29,30]. Thus, it is necessary and timely to summarize these progress.

2.1. Hydrogen evolution reaction (HER)

The HER progress involves two-electron transfer. Whether in an acidic or alkaline electrolyte, the reaction follows either the Volmer- Heyrovsky or Volmer-Tafel mechanism. To date, the commercial Pt/C is still an excellent HER catalyst. However, the high cost restricts its large-scale applications. Thus, it is highly desirable to develop new efficient and stable non-noble metal catalysts that can compete with the commercial Pt/C catalyst.

Li et al. [31] introduced W species to the UiO-66-NH2, where the W species were coordinated with the uncoordinated ammonia group in 2018. After pyrolysis and HF etching, a W-SAC catalyst was obtained. The high-angle annular dark filed-scanning transmission electron mi-croscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy analysis demonstrated that W species dispersed in atomic level and W1N1C3 moiety might exist (Fig. 1a–b). The density functional theory (DFT) calculation indicated that W1N1C3 moiety was a catalytically-active center in HER progress. Through electrochemical measurements, the W-SAC catalyst showed a Tafel slope of 53 mV dec-1

and an overpotential of 85 mV with a current density of 10 mA cm− 2,

which was similar to the commercial Pt/C catalyst (Fig. 1c–h). The present study gives a novel insight into efficient HER electrocatalysts.

2.2. Oxygen evolution reaction (OER)

Since the OER involves four proton-electron transfer steps, the re-action dynamics are much slower than that of HER. In recent years, several reports indicate that MOFs and their derived materials can be used as high-efficiency electrocatalysts in OER. However, the structur-e–activity relationship of MOFs and their derived materials electro-catalysts are not very clear [32–35]. In particular, how the active species of intermediates affect the catalytic activity together with their origin requires more characterization at the atomic level.

As shown in Fig. 2a, Li group prepared a linker-missing layered- pillared MOF (CoBDC-Fc) by the introduction of various linkers [36]. The DFT calculation showed that terephthalic acid linker could be replaced by hydroxyl-ferrocene, where the electronic structure of MOF could be adjusted and controlled to improve the conductivity. Moreover, CoBDC-Fc can enhance the adsorption ability of intermediate OH* and decrease the activation energy of OER limited step (Fig. 2b–c). As dis-played in Fig. 3d–g, the linkers-missing self-supported MOF nanoarrays displayed outstanding OER efficiency with ultralow overpotential (241 mV) at 100 mA cm− 2. Since then, the strategies to regulate electronic structures by introducing missing linkers have been widely adopted in the design of MOF electrocatalysts. In 2020, Tang et al. [37] selected bimetallic Ni0.5Co0.5-MOF-74 nanocrystals with high OER activity as the model catalyst. The high-resolusion transmission electron microcopy (HRTEM) study and in-situ XAS analysis confirmed the structure trans-formation of Ni0.5Co0.5-MOF-74 in OER progress, where there was a two- phase structural conversion at the metal node and both Ni0.5Co0.5(OH)2 and Ni0.5Co0.5OOH0.75 were formed under lower and higher applied potential, respectively. The further study suggested that the in-situ formed Ni0.5Co0.5OOH0.75/ MOF-74 had a higher OER activity. Besides, they also found that the ratio of Ni and Co in Ni0.5Co0.5-MOF-74 could control the two-phase structural transformation through adjusting the phase transition potentials of the Ni-O and Co-O parts, thus optimizing the activity of catalysts. Based on this discovery, the authors also pre-pared a novel Ni0.9Fe0.1-MOF-74 with low overpotentials of 198 mV and 231 mV at the current density of 10 mA cm− 2 and 20 mA cm− 2, respectively. This work provides a clear understanding of the basic structural transformation processes in OER processes, which is impor-tant to help designing novel highly active OER electrocatalysts in the future.

2.3. Oxygen reduction reaction (ORR)

The ORR is a multi-electron transfer reaction with a more compli-cated reaction mechanism comparing to HER. ORRs can be divided into two reaction pathways: one is a four-electron transfer reaction to pro-duce water and the other is a two-electron transfer reaction to form a hydrogen peroxide intermediate, which can continue to perform a two- electrons transfer to form water or directly evolve H2O2 from the elec-trolyte. Recently, MOFs and their derived materials as ORR catalysts have attracted more researchers’ interests by considering their large specific surface areas, adjustable structures, excellent porosities, and clear metal active centers [38,39].

