Characterizing Electrocatalysts with Scanning Electrochemical

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Kinetics, Catalysis, and Reaction Engineering

Characterizing Electrocatalysts with Scanning Electrochemical Microscopy Dylan T. Jantz, and Kevin C. Leonard Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00922 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Industrial & Engineering Chemistry Research

Characterizing Electrocatalysts with Scanning Electrochemical Microscopy Dylan T. Jantz and Kevin C. Leonard⇤ Center for Environmentally Beneficial Catalysis, Department of Chemical & Petroleum Engineering, The University of Kansas, Lawrence, KS, USA E-mail: [email protected] Phone: +1 785-864-1437 Abstract Scanning Electrochemical Microscopy (SECM) is a “non-contact” scanning probe technique capable of providing chemical and/or topographic information about surfaces immersed in a solution. It is a powerful in-situ and operando tool for obtaining insights into electrocatalytic rates and mechanisms. Herein, examples are given on how SECM can be used to characterize (1) hydrogen oxidation reaction electrocatalysts for fuel-cell applications, (2) oxygen evolution reaction electrocatalysts for water splitting applications, and (3) electrochemical CO2 -reducing catalysts. We also report a new operando method of SECM in which we can separate electrochemical reduction of CO2 to CO in aqueous media from parasitic hydrogen evolution as a function of applied potential. Understanding hydrogen evolution suppression is a major challenge in the intelligent design of CO2 -reducing electrocatalysts. Via this multireactional SECM technique, we observed the optimal potential window for electrochemically reducing CO2 to CO with high selectivity. At potentials lower than this optimal window, CO2 reduction rates were small, but at potentials higher this this optimal window, water reduction to H2

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dominated the electrochemical conversions. This observation is something that traditional voltammetry alone is not able to resolve. We anticipate that a wider adoption of SECM by the catalysis community will allow for the operando characterization of many types of catalysts, and holds the potential to provide unprecedented insights into the composition/activity and structure/activity relationships.

Introduction A grand challenge in catalysis science is developing the ability to intelligently design catalysts for a specific reaction with high efficiency and selectivity. In order to make this a reality, a better understanding is needed for both the composition/activity and structure/activity relationships. To obtain this understanding, there has been a recent upsurge in the development of new in-situ and operando tools for catalyst characterization. 1–3 While there has been a plethora of recent developments for in-situ and operando techniques for gas-phase catalysis (e.g. Environmental TEM, ambient-pressure XPS, etc.), additional techniques are still needed for catalysts that operate in the liquid-phase. However, developing such tools that operate in the liquid phase are challenging due to the complex environment in which liquid-phase catalysis occurs. A major class of catalysts that always operate in the liquid phase are electrocatalysts. This is because electrocatalysts need to operate in high conductivity environments, which are only a↵ordable in the liquid phase. In recent years, there has been an explosion of research in electrocatalyst due to their green energy applications, such as hydrogen generation, fuel-cells, air-batteries, CO2 -fixation, and ammonia production. 4–7 However, intelligent design of heterogeneous electrocatalysts is still not possible due to a lack of understanding of fundamental catalytic relationships. If better understanding of how composition and structure a↵ect activity of electrocatalyst, major advances could be made in many green energy applications. To obtain this, researchers in catalysis development need to utilize and develop new in-situ and operando tools that operate in the liquid phase. A liquid-phase in-situ/operando technique that our group utilizes for electrocatalyst char2


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acterization is Scanning Electrochemical Microscopy (SECM). SECM was invented by Allen Bard, in 1989. 8,9 SECM is an “non-contact” scanning probe technique capable of providing chemical and/or topographic information about surfaces immersed in a solution. SECM utilizes an ultramicroelectrode “UME” or nanoelectrode as the probe (a.k.a. the SECM tip electrode), and SECM measurements are made by monitoring electrochemical reactions that occur on the SECM tip electrode. Unlike other scanning probe techniques (e.g. AFM 10,11 or STM 12 ), the probe does not contact the surface. Instead, the probe hovers 10’s-100’s of nm above the surface, which allows for the electrochemical reactions to occur on the tip electrode. This gives SECM the unique capability to make measurements that are not possible with other scanning probe techniques. SECM has been frequently used to study heterogeneous and homogeneous reaction kinetics, semiconductor surfaces, biological systems, and single particle detection. 13–20 However, there are a wide ranging applications where SECM is underutilized. SECM has several modes of operation, allowing for a single instrument to perform multiple types of measurements. The most well-known mode of SECM is its imaging mode, which can create in-situ chemical reactivity maps of catalysts while they are operating in the solution phase. 21–25 In the imaging mode, the SECM tip electrode is raster-scanned across the catalytic surface, and the current on the electrode tip is measured as a function of position. There are two types of SECM imaging modes. The first is the topographic imaging mode, sometimes referred to as the “feedback” imaging mode. In topography imaging, a redox mediator (i.e. a compound that can undergo reversible, outer-sphere oxidization and/or reduction reactions, e.g. ferrocene) is added to the solution, and the redox mediator is either oxidized or reduced on the SECM tip electrode. Because the SECM tip electrode is only 10’s to 100’s of nm above the surface, the di↵usion of the redox mediator towards the SECM tip electrode will be hindered by the substrate as it is scanned across the surface. Conversely, if the substrate is electrically conductive (and moreover, if an electrical potential is applied to the substrate) the reverse electrochemical reaction can happen on the substrate, which would

