Article This article is licensed under CC-BY 4.0 www.acsnano.org Electrochemical Stability of Rhodium−Platinum Core−Shell Nanoparticles: An Identical Location Scanning Transmission Electron Microscopy Study Miquel Vega-Paredes,* Raquel Aymerich-Armengol, Daniel Arenas Esteban, Sara Martí-Sánchez, Sara Bals, Christina Scheu,* and Alba Garzón Manjón* Cite This: ACS Nano 2023, 17, 16943−16951 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Rhodium−platinum core−shell nanoparticles on a carbon support (Rh@Pt/C NPs) are promising candidates as anode catalysts for polymer electrolyte membrane fuel cells. However, their electrochemical stability needs to be further explored for successful application in commercial fuel cells. Here we employ identical location scanning transmission electron microscopy to track the morphological and composi- tional changes of Rh@Pt/C NPs during potential cycling (10 000 cycles, 0.06−0.8 VRHE, 0.5 H2SO4) down to the atomic level, which are then used for understanding the current evolution occurring during the potential cycles. Our results reveal a high stability of the Rh@Pt/C system and point toward particle detachment from the carbon support as the main degradation mechanism. KEYWORDS: fuel cells, catalysts, identical location, degradation, platinum, rhodium, core−shell P t−Rh-based materials have attracted a great deal of attention in the last years due to their wide range of catalytic applications, including the preferential oxida- tion of CO in hydrogen,1,2 the control of NOx and CO emissions from car exhaust,3 the hydrogen evolution reaction,4 transition metal, such as Ni or Co, the preferential dissolution of these metals can take place.17,18 This process also affects Pt−Ru-based catalysts,19−21 the materials of choice for CO- tolerant PEMFC anodes due to their superior catalytic activity,22,23 which impacts greatly their performance toward formic acid oxidation,5,6 and the hydrogenation of organic the HOR. Therefore, it is still necessary to find a durable anode compounds,7,8 among others. Furthermore, Pt−Rh-based catalyst that can boost the commercial viability of PEMFCs.22 materials have been explored as catalysts for proton exchange In this context, Pt−Rh catalysts are promising candidates to membrane fuel cells (PEMFCs), both in the cathode9 and in replace Pt−Ru-based anodes on PEMFCs if their stability the anode, where they have been investigated for the oxidation under operating conditions is higher, as hinted by the Pourbaix of methanol10,11 and ethanol10−12 or as CO-tolerant hydrogen diagrams of Ru and Rh.24 However, their electrochemical oxidation reaction (HOR) catalysts.13,14 stability needs to be further explored since not much is known Despite these investigations, the understanding of the electrochemical stability and degradation mechanisms affecting about the degradation mechanisms affecting Pt−Rh catalysts Pt−Rh catalysts and the resulting impact on their catalytic under fuel cell conditions. activity remains limited. This is especially relevant for PEMFCs, since limited catalyst stability is one of the central Received: May 5, 2023 factors limiting their widespread commercialization.15 Particle Accepted: August 16, 2023 dissolution, agglomeration, Ostwald ripening, and particle Published: August 21, 2023 detachment have been reported as some of the main phenomena affecting state-of-the-art Pt-based PEMFC cata- lysts.16 Moreover, when Pt is alloyed with a less stable © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acsnano.3c04039 16943 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article Figure 1. Structural and compositional characterization of the as-synthesized Rh@Pt/C NPs. (a) HAADF-STEM micrograph of a faceted particle visualized along the [011] ZA. Its FFT is shown as an inset. (b) 3D atomic model of that particle that deviates from the thermodynamic equilibrium shape, shown in (c). (d) HAADF-STEM micrograph, (e) its corresponding energy dispersive X-ray spectroscopy (EDS) composition map, and (f) intensity profile along a NP with a 10-monolayer-thick Pt shell. In this work we study the stability of Rh−Pt core−shell atomic number of Pt compared to Rh (ZPt = 78, ZRh = 45) nanoparticles on a turbostratic carbon support (Rh@Pt/C results in Pt atoms scattering electrons more strongly and NPs) during electrochemical cycling (10 000 cycles, 0.06−0.8 appearing brighter in the high angle annular dark field VRHE, 0.5 H2SO4) by identical location scanning transmission (HAADF) images.37 Therefore, the