Scanning Electrochemical Cell Microscopy Investigation of Single ZIF‐Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media

Abstract “Single entity” measurements are central for an improved understanding of the function of nanoparticle‐based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF‐67)‐derived Co−N‐doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non‐interfering Os complex as free‐diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s−1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects.

Abstract: "Single entity" measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value.W er eport as canning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived CoÀN-doped Cc omposite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of an on-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone.T he turnover frequency (TOF) is 0.25 to 1.5 s À1 at potentials between 1.7 and 1.8 Vv s. RHE, respectively,b ased on the number of Co atoms in each particle.C onsistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects.
Recent advancement in earth-abundant, non-noble metal based electrocatalysts has stimulated visions for their utilization in sustainable energy infrastructure. [1] Ther ational development of effective electrocatalysts requires af undamental understanding of the intrinsic structure-reactivity relationship with regard to individual catalyst particles and their averaged ensemble properties. [2] Accordingly,significant efforts have been made to analyze individual electrocatalytic profiles using nanoprobe techniques,f or the extraction of current responses at small physical dimensions. [3] Thedroplet cell-based scanning electrochemical cell microscopy (SECCM) technique is one of the powerful local electrochemical tools extensively explored by Unwin and co-workers to visualize heterogeneous electron transfer processes on single-crystal and polycrystalline surfaces. [4,5] SECCM offers high-resolution insight into the structure-activity relationship at the nanoscale of ah eterogeneous electrochemical interface. [6,7] However,d ue to the necessary wetting by the positioned nanodroplet cell and the inherent difficulties of surface wetting using highly alkaline electrolytes,SECCM has been rarely applied for the investigation of electrocatalytic reactions of non-noble metal catalysts,which requires the use of alkaline electrolytes owing to their limited stability in acidic media. [8] We recently determined the OER activity of as ingle Co À N/C nanocomposite particle on top of an anoelectrode and found aTOF of about 5s À1 per Co atom, under non-limiting mass transport conditions. [9] Here,w er eport ac omplementary,f aster, sequential, and more pragmatic approach to derive the electrochemical response of one to small numbers of ZIF-67-derived composite nanoparticles using SECCM.
Thec omposite nanoparticles are inspired by carbonsupported non-noble metal electrocatalysts. [10] Thew elldefined molecular structure and morphology of the ZIF-67 nanocrystals allow precise quantification of the number of Co atoms in the N-doped carbon composite (Co À N/C). [11] The poor stability of the commonly used miniaturized Ag-based "quasi-reference/counter electrode (QRCE)" systems,h ere chloridized silver wires,a th igher current densities and at alkaline pH values leads to aconsiderable drift in the working electrode potential making the nanoscale voltammetric measurements unreliable. [12] Here,weintroduce aspecifically designed non-interfering,r eversible redox compound as an internal potential standard to account for the potential drift during SECCM measurements at alkaline conditions.M oreover, the voltammetric response of the pH-independent redox conversion of an Os complex ( Figure 1a)w as also used to derive the relative wetted electroactive surface area at each landing site of the SECCM tip.
Initially,ZIF-67 nanocrystals with asize ranging from 165 to 235 nm were deposited on ag lassy carbon (GC) plate (Scheme 1a). TheG Cp late was pulled out of the ZIF-67 solution stepwise to form ag radient in surface coverage of ZIF-67 nanocrystals (for details see Section 2, Supporting Information), and the laterally heterogeneous surface coverage of the ZIF-67 nanocrystals was verified by means of scanning electron microscopy (SEM). Structure and phase purity of the ZIF-67 nanocrystals were confirmed by powder X-ray diffractometry and comparison with the simulated ZIF-67 crystal structure [13] (Figure S1, Supporting Information). Thef ormation regions with varying ZIF-67 nanocrystal densities (from single to multiple units in the landing area of the formed nanodroplet) is critical to derive the response of individual ZIF-67-derived nanocomposite particles as well as of few particle ensembles in as ingle experiment. TheG C plate modified with ZIF-67 particles was heat-treated under an inert reducing atmosphere (H 2 /Ar) to pyrolytically transform ZIF-67 nanocrystals into Co À N/C nanocomposites (Scheme 1b). Local voltammetric measurements were performed by means of hopping-mode SECCM with apredefined movement in x-a nd y-direction of 7 mmb etween measurement areas.T he approach was performed with the SECCM tip being vibrated normal to the sample surface (Scheme 1c). During the approach, the currents between the two barrels of the capillary and between one of the barrels and the sample surface were simultaneously monitored. In particular,the AC component of the currents induced by the vertical vibration was used as the feedback signal. Asudden increase in the AC signal was indicative of contact between the meniscus at the end of the nanopipette and the sample surface as previously described [14] (for details see Section 4, Supporting Information). Initial SECCM experiments using chloridized Ag wires as the QRCE displayed an irregular potential drift likely due the formation of Ag hydroxide during polarization in alkaline solution. We employed Pt wires instead and monitored and corrected for possible potential drifts by adding asoluble Os complex (Figure 1a)t ot he electrolyte within the capillaries as an internal reference system. With af ormal potential of 1.35 Vv s. RHE for the Os 3+/2+ redox conversion, which is % 350 mV more cathodic than the potential at which measurable OER activity can be observed (Figure 1b), no interference with electrocatalysis is supposed. Ad oublebarrel theta nanopipette pulled to an arrow elliptical tip (diameter of for example,1.63 mmor0.78 mm) was filled with asolution containing 0.1 mm Os 2+/3+ complex in 50 mm KOH. These concentrations were optimized to minimize precipitation of KOHonthe capillary landing positions,following the electrolyte exposure over prolonged experimental time scale ( Figure S2). AP tw ire (0.3 mm diameter) was inserted into each barrel and af ixed bias potential of 20 mV was applied between the two electrodes.U pon each landing of the SECCM tip on the sample surface,t hree cyclic voltammograms (CV) at different scan rates were recorded in apotential range from 0.8 to 1.65 Vvs. RHE. Afterwards,alinear sweep voltammogram (LSV) was recorded with as can rate of 200 mV s À1 using an extended potential window to derive the OER response from each of the locations as shown in Figure 1b.I nasingle array scan with multiple tip landings, electrochemical response profiles from % 75 %o ft he locations could be obtained. All voltammograms showed the redox peaks corresponding to the reversible electrochemistry for the Os 3+/2+ couple ( Figure S3, Supporting Information).
