Atomic Scale Insights Into Electrochemical Dissolution of Janus Pt

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Atomic Scale Insights Into Electrochemical Dissolution of Janus Pt-SnO2 Nanoparticles in the Presence of Ethanol in Acidic Media: IL-STEM and EFC-ICP-MS Study Primož Jovanovi#, Francisco Ruiz-Zepeda, Martin Šala, and Nejc Hodnik J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02104 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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The Journal of Physical Chemistry

Atomic Scale Insights Into Electrochemical Dissolution of Janus Pt-SnO2 Nanoparticles in the Presence of Ethanol in Acidic Media: ILSTEM and EFC-ICP-MS Study Primož Jovanovič1, Francisco Ruiz-Zepeda*2, Martin Šala1, Nejc Hodnik*3 1

Department of Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana,

Slovenia 2

Department of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana,

Slovenia 3

Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19,

SI-1000, Ljubljana, Slovenia *Corresponding Authors: E-mail: [email protected], [email protected]; Tel.: +386 1 4760 212

Abstract The studies on Pt or Pt-alloy based catalysts for ethanol oxidation have been focused mostly on their activity and with much less emphasis on the stability. Dissolution of commercial PtSnO2/C electrocatalyst for direct ethanol fuel cells was investigated at the atomic scale by employing two advanced electrochemical characterization techniques: (i) an Identical Location Scanning Electron Transmission Microscopy (IL-STEM) and (ii) Electrochemical Flow Cell connected to Inductively Coupled Mass Spectrometry (EFC-ICP-MS). IL-STEM provides electrochemically induced atomic scale insights into morphological and structural changes of Pt-SnO2/C Janus type nanoparticles and the second methodological configuration enables potential- and time-resolved dissolution monitoring of individual metal counterparts with extremely high sensitivity, even in the presence of ethanol. We observe that Sn is mostly dissolving in the anodic potential ramp, which is as a rule not affected by the presence of ethanol. Surprisingly, Pt dissolution gets dramatically enhanced in the presence of ethanol with the onset already at 0.57 V. Compared to the experiment without ethanol the onset is at 1.1 V, which is typical for Pt. We discuss the possible mechanisms governing these alterations.

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Introduction

It is known that Pt bimetallic electrocatalysts seldom express enhanced activities compared to their pure platinum analogues. The superior activity can originate from at least four different effects. (i) The electronic or ligand effect is caused by the changes in the electronic band structures of active surface (usually Pt), hence influencing the binding energy between the metal surface and adsorbate molecules.1,2 (ii) The geometric effect is produced by strain of active surface atoms caused by the lattice mismatch of the atoms underneath.1,3 (iii) An ensemble effect arises when small groups (ensembles) of surface metal atoms act as preferential and/or specific adsorption sites for molecules.4–7. And (iv) the effect of bifunctionality of the surface with two active centers each needed to effectively run the Langmuir–Hinshelwood mechanism type reactions as are methanol and ethanol oxidations.4–7 It is, therefore, safe to conclude that the structure of both metals in the nanoparticles influence the reaction rate. In this respect, Pt-based bimetallics as a representative electrocatalysts in direct ethanol fuel cells (DEFCs) are no exception. Namely, composites such as Pt-Ru and PtSn are considered to be the most active materials toward methanol (MOR) and ethanol oxidation reaction (EOR).8–13 Ru and Sn have been recognized as a co-catalyst, because of their ability to adsorb and activate water to form OHad species at relatively low potentials, which are needed to oxidize strongly bounded blocking intermediate CO on the Pt surface.14,15 Ru and Sn can also induce a possible useful electronic and geometric effects on Pt catalyst surface.9,11,16–20 Interestingly, it was shown that alloyed Pt-Sn is more active compared to unalloyed Pt-Sn electrocatalysts. However, there is a tradeoff on account of the selectivity, where the first one leads only to the acetic acid formation, lowering the efficiency and the latter to CO2.21 Through intensive research activities in recent decades, electrocatalytic activity of EOR catalysts has already provided significant progress. However, a further increase in activity and selectivity is needed for the DEFC to become commercially viable. Durability issues, on the other hand, are still relatively unexplored and also provide a major challenge. In this sense, the initial and prolonged performance of the catalyst under reaction environment conditions represents one of the bottlenecks for future applications and is also fundamentally intriguing.22 Metal migration and surface segregation23, as well as dissolution and dealloying, can induce alterations in the surface structure and composition, and thus on the reaction rate. Understanding of which of these mechanisms is governing the degradation is very important in the development of the new more stable electrocatalysts.


