Operando Spectroscopy of Nanoscopic Metal/Covalent Organic Framework Electrocatalysts Nanoscale

Showcasing research from Prof. Nikolay Kornienko at the University of Montreal, Department of Chemistry, As featured in: Montreal, Canada.

Volume 13 Number 3 21 January 2021 Pages 1369-2046 Operando spectroscopy of nanoscopic metal/covalent organic framework electrocatalysts Nanoscale

rsc.li/nanoscale Metal and covalent organic frameworks (MOFs and COFs) are increasingly ﬁ nding exceptional utility in electrocatalytic systems. In order to obtain insights into their function, mechanism and dynamics under electrocatalytic conditions, operando spectroscopy, which is performed as the catalyst is functioning, has been increasingly applied. This review highlighted emerging research in recent years that have ISSN 2040-3372

PAPER Naohiro Kameta and Wuxiao Ding Stacking of nanorings to generate nanotubes for used operando spectroscopic techniques to investigate acceleration of protein refolding electrocatalytic MOFs and COFs. See Nikolay Kornienko, Nanoscale , 2021, 13 , 1507.

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Operando spectroscopy of nanoscopic metal/ covalent organic framework electrocatalysts Cite this: Nanoscale, 2021, 13, 1507 Nikolay Kornienko

Metal and covalent organic frameworks (MOFs and COFs) are increasingly ﬁnding exceptional utility in electrocatalytic systems. Their chemically deﬁned porous nature grants them key functions that may enhance their electrocatalytic performance relative to conventional molecular or heterogeneous materials. In order to obtain insights into their function, mechanism, and dynamics under electrocatalytic conditions, operando spectroscopy, that which is performed as the catalyst is functioning, has been increasingly applied. This mini review highlights several key works emerging in recent years that have used various operando spectroscopic techniques, namely UV-vis absorption, Raman, Infrared, and X-ray Received 20th October 2020, absorption spectroscopy, to investigate electrocatalytic MOFs and COFs. A brief introduction to each Accepted 13th November 2020 technique and how it was applied to investigate MOF/COF-based electrolytic systems is detailed. The DOI: 10.1039/d0nr07508f unique set of data obtained, interpretations made, and progress attained all point to the power of oper- rsc.li/nanoscale ando spectroscopy in truly opening the functionality of MOFs and COFs across many aspects of catalysis.

Introduction grafted polymers17), and heterogenized homogeneous catalysts (ex. graphite conjugated porphyrins18). As research into renewable energy becomes increasingly Within this framework, one group of hybrid systems that urgent, more emphasis is placed on the development of particularly stands out is metal and covalent organic frame- electrochemical technologies and the electrocatalysts that works (MOFs and COFs). MOFs and COFs are crystalline, per- drive them.1,2 Such systems carry the potential to be comple- manently porous systems featuring metal nodes (in the case of tely powered by renewable-derived electricity and simul- MOFs) linked together by organic species.19 Their appeal in

Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM. taneously be economically competitive with established routes terms of electrocatalyst design lies in their exceptional tune- to meet many of society’s energy, fuel, and material demands.3 ability, in which the nodes and organic likers can be tuned, In fact, the scope of electrosynthetic systems is also being sig- and therefore the porosity and hydrophilicity can be rationally 20,21 nificantly expanded, ranging from water electrolysis and CO2 designed a priori. Further, catalytically active sites, 4,5 6–8 9,10 reduction, to biomass valorization, N2 fixation, per- whether they are situated on the organic linker or on coordina- oxide electrosynthesis,11 methane oxidation12 and eventually tively-unsaturated sites on the inorganic node, can be designed to a whole host of reactions that may electrify the chemical with atomic precision. Finally, the pockets within the MOF/ industry.13,14 COF framework can be functionalized to act in an enzyme- Within the context of electrocatalyst research, traditionally mimetic manner through secondary coordination sphere catalysts have been divided between molecular and hetero- effects.22,23 Here, the catalytic pockets stabilize intermediates, geneous, with each featuring a unique set of strengths and minimize reorganization energies and thermodynamically limitations. In contrast to these defined boundaries, many and/or kinetically direct the reaction to efficiently proceed works are beginning to emerge using hybrid systems that in solely through the desired pathway. – various capacities blend aspects of molecular and homo- To this end, operando spectroscopy,24 29 that which is per- geneous catalysts to generate functional systems that combine formed as the catalyst is functioning, is playing a substantial their advantages and minimize drawbacks.15 Notable examples role in aiding researchers in their efforts to fully understand of these systems include nanoparticles exhibiting functional the function of electrocatalytic MOFs and COFs, and from this molecular entities grafted onto their surfaces (ex. Cu/arylpyri- understanding, develop design rules towards the construction dinium interfaces16), polymeric catalysts (ex. Co-terpyridine of next-generation systems. The primary spectroscopic techniques include, but are not limited to, UV-Vis absorption, Raman, infrared, and X-ray absorption spectroscopy (Fig. 1). Department of Chemistry, Université de Montréal, 1375 Avenue Thérèse-Lavoie-Roux, Key insights obtained include the determination of the catalyst Montréal, QC H2 V 0B3, Canada. E-mail: [email protected] redox state, detection of reaction intermediates, and the

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Fig. 1 Operando spectroscopy techniques and illustrations of typical spectroelectrochemical cells utilized to probe the function of electrocatalytic frameworks.

unveiling of charge transfer pathways. Often, such information element specific spectroscopy that can inform researchers on is not obtainable through the standard array of electroanalytic both the elemental electronic state, through analysis of the techniques or through ex situ measurements performed before shape/position of absorption peaks, and on the element’slocal or after catalysis. chemical environment, through investigation of the extended In general, UV-Vis absorption spectroscopy is the easiest to fine structure of the absorption.34,35 Through this combination, use amongst the techniques listed. Its main use is to probe the the depth of information on catalyst behavior is exceptionally

Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM. catalyst electronic structure through measuring electronic tran- high. The limitations of XAS is that this technique usually sitions of molecular species within MOFs induced by the cannot probe reaction intermediates and that specialized facili- absorption of UV or visible light.30 Generally, it is highly sensi- ties that supply high-intensity X-ray light are necessary. tive and its main limitation is that it cannot measure reaction This mini review aims to highlight several key works in intermediates or provide direct structural information about recent years that have utilized various operando spectroscopic MOFs. Raman spectroscopy measures inelastically scattered techniques to shed light on nanoscale electrocatalytic MOFs/ light that has lost energy to excite molecular vibrations that COFs and to lay out promising avenues to pursue in order to feature changes in molecular polarizability.26,31 The resultant fully take advantage of this exciting array of materials. information can inform on catalyst redox state, catalyst structure, and reaction intermediates. Its low sensitivity often makes the use of resonance Raman or surface-enhanced Raman UV-Vis absorption spectroscopy necessary. This can also be used advantageously as the former can selectively probe a chromophore (e.g. porphyrin) and the UV-Vis absorption spectroscopy is arguably the most accessible latter is exceptionally surface sensitive (nm level) that is sensi- technique mentioned in this text as most laboratories are tive enough to detect sub-monolayer concentrations of mole- equipped with a suitable spectrometer capable of carrying out cular species. Infrared spectroscopy also measures molecular these measurements. Often, this is performed in transmission vibrations that exhibit changes in the molecular dipole mode with the catalyst grown on a transparent conductive sub- 32,33 moment. This can also inform of MOF structural dynamics strate. By monitoring characteristic features in the absorption and reaction intermediates. In the most commonly used con- spectrum, both the MOF/COF redox state in steady state configuration (attenuated total reflection mode), the infrared ditions as well as the rate of which charge moves through the source penetrates approx. 1 µm deep into the electrolyte. IR framework can be recorded. spectroscopy does not require any resonance or surface A Co-porphyrin based MOF (CoPIZA) was first evaluated enhancement to obtain high-quality spectra. Finally, XAS is an with UV-Vis to understand the nature of its redox processes.36