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increase mass transfer of the redox mediator to the SECM tip electrode. In either event, the current measured on the SECM tip electrode will be a related to the the tip/substrate distance. Thus, maps of the SECM tip current as a function of position will obtain the topography of the catalyst as it is operating in the solution phase. The second type of SECM imaging is the reactivity imaging mode. This mode is most unique to SECM. Here, products that are generated on catalytic substrate are electrochemically “collected” on the SECM tip electrode by either oxidizing or reducing them. For example, if one was obtaining an electrochemical reactivity map of a hydrogen evolving electrocatalyst, one would use a Pt SECM tip electrode and perform electrochemical hydrogen oxidation on the tip electrode. Then by raster-scanning the SECM tip electrode over the surface, a map of the tip current versus position would reveal local areas of higher product concentration, and thus result in a reactivity map of the catalyst surface. In addition to imaging, SECM has several non-imaging modes of operation where the SECM tip electrode is placed over the substrate being analyzed and chemistry is performed inside the “nano gap.” In these non-imaging mode experiments, this “nano gap” can range in size anywhere from 10’s of nm to 10’s of µm depending on the electrode size and type of measurement being performed. 26–28 One of these modes is referred to as the feedback mode, which is used to measure fast, homogeneous electron-transfer rate kinetics. 29–31 By performing this electrochemistry inside the “nano gap”, the mass-transfer limitation can be overcome, allowing for the determination of electron transfer kinetic rate constants that are not possible to measure with traditional voltammetry experiments. Another “nano gap” mode is the generation/collection mode (either substrate generation/tip collection or tip generation/substrate collection). 32–34 Similar to the reactivity imaging mode, a product generated by a catalytic substrate is electrochemically collected on the SECM tip electrode. This allows for in-situ product detection of catalyst and very small scales without needing to perform ex-situ product characterization techniques (e.g. NMR, GC, HPLC, etc.). The final “nano gap” mode (which is typically

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performed in a “micro-gap”) is an electrochemical titration techniques referred to as surfaceinterrogation SECM, or SI-SECM. 27,35,36 SI-SECM is a very powerful operando technique for catalyst characterization. With SI-SECM it is possible to (1) quantify the activity site densities of catalysts, (2) accurately determine electron-transfer kinetic rate constants of catalytic active sites, and (3) quantify the rate constants of di↵erent active sites on a catalyst surface to distinguish between fast and slow sites. SI-SECM utilizes a titrant that can be activated via applying an electrochemical potential on the SECM tip electrode. When surface-adsorbed species are present on the catalytic substrate, the titration of said surface species produces positive feedback current on the tip until all surface-adsorbed species on the substrate are consumed. Thus by measuring the SECM tip current during the titration, it is possible to quantify the number of surface-adsorbed species, which in turn results in the number of active sites on the catalyst. Performing time-dependent titrations, it is possible to obtain the kinetics of the active sites, and separate fast sites from slow sites. 37 Because SECM has been so under utilized in catalyst characterization field, this manuscript will first highlight some of the recent work performed by our group on electrocatalyst characterization utilizing the di↵erent modes of SECM. Second, we report a new SECM technique that is capable of separating two di↵erent products evolving from a CO2 -reducing catalyst in aqueous media. This reaction is of great importance for the conversion of carbon dioxide into value-added fuels and chemicals. 38–40 However, electrochemical CO2 reduction always competes with the hydrogen evolution reaction in aqueous media. 41 Thus, an operando technique which can characterize the potential dependent selectivity of the catalyst which greatly aid in understanding selectivity of CO2 -reducing catalyst. Here we show that SECM can be this operando technique and provide unprecedented insights into CO2 -reducing electrocatalysts.