As displayed in Fig. 3a–b, Feng et al. [40] synthesized a layered copper phthalocyanine-based two-dimensional (2D) conductive MOF (PCCu-O8-Co) through a solvent thermal method in 2019. Further research indicated that the composite of PCCu-O8-Co and carbon nanotubes displayed an ultrahigh ORR catalytic activity, where the half- wave potential was 0.83 V (vs RHE), the number of transfer electron was 3.93, and the limit current was 5.3 mA cm− 2 (Fig. 3c–f). The planar CoO4 junction was proven to be the active center by X-ray asorbption near- edge structure (XANES) measurement, confirming the existence of an-tibonding orbital monads in Co atoms (Fig. 3g–h). These results further Scheme 1. Engineering MOFs and their derived materials for electrocatalysis.

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suggest that these single electrons play a major role in the improvement of catalytic activity, which is beneficial to the breaking of the bond of M− OH– and the exchange reaction of O2–/OH– to obtain a high elec-trocatalytic activity. The in situ Raman spectroscopy analysis and DFT calculations further confirmed that Co sites were the active sites of ORR. This work elucidates that the construction of efficient active centers plays an important role in achieving highly efficient ORR catalysts. Recently, Zhou group reported a new design strategy, namely the combination of metalloporphyrin and Zr-chains, where metal-loporphyrin provided redox-active sites and Zr-chains promoted chem-ical stability (Fig. 3i) [41].

As shown in Fig. 3j–k, based on their previous works, they found that the ORR catalytic activity of PCN MOFs could be regulated by adjusting the distance between active sites by the Zr chain. The PCN-226 exhibited that the best ORR activity, which showed a maximum power density

(133 mW cm− 2) and the stability superior to that of commercial catalysts when assembled in a Zn-air battery (Fig. 3m–o). DFT calculations also demonstrated that the suitable spacing between the two metal active sites possessed an excellent ORR activity, which affected the density of the active sites and the thermodynamics. Therefore, based on this strategy, we can design the suitable distance between active sites by regulating and controlling the length of the chain to obtain outstanding ORR activity. This gives valuable insight into the design of stable and efficient MOF electrocatalysts in the future.

2.4. CO2 reduction reaction (CO2RR)

The CO2RR is an attractive energy conversion and storage method that can convert the greenhouse gas into carbon-based fuels using electricity. Compared with water electrolysis, fuel cell, and Zn-air

Fig. 1. (a) TEM images of W-SAC. (b) K3-weight FT-EXAFS curves of the W-SAC. (c-f) Polarization curves, overpotential, Tafel plots and Nyquist plots of different catalysts. (g) TOF value of W-SAC. (h) The stability test of W-SAC. Represented with permission from ref. 31 Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 2. (a) Modulating electronic structure of MOFs. (b-c) DOS and Gibbs free energy calculation of CoBDC and CoBDC-Fc. (d-g) The performance of OER tests. Represented with permission from ref. 36 Copyright 2019, Spring Nature.

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battery, the electrolysis technology to convert CO2 into high energy density and high value-added carbon-based fuels are not mature yet. Meanwhile, the complex reaction progress (multi-electron transfer re-action and competitive hydrogen evolution reaction) severely restricted its development. To improve reaction efficiency and reduce the occur-rence of competitive reactions, efficient catalysts play an important role in this process. Recent studies have shown that porphyrin-based MOFs exhibit high metal-center density and improved electronic conductivity, indicating the potential applications in CO2RR.

Recently, Feng et al. [42] synthesized a bimetallic layered conju-gated MOF electrocatalyst (PcCu-O8-Zn), where the Cu-phthalocyanine was linked with zinc-bis (dihydroxy) complex (Fig. 4a–b). The PcCu- O8-Zn mixed with carbon nano tube (CNT) showed outstanding CO2RR catalytic activity. The selectivity of CO was up to 88% and the TOF values were 0.39 s− 1, surpassing the previously reported MOF-based catalysts. Moreover, the molar ratio of H2/CO was controlled by regu-lating the metal sites and applied potentials (Fig. 4c–f). The operando surface enhanced infrared absorption (SEIRA), XAS analysis, and DFT calculation demonstrated that ZnO4 units are the catalytically active centers of CO2RR while CuN4 units boost the proton and electron transfer (Fig. 4g–j). This work provides a new design strategy using the bimetallic synergetic effect to synthesize some efficient MOF electro-catalysts. As illustrated in Fig. 4k–m, Jiang et al. [43] developed a general synthetic strategy towards single-atom catalysts (SACs) derived from porphyrin-based MOFs. Among them, Ni-N-C SAC showed a faradic

efficiency of 96.8% for CO. Even at 15% or 30% concentration of CO2, it still kept excellent faradic efficiency, which presented the potential for industrial-scale production (Fig. 4n–o). DFT calculations demonstrated that the activity center of Ni-N-C boosted CO2RR and suppressed the HER (Fig. 4p–q).