bright shell surrounding electron microscopy (IL-STEM).25 This particular nanostruc- the dark core in the particle in Figure 1a is indicative of a Rh- ture was selected because similar core−shell NPs have been core Pt-shell particle. The fast Fourier transform (FFT; inset shown to have a superior performance than the alloyed Figure 1a) shows that both the shell and the core are face- counterparts.1,26,27 IL-(S)TEM is a powerful tool for studying centered cubic and oriented along the [011] zone axis (ZA). local changes down to the atomic level in nanostructured The Rh@Pt/C NPs investigated in this work are highly faceted catalysts, which has been extensively used for gaining insights and present the lowest energy {111} and {100} facets into the degradation of electrocatalysts 28−32 or their (Supporting Information (SI) Figure FS1).38 However, their support29,33 under fuel cell conditions. In IL-(S)TEM, the shape deviates from that of the thermodynamic equilibrium same region of interest can be investigated before and after (Figure 1b,c), as they present asymmetric facets that distort the electrochemical testing, allowing correlation of the changes in 3D structure from the expected cuboctahedron. The shell the particles with the catalytic activity. This technique solves some of the limitations of ex situ (S)TEM, in which only thickness is measured to be between three (Figure 1a) and 10 general statistical insights are possible, which might fail to (Figure 1d−f) Pt monolayers. Nonetheless, even in the reflect the exact changes taking place in the system.34 In particles with a shell of only three Pt monolayers, the Rh addition, it also presents some advantages compared with in core appears to be fully encapsulated, which is desirable since situ electrochemical liquid cell (S)TEM, in which the electron an incomplete core coverage would result in the less stable Rh beam induced radiolysis of the electrolyte can produce being exposed to the electrolyte during cycling/operation, and unwanted artifactual reactions,35 as well as having its special therefore in a lower stability.20 resolution limited by the presence of a thick liquid layer.36 Since the encapsulation of the core is a 3D phenomenon, the Our results indicate that the investigated Rh@Pt/C NPs are NPs were further investigated by high-resolution electron stable systems and point toward particle detachment as the tomography (Figure 2). In Figure 2a, the intensity-based main degradation mechanism taking place during potential segmentation of a reconstructed Rh@Pt/C NP at atomic cycling. resolution is shown. The low-intensity voxels are assigned to Rh, whereas those with high intensity correspond to Pt (Figure RESULTS AND DISCUSSION 2b). An animated version of the reconstruction and orthoslices Characterization of the As-Synthesized Rh@Pt NPs. through the 3D data set can be found in the SI. These results The as-synthesized Rh@Pt/C NPs were characterized by confirm that the Rh core is completely surrounded by the Pt means of (S)TEM in order to confirm the core−shell structure shell, and therefore a high electrochemical stability can be and study the atomic arrangement (Figure 1). The higher expected. 16944 https://doi.org/10.1021/acsnano.3c04039 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article NPs without any binder, large groups of Rh@Pt/C NPs not properly attached to the TEM grid can also get removed (Figure FS2), which partially explains the loss of current observed in Figure 3. Nonetheless, loss of current is still observed for the voltammograms performed on Rh@Pt/C- glassy carbon (Figure FS3a) to a smaller extent. Since that electrode contained Nafion as a binder, the removal of large groups of Rh@Pt/C NPs cannot solely explain the loss of activity of Rh@Pt/C NPs, and other degradation mechanisms need to be considered. Identical Location STEM. In order to understand the causes behind the current degradation observed in Figure 3, Figure 2. High-resolution tomography of as-synthesized Rh@Pt/C IL-STEM experiments were performed. Figure 4 shows how a NPs. (a) Segmented 3D volume and (b) orthoslice of the reconstruction. Electrochemical Accelerated Stress Tests. After the initial characterization of the Rh@Pt NPs, accelerated stress tests (ASTs) were carried out, as described in the Experimental Methods. The upper potential limit of 0.8 VRHE differs from ASTs found in the literature, in which higher upper potential limits are used for studying the degradation associated with start-up or shut-down