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Due to high lateral resolution and chemical sensitive z-contrast imaging technique (High Angle Annular Dark Field) Cs-corrected electron microscopy is recognized as a perfect tool to study localized morphological, structural and compositional changes of Pt or other heavy metal element based nanoparticles.24,25 Direct observation and tracking of these changes, usually connected to the electrocatalysts degradation, are possible with the advanced characterization method named identical location electron microscopy (IL-TEM).26–30 It enables tracking of morphological surface changes on a nanometer scale on identical locations of an electrocatalyst material before and after electrochemical aging. Compared to usual imaging of random parts of material with certain statistical uncertainty, it provides direct proof of the studied events. Thus, sound conclusions on complex degradation phenomena of the Pt-based particle can be drawn: detachment, platinum dissolution, dealloying, carbon corrosion, agglomeration, reshaping and Ostwald ripening.26,27 IL-TEM is now well established in the PEM-FC community where usual degradation studies include various types of electrocatalysts, different potential cycling protocols.26–31 TEM microscope enables many different techniques like Electron Energy Loss Spectroscopy (EELS), Energy-Dispersive Xray Spectroscopy (EDX), Tomography and high-resolution High Angle Annular Hark Field Scanning Transmission Electron Microscopy (HAADF-STEM), thus IL-TEM is defined as a toolbox approach.26,27 Identical location is a straightforward and affordable method and can be recognized as a general way to study any morphological and structural change of electron microscopy compatible samples like thermal synthesis treatment32 and gas reaction, etc. Therefore we expect IL-TEM popularity to rapidly grow also through other scientific and technological areas besides Pt electrocatalysis. In recent years precise online dissolution of platinum group metals (PGM) has been extensively studied - either in the form of polycrystalline disks8,33–39 or as a nanoparticulate catalysts.40–47 Through the introduction of the online analytics that utilize inductively coupled plasma mass spectrometry (ICP-MS) high sensitivity was enabled. It has been shown that the electrochemical dissolution of PGM is predominantly a transient phenomenon occurring due to the interplay of PGM oxidation and reduction processes. These processes can be manipulated by changing the electrochemical treatment (scan rate, anodic and cathodic potential window), gas atmosphere, electrolyte, the thickness of the catalyst layer as well as particle size.38,40,42,48–51 Significantly less explored is the dissolution of PGM based alloys. This phenomenon is intensively gaining in interest and importance.44,52–55 Corrosion of metallic electrocatalyst has predominantly been investigated under plain conditions excluding the possible effects of reaction environment. Few recent studies have taken into consideration 3 ACS Paragon Plus Environment


different effects of real conditions such as gas atmosphere50,54, temperature8, support56 and fuel.42 These clearly display that corrosion process can significantly change under the influence of mentioned parameters. In the present work, for the first time, atomic scale identical location scanning transmission electron microscopy and a highly sensitive time- and potential-resolved dissolution of commercial benchmark bimetallic Janus type Pt-SnO2/C electrocatalyst is investigated in the presence of ethanol.

