Scanning probe microscopy for electrocatalysis

Electrochemical energy storage and conversion devices have emerged as sustainable energy provision schemes. A major challenge for practical application is to develop high-performance, low-cost, and stable electrocatalysts to advance their overall efficiency. In-depth understanding of electrocatalysis mechanisms to clarify how electrocatalysts structure affect local activity and how electrochemical reactions proceed at the electrode-electrolyte interfaces is required to guide the rational design of advanced electrocatalysts. Scanning probe microscopy (SPM) can perform nanoscale surface property and local activity measurements under realistic conditions in real time and in operando, thus providing a powerful tool to study these electrocatalysis processes. This review provides an overview of the development and application of SPM techniques in electrocatalysis. We expect it to provide new insights into the study of electrocatalysis mechanisms and guide the rational design of novel electrocatalysts for speeding up the commercial application of renewable technologies. The development of high-efficiency energy storage and conversion devices requires a deeper understanding of the structure-activity relationship of electrocatalysts, material transformations during electrocatalysis processes, and complex electrochemical processes at electrode-electrolyte interfaces. Scanning probe microscopy (SPM) is a powerful tool for visualizing the surface properties and localized electrochemical activity down to the atomic scale in situ and even operando, thus plays an essential role in studying heterogeneous electrocatalysis mechanisms. We summarize recent advancements in SPM for investigating energy-related electrocatalysis based on three unique characteristics of SPM—surface property imaging, in situ/operando monitoring, and nanoscale electrochemical mapping，outline the application of SPM in investigating the structure-activity relationship, material transformations, and electrochemical processes. The specific SPM techniques discussed here include scanning tunneling microscopy, atomic force microscopy, scanning electrochemical microscopy, scanning ion conductance microscopy, and scanning electrochemical cell microscopy. Finally, the opportunities and challenges of SPM in electrocatalysis are discussed. The development of high-efficiency energy storage and conversion devices requires a deeper understanding of the structure-activity relationship of electrocatalysts, material transformations during electrocatalysis processes, and complex electrochemical processes at electrode-electrolyte interfaces. Scanning probe microscopy (SPM) is a powerful tool for visualizing the surface properties and localized electrochemical activity down to the atomic scale in situ and even operando, thus plays an essential role in studying heterogeneous electrocatalysis mechanisms. We summarize recent advancements in SPM for investigating energy-related electrocatalysis based on three unique characteristics of SPM—surface property imaging, in situ/operando monitoring, and nanoscale electrochemical mapping，outline the application of SPM in investigating the structure-activity relationship, material transformations, and electrochemical processes. The specific SPM techniques discussed here include scanning tunneling microscopy, atomic force microscopy, scanning electrochemical microscopy, scanning ion conductance microscopy, and scanning electrochemical cell microscopy. Finally, the opportunities and challenges of SPM in electrocatalysis are discussed. 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To this end, characterization tools that enable high-resolution surface renovation under reaction conditions and local activity measurement are desired to clarify how electrocatalyst structure affects local activity and how electrochemical reactions proceed at the electrode-electrolyte interfaces during electrochemical measurements.12Wang X. Cai Z.-F. Wang D. Wan L.-J. Investigation of catalytic reactions on electrode surface by scanning tunneling microscopy.Sci. Sin. Chim. 2018; 49: 470-479Crossref Scopus (0) Google Scholar For example, transmission electron microscopy (TEM) is capable of investigating electrocatalyst structure at the atomic level in real time.13Fan Z. Zhang L. Baumann D. Mei L. Yao Y. Duan X. Shi Y. Huang J. Huang Y. Duan X. In situ transmission electron microscopy for energy materials and devices.Adv. 