events.30,39 However, the development of system strategies mitigates the degradation during these events,40,41 making a lower upper potential limit more suitable for studying anode catalyst degradation studies.21 Figure 3 shows the changes in the Rh@Pt/C-TEM grid voltammograms that occurred during the potential cycles. A Figure 4. HAADF-STEM micrographs showing a region of the sample at (a) 0, (b) 1000, (c) 4000, and (d) 10 000 AST cycles. The different colored arrows indicate particle movement (purple), particle reattachment from another region (blue), and particle reprecipitation from dissolved species (green). Figure 3. Voltammograms showing the changes in the Rh@Pt/C NPs’ activity during the potential cycles. representative region of the sample changed during the potential cycles. In the SI, the evolutions of other regions are provided (Figures FS4, FS5). No clear particle growth or cyclic voltammetry recorded with the bare glassy carbon agglomeration can be seen, and a constant mean particle size electrode is also provided as blank current. The comparatively (represented by the equivalent diameter) of ∼5.9 nm is found higher current obtained after adding the TEM grid confirms during the cycles. The particle size distribution histograms are the correct electrical connection of the grid to the electrode. provided in the SI (Figure FS6). A similar particle size In the initial cycles oxidation and reduction peaks are distribution was found for the particles cycled directly after present at 0.65 VRHE and 0.55 VRHE, respectively, which can be drop casting on the glassy carbon electrode (Figure FS3d). attributed to Pt.42 Additionally, a peak in the hydrogen The fact that the mean particle size does not increase during underpotential deposition region is also seen. During the ASTs is indicative of the stability of the particles under cycling potential cycles, the cyclic voltammetry curves flatten conditions. Thus, the observed loss of current of the catalytic progressively, and after 10 000 cycles only capacitive current material cannot be attributed to a decrease in the electro- can be observed, which can be attributed to the carbon chemically active surface area (ECSA) of the catalysts derived support.43 This is indicative of a loss of active catalyst during from NP growth. Nonetheless, small-particle nucleation is the potential cycles, which could be caused by the common observed after 10 000 cycles (Figure 4, green arrows). degradation mechanisms of fuel cell catalysts (particle Although it is possible that part of the small particles could dissolution, agglomeration, Ostwald ripening, and particle be originating from redeposition of Pt ionic species dissolved detachment from the carbon support). Moreover, since the from the Pt-wire counter electrode, small-particle nucleation cycles are performed on a TEM grid loaded with Rh@Pt/C was also observed when a glassy carbon counter electrode was 16945 https://doi.org/10.1021/acsnano.3c04039 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article Figure 5. Particle detachment due to C support corrosion. (a) Rh@Pt particles found after 1000 ASTs without a carbon support (blue arrows) that have reattached from other regions of the sample. HAADF micrograph (b) and EDS composition map (c) of one particle. (d) Bright-field STEM micrograph, showing the region where the EELS spectra (e) were taken at 0 and 10 000 cycles. used (Figure FS7). The presence of small Pt particles was also the Rh@Pt/C NPs have a tendency of decreasing their reported in previous IL-STEM studies on Pt−Ni NPs and was distance and would eventually get aggregated if more cycles are attributed to the dissolution and reprecipitation of catalytic performed, low-magnification HAADF-STEM tomography species.44 Although the authors claimed that these dissolved experiments at different points of the potential cycles (0, species could migrate on the carbon support and redeposit 1000, 4000, 10 000) were carried out. In the SI, the animated onto other particles, resulting in Ostwald ripening, this is not movies of the segmented reconstructed volumes are provided. observed in the mean particle size of the Rh@Pt/C NPs, which Similarly to the HAADF micrographs, the low-magnification indicates that this Ostwald ripening is not significant in our 3D reconstructions also show particle movement during the system for the chosen conditions. Besides the constant mean potential cycles, which results in small fluctuations in the particle size, X-ray diffraction experiments