Methods Commercial catalyst based on bimetallic nanoparticles Pt-SnO2/C with Pt/Sn ration 3/1 and 3 nm average particle size deposited on a high surface area carbon (Vulcan XC72) with a metal loading of 20 wt% was investigated. The catalyst was produced by E-TEK and was used as reference materials in the electrochemical ethanol oxidation studies12,21,57 where further details and characterization can be found. Electron microscopy investigations were done in a JEM-ARM200CF microscope, equipped with a SSD JEOL EDX spectrometer and a Gatan GIF Quantum electron energy-loss spectrometer (EELS). The operation voltage was set to 80 kV and the approx. probe current used was 14 pA. Samples were prepared by diluting the powder in ethanol and depositing the solution with a pipette in lacey carbon coated coper grids. Some HAADF images were denoised using a nonlinear filter.58 The procedure for IL-STEM was performed similarly to that reported in31. The electrocatalyst suspension (1 mg/mL) was ultrasonicated for 15 minutes and diluted 10 times (100 uL of the suspension, 900 uL of miliQ water). After additional 5 minutes of ultrasonification (diluted suspension), 5 uL of the suspension was drop casted on a gold finder grid (Quantifoil). Once dried, the grid was inspected under TEM. Several spots were identified and imaged in TEM and STEM at different magnifications. The spending time on examining each particle was minimized in order to avoid any influence of the beam on the particle structure or any possible beam damage. The grid was afterwards removed from the microscope to perform the electrochemistry as following: the grid was mounted on a gold plated tweezers, which served as a working electrode. Electrochemical tretment was perfomed in a two-compartment electrochemical cell in a Ar saturated mixture of 0.1 M C2H5OH and 0.1 M HClO4 (purity > 99.99 %, Merck) electrolyte with conventional three-electrode system controlled by a potentiostat (Autolab PGSTAT 30 (Eco Chemie, Utrecht). The electrocatalyst on the Au-grid was electrochemically cycled 1000 times between 0.05 V and 0.8 V (vs. RHE) with a scan 4 ACS Paragon Plus Environment

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rate of 820 mV/s. After electrochemical biasing, Au-grid was dipped into fresh miliQ water and left to dry at room temperature. Once dried, the grid was once again inspected under TEM, tracking the areas previously imaged and identified by the letters. The images were then taken under the same conditions as the previous ones. Electrochemical flow cell (EFC) coupled with ICP-MS setup was already introduced in our previous

publications.29,40,42,44,52,53,56,59

Shortly,

an

in-house

made

TECAPEEK

electrochemical flow cell with a silicon gasket (1.1 mm thickness) was coupled with an Agilent 7500ce ICP-MS instrument (Agilent Technologies, Palo Alto, USA) equipped with a MicroMist glass concentric nebulizer and a Peltiercooled Scott-type double-pass quartz spray chamber. A forward radio frequency power of 1500 W was used with the following Ar gas flows: carrier 0.85 L/min, makeup 0.28 L/min, plasma 1 L/min, and cooling 15 L/min. 0.1 mol/L HClO4 (Aldrich 70%, 99.999% trace metals basis) acid was used as electrolyte carrier medium. Solutions were pumped at 263 µL/min using syringe pump (WPI sp100i). The catalyst thin film preparation consisted of a catalyst suspension in milli-Q water (18.2 MΩ/cm-1) with a concentration of 1 mg/mL. Suspension was cast dropped over one of the glassy carbon electrodes and stabilized by a 5 µL of Nafion diluted by isopropanol (volume ratio of 1/50). The second glassy carbon electrode was used as a counter electrode. The order of the working (WE) and counter electrode (CE) was adjusted so that WE was positioned after CE in the direction of electrolyte flow in order to avoid re-deposition. The total amount of catalyst material was 10 µg. The experimental procedure consisted of potentiodynamic treatment with a scan rate of 5 mV/s from 0.05 V to three upper potential limits: 0.8, 1.0 and 1.3 V vs. RHE. Experiments in the presence of ethanol were performed in a mixture of 0.1 M C2H5OH and 0.1 M HClO4.