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On the other hand, electrocatalyst performance is typically measured by macroscopic electrochemical techniques that quantify electrochemical activity in a surface-averaged manner, obscuring vital information on the activity distribution of single entities or surface structural and compositional heterogeneity (e.g., steps and defects).Alternatively, SPM can obtain surface structure information down to the atomic scale under realistic conditions, in solution and with potential control, and allows for electrochemical activity measurements with high spatiotemporal resolution, making it a valuable tool to study electrocatalysis processes.15Liang Y. Pfisterer J.H.K. McLaughlin D. Csoklich C. Seidl L. Bandarenka A.S. Schneider O. Electrochemical scanning probe microscopies in electrocatalysis.Small Methods. 2019; 3: 1800387Crossref Scopus (17) Google Scholar,16Gewirth A.A. Niece B.K. Electrochemical applications of in situ scanning probe microscopy.Chem. Rev. 1997; 97: 1129-1162Crossref PubMed Google Scholar SPM utilizes the interaction between a scanning probe and surface to track or interrogate the sample properties. It can be subdivided into a series of techniques according to the detection principle. The SPM techniques that are integral for studying electrocatalysis include scanning tunneling microscopy (STM), atomic force microscopy (AFM), scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), and scanning electrochemical cell microscopy (SECCM). STM and AFM are the two most used SPM methods, and they can provide critical structural information down to atomic resolution and are compatible with electrocatalysis conditions.17Binnig G. Rohrer H. Gerber C. Weibel E. Surface studies by scanning tunneling microscopy.Phys. Rev. Lett. 1982; 49: 57Crossref Scopus (0) Google Scholar, 18Binnig G. Rohrer H. Gerber C. Weibel E. Tunneling through a controllable vacuum gap.Appl. Phys. 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Nanoscale surface structure–activity in electrochemistry and electrocatalysis.J. Am. Chem. Soc. 2018; 141: 2179-2193Crossref PubMed Scopus (85) Google Scholar,22Bentley C.L. Edmondson J. Meloni G.N. Perry D. Shkirskiy V. Unwin P.R. Nanoscale electrochemical mapping.Anal. Chem. 2018; 91: 84-108Crossref PubMed Scopus (71) Google Scholar thus providing valuable insights into guiding improvement and innovation of electrochemical energy storage and conversion devices.In this review, we introduce SPM techniques and their working principles before discussing their applications in electrocatalysis focusing on their contextual advantages and disadvantages. From this perspective, we discuss impactful advancements in understanding the structure-activity relationship, material transformations, and electrochemical processes driven by SPM development. The applications in electrocatalysis are discussed separately according to surface property imaging, in situ/operando monitoring, and nanoscale electrochemical mapping (Scheme 1). Finally, the challenges and future development trends of SPM in electrocatalysis are further discussed. It is expected that, by reviewing the implementation of SPM technologies in the field of electrocatalysis, new insights into the study of electrocatalytic mechanism and the design of electrocatalysts could be provided.SPM development and working principleSPM is a microscopy family that started with the invention of STM by Gerd Binning and Heinrich Rohrer in 1982 to image the topography of surfaces at the atomic level.17Binnig G. Rohrer H. Gerber C. Weibel E. Surface studies by scanning tunneling microscopy.Phys. Rev. Lett. 1982; 49: 57Crossref Scopus (0) Google Scholar,18Binnig G. Rohrer H. Gerber C. Weibel E. Tunneling through a controllable vacuum gap.Appl. Phys. Lett. 1982; 40: 178-180Crossref Scopus (0) Google Scholar Originally it was operated strictly in a vacuum, but it has been extended to operate under both ambient atmosphere and liquid conditions.23Sonnenfeld R. Hansma P.K. Atomic-resolution microscopy in water.Science. 1986; 232: 211-213Crossref PubMed Google Scholar,24Drake B. Sonnenfeld R. Schneir J. Hansma P. Scanning tunneling microscopy of processes at liquid-solid interfaces.Surf. Sci. 1987; 181: 92-97Crossref Scopus (68) Google Scholar On the basis of STM, Itaya and Tomita developed EC-STM for imaging surfaces under electrochemical conditions.25Itaya K. Tomita E. Scanning tunneling microscope for electrochemistry—a new concept for the in situ scanning tunneling microscope in electrolyte solutions.Surf. Sci. 1988; 201: L507-L512Crossref Scopus (0) Google Scholar Considering that STM is only applicable to conductive samples, AFM was developed based on force measurement in 1986 to study any surface.26Binnig G. Quate C.F. Gerber C. Atomic force microscope.Phys. Rev. Lett. 1986; 56: 930-933Crossref PubMed Scopus (11929) Google Scholar Similarly to EC-STM, EC-AFM was invented soon after to image surfaces during electrochemical experiments.27Manne S. Massie J. Elings V. Hansma P. Gewirth A. Electrochemistry on a gold surface observed with the atomic force microscope.J. Vac. Sci. Technol. B. 1991; 9: 