2.5. N2 Reduction reaction (NRR)

Ammonia is one of the chemical products, which is synthesized by the Haber-Bosch method at high temperatures and pressures. Hence, it is of great significance to find a green synthetic route for sustainable development. The NRR can convert nitrogen into ammonia gas at room temperature and pressure, which has attracted much attention in recent years [44,45]. Nowadays, the technology of electrochemical synthesis of ammonia is still in the early stages of exploration. There are also few reports on the application of MOFs in NRR.

In 2020, Luo et al. [46] synthesized a series of N-doped carbon Co3O4 NRR electrocatalyst with a core–shell structure derived from ZIF-67 (Fig. 5a–f). As depicted in Fig. 5g–h, Co3O4@NCs exhibited a high yield of ammonia (42.58 ug h− 1 mgcat

-1 ) and good Faraday efficiency (8.5% at − 0.2 V vs RHE). They found that Co3O4 with abundant oxygen vacancies and N-doped carbon boosted the performance of NRR. Moreover, the core–shell structure also promoted the electrocatalytic activity for NRR. The work offered a new direction in exploring and developing MOFs-derived NRR electrocatalysts in the future. In the same

Fig. 3. (a) structure of PcCu-O8-M. (b) TEM image of PcCu-O8-Co. (c-f) ORR test of PcCu-O8-Co/CNT. (g-h) Cu and Co K-edge XANES spectra. Reproduced with permission from ref. 37 Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (i) the structure of PCN-226. (j-k) ORR test of catalysts. (m-o) the Zn-air battery, discharge and charge cycling curves, polarization curves and power density. Reproduced with permission from ref. 41 Copyright 2020, American Chemi-cal Society.

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year, Qiao et al. reported that a nano rod-like Bi-MOF was reduced into Bi0 nanoparticles under in situ electrochemical measurements, which exhibited excellent NRR performance [47]. Some in situ advanced equipment such as in situ Raman spectroscopy, electron microscopy, and online differential electrochemical mass spectrometry, were employed to study the chemical and structural transformation of the Bi species. This work provides a feasible strategy to identify the active sites and

reaction mechanism.

3. Conclusions and outlook

So far, the application of MOFs and their derived materials in elec-trocatalysis are of great interest for researchers due to their high surface area, porosity, and adjustable structures. However, there are still some

Fig. 4. (a-b) The structure and TEM image of PcCu-O8-Zn. (c-f) CO2RR performance tests of various catalysts. (g) Zn K-edge XANES and Fourier transform EXAFS spectra of various catalysts. (i) Gibbs free energy diagram of PcCu-O8-Zn. (j) Proposed reaction process of HER and CO2RR. Reproduced with permission from ref. 39 Copyright 2020, Spring Nature. (k-m) TEM image of Ni-PCN-222 and Ni-N-C. (n-o) LSV curves and FEs of various M− N− C. (p-q) DFT calculations of various M− N− C catalysts. Represented with permission from ref. 43 Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 5. (a-f) SEM and TEM images of Co3O4@NC-5, Co3O4@NC-10, Co3O4@NC-15. (g) LSV curves of different catalysts. (h) The NRR performance of different electrocatalysts. Represented with permission from ref. 46 Copyright 2019, American Chemical Society.

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challenges to be addressed. Firstly, it is of great importance to develop novel MOFs to solve the inferior conductivity and stability of MOFs. Secondly, MOFs are prone to irreversible transformation and the real active sites are easily ignored at high voltage or current. Therefore, advanced in-situ characterization techniques are required to understand the electrocatalytic mechanism. Thirdly, some uncontrollable factors in the carbonization process such as the collapse of the porous structures and agglomeration of metal particles need to be resolved. Finally, the ligands of some MOFs are extremely expensive, which limits the in-dustrial application of MOFs. Thus, developing economic ligands to design efficient and durable MOF-based electrocatalysts have great application potential. In addition, the stability of these MOFs and their derivatives in acidic electrolyte needs to be further improved. To sum up, the rapid development of MOFs and their derivatives provides many new opportunities for electrocatalysis, which will greatly promote the advance of energy conversion technologies.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Natural Science Foundation of Zhejiang Province (No. LY20E020001). QZ thanks the support from starting funds from the City University of Hongkong.

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