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(b) (a)

(c)

Figure 1: (a) Schematic showing the necessary components of a Scanning Electrochemical Microscope. SEM images of a SECM tip nanoelectrode showing (b) a high quality electrode and (c) an electrode that is damaged. Reproduced with permission from reference 22. Copyright 2016 American Chemical Society.

Results and Discussion SECM Instrumentation In order for a researcher to utilize SECM, one needs the proper instrumentation. SECMs can be purchased commercially from a few di↵erent suppliers (e.g. CH Instruments, Ametek Scientific Instruments, Bruker, etc.) or they can be assembled from commercial components. Figure 1 shows a typical schematic for a Scanning Electrochemical Microscope. 22 An SECM has three main components – a motion control system for moving the SECM tip electrode, a potentiostat for monitoring the SECM tip current, and software which can interface the motion control system with the potentiostat. The SECM tip electrode is mounted to the motion control system so that the tip electrode can be position in close proximity to the substrate being investigated. The motion control system typically has 3-axis control so that both imaging and nano-gap electrochemistry can be performed on the same instrument. In our group, we have constructed a custom high-resolution SECM system using high-precision

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motion controllers. 42 The fine control nanopositioner moves the tip electrode in the X, Y, and Z directions with a spatial resolution of 0.4 nm. The entire system uses custom software developed with LabVIEW. The instrument in our group also utilizes innovative software to intelligently control the tip electrode using a fuzzy logic control algorithm. The potentiostat can be any commercial potentiostat, but typically a SECM uses a bi-potentiostat with two working electrodes. This allows two separate potentials to be applied to the SECM tip electrode and substrate, respectively, which is necessary for imaging, feedback experiments, and SI-SECM. Other components that are needed for SECM are (1) a chamber to house the SECM, which can act as a faraday cage and a controlled temperature/humidity chamber (specifically for nanoscale SECM) 22 (2) a tilt adjustable stage to provide good tip/substrate alignment, and (3) a vibration isolation table. Details on the specifics on these components can be found the seminal book by Bard and Mirkin. 43 The most important part of successfully carrying out an SECM experiment is having a good SECM tip electrode (Fig. 1b). Unlike other scanning-probe techniques, there are additional requirements on the SECM tip electrode. First and foremost, the SECM tip electrode needs to be insulating on the sides of the electrode. This is because measurements need to be made between the tip and the substrate, and if the SECM tip electrode is not insulated then reactions will occur on the sides of the electrode and mask any reactions that are occurring within the “nano gap”. Typically SECM electrodes contain a disk electrode which is surrounded by glass (Fig. 1b). While SECM tip electrodes can be purchased commercially (e.g. CH Instruments), they are typically not small enough to do nanometer-scale SECM experiments. Thus, most researchers that use SECM fabricate their own tip electrodes which also allows for tip size and RG customization. Nanoelectrode tips that are necessary for nanometer-scale SECM experiments can be damaged easily (Fig. 1c) from tip crashing and static charge. There have been plenty of research on di↵erent methods of fabricating SECM tip electrodes, 44–46 but generally they are made by pulling a microwire inside a glass capillary using a laser puller, such that the glass becomes adhered to the microwire.

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In addition, it is critically important to have the disk electrode perfectly aligned with the insulation sheath (not protruding or recessed as in Fig. 1c). Without such alignment, small tip/substrate distances can not be achieved, which will lower the overall resolution of the SECM experiment.

SECM Imaging (a)

(b)

(c)

Figure 2: (a) Schematic showing both the topography imaging mode (left) and reactivity imaging mode (left) of SECM. (b) SECM topography image and (c) HOR reactivity image of Pt nanoparticles deposited on the step edges of HOPG. Reproduced with permission from reference 21. Copyright 2016 American Chemical Society. As stated above, understanding the relationship between the structure and the reactivity of catalytic nanoparticles is important to achieve higher efficiencies in catalytic systems. 8