on the Rh@Pt/C average nearest neighbor distance (Figure FS8). However, no NPs (Figure FS3b) also reveal that the crystalline structure of clear trend can be discerned, meaning that the Rh@Pt/C NPs the NPs did not change during the potential cycles. are not significantly aggregated during the ASTs. Therefore, During all of the AST cycles, particle movement (Figures 4, particle aggregation/agglomeration can be excluded as the FS4, and FS5, purple arrows) can be observed. This is a well- main degradation mechanisms of these particles. documented phenomenon occurring in PEMFCs,45−48 which Besides particle movement, another consequence of the is caused by changes in the underlaying carbon support (i.e., turbostratic carbon support corrosion is particle detachment, the turbostratic carbon). Carbon corrosion is thermodynami- which results in the loss of catalyst material with the cally possible at voltages E > 0.207 VRHE, and although it is corresponding drop in ECSA and PEMFC performance. kinetically slow at typical PEMFC potentials, it is known to be During IL-STEM experiments, the detached particles can (i) catalyzed by the Pt present in the catalyst NPs.48 This particle reattach in another region containing Rh@Pt/NPs, (ii) movement can result in particle aggregation and coalescence, reattach on the TEM grid, and (iii) be washed out by the which decrease the ECSA of the catalyst and can be one of the electrolyte. Examples of particle reattachment from a different main degradation mechanisms behind the loss of performance region can be seen in Figure 4 (blue arrows), where particles in PEMFCs. However, the particle movement observed for the that are not present at 0 cycles appear after 1000 or 4000 Rh@Pt/C NPs during the potential cycles is subtle. Since the cycles. Lower magnification images were also checked to movement can take place both in and out of plane, from the discard the possibility of particle migration from a neighboring 2D projections, it cannot be properly quantified. To assess if region. Particle reattachment on the TEM grid can also be 16946 https://doi.org/10.1021/acsnano.3c04039 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article Figure 6. Composition changes in Rh@Pt/C NPs during ASTs. HAADF-STEM micrographs of the same region are shown at 0 (a) and 10 000 (b) potential cycles. As insets, the EDS composition maps of the central NP are provided. (c) Evolution of the Pt/Rh at. % ratio quantified from the EDS composition maps for 11 different particles. observed frequently, since after potential cycling Rh@Pt instance, in bimetallic Pt−Ru particles preferential dissolution particles without a turbostratic carbon support can be found of Ru can take place.19 Even in core−shell NPs, Ru-core on the TEM grid (Figure 5a−c). Considering that all of the dissolution can take place if the Pt-shell is not fully covering particles were found deposited on the carbon support before the core20 or by thermally induced shape fluctuations of the the cycles, this is a clear indication of particle detachment from shell,54 both of which result in the less stable core metal being the carbon support and reattachment on the TEM grid. To see exposed to the electrolyte, with its consequent dissolution. better the lack of turbostratic carbon support, intensity Energy dispersive X-ray spectroscopy (EDS) spectral images saturated micrographs of the area depicted in Figure 5 are were acquired during the ASTs to study the changes in the provided in the SI (Figure FS9), together with carbon- elemental composition and distribution of the Rh@Pt/C NPs. supported Rh@Pt/C NPs for comparison. Notice that when In Figure 6, the elemental distribution maps of a Rh@Pt/C NP the carbon support is present, it can be clearly distinguished are shown. EDS maps of other particles are also provided in from the amorphous carbon from the TEM due to the the SI (Figure FS12). These results reveal an identical core− presence of graphitic (0001) planes. shell structure before and after the ASTs, indicating the In the SI (Figure FS10), two examples of particles being compositional stability of the NP system. Moreover, no Rh detached from the turbostratic carbon support are provided. preferential dissolution is detected, since the composition of These results indicate that particle detachment from the the particles remains unaltered during potential cycling, with carbon support is a relatively frequent phenomenon in Rh@ only small variations of up to 3 at. % (near the sensitivity of the Pt/C