950-954Crossref Google Scholar Both EC-STM and EC-AFM can image samples with unmatched resolution and allow correlation to electrochemical experiments, but fails to provide quantitative and localized electrochemical information. A new direction was started with the introduction of SECM in 1989 to perform localized electrochemical measurements.28Bard A.J. Fan F.R.F. Kwak J. Lev O. Scanning electrochemical microscopy. Introduction and principles.Anal. Chem. 1989; 61: 132-138Crossref Scopus (966) Google Scholar Later, SECCM was developed to perform localized measurements directly on an electrode material. SICM is a general method that can be used to sense the local ionic environment that is gaining momentum for electrocatalysis due to its versatility.29Kang M. Momotenko D. Page A. Perry D. Unwin P.R. Frontiers in nanoscale electrochemical imaging: faster, multifunctional, and ultrasensitive.Langmuir. 2016; 32: 7993-8008Crossref PubMed Scopus (53) Google Scholar, 30Lipson A.L. Ginder R.S. Hersam M.C. Nanoscale in situ characterization of Li-ion battery electrochemistry via scanning ion conductance microscopy.Adv. Mater. 2011; 23: 5613-5617Crossref PubMed Scopus (59) Google Scholar, 31Ebejer N. Schnippering M. Colburn A.W. Edwards M.A. Unwin P.R. Localized high resolution electrochemistry and multifunctional imaging: scanning electrochemical cell microscopy.Anal. Chem. 2010; 82: 9141-9145Crossref PubMed Scopus (168) Google Scholar, 32Ebejer N. Güell A.G. Lai S.C.S. McKelvey K. Snowden M.E. Unwin P.R. Scanning electrochemical cell microscopy: a versatile technique for nanoscale electrochemistry and functional imaging.Annu. Rev. Anal. Chem. 2013; 6: 329-351Crossref PubMed Scopus (0) Google ScholarAll these techniques are SPMs due to the similar scanning mechanism in which a probe is scanned across a surface, and the probe-sample interaction is monitored. A schematic of a generic SPM setup for investigating electrochemical systems is presented in Figure 1. The fundamental components across all SPM methods are probe, scanner, and feedback system. The probe senses changes in the local environment, typically due to probe-surface interactions. The scanner actuates the probe and/or sample by piezo electric motors, or mechanical motors for coarse movement, in response to a voltage change applied across piezo electric materials that expands or retracts accordingly. Different SPMs utilize different probes and detect different probe-sample interactions as the signal. The specific working principles of the SPM techniques mentioned above are introduced below.Figure 1Schematic illustration of an SPM setup for electrochemical systemsShow full captionThe probe details and probe-sample interaction of different SPM techniques are shown below the setup. RE and CE represent the reference electrode and counter electrode, respectively. The transformation from O to R refers to an electrochemical reduction process. The dotted lines indicate that the connection is not always required, depending on the type of SPM. To be specific, the connection from the potentiostat to the probe is not required in EC-AFM; external CE is not necessary in SICM; in SECCM, both external RE and CE are not required.View Large Image Figure ViewerDownload Hi-res image Download (PPT)STM and EC-STMSTM uses a sharp metallic tip as the probe for scanning over a conducting sample without making physical contact, detecting the tunneling current between the probe and sample surface for positional feedback.17Binnig G. Rohrer H. Gerber C. Weibel E. Surface studies by scanning tunneling microscopy.Phys. Rev. Lett. 1982; 49: 57Crossref Scopus (0) Google Scholar,18Binnig G. Rohrer H. Gerber C. Weibel E. Tunneling through a controllable vacuum gap.Appl. Phys. Lett. 1982; 40: 178-180Crossref Scopus (0) Google Scholar When the probe is brought very close to the surface, a bias applied between the probe and sample permits electrons to tunnel across the tunnel barrier between them, and the resulting tunneling current increases exponentially with decreasing tip-sample distance. When the tunneling current is set to be constant, the tip tracks the sample surface morphology under the control of the feedback system. It is important to point out that STM does not measure physical height since the electric current depends on the sample geometry and local density of states (LDOS).33Voigtländer B. Scanning Probe Microscopy: Atomic Force Microscopy and Scanning Tunneling Microscopy. Springer, 2015Crossref Google Scholar The LDOS can be determined by holding the tip in a constant position above the surface, varying the voltage between the tip and the sample, and recording the current changes.34Feenstra R.M. Stroscio J.A. Fein A.P. Tunneling spectroscopy of the Si(111)2 × 1 surface.Surf. Sci. 1987; 181: 295-306Crossref Scopus (506) Google