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There exists a great challenge to study these relationships at the individual nanoparticle level. To address this challenge, we developed, in collaboration with the Bard group, 21 an approach that uses nanometer-scale SECM to study the geometric properties and catalytic activity of individual Pt nanoparticles for the hydrogen oxidation reaction (HOR). Here, Pt nanoparticles with a few tens to a hundred nm radius were electrodeposited on a highly oriented pyrolytic graphite (HOPG) surface via nucleation and growth. Using this method, the Pt nanoparticles are preferentially deposited along the step edges of HOPG. A welldefined nanometer-sized tip with comparable dimensions to the nanoparticles, combined with a nm-scale tip/substrate distance, enabled us to achieve topography and reactivity images at nanometer-scale and study fast electron-transfer kinetics (Fig. 2a). The topography image was measured using an outer-sphere redox mediator, ferrocene-trimethylamine, FcTMA+ . Here, FcTMA+ was oxidized to FcTMA2+ on the SECM tip electrode and the generated FcTMA2+ was reduced back to FcTMA+ on the substrate due to a potential applied to the HOPG electrode. This will increase the mass transfer of the FcTMA+ back to the tip electrode, causing an increase in the measured current. However, since the tip/substrate distance will be smaller when the SECM tip electrode is over the Pt nanoparticle compared to when the tip is over the the HOPG surface, there will be an enhancement of the tip current when the tip is over a Pt nanoparticle. Thus, a map of the tip current as a function of position will reveal the topography of the substrate electrode. Figure 2b shows the topographic image of the Pt nanoparticles in solution. Here we can see that the Pt nanoparticles stay on the HOPG step edge and that they do not aggregate in solution under an applied potential. Once a topography image is taken, it is next possible to obtain a reactivity image. Figure 2c shows the reactivity image for the electrochemical hydrogen oxidation reaction (HOR) – an important reaction for fuel-cell applications. To obtain the reactivity image, the protons that are produced via the HOR reaction are reduced on the Pt-tip electrode. Thus, areas with higher proton concentration will result in higher tip currents, and plotting the tip current vs. position will result in an electrochemical reactivity map. In addition, quantitative results

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can be obtained from these reactivity maps. Here, we performed numerical simulations on the topography image and determined that the radii of the 5 nanoparticles (labeled 1-5) were 63, 63, 108, 76, 117 nm respectively. Again with the aid of numerical simulations, we measured a large e↵ective rate constant for the HOR (k0e↵ of

2 cm/s as a lower limit at

each of the Pt nanoparticles). We did not observe any di↵erence in the catalytic activity for the di↵erent shaped nanoparticles. However, this technique allows for the size, shape, spatial orientation, and the catalytic activity of catalytic nanoparticles to be determined at an individual level.

Generation/Collection SECM on CO2 -Reducing Electrocatalysts (a)

ETip

+0.2V

(d)

(b)

Pt SECM Electrode

e-

(a)

ETip

(b)

+0.2V

(c) (c)

Pt SECM Electrode

(e)

HCOO- CO CO2 FEP Mask

FEP Maske

-

In0-In2O3 Composite on FTO ESub

HCOO- CO CO2 FEP Mask

FEP Mask

In0-In2O3 Composite on FTO ESub

Figure 3: Substrate generation/tip collection SECM results on a In0 -In2 O3 CO2 producing electrocatalyst. (a) Schematic showing the SECM experiment. Cyclic voltammetry data on the (b) In0 -In2 O3 electrode and (c) a polycrystalline In foil electrode in Ar-saturated and CO2 -saturated 0.1M Na2 SO4 at 50 mV-1 . (d) Collection current of CO, H2 , and COOH – at +0.2V vs Ag/AgCl on a Pt electrode showing that only CO and H2 can be collected. (e) Substrate generation/tip collection results showing that the In0 -In2 O3 electrode produces CO at relatively low overpotentials. Reproduced from Ref. 28 with permission from the Royal Society of Chemistry. As stated above, the “nano gap” electrochemistry mode of SECM can be used for in-situ product detection. Recently our group reported a In0 -In2 O3 composite material that can electrochemical reduce CO2 to CO at relatively low overpotentials. 28 In order to determine 10