NPs. The degradation of the carbon support was further technique) that do not follow a clear trend (Figure 6c). This is confirmed by electron energy loss spectroscopy (EELS; Figure in good agreement with the HR-STEM observations and EDS 5d,e). The EELS spectrum of the carbon K-edge before cycling composition maps performed on particles cycled directly after is typical for graphitic carbon materials, with peaks drop casting on the glassy carbon electrode (Figure FS3c), corresponding to transitions to antibonding π* states (∼285 which further confirm the preservation of the core−shell eV) and antibonding σ* states (∼292 eV).49 After 10 000 structure. cycles, the intensity of these peaks decreased, demonstrating an This was further confirmed in lower magnification EDS amorphization of the carbon support.50 Nonetheless, pro- experiments in order to increase the statistics in the measures nounced π* and σ* peaks are still observed, indicating that the (Figure FS13). Figure FS13 corroborates that there are no amorphization is partial and explaining the moderate particle significant changes in the elemental composition during the movement and detachment observed in the IL-STEM ASTs. This is most likely caused by the full Rh core experiments. A similar partial amorphization was seen in a encapsulation (Figure 1, Figure 2) and by the higher region not previously exposed to the electron beam (Figure electrochemical stability of Rh than Ru.24 FS11), ruling out possible electron beam effects, and for the Even though the size and composition of Rh@Pt/C NPs carbon support on the Rh@Pt/C NPs cycled directly after remained practically constant during potential cycles, a careful drop casting on the glassy carbon electrode (Figure FS3e). analysis of high-resolution STEM micrographs shows that As previously discussed, no particle aggregation/agglomer- changes at the atomic level took place (Figure 7, Figure FS14). ation, significant catalyst dissolution, or Ostwald ripening Both dissolution (i.e., atoms present at 0 cycles that are not (Figure 4, Figure FS6) is observed in the IL-STEM present after 10 000 cycles) and redeposition (i.e., atoms not experiments. Therefore, the lower current observed in Figure present in the as-synthesized particle, that appear after the 3 and Figure FS3a upon electrochemical cycles is assigned to ASTs) of atomic columns can be observed. These two be predominantly caused by particle detachment from the phenomena occur in the surface steps involving atoms with turbostratic carbon support. Efforts for increasing the stability unsaturated bonds. A similar behavior was reported by Rasouli of Rh@Pt/C particles should focus on enhancing the support− et al.44 for Pt−Ni NPs in more oxidative conditions (0.6−1.1 NP interaction, which could be achieved by functionalizing the VRHE), in which the atoms on high-energy sites (steps and support51 or by replacing it with another material, such as kinks) dissolved during potential cycling. Our results indicate oxide-52 or graphene53-based supports. that in the case of Rh@Pt/C NPs atomic column dissolution Besides the previously mentioned degradation mechanisms, can take place, even under the milder conditions used for our in bimetallic systems, other phenomena can occur. For ASTs. Beyond dissolving in the electrolyte, these species can 16947 https://doi.org/10.1021/acsnano.3c04039 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article (RHE, Gaskatel) were respectively used. The system was controlled by a Gamry 600 reference potentiostat. Before the ASTs, a 0.5 M sulfuric acid electrolyte (H2SO4, Suprapur, Sigma-Aldrich) was purged for 30 min with argon. The potential window was chosen between 0.06 VRHE and 0.8 VRHE, and a total of 10 000 cycles were carried out, conditions previously used for studying the degradation of an anode catalyst in PEMFCs.20,21 The Rh@Pt/C-glassy carbon working electrode was obtained by drop casting 10 μL of an ink (1 mg of Rh@Pt/C + 30 μL of Nafion + 2 mL of isopropanol) in a polished glassy carbon electrode. After the cycles, the electrode was scratched to detach some of the cycled Rh@ Pt/C NPs, which were later characterized using X-ray diffraction (XRD) and STEM. Additionally, to study how individual Rh@Pt/C NPs changed during potential cycles in identical location conditions, 10 μL of a 0.28 mg/mL dispersion of Rh@Pt/C on deionized (DI)-water (0.055 μS/cm) was drop cast onto a holey carbon-coated Au TEM finder Figure 7. Atomic scale changes in Rh@Pt/C NPs after 10 000 grid and left