Potential drifts of up to 40 mV were measured in asingle experiment (Figure 1c). Thep otential-corrected LSV curves were further corrected with respect to the capacitive doublelayer charging current (details in Section 4, Supporting Information). Foraconclusive demonstration of the sequential activity mapping, SEM images of the electrochemically evaluated area were complementarily used as a"roadmap" to correlate the number of ZIF-derived nanocomposite particles at each spot with the corresponding electrochemical response (Figure 2a). Figure 2b shows the increase in the number of Co atoms with the number of pyrolyzed ZIF-67 units.T he number of Co atoms (mol Co )w ithin the nanocomposite particles was derived using the simulated ZIF-67 crystallographic information considering the volume shrinkage during pyrolysis (details in Section 5, Supporting Information).
Thegeometric surface area wetted by the droplet formed upon contact of the SECCM capillary with the surface is generally assessed by measuring the size of the meniscus footprint produced during the scan, for example,bymeans of SEM. [4,7] However,i na lkaline solution the geometric area is highly influenced by the wetting properties of the sample at the applied electrochemical potential. [15] Determination of the exact areal contribution of individual pyrolyzed ZIF-67 units was intricate owing to their small dimensions and varying interparticle distances.T herefore,t he voltammograms were normalized with respect to their electrochemically active surface area, which was derived from the Os 3+/2+ oxidation peak of the LSV using the Randles-Sevcik equation [16] (details in Section 4, Supporting Information).
Thev oltammograms together with the corresponding SEM images showed negligible OER activity of the bare GC surface in the absence of nanocomposite particles (Figure 2c). LSVs showed substantially higher anodic OER currents each time the nanoprobe landed on an area in which an anocomposite particle or particle ensemble was present. A comparison of the catalytic OER activity at the measurement locations shown in Figure 2a indicated an increase in the OER activity with the number of nanocomposite units (Figure 2d). Thel ocations enclosing two and three Co À N/C particles displayed as imilar electrochemical activity (Figure 2d), which may be due to different particle sizes [17] ( Figure 2b). Thei ntrinsic electrocatalytic activity of pyrolyzed ZIF-67 composite particles was further evaluated in terms of the turnover frequency( TOF). Locations with as ingle nanoparticle exhibited marginal OER activity as compared with the bare carbon surface (Figure 2d and Figure S9, Supporting Information) and were therefore excluded from TOFe valuation. Thec ontribution of the carbon surface to the total measured current on each spot was subtracted to investigate the catalytic activity of only the Co À N/C nanocomposite particles (calculation details in Section 6, Supporting Information).
TheTOF derived at three different potentials (1.70, 1.75, and 1.80 Vv s. RHE) reflected the expected increase in the OER activity with increasing anodic potentials,with values of about 0.25 to 1.5 s À1 per Co atom (Figure 2e). TheO ER activity is influenced by interfacial pH, local O 2 supersaturation, and blocking of the catalytic surface by O 2 gas bubbles. [18] Thed ecrease in TOF with an increase in the number of nanocomposite particles reflects the increasing variations in the chemical environment within the droplet. As patially resolved electrochemical movie was compiled using an array scan comprising about 150 measurement areas and hence % 150 LSVs in the potential range from 1t o1 .8 Vv s. RHE. Thee lectrochemical reactivity maps derived at potentials in the OER region are in good agreement with the particle coverage on the GC surface,a sc onfirmed using the complementary SEM images ( Figure 3).
In conclusion, we report av ersatile,e ffective,h ighthroughput SECCM measurement to derive the electrochemical reactivity of as ingle to several individual CoÀN/C composite nanoparticles in as ingle experiment at alkaline conditions.U sing af ree-diffusing redox compound as an internal potential standard allowed to compensate not only for potential drifts but also to derive the wetted surface area for each individual droplet formed by the landing of the SECCM tip as the basis to derive the OER activity and the TOFofindividual CoÀN/C nanocomposite particles.