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the potential dependent CO production, one could perform bulk electrolysis experiments over a wide range of potentials and collect gas products for analysis by gas chromatography. However, this can be a very time intensive experiment. Instead, we utilized a novel mode of Scanning Electrochemical Microscopy where CO can be selectively detected from formate on the tip of a SECM electrode in-situ during the voltammetry experiment inside of a “micro gap”(Fig. 3a). We determined that it was possible to selectively oxidize CO and not formate on a Pt electrode at +0.2 V vs Ag/AgCl (Fig. 3d). Thus, we positioned a Pt microelectrode c.a. 10 µm over a masked In0 -In2 O3 composite electrode. This mask had a c.a. 100 µm diameter center void so that any CO generated by the In0 -In2 O3 electrode could be collected on the Pt microelectrode. After the tip electrode was positioned, a potential pulse was applied to the In0 -In2 O3 composite electrode for 20 s per potential over a wide range of potentials to generate the CO2 reduction products. After the 20 s potential pulse, the In0 -In2 O3 composite electrode was brought to open-circuit while simultaneously applying +0.2 V vs Ag/AgCl to the Pt tip electrode. Performing this experiment allows us to electrochemically collect any CO that is produced on the catalytic electrode because formate is not electrochemically active at this potential. Figures 3b and 3c show the voltammetry data for the In0 -In2 O3 composite electrode and a polycrystalline In foil electrode in both CO2 -saturated and Ar-saturated 0.1M Na2 SO4 . Here we can see that only the In0 -In2 O3 electrode in CO2 -saturated electrolyte is electrochemically active. To verity that this reaction is producing CO, the substrate generation/tip collection experiment was performed. By integrating the net current versus time, we are able quantify the amount of charge collected on the Pt tip electrode as a function of applied potential on the In0 -In2 O3 composite electrode (Fig. 3e). Comparing Fig. 3e to the voltammetry data shows several insights about the electrochemistry of the In0 -In2 O3 composite electrode in CO2 saturated Na2 SO4 . First, because we observed no collection on the Pt tip electrode in the potential range of -0.6 V to -0.9 V vs Ag/AgCl, we can conclude that the redox feature that

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we observed at that potential in the voltammetry is not producing either CO or H2 . Secondly, the onset and increase in collection on the Pt tip after -1.0 V vs Ag/AgCl, as we observed in the voltammogram, proves that the onset wave at -1.0 V vs Ag/AgCl is in fact electrocatalysis of CO2 . Moreover, since we are able to collect the product on the Pt tip electrode, it shows that formate can not be the major product, because it is not electrochemically active on Pt at +0.2 V vs Ag/AgCl. SECM experiments on the In0 -In2 O3 composite electrode in Ar-saturated Na2 SO4 show no collection of products, even though H2 oxidation will occur at +0.2 V vs Ag/AgCl. This suggests that the In0 -In2 O3 electrocatalyst is highly selective for producing carbon-based products. Finally, SECM measurements on the In foil electrode show no collection of products in either CO2 -saturated or Ar-saturated electrolytes, demonstrating that the In-foil electrode does not produce CO within the potential range investigated. While most researchers who study CO2 -reducing electrocatalyst relay on ex-situ product detection, we feel that in-situ product detection via SECM could be a very powerful method for determining potential dependent product formation.

Surface Interrogation SECM on OER Electrocatalysts A major e↵ort in catalyst characterization studies, is to characterize the kinetics of the active sites. In a study on nickel-iron oxyhydroxide catalysts, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) as an operando method to directly measure active site kinetics. 37 Traditionally, the layered-double hydroxide structure of nickeliron oxyhydroxide is thought to have “fast” Fe sites and “slow” Ni sites. In fact, the Bard group used SI-SECM to probe the kinetics of the active sites on iron, nickel, and nickeliron (oxy)hydroxides. In Bard’s study, they in fact observed that two types of catalytic sites existed for nickel-iron oxyhydroxide; fast sites with a kinetic rate constant of 1.70 s-1 per site and slow sites with a kinetic rate constant of 0.056 s-1 per site. 14 Our study focused on amorphous nickel-iron oxyhydroxides. Because our amorphous materials had very little nickel, iron segregation, we used SI-SECM to determine the kinetic 12


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(a)

(b)

(c)

k = 1.85 s-1

Figure 4: (a) Schematic of the SI-SECM experiment to probe the kinetics of active sites on a nickel-iron OER electrocatalyst. (b) Current vs time data of the glassy carbon tip electrode at di↵erent delay times showing the enhancement of current when surface active species are present. (c) Kinetic analysis of the SI-SECM results showing that the amorphous nickel-iron oxide OER catalyst had a single rate constant with a value of 1.85 s-1 . Reproduced from Ref. 37 with permission from the Royal Society of Chemistry.