drying overnight. After initial characterization of the cycles. The same particle at 0 (a, b) and 10 000 (c, d) cycles is Rh@Pt/C NPs, this TEM grid was fixed on a glassy carbon electrode shown. The atomic column positions of the outermost three with a holey Teflon cap and used as a working electrode on the same atomic layers are indicated by yellow dots. The pink dots indicate three-electrode setup. This allowed for tracking changes in specific those atomic columns that dissolved during the ASTs, while the regions (and particles within those regions) between 0, 1000, 4000, blue dots indicate those that got redeposited. and 10 000 potential cycles. (S)TEM Characterization. High-resolution (HR)-TEM micro- either redeposit on previously existing particles or nucleate graphs of the as-synthesized Rh@Pt/C NPs were obtained in an forming small particles, as seen in Figure 4 (green arrows), image-corrected Titan Themis microscope (Thermo Fisher Scientific) explaining their origin. operated at 300 kV. The corrector was set to introduce negative To sum up, our results indicate that Rh@Pt/C NPs are very spherical aberration on the objective lens, which together with an overfocus results in bright atom contrast.55 The TEM images were stable systems under the studied conditions, as opposed to recorded on a CMOS 4k × 4k camera. comparable Ru@Pt/C NPs.20,21 Moreover, they also point out The Rh@Pt/C NPs were characterized as synthesized and after that particle detachment from the turbostratic carbon support 1000, 4000, and 10 000 potential cycles by means of STEM. STEM is the main phenomenon causing the loss of active catalyst micrographs were acquired in a probe-corrected Titan Themis material in Rh@Pt/C NPs during ASTs. Therefore, efforts for microscope (Thermo Fisher Scientific) operated at 300 kV by using increasing the stability should focus on enhancing the carbon an HAADF detector. A convergence angle of 23.8 mrad was used, support−particle interaction or on different support materials. resulting in a probe of around 0.1 nm. 3D atomic models were constructed from the STEM micrographs using the Rhodius CONCLUSIONS software.56,57 The equilibrium thermodynamic shape was built taking as a reference the Wulff construction for Rh.58 EDS spectral images In the present work, the stability of Rh@Pt/C NPs obtained were acquired to study the chemical composition and elemental via a two-step polyol synthesis was explored. After initial distribution of Rh@Pt/C NPs. To minimize the electron-beam- characterization of the particles showing a complete Rh-core induced damage on the particles, the acquisition time of the EDS encapsulation, accelerated stress tests in the form of potential spectral images was limited to 5 min and were only taken on the as- cycles (0.06−0.8 VRHE, 10 000 cycles) were performed on a synthesized NPs and at the end of the AST (after 10 000 cycles). bulk electrode and on a TEM grid. The loss of activity seen in Moreover, HAADF micrographs before and after the acquisition were the voltammograms during the potential cycles was correlated taken and compared to rule out electron-beam-induced morpho- to the morphological and compositional changes observed on logical changes. Quantification from the EDS spectral images was the TEM grid in IL-STEM experiments. performed with the Cliff-Lorimer method.59 EELS data were acquired in STEM mode with a dispersion of 0.100 eV per channel and a pixel The main degradation mechanism affecting the Rh@Pt/C acquisition time of 1 s using the Quantum Gatan imaging filter. All of NPs was found to be particle detachment from the carbon the spectra were acquired on NPs lying on holes of the TEM grid, to support. Although atomic column dissolution and redeposition avoid the contribution from the carbon of the grid. For the high- were observed, no significant changes in the particle size or resolution STEM-HAADF tomography, a “cubed” aberration- composition were detected. corrected Thermo Fisher Titan X-Ant-EM operating at 300 kV was Our results indicate that Rh@Pt/C particles are very stable utilized to acquire the HAADF-STEM projections. In addition, to under the tested electrochemical conditions and point that study the nearest neighbor distance variation during potential cycles, efforts for improving even further the stability should focus low-magnification tomography was performed. More experimental toward enhancing the support−particle interaction. details can be found in the Supporting Information. X-ray Diffraction Experiments. The