Results and discussion Transmission electron microscopy investigation The morphology of the particles consisted of a dumbbell shape with an asymmetric character. The shape is typical of a special case of a Janus particle (JPs), also known as “snowman”.60 The composition of the JPs consisted of Pt and SnO2 as confirmed by the measured interlattice distance of the Janus nanoparticle (Fig. 1a), corresponding to {111} lattice spacing of Pt, and {101} of SnO2. The difference in contrast in the HAADF image (contrast Z) in Fig. 1a also revealed the duality of the particle. EELS maps show signals from Sn M4,5 and O K edges, overlaid also with the HAADF signal in Fig. 1b. More on the compositional maps and on the EELS spectra of the SnO2 phase61 is described in Fig. S1 in SI. The particles were 5 ACS Paragon Plus Environment


homogeneously distributed over the carbon support. The average size of each component was around 2-4 nm, in some cases with pronounced asymmetric sizes. An overview picture of the distribution can be consulted on Fig. S2 from Supp. Info.

Figure 1: (a) HAADF images of Pt-SnO2 nanoparticles with Janus shape in the form of dumbbell or “snowman”. Measured lattice spacing corresponds to 2.28 Å for Pt {111} and 2.6 Å for SnO2 {101}. The higher contrast part corresponds to Pt composition, while the part with lower contrast corresponds to SnO2. (b) HAADF signal plus EELS maps of Sn and O, and the overlay of all three signals.

Time- and potential-resolved dissolution of Sn Potentiodynamically induced dissolution was monitored by gradually increasing the upper potential limit (UPL). We note that prior to electrochemical experiment the Sn is in the form of SnO2 and Pt is partially oxidized also. The presence of oxides is clearly indicated at the beginning of potentiodynamic experiment when electrochemical contact is established at 0.05 V, where an increase in dissolution of both metals can be detected (Fig. 2a). This corresponds to: (i) a disruptive electrochemical reduction of the surface/subsurface oxides (also referred to as cathodic dissolution), which were formed during air exposure prior experiment62,63 and (ii) a pure chemical dissolution of SnO2 to Sn4+ out of a nanostructured composite of Pt and SnO2 (Supp. Info. for possible dissolutions reactions and their Nernstian potentials are in Table S1). 6 ACS Paragon Plus Environment

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In an experiment without ethanol with the UPL of 0.8 V only Sn dissolution profiles are detected. This is expected since Pt is presumably stable in this potential range.40 Sn dissolution onsets (labeled as peak A) at approximately 0.55 V, which can be ascribed to (in terms of conventional thermodynamic Pourbaix diagrams) only the formation of SnO3-2 species (Supp. Info. Table S1).64 Thermodynamically the solubility of these species is extremely low, hence their abundance in the downstream electrolyte should be ascribed to the transient dissolution mechanism triggered by repetitive oxide reduction and formation, which has a destructive character as reported for several other metals.36 As seen in the Fig. 2b onset of Sn dissolution shifts anodically within the three consecutive cycles. This shift is ascribed to a consumption of less stable Sn species. Since Sn is in the form of as-synthesized SnO2, we presume that with each cycle it is getting more stable. We note that no activation cycles were performed before this experiment. Potentially, if Sn would be alloyed with Pt it could become more stable since alloying metals with noble components increases metals electrochemical dissolution potential.52,53 Onsets of dissolution can be demonstrated unambiguously by plotting cyclic voltammograms one over another and corresponding dissolution profiles on the same potential axis (Fig. 3a). In accordance also the intensity of the dissolution is decreasing within each consecutive cycle. This is in line with the removal of less stable Sn surface/subsurface defects from SnO2 composite part (Fig. 2b,c, Fig. 3a,b). In the presence of ethanol dissolution of Sn is virtually the same when cycling till UPL 0.8 V, indicating that ethanol itself and the proceeding of EOR has no effect on Sn dissolution. Since, due to catalytic effect of SnO2, EOR onsets even below 0.55 V Sn dissolution is just part of the process of stabilization of SnO2 in the composite. It is interesting to compare this to the contrasted situation in the analogues case of MOR and Ru dissolution out of Pt-Ru.42 The main difference is that PtRu is in a form of an alloy in that report and that Ru is more noble/stable compared to Sn. Sn dissolution trend, however, gets altered when the UPL is increased to 1 V (Fig. 2c). The shape of dissolution profile stays the same, however in the presence of ethanol Sn removal is increased. This is most likely related to the EOR. However, since Pt and Sn are not alloyed, the effect is not as severe as in the case of alloy Pt-Ru where all of the atoms are in direct contact. Therefore, different dissolution mechanism must be at play for the Pt-Sn case. Interestingly, at UPL of 1.3 V dissolution profiles are again similar in the intensity and shape (Fig. 2d). Nevertheless, careful observation reveals that in the presence of ethanol dissolution peak A2 and C1 are a bit more pronounced (further discussion can be found below).