Scholar,35Hasegawa Y. Avouris P. Direct observation of standing wave formation at surface steps using scanning tunneling spectroscopy.Phys. Rev. Lett. 1993; 71: 1071-1074Crossref PubMed Scopus (575) Google ScholarEC-STM combines the classical STM setup with electrochemical measurements,25Itaya K. Tomita E. Scanning tunneling microscope for electrochemistry—a new concept for the in situ scanning tunneling microscope in electrolyte solutions.Surf. Sci. 1988; 201: L507-L512Crossref Scopus (0) Google Scholar in which a bipotentiostat is used to measure and control the current and potential of probe and sample against a reference electrode (RE) independently, and a counter electrode (CE) is used for closing the circuit. Thus, an external voltage can be used to control the electrochemical reaction, while the topography of the sample surface is imaged.AFM and EC-AFMIn AFM, a cantilever with a sharp tip is used as the probe and the interactive forces between the tip and sample are measured. When the probe scans over a sample, the tip-surface interaction force bends the cantilever according to Hooke’s law or changes the kinetics of the cantilever, and the measured cantilever deflection allows to generate the surface topography map.26Binnig G. Quate C.F. Gerber C. Atomic force microscope.Phys. Rev. Lett. 1986; 56: 930-933Crossref PubMed Scopus (11929) Google Scholar The forces measured in AFM include chemical forces, friction forces, van der Waals forces, capillary forces, and electrostatic and magnetic forces. Kelvin probe force microscopy (KPFM) is based on the long-range electrostatic force by combining AFM and the Kelvin probe technique.36Nonnenmacher M. o’Boyle M. Wickramasinghe H.K. Kelvin probe force microscopy.Appl. Phys. Lett. 1991; 58: 2921-2923Crossref Scopus (0) Google Scholar,37Baikie I. Estrup P. Low cost PC based scanning Kelvin probe.Rev. Sci. Instrum. 1998; 69: 3902-3907Crossref Scopus (132) Google Scholar It senses the contact potential difference (CPD) between the conductive probe and sample due to the work function (WF) difference, by adjusting the voltage on the probe, the CPD can be compensated and measured. Therefore, the local surface potential of the sample can be determined.EC-AFM combines a classic AFM setup with a three-electrode electrochemical cell to perform topographical measurements during electrocatalysis.27Manne S. Massie J. Elings V. Hansma P. Gewirth A. Electrochemistry on a gold surface observed with the atomic force microscope.J. Vac. Sci. Technol. B. 1991; 9: 950-954Crossref Google Scholar A potentiostat is used to measure/control the current and potential of the sample while the AFM probe can monitor surface topography.SECMSECM uses an ultramicroelectrode (UME) as the probe to interrogate chemical species locally over an electrolyte-immersed sample. A bipotentiostat is used to control and measure the faradic current and/or potential of the probe and/or sample.28Bard A.J. Fan F.R.F. Kwak J. Lev O. Scanning electrochemical microscopy. Introduction and principles.Anal. Chem. 1989; 61: 132-138Crossref Scopus (966) Google Scholar,38Engstrom R.C. Pharr C.M. Scanning electrochemical microscopy.Anal. Chem. 1989; 61: 1099A-1104ACrossref PubMed Google Scholar The current response is affected by the probe-sample distance and the sample properties underneath. SECM has various operating modes and we cover the feedback mode and the generation-collection (G/C) mode.39Lai S.C. Macpherson J.V. Unwin P. In situ scanning electrochemical probe microscopy for energy applications.MRS Bull. 2012; 37: 668Crossref Scopus (29) Google Scholar In the feedback mode, the response current at the probe is recorded as a function of tip-to-substrate distance and surface activity, with a redox mediator present in the bulk solution. The tip current decreases when the probe approaches an insulating material since the diffusion of the oxidized form of the redox mediator (O) is inhibited (negative feedback); whereas the current increases if the probe approaches a conducting substrate since the reduced form of the redox mediator (R) can be regenerated at the surface (positive feedback). In the generation/collection (G/C) mode, a species is electrogenerated at the probe and collected at the substrate (tip generation/substrate collection [TG/SC] mode) or vice versa (substrate generation/tip collection [SG/TC] mode).SICMIn a typical SICM setup, an electrolyte-filled pipette with a back-inserted quasi-reference CE (QRCE) serves as probe to scan over a sample immersed in the electrolyte. The probe is fabricated from a glass capillary that is heated and pulled to produce a pipette with a sharp tip. A second QRCE placed in the bath solution serves as the RE and closes the circuit.40Hansma P. Drake B. Marti O. Gould S. Prater C. The scanning ion-conductance microscope.Science. 