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. rsc.li/nanoscale Registered charity number: 207890 Nanoscale View Article Online MINIREVIEW View Journal | View Issue 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 tech- niques 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 This journal is © The Royal Society of Chemistry 2021 Nanoscale,2021,13,1507–1514 | 1507 View Article Online Minireview Nanoscale 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 struc- ture, 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 con- figuration (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 1508 | Nanoscale,2021,13,1507–1514 This journal is © The Royal Society of Chemistry 2021 View Article Online Nanoscale