Figure 2: Comparison of Sn dissolution profiles in the presence (orange curves) and absence (red curves) of 0.1 M ethanol-induced by potentiodynamic treatment of Pt-SnO2/C. a) Magnification of Pt and Sn dissolution profiles at the beginning of potentiodynamic experiment when cycling to UPL of 0.8 V. Sn dissolution profiles by gradually increasing UPL to b) 0.8 V, c) 1.0 V and d) 1.3 V.

Under these electrochemical conditions several significant features are monitored: i) the onset of Sn dissolution is shifted towards higher potentials (from 0.57 V to approximately 0.8 V), ii) Sn dissolution profiles have similar shapes at all UPL and with and without ethanol, iii) Sn dissolution is to a certain degree enhanced in the presence of ethanol, iv) additional peak appears in the anodic dissolution profile (labeled as A2) and v) additional peak appears in the cathodic dissolution profile regardless of ethanol presence (labelled a peak C1). Phenomena i) is in line with the dissolution of less stable parts of the SnO2 structure. We refer to this phenomena as stabilization. Feature ii) indicates that the presence of ethanol does not have much impact on the Sn corrosion. A small increase of Sn dissolution observed in the features iii) and iv) seems to indicate that also another driving force for Sn dissolution is in play besides the transient formation of SnO3-2. Most likely it is related to the corrosion at the interface between Sn and Pt and to the ongoing EOR. In the following section, it will be demonstrated that this is directly related to Pt dissolution that is ongoing simultaneously with Sn dissolution (see section Time and potentially resolved dissolution-Pt case). When the UPL 8 ACS Paragon Plus Environment

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is increased to 1.3 V additional anodic and cathodic Sn dissolution peak can be noticed (peak A2-feature iv) and C1-feature v) in Fig. 3c). These are directly related to the aggressive type of Pt corrosion referred as transient dissolution.37,40 It is caused by both the formation and the reduction of Pt oxide via place exchange mechanism. As a result of this roughening of the Pt surface and subsurface and subsequent Pt dissolution new Sn atoms at the Pt-Sn interface get exposed. These are then more prone to the dissolution (please again refer to section Time and

potentially resolved dissolution-Pt case for detailed interpretation). In order to present cathodic Sn dissolution peak C1, Sn and Pt dissolution profiles are plotted together (Fig. 4a). Pt dissolution signal in the electrolyte without ethanol is detected once the UPL of 1.3 V is reached. We note that in the case of cycling till 0.8 V and 1.0 V no Pt dissolution signal is detected (see Supp. Info. Fig. S3).



Figure 3: Time-resolved potentiodynamic experiment plotted against Sn ICP-MS response and corresponding cyclic voltammogram of Pt-SnO2/C. Second dissolution cycle is shown.

Time- and potential-resolved dissolution of Pt

Interestingly, in comparison to Sn, Pt dissolution is dramatically enhanced in the presence of ethanol (Fig. 4b-d and Supp. Info. Fig. S3). Well-defined dissolution peak is observed already when cycling to the UPL of 0.8 V (labeled as peak A1 in Fig. 4b). So far the lowest UPL of platinum dissolution in the acidic electrolyte is 0.65 V vs RHE as reported by Komanitsky et al. for Pt single crystals.65 In both cases, accumulation was performed before the ICP-MS measurement in order to increase the sensitivity. In our case, the onset of Pt dissolution in the presence of ethanol, without any accumulation step, is positioned already at approximately 10 ACS Paragon Plus Environment