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rate constant of the active sites. Here Fe(III)-TEA was used as the redox mediator, which was reduced to Fe(II)-TEA on a glassy carbon SECM tip electrode (Fig. 4a). This Fe(II)TEA species is able to react with active NiIV or FeIV sites, thus when these active sites are present on the electrode surface, an enhanced current will be measured on the tip electrode (Fig. 4a). By measuring this current enhancement at di↵erent delay-times, it is possible to determine the lifetime of these active sites, i.e. the kinetics of how these reactive sites react with hydroxide ions to produce oxygen. Our findings show that the amorphous (Ni,Fe) oxide material had a lower overpotential for the OER compared to the crystalline LDH nickel-iron oxyhydroxide. Our SI-SECM experiments on the crystal-derived sample showed the existence of “fast” and “slow” sites – kinetic rate constants of 1.3 s-1 and 0.05 s-1 , respectively. These results are in very good agreement with the study recently performed by Bard and co-workers. 14 However, SI-SECM experiments on the nanoamorphous nickel-iron structure showed only one type of site, and the kinetic rate constant of this site (1.9 s-1 ) matched well with the kinetics of the fast site on the crystal-derived structure (Figs. 4b and 4c). This finding suggests that welldispersed, amorphous materials may have higher catalytic activity because they exhibit only the fast catalytic sites. However, this finding is only made possible by the operando SI-SECM technique that we employed.

Multireactional SECM on CO2 -Reducing Electrocatalyst in Aqueous Media As mentioned above, SECM is a powerful tool for in-situ and operando product detection, but a question remains on how selective the collection of products on the SECM tip electrode can be. For example, researchers who study electrocatalyst typically use voltammetry experiments (e.g. cyclic voltammetry) in the study of electron-transfer processes. However, a drawback of using a single electrode for voltammetry is that the measured current at a given potential is the sum of all electrochemical reactions that are occurring on the electrode. 14


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Thus, on an electrode where multiple electrochemical reactions occur simultaneously (i.e. a multireactional electrochemical interface), it is impossible to separate the individual partial currents in order to study the individual reactions using traditional voltammetry techniques. Recently, Leonard and Bard 47 developed a multireactional mode of scanning electrochemical microscopy (MR-SECM) that is able to separate the partial currents of a multireactional interface. MR-SECM is able to separate the partial currents by using a secondary SECM tip electrode to selectively collect only one of the products, overcoming the limitation of ! single electrode voltammetry. The initial MR-SECM work 47 separated the E1,p E2,p multireactional interface (i.e. a multireactional interface comprising one reduction reaction and one oxidation reaction) of hydrogen evolution occurring simultaneously with Mn corrosion. Currently, one of the most highly studied multireactional electrochemical interface are electrodes that reduce CO2 in aqueous or protic solvents. 48–55 This reaction is of great importance for conversion of carbon dioxide into value-added chemicals or fuels. 56–58 However, the potential range where electrochemical CO2 reduction occurs typically overlaps with the ! ! hydrogen evolution reaction (HER) resulting in a E1,p E2,p (i.e. two reduction) multireactional interface. Thus, when cyclic voltammetry is used to study electrochemical CO2 reduction in the presence of H+ or H2 O, the measured current is sum of both reduction reactions. Thus, unless you have a material that is very inert to hydrogen evolution (e.g. indium as described above), it is impossible to directly distinguish between the two reactions by the voltammetry alone. Traditionally, the CO2 reduction is separated from the hydrogen evolution reaction (HER) by comparing both voltammetry experiments and bulk electrolysis experiments where the electrolyte is CO2 -saturated to electrolytes that are Ar-saturated. 59,60 However, this traditional method of performing separate experiments is not only time and labor intensive, but it prohibits direct understanding of interactions between the HER and CO2 reduction. Here we report, for the first time, the use of MR-SECM to in-situ deconvolute electrochemical CO2 reduction from electrochemical hydrogen evolution on a gold electrode. Gold

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electrodes where chosen for this study because they are known to be highly selective towards carbon monoxide (CO), 61–65 but are also fairly efficient at the hydrogen evolution 66 , as shown in ex-situ gas chromatography results (Fig. S1b).