1989; 243: 641-643Crossref PubMed Google Scholar Ionic current is passed through the pipette pore as a bias is applied between two QRCEs, and the ionic current decreases as the probe is moved toward a surface due to hindered ion flow. The ion current serves as the feedback signal to adjust the position of the probe during scanning for topographical mapping. It is important to note that the ion current response is also sensitive to the interfacial properties, particularly the surface charge and electrochemical reactions at the interface. These interfacial properties will induce local ionic current changes, which can be detected and characterized by using appropriate bias between the two QRCEs and finally be converted to surface charge or local activity mapping. It is also possible to collect topography and surface charge/activity with bias-modulated (BM) SICM simultaneously by carefully tuning the alternative current signals that arise when the potential is oscillated.41Page A. Perry D. Unwin P.R. Multifunctional scanning ion conductance microscopy.Proc. Math. Phys. Eng. Sci. 2017; 473: 20160889PubMed Google ScholarSECCMIn SECCM, an electrolyte-filled (dual-channel) pipette with a back-inserted QRCE(s) serves as the probe by forming a droplet cell upon meniscus contact between a droplet hanging from the probe with the underlying substrate.31Ebejer N. Schnippering M. Colburn A.W. Edwards M.A. Unwin P.R. Localized high resolution electrochemistry and multifunctional imaging: scanning electrochemical cell microscopy.Anal. Chem. 2010; 82: 9141-9145Crossref PubMed Scopus (168) Google Scholar The ionic current induced by applying a bias between the QRCEs can be used as the feedback signal to obtain height profile. Sweeping the potential (cyclic voltammogram [CV] or linear-sweep voltammogram [LSV]) between the sample and the QRCEs can trigger a surface redox process, and the as-geneated electrochemical current flow can be measured at each pixel to obtain an electrochemical activity map. It was demonstrated that, for electrocatalytic materials, the dual-channel probe can be simplified to a single-channel configuration with only one back-inserted QRCE based on an earlier scanning micropipet contact method.42Williams C.G. Edwards M.A. Colley A.L. Macpherson J.V. Unwin P.R. Scanning micropipet contact method for high-resolution imaging of electrode surface redox activity.Anal. Chem. 2009; 81: 2486-2495Crossref PubMed Scopus (146) Google Scholar In single-channel SECCM, an appropiate voltage is applied between the sample and QRCE during the approaching process, once the droplet contacts the sample surface to build up an electrochemical cell with two electrodes, the target reaction can be triggered to generate a current response for topographic feedback, then the tip is held at this position to perform electrochemical measurements, and topographical and electrochemical mapping can be obtained synchronously.43Daviddi E. Gonos K.L. Colburn A.W. Bentley C.L. Unwin P.R. Scanning electrochemical cell microscopy (SECCM) chronopotentiometry: development and applications in electroanalysis and electrocatalysis.Anal. Chem. 2019; 91: 9229-9237Crossref PubMed Scopus (19) Google ScholarApplication in electrocatalysisSurface property imagingThe catalytic efficiency of electrocatalysts is mostly determined by their surface properties because electrocatalysis processes are usually surface dominated.44Wang X. Vasileff A. Jiao Y. Zheng Y. Qiao S.-Z. Electronic and structural engineering of carbon-based metal-free electrocatalysts for water splitting.Adv. Mater. 2019; 31: 1803625Crossref Scopus (133) Google Scholar Therefore, optimization of electrocatalysts has been achieved by surface engineering, such as crystal facet controlling, heteroatom doping, defect engineering, and interface engineering.45Jiao Y. Zheng Y. Jaroniec M. Qiao S.Z. Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions.Chem. Soc. 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We present fundamental investigations of materials properties with SPM and give examples of how they cooperate electrochemical measurements to illustrate structure-activity relationships.Imaging surface topographyAll SPM techniques provide, in principle, the basic functionality of imaging surface topography, where STM and AFM serves as the state-of-the-art methods with down to atomic resolution in three dimensions. The unmatched resolution makes STM and AFM widely used to investigate nanoscale defects, crystal facets, grain boundaries, size, shape, terraces, thickness, and roughness of electrocatalysts, which are important elements in determining the electrocatalytic performance.49Dou S. Wang X. Wang S. Rational design of transition metal-based materials for highly efficient electrocatalysis.Small Methods. 2019; 3: 1800211Crossref Scopu