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0.57 V (Fig. 4b), which is the lowest reported potential for Pt anodic dissolution. That is excluding cathodic Pt dissolution phenomena that was observed also below 0.4 V.37 In general Pt transient dissolution is consisted of the so-called anodic and cathodic counterpart, where the former is attributed to the formation of Pt subsurface oxide and the latter to its reduction.37,38,40 Therefore without the anodic part (oxide formation), there is no cathodic counterpart (oxide reduction). The early onset of Pt dissolution, when cycling till 0.8 V where no oxide is formed, indicates that observed dissolution is not induced by formation of oxides. Since this trend is not observed in the results of Fig. 4b-d a different mechanism is at play. Importantly, in the presence of ethanol the onset of EOR, Pt and Sn dissolution commence simultaneously (Fig. 4b,c), which indicates that the processes are connected. Sn is, however, dissolving also in the absence of ethanol, therefore we can assume, as discussed before, that some parts of SnO2 are thermodynamically unstable. For that reason, one possible mechanism would be that Sn removal at the interface of Pt and Sn parts (“snowman's neck”) is most likely undermining Pt that thus forms newly exposed low coordinated sites that are prone to direct dissolution. However, this is only observed in the presence of ethanol. Along with the fact that metals are not alloyed and the interface between the two metal is not that big, only this mechanism alone is most likely not the be decisive for the corrosion. We presume that in the absence of ethanol Pt sites are most likely protected against dissolution by adsorbed species from the electrolyte. At EOR onset potential (below 0.55 V vs. RHE) this is most likely OHad. In the presence of ethanol OHad is getting consumed by EOR via Langmuir– Hinshelwood mechanism. Due to spillover effect this can affect all of the surface Pt atoms. After that “unprotected” Pt sites are exposed to either ethanol or EOR intermediates and side products such as acetaldehyde and acetic acid that have been shown to enhance Pt dissolution.66–70 This is in contrast to analogues case of MOR and Pt dissolution out of PtRu.42 The main difference is that in that report PtRu is forming an alloy, thus there is direct contact of almost all atoms, and that Ru is more stable compared to Sn. Presumably formed OHad on Ru can directly oxidize CO at the Pt whereas OHad formed on Sn must spill-over to Pt to remove CO. The effect of organic compounds on Pt stability has rarely been reported apart from our recent studies of methanol case42,13 and the work on the effect of formic acid.71

A detailed inspection of Pt dissolution profiles in the case of 1.3 V UPL shows that anodic dissolution consists of an additional peak (labeled as A2 in Fig. 4d). Dissolution maxima A2 is well known from the literature and is ascribed to the so-called place exchange mechanism during formation of Pt oxide that onsets at approximately 1.1 V vs RHE.37,38,72 It can be 11 ACS Paragon Plus Environment


observed also in the experiment without ethanol and is again not connected to EOR. This is also confirmed by plotting cyclic voltammogram and corresponding dissolution profiles on the same potential axis (Fig. 5). Dissolution peak A1, on the other hand, should be related to the EOR as in the case of cycling till 0.8 V and 1.0 V. Interestingly the onset of Pt dissolution when cycling till 1.3 V in ethanol is positioned at 0.61 V, which is approximately 250 mV less anodic than in the case of UPL of 1.0 V. Again, if Pt anodic dissolution is compared to Sn dissolution direct correlation is observed indicating that Sn dissolution also somehow influences the Pt removal. This means that Pt dissolution is inherently connected to the Sn corrosion and EOR. Presumably, higher Sn dissolution results in the higher exposure of new Pt sites at the “snowman's neck” along with the higher rate of EOR, which is removing protective OHad from Pt surface and consequently increased Pt dissolution (Fig. 5).

Figure 4: Time and potentially resolved dissolution profiles of Sn (a) and Pt (a-d) at different upper potential limits out of Pt-SnO2/C. Upper potential limits in (a) and (d) is 1.3 V, in (b) it is 0.8 V and (c) it is 1 V.