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The steady-state absorption spectra and in particular, the spec- (CV) measurements due to large capacitive currents and slug- tral changes corresponding to the porphyrin redox states, were gish charge transport through MOF/COF films. used to extract out the redox potential of the Co units in this MOF. Beyond recording the redox potentials corresponding to the Co(II/III) couple, the authors also recorded the apparent Raman spectroscopy diﬀusion coeﬃcient of charge passing through their MOF by fitting their time-dependent Co(II/III) redox change to a modi- Raman spectroscopy measures inelastically scattered light upon − fied Cottrell equation. The derived value of 7.55 (±0.05) × 10 14 irradiating a sample with a visible or ultraviolet laser. Raman − cm2 s 1 corresponded to electrons hopping between redox- active vibrations are those that feature a change in the polarizabil- active porphyrin sites. This seminal work laid out a foundation ity in the material/molecule. A limitation to this technique is the for spectroelectrochemically evaluating electrochromic MOF small fraction of photons that are inelastically scattered (1 out of films. A similar measurement was subsequently performed on every 106) and thus, techniques to enhance this such as reso- Co-porphyrin bearing conductive COFs to demonstrate that nance Raman (RR)40 or surface-enhanced Raman spectroscopy electrons transport through at approximately 1–2 orders of (SERS)41 are often employed. The advantage of Raman spec- magnitude faster in this case.37 troscopy, especially in the context of operando experiments, is Under steady state conditions, a Co-porphyrin containing that the simple setup (Fig. 3b), often operated in reflection mode,

MOF, active towards CO2 reduction to CO, was investigated is highly amenable to a host of reactor designs. Often either a with UV-Vis absorption spectroscopy.38 By recording the long working distance objective or a water immersion objective is characteristic absorption of the Co-porphyrin, the authors utilized, situated just above the catalyst-containing working elec- determined that a Co(I) species was built up under catalytic trode. Depending on the exact system, both catalyst structure, conditions and that this was the species responsible for redox state, and reaction intermediates can be deciphered as a

initially reacting with CO2. Recently, a MOF featuring catalytic function of time, voltage, or other relevant parameters. cobaloxime linkers was synthesized and found to be active Recently RR was utilized to identify active sites within a towards the hydrogen evolution reaction (HER).39 The authors phthalocyanine-based Co–Cu bimetallic MOF active towards used UV-Vis absorption spectroscopy to demonstrate the rever- the oxygen reduction reaction (ORR), a key component in 42 sibility of the Co species between the Co(I) and Co(II) redox various types of fuel cells. The spectra, acquired under simu- states, and a lack of reversibility between the Co(I) and Co(III) lated reaction conditions in an alkaline electrolyte, revealed O – states (Fig. 2b d). These measurements are particularly impor- and OH adsorbed onto the Co species of the Co–O4 sites in tant as redox transitions and the evaluation of reversibility are conjunction with the redox changes of the Co between the

not always evident through conventional cyclic voltammetry Co(III) and Co(II) states. The Cu–N4 sites within the phthalo- Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM.

Fig. 2 Typical setup for UV-vis spectroscopy (a). A cobaloxime-based MOF (b) undergoes transitions between Co(III), Co(II), and Co(I), all of which can be probed through analysis of its voltage-dependent UV-Vis absorption spectra (c). Reversibility is demonstrated by monitoring the absorption at 520 and 670 nm (d). Reprinted with permission from ref. 39: S. Roy, Z. Huang, A. Bhunia, A. Castner, A. K. Gupta, X. Zou and S. Ott, J. Am. Chem. Soc., 2019, 141, 15942–15950. Copyright 2019 American Chemical Society.