Figure 5: Cyclic voltammograms of 200 µm Au electrode in Ar-saturated (blue), and CO2 saturated (red) 0.1 M Na2 SO4 at pH 4.3 and 5 mV/s. Figure 5 shows the cyclic voltammetry data of a polycrystalline Au electrode in both CO2 saturated and Ar-saturated Na2 SO4 . In the Ar-saturated solution, only the HER occurs, while in the CO2 -saturated solution, both the HER and electrochemical CO2 reduction occur. The potential for CO2 reduction to CO and H2 O reduction to H2 both occur at potentials more negative than -1.0 V vs Ag/AgCl, indicated by the sharp onset of catalytic current in both cyclic voltammogram curves. However, there is a problem when one tries to separate the partial current for each reaction, because a simple subtraction of the two voltammograms would not lead to insights of the interplay between these reactions when they occur simultaneously. Scheme 1 depicts our MR-SECM approach for in-situ separation of electrochemical CO2 reduction from the HER. For this MR-SECM, a platinum tip electrode was used to selectively

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Scheme 1: Scheme depicting the MR-SECM technique showing the selective collection of (a) H2 via constant potential and (b) CO via linear sweep cyclic voltammetry. collect products from the simultaneous HER and CO2 reduction in-situ during voltammetry experiments. It is well known that Pt readily adsorbs CO, 67 which can be electrochemically stripped, providing an indicator for CO exists in solution (Scheme 1b). To demonstrate this, a Na2 SO4 solution was bubbled with the known products (CO and H2 ) and a potential sweep from 0.0 to +1.0 V vs Ag/AgCl was performed using a platinum working electrode. Figure 6c shows that when CO is in the solution, a potential sweep electrochemically strips the adsorbed CO from the Pt surface, indicating that CO is in solution. To demonstrate that a Pt electrode can also selectively collect H2 , a similar test was done where a Na2 SO4 solution was bubbled with the known products and chronoamperometry was performed using a Pt working electrode. At a constant potential of +0.0 V vs Ag/AgCl, Pt will not oxidize any of the expected products (Fig. 6a). However, applying a constant potential of +0.1 V vs Ag/AgCl on Pt significantly oxidizes H2 , while not a↵ecting any other expected products (Fig. 6b). Thus, by performing chronoamperometry at +0.1V vs Ag/AgCl, a Pt SECM tip electrode can selectively detect H2 , and then by performing CV a Pt electrode can selectively 17



detect CO. This ability to will allow us to utilize a Pt SECM tip electrode to perform MRSECM and separate H2 production from CO production.

Figure 6: Chronoamperometry at (a) 0.0 V and (b) +0.1 V and (c) cyclic voltammetry of a 200 µm Pt electrode in Ar-saturated (blue), CO-saturated (red), H2 -saturated (green), and Formate-saturated (pink) 0.1 M Na2 SO4 . To determine the amount of time necessary to fill the tip/substrate gap with products and provide a visualization for the MR-SECM experiment, we developed a COMSOL model showing the concentration profile of the reduced products within the tip/substrate gap. First, the Au substrate electrode generates a reduced product (either from CO2 reduction or the HER) for 20 seconds, shown in Figs. 7a and 7b. After 20 seconds, the potential is removed from the Au substrate so reduction products stop forming. Simultaneously, a potential is applied on the Pt tip electrode such that the reduced products will be oxidized on the tip, shown in Figs. 7c and 7d. It is necessary to determine appropriate generation/collection times such that the tip/substrate gap will be completely cleared of reduction products before the next cycle begins. Figures 8 and 9 show the results of the MR-SECM experiment. Collection of H2 from the hydrogen evolution reaction was measured first. A potential pulse was applied to the Au substrate for 20 s over a wide range of potentials. After each 20 s potential pulse, the Au electrode was turned o↵ while the Pt tip potential was turned on to +0.1 V vs Ag/AgCl, thus collecting any H2 that was formed on the Au. Figures 8b and 8c show the raw current versus time for the collection on the Pt tip electrode. Each curve on these figures represent the applied potential on the Au substrate while the measured current is that of

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Figure 7: COMSOL simulations of Multireactional SECM di↵usion profiles with a 35 µm tip/substrate distance at (a) 5 sec, (b) 15 sec, (c) 21 sec and (d) 40 sec. the Pt tip electrode. As the applied potential on the Au electrode becomes more negative, there will be more reduced products in the tip/substrate gap for the Pt tip electrode to oxidize, resulting in a higher current. Integrating the net current allows for determination of the charge transferred on the Pt electrode, indicating that an electrochemical reaction has occurred. Figure 8a shows the amount of charge transferred on the Pt tip as a function of applied potential on the Au substrate. This was done independently for CO2 -saturated and Ar-saturated solutions. The similarity between the two curves shown in Fig. 8a suggests that CO2 does not a↵ect the HER for a Au electrode. These curves also show that H2 begins to form around -1.1 V vs Ag/AgCl, which matches the large catalytic peak from the CV in Fig. 5. Collection of CO2 reduction products followed a similar process to that of the HER, providing the same 20 s pulse. However, after each 20 s pulse, cyclic voltammetry was performed on the Pt tip from -0.1 V to +0.8 V vs Ag/AgCl to strip the adsorbed CO. Figure 9b shows the raw current for the collection Pt tip electrode versus the potential applied to the Au substrate electrode. As the applied potential on the Au electrode becomes more negative, the CO stripping peak grows larger, indicating that more CO has adsorbed on the Pt tip. Integrating the area under the peak allows for determination of the charge 19