XRD experiments were EXPERIMENTAL METHODS performed on a Rikaku Smartlab 9kw diffractometer, using a micro area optics setup and Cu Kα radiation as X-ray source. The cycled Rh@Pt/C NP Synthesis. Carbon-supported Rh@Pt/C NPs were particles for the XRD experiments were obtained by scratching the prepared via a two-step polyol synthesis. The detailed synthesis cycled Rh@Pt/C-glassy carbon working electrode. protocol has been previously described elsewhere.21 A nominal catalyst loading of 21.1 wt % Rh and 20.0 wt % Pt was used. The carbon support was Cabot FCX 400. ASSOCIATED CONTENT Electrochemical Cycling. To study the degradation of the Rh@ Pt/C NPs, these were subjected to ASTs by cyclic voltammetry * sı Supporting Information acquired in a three-electrode setup. As counter and reference The Supporting Information is available free of charge at electrodes, a Pt wire (Redoxme) and a reversible hydrogen electrode https://pubs.acs.org/doi/10.1021/acsnano.3c04039. 16948 https://doi.org/10.1021/acsnano.3c04039 ACS Nano 2023, 17, 16943−16951 ACS Nano www.acsnano.org Article Additional details on tomography experiments and their scientific support, and contributed to the interpretation of supplementary figures referenced in the manuscript the results. All authors contributed to the manuscript. All (DOCX) authors have given approval to the final version of the Animated movie with atomic 3D model of experimental manuscript. particle (AVI) Funding Animated movie with atomic 3D model of particle in Open access funded by Max Planck Society. This research thermodynamic equilibrium (AVI) project (FKZ 03ETB018) was supported by the Federal Animated movie with segmented high-resolution tomo- Ministry for Economic Affairs and Climate Action (BMWi) graphic reconstruction and orthoslice cut of as- based on a decision taken by the German Bundestag. This synthesized Rh@Pt/C NPs (MP4) project has received funding from the European Union’s Animated movie with segmented low-resolution tomo- Horizon 2020 research and innovation program under grant graphic reconstruction of Rh@Pt/C NPs at 0 potential agreement No 823717 − ESTEEM3. cycles (AVI) Notes Animated movie with segmented low-resolution tomo- The authors declare no competing financial interest. graphic reconstruction of Rh@Pt/C NPs at 1000 potential cycles (AVI) ACKNOWLEDGMENTS Animated movie with segmented low-resolution tomo- graphic reconstruction of Rh@Pt/C NPs at 4000 The authors would like to thank C. Bodirsky for providing the potential cycles (AVI) samples, N. Rivas Rivas for his corrections on the manuscript, Animated movie with segmented low-resolution tomo- and D. Chatain for providing her expertise on the equilibrium graphic reconstruction of Rh@Pt/C NPs at 10 000 shape of nanoparticles. Special thanks to B. Breitbach for potential cycles (AVI) performing the XRD experiments. A.G.M. acknowledges the Grant RYC2021-033479- I funded by MCIN/AEI/10.13039/ 501100011033 and, as appropriate, by European Union AUTHOR INFORMATION NextGenerationEU/PRTR. Corresponding Authors Miquel Vega-Paredes − Max-Planck-Institut für REFERENCES Eisenforschung GmbH (MPIE), 40237 Düsseldorf, (1) Alayoglu, S.; Eichhorn, B. Rh-Pt Bimetallic Catalysts: Synthesis, Germany; orcid.org/0000-0001-9916-0167; Characterization, and Catalysis of Core-Shell, Alloy, and Mono- Email: m.vega@mpie.de metallic Nanoparticles. J. Am. Chem. Soc. 2008, 130 (51), 17479− Christina Scheu − Max-Planck-Institut für Eisenforschung 17486. GmbH (MPIE), 40237 Düsseldorf, Germany; orcid.org/ (2) Nilekar, A. U.; Alayoglu, S.; Eichhorn, B.; Mavrikakis, M. 0000-0001-7916-1533; Email: scheu@mpie.de Preferential CO Oxidation in Hydrogen: Reactivity of Core-Shell Alba Garzón Manjón − Max-Planck-Institut für Nanoparticles. J. Am. Chem. Soc. 2010, 132 (21), 7418−7428. Eisenforschung GmbH (MPIE), 40237 Düsseldorf, Germany; (3) Twigg, M. V. Catalytic Control of Emissions from Cars. Catal. Present Address: Catalan Institute of Nanoscience and Today 2011, 163 (1), 33−41. Nanotechnology (ICN2), CSIC and BIST, Campus UAB, (4) Zou, Y.; Goei, R.; Ong, S. A.; Ong, A. J.; Huang, J.; Tok, A. I. Y. Development of Core-Shell Rh@Pt and Rh@Ir Nanoparticle Thin 08193 Bellaterra, Spain; Email: alba.garzon@icn2.cat Film Using Atomic Layer Deposition for HER Electrocatalysis Authors Applications. Processes 2022, 10 (5), 1008. Raquel Aymerich-Armengol − Max-Planck-Institut für (5) Harak, E. W.; Koczkur, K. M.; Harak, D. W.; Patton, P.; Skrabalak, S. 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