When cycling till the UPL of 1.3 V Pt dissolution shape is consisted of a cathodic counterpart as well (Fig. 4d and Fig. 5) as expected due to reduction of the oxide formed in the anodic scan.37,38,40 The more enhanced cathodic Pt dissolution in the presence of ethanol in 12 ACS Paragon Plus Environment

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comparison to the non-ethanol case is in accordance with the scarce literature reports on the effect of organic molecules on Pt dissolution. These studies show that redeposition of Pt ions is inhibited due to the Pt surface blockage by adsorbed organic molecules or its intermediates like CO, which increases the overall cathodic Pt dissolution.42,50

Figure 5: Cyclic voltammograms (up) and corresponding Pt ICP-MS dissolution profiles (down) on the same potential axis of Pt-SnO2/C. Second dissolution cycle is shown.

We performed a stability test of 1000 cycles till 0.8 V in order to see if there is any structural difference on the catalyst. As can be seen in Fig. 6a,b. IL-STEM reveals two important observations. The first one is the creation of Pt single atoms (Fig. 6b). This can occur due to the dissolution of Pt and Sn that get deposited and subsequently coordinated in the carbon matrix via the process of metalation.73,74 However, due to the higher contrast in the image, we presume that the atoms are Pt. Therefore observations recorded with ICP-MS are confirmed by IL-STEM studies. Secondly, it can also be seen, that in this case, single atoms do not move due to electron beam exposure, discarding then knock on radiation damage that will strip out atoms from the particle during the observation time. This is very important since one could ascribe this to the beam damage.31 We also want to note that without atomic resolution we would not be able to observe any of these changes. More IL-STEM figures confirming the observed phenomena can be found in Supp. Info. Fig. S4 to S8.



Figure 6: IL-STEM images of the Pt-SnO2 “snowman” type nanoparticles; a) before and b) after 1000 electrochemical cycles between 0.05 - 0.8 V vs. RHE with 820 mV/s. Consecutive 3 images increasing in magnification (8Mx, 10Mx and 12Mx) were recorded each after 10 seconds of e-beam exposure in order to exclude any major beam effects. It is clear that single atoms are not initially present and are not being formed before degradation due to the beam exposure; additionally, single atoms practically do not move as can be seen in the after degradation image.

Conclusions We present the first potential- and time-resolved dissolution and identical location electron microscopy study of commercial Pt-SnO2/C electrocatalyst with Janus type, also referred as snowman shape, in the presence of ethanol with atomic resolution. The employment of extremely sensitive downstream analytics provided several new physical insights. Namely, Sn dissolution is predominantly proceeding via formation of SnO32- and is not significantly altered by the presence of ethanol. On the contrary, Pt dissolution is considerably increased and onsets already at 0.57 V when the ethanol is present. This is proven to be related to the ongoing EOR where the formation of reaction intermediates acetaldehyde and acetic acid is presumably causing Pt dissolution. This interplay is confirmed by overlaying with Sn dissolution profiles and OER polarization curves. Namely, increase in Sn dissolution and higher EOR rates cause Pt to corrode at relatively low potentials. This is also proved by the 14 ACS Paragon Plus Environment

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atomically resolved IL-STEM observations where Pt single atoms are clearly distinguished after cycling till 0.8 V vs. RHE, where Pt is supposed to be thermodynamically stable. Results of this investigation can be exploited to design more stable electrocatalyst and potentially also novel strategies for platinum recycling.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Figure S1A,B,C: EELS spectrum, HAADF and EELS maps and TEM images. Table S1: Sn based thermodynamic electrochemical reactions. Figure S3: ICP-MS dissolution profiles and cyclic voltammograms. Figure S4-S8: IL-STEM and HAADF and BF images of PtSnO2.

Acknowledgment Financial support from the Slovenian Research Agency (ARRS) through the Research Core Funding Programmes P2-0152, P2-0393, P1-0034 and Project Z2-8161 is also fully acknowledged.

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TOC graphic:


Atomic Scale Insights Into Electrochemical Dissolution of Janus Pt

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