Figure 8: (a) Charge density of H2 collection on a 200 µm Pt tip electrode at +0.1 V vs Ag/AgCl in Ar-saturated (blue) and CO2 -saturated (red) 0.1 M Na2 SO4 at pH 4.3 and the corresponding raw MR-SECM collection currents of (b) Ar-saturated and (c) CO2 -saturated solutions via chronoamperometry.

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transferred on the Pt tip, as shown in Fig. 9a. This curve shows that CO stripping begins to occur around -0.9 V vs Ag/AgCl, which shows that is the potential when CO2 reduction occurs on a gold electrode. It is also important to note that at potentials more negative than -1.2 V vs Ag/AgCl, CO2 reduction begins to decrease, which is not possible to detect via traditional cyclic voltammetry on a gold electrode. Thus, the optimum potential window to perform CO2 reduction on a Au electrode in aqueous media is -0.9 to -1.1 V vs Ag/AgCl. At potentials lower that -0.9 V vs Ag/AgCl, CO2 reduction is too low, but at potential above -1.1 V vs Ag/AgCl, CO2 reduction begins to be dominated by the hydrogen evolution reaction.

Figure 9: (a) Charge density of CO stripping on a 200 µm Pt tip electrode in CO2 -saturated 0.1 M Na2 SO4 at pH 4.3 and (b) the corresponding raw MR-SECM collection currents via cyclic voltammetry from -0.1V to +0.8V vs Ag/AgCl.

Conclusions and Future Direction Here we demonstrated the ways in which Scanning Electrochemical Microscopy can be utilized for the in-situ and operando characterization of electrocatalyst. Since SECM is an electrochemical technique, it is typically only utilized by electrochemists. However, there is an immediate need for innovative technologies to better understand mechanisms for chemocatalysts that utilize emerging feedstocks and operate in the liquid-phase. Lack of such analytical methodology is impeding our fundamental understanding of catalytic mechanisms, 21



and is inhibiting catalyst development in the areas of renewable energy, biomass conversion, and CO2 and methane activation. Moreover, in-situ and operando techniques are needed to (1) better understand of adsorption mechanisms and structure e↵ects to identify bottlenecks in catalytic processes, and (2) bridge the gap between the conditions at which catalyst characterization measurements are taken and the conditions at which the catalysts need to operate. SECM holds the potential to be a unique instrument that could benefit the entire catalysis community by uncovering new insights into catalytic mechanisms and intermediates. Even though the reactions that occur on the tip electrode are electrochemical, the reactions that occur on the substrate electrode need not be electrochemical, as long as the liquid phase has enough conductivity to support the tip reactions. Thus, we envision the imaging, generation/collection, surface-interrogation, and multireactional modes of SECM could be adopted by researchers studying chemocatalytic mechanisms in the liquid phase.

Acknowledgement This work was funded through The Army Research Office Young Investigator Grant No. 66446-CH-YIP (Award Number W911NF-17-1-0098).

Supporting Information Available Experimental procedures describing the multireactional SECM technique can be found in the Supporting Information File. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry

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Kevin C. Leonard is an Assistant Professor in the Chemical and Petroleum Engineering Department at the University of Kansas. Kevin received his Ph.D. in Materials Science from The University of Wisconsin - Madison in 2011 (Advisor: Marc A. Anderson), and subsequently was a postdoctoral fellow in Prof. Allen J. Bard’s group at the University of Texas at Austin. Kevin joined the faculty at the University of Kansas in 2013, and his current research interests include heterogeneous electrocatalysis, electrocatalytic nanomaterials, and catalytic imaging via Scanning Electrochemical Microscopy. Kevin has been recently recognized for both his teaching (Raymond Oenbring Teaching Award and Don Green Faculty Fellowship for Teaching Excellence) and his research (Army Research Office Young Investigator Award and Miller Distinguished Service Award for Research) at the University of Kansas.


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Characterizing Electrocatalysts with Scanning Electrochemical

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