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Fig. 3 Spectroelectrochemical setup used for operando UV-Vis absorption (a) and Raman (b) spectroscopies. UV-vis absorption (c) and Raman (d) spectra point to the structural evolution of the ZIF-67 structure into CoOOH as the true water oxidation catalyst (e) under applied oxidizing voltages. Reprinted with permission from ref. 43: W. Zheng, M. Liu and L. Y. S. Lee, ACS Catal., 2020, 10, 81–92. Copyright 2020 American Chemical Society.

cyanine units were deemed instead to be catalytically inert in energy is used to excite vibrational modes featuring a change

Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM. this case. A combination of UV-Vis absorption and Raman spec- in the molecule’s dipole moment. In the context of operando troscopies unveiled structural dynamics within ZIF-67 water oxi- electrochemical experiments, IR spectra are usually acquired dation catalysts (Fig. 3a and b).43 The starting material, ZIF-67, in an attenuated total reflection (ATR) setup in which a catalyst featured Co atoms coordinated to the four nitrogens of the film is deposited onto an ATR waveguide and the IR light 2-methylimidazole linkers in a tetrahedral geometry. As the comes from below and reflects back to the detector (Fig. 4a).44 potential was poised in the positive direction in which water oxi- This configuration enables researchers to selectively measure dation begins to occur, features in both, the UV-Vis absorption within 1 µm of the waveguide/catalyst-electrolyte interface and and Raman spectra attrivuted to the ZIF-67 began to disappear avoids having the solvent signal completely overwhelm the

(Fig. 3c and d). In turn, Co-(OH)2, and finally CoOOH signatures spectra. For higher surface-sensitivity, surface-enhanced IR emerged in the spectra. This data, in conjunction with ex situ absorption spectroscopy (SEIRAS) can be performed with the X-ray diﬀraction and X-ray photoelectron measurements, lead use of nanostructured metal films.45 the authors to conclude that the ZIF-67 first undergoes a series IR spectroscopy was used to probe the accessibility of mole- − of ligand substitutions, replacing imidazole to OH ,thenfinally cular H2 evolving catalysts grafted into amino functionalized 46 goes through a phase transition to yield the CoOOH catalyst MIL-101(Cr) MOFs. The reduction of the catalyst, [Fe2(cbdt) (Fig. 3e). The exact nature of these steps was deemed important (CO)6] ([FeFe], cbdt = 3-carboxybenzene-1,2-dithiolate) with as the two types of CoOOH formed exhibited significantly chemical reductants was monitored over time to probe the diﬀerent water oxidation activities. functional accessibility of these species deep within the MOF structure. Complete reduction, signified by several key marker bands in the IR spectrum, took place within the hours and at Infrared spectroscopy short time scales was found to be hindered by the ions of the reduced catalyst and oxidized reductant formed within the Infrared spectroscopy, highly complementary to Raman spec- pores. This work pressed for careful consideration of mass troscopy, records light absorbed in the infrared region whose transport in the design of catalytic MOFs.

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Fig. 4 Experimental setup used for operando Raman and IR spectroelectrochemical measurements (a). Raman (b and c) and IR (d) data suggest that the redox change of the porphyrin units is accompanied by reversible changes in ligation and partial detachment of the carboxylate units on the porphyrins (e). Reprinted with permission from ref. 48: N. Heidary, M. Morency, D. Chartrand, K. H. Ly, R. Iftimie and N. Kornienko, J. Am. Chem. Soc., 2020, 142, 12382–12393. Copyright 2020 American Chemical Society.

A series of bimetallic MOFs, comprised of Cu/Zn phthalo- As mentioned in the introduction, a key strength of electro-

cyanine units with Cu/Zn–N4 coordination linked via Zn/Cu-O4 catalytic MOF systems is that functional units may be grafted clusters, was found to be exceptionally active towards electro- in to promote catalysis via secondary coordination sphere 47 ﬀ – chemical CO2 conversion to CO. The MOF was deposited e ects. To this end, pyridine units were grafted into a metal onto a roughened gold surface and SEIRAS was subsequently organic layer containing Co-porphyrins as the catalytic site.49

used to probe the mechanism of this system. A CuO4-H inter- The protonation of the pyridine to pyridinium under catalytic −1 mediate was detected at 1850 cm , as were CuN4-CO and conditions was captured with operando IR spectroscopy and it −1 CuO4-CO species at 1933 and 2071 cm , respectively. was this species that was thought to stabilize the intermediates

Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM. However, no intermediates were detected on the CuN4 sites, of the CO2 as it was reduced to CO, thereby enhancing the rate nor any on either of the Zn sites, possibly due to their transi- of this process. ent nature. This led to the interpretation the Cu is active for Finally, the mechanism of bifunctional MOFs, catalytic proton adsorption, which either facilitates hydrogen evolution towards oxygen reduction and water oxidation, was deciphered 50 or protonation of the CO2 intermediates situated on the Zn with operando IR spectroscopy. The series of ultrathin sites in a synergistic manner. (20 nm) MOFs, featuring octahedral Ni- and Fe-oxo nodes and A combined UV-Vis, Raman and IR study was used to unveil 2,6-naphthalenedicarboxylate linkers, were solvothermally syn- the structural reorganization of a Mn-porphyrin containing thesized and subsequently irradiated with UV light to induce MOF thin film under electrochemical bias.48 First, UV-Vis lattice strain within their structure. This modulated their elec- absorbance measurements, following the spectroscopic signa- tronic structure and consequently their catalytic activity. Under tures of the Mn(II/III) redox couple, showed that the complete both oxygen reduction and water oxidation conditions, a band − reduction of the system occurred within several seconds but at 1048 cm 1 was evident which corresponded to the super- the re-oxidation, while being fully reversible, took tens of oxide *OOH species built up under steady state conditions. minutes. Raman and IR measurements (Fig. 4a–d) revealed This observation, coupled to isotope-labeled experiments and that the redox process was coupled with a ligation change and operando XAS measurements, was key to elucidate the com- unveiled the partial detachment of the porphyrin carboxylate plete catalytic cycle of this system for both reactions. units to the Zn centers, the latter of which matched the time- scales of the rapid reduction/slow oxidation. This allowed the researchers to put forth a model of how this system restruc- X-Ray absorption spectroscopy tured under applied bias (e). IR was further extended to probe −• the CO2 and CO intermediates of this system en route to CO2 X-Ray absorption spectroscopy (XAS) uses high energy X-ray to CO conversion, enabling the construction of a catalytic reac- irradiation to probe electronic transitions between core and tion mechanisms. valence levels of the species of interest. This technique is

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element specific and can be used to both, probe the electronic experiments to arrive at a more conclusive data set and structure of the element of interest by examining the shape interpretation. and energy position of the near-edge region (XANES) or to It must be mentioned that additional operando techniques probe the element’s chemical environment by analyzing the not mentioned in this mini review have also been used to extended fine structure region that extends several hundred eV investigate electrocatalytic systems. These include X-ray photo- past the edge (EXAFS). The depth of information provided is electron spectroscopy, X-ray diﬀraction, X-ray scattering, and largely why these techniques are used across many areas of cat- quartz-crystal microbalance experiments, as well as spatially alysis. However, the significant drawback to XAS is that high resolved techniques such as electrochemical microscopy and intensities of X-rays are required and only available in special- tip-enhanced Raman spectroscopy. Translating these to MOF ized facilities. In a typical XAS setup employing hard X-rays and COF catalysts stands to further unveil the interesting com- (above 5 keV), the catalyst/working electrode is placed in the plexity of their function. Finally, researchers should continu- path of the X-ray source in a thin electrochemical cell and ally push to narrow the gap between the conditions used for either reflection or transmission mode may be used to collect spectroelectrochemical experiments and those in the actual signals. In the case of soft X-rays (below 5 keV), a high-vacuum catalytic experiments. A notable example lies within the field

environment is used with the catalyst deposited on one side of of CO2 reduction in which most spectroscopy is conducted a conductive ultrathin window/electrode with electrolyte on within standard 3-electrode cells while catalytic experiments one side and vacuum on the other. are increasingly being performed in gas-diffusion electrodes XAS was recently used to highlight differences in electronic and flow cells. Beyond electrocatalysis, MOFs and COFs just structure between a series of Co-Porphyrin containing COFs been started to be applied in electrochemical contexts in the and the isolated porphyrin linkers active towards the reduction last handful of years, including the area of supercapacitors,54 37,51 55 56 of CO2 to CO. An examination of the K-edge and L-edge batteries, and sensors and are already making sizeable spectra indicated that the COF structure imbibed an electron- impacts. Their versatility coupled to a greater understanding withdrawing effect to the Co active site within the porphyrin of how they function is envisioned to truly open up their use units, in this case beneficial for catalysis. Furthermore, the these fields.

reduction of Co(II)toCo(I) under CO2 reduction conditions was captured, successfully identifying the active species. XAS was also crucial in the investigation of NiCo bimetallic Conﬂicts of interest ultrathin (3 nm) MOF water oxidation catalysts.52 The ultrathin MOFs were much more active than their bulk counterparts There are no conflicts to declare. and this observation warranted a XAS study to explain. XANES at the Ni and Co K-edges, in conjunction with DFT simu- lations, was used to identify the undercoordinated sites Acknowledgements involved in the catalysis. Operando XANES measurements N. K. acknowledges NSERC Discovery Grant RGPIN-2019- Published on 13 November 2020. Downloaded 10/1/2021 12:16:32 AM. revealed that a higher fraction of Ni and Co atoms were oxi- 05927. dized under positive water oxidation potentials in the ultrathin MOFs relative to the bulk analogues, highlighting the catalytic benefits of the ultrathin geometry. Recently, NiCo-MOF-74 and References NiFe-MOF-74 MOFs were probed with XAS as they catalyst water oxidation.53 Through analysis of operando EXAFS 1 V. R. Stamenkovic, D. Strmcnik, P. P. Lopes and spectra, the authors found a reversible oxyhydroxide-hydroxide N. M. Markovic, Nat. Mater., 2017, 16,57–69. structural transformation between catalytic and pre-catalytic 2 R. M. Bullock, J. G. Chen, L. Gagliardi, P. J. Chirik, conditions. These materials behaved in contrast to what has O. K. Farha, C. H. Hendon, C. W. Jones, J. A. Keith, been established for several porous catalysts, opening up path- J. Klosin, S. D. Minteer, R. H. Morris, A. T. Radosevich, ways to further explore new modes of function with MOFs. T. B. Rauchfuss, N. A. Strotman, A. Vojvodic, T. R. Ward, J. Y. Yang and Y. Surendranath, Science, 2020, 369, eabc3183. Concluding remarks 3 P. De Luna, C. Hahn, D. Higgins, S. A. Jaﬀer, T. F. Jaramillo and E. H. Sargent, Science, 2019, 364, eaav3506. This mini review highlighted how operando spectroscopy can 4 M. B. Ross, P. De Luna, Y. Li, C.-T. Dinh, D. Kim, P. Yang be leveraged to understand and fully harness the capacities of and E. H. Sargent, Nat. Catal., 2019, 2, 648–658. MOFs and COFs in the context of electrocatalysis. While detail- 5 Y. Y. Birdja, E. Pérez-Gallent, M. C. Figueiredo, A. J. Göttle, ing every study utilizing operando spectroscopy is out of the F. Calle-Vallejo and M. T. M. Koper, Nat. Energy, 2019, 4, scope of this work, several key investigations in recent years 732–745. were detailed. Each technique provides the researchers with a 6 K. Li and Y. Sun, Chem. – Eur. J., 2018, 24, 18258–18270. unique set of information and similarly has its own limit- 7 L. Du, Y. Shao, J. Sun, G. Yin, C. Du and Y. Wang, Catal. ations. Therefore, many studies combine two or more operando Sci. Technol., 2018, 8, 3216–3232.

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