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1 The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters Markus Nesselberger 1, Melanie Roefzaad 1, R. Fayçal Hamou 2, P. Ulrich Biedermann 2, Florian F. Schweinberger 3, Sebastian Kunz 3, Katrin Schloegl 1, Gustav K.H. Wiberg 1, Sean Ashton 1, Ueli Heiz 3, Karl J.J. Mayrhofer 2, Matthias Arenz 1 * 1 Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø Denmark 2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, D Düsseldorf, Germany 3 Lehrstuhl für Physikalische Chemie, Zentralinstitut für Katalyseforschung, Technische Universität München, Lichtenbergstr. 4, D Garching, Germany Corresponding author: m.arenz@chem.ku.dk I: Effect of the EDL potential distribution on the ORR In order to demonstrate the effect of the EDL structure and potential distribution on the specific activity (SA) of the oxygen reduction reaction (ORR), it is important to scrutinize the effect of additional parameters governing the structure of the EDL (in comparison to the EDL overlap effect). Therefore the effect of the electrolyte concentration, which determines the potential drop at the compact and the diffuse layers, is investigated and its influence on the specific activity of a flat platinum surface is analysed. The impact of the ion concentration on the potential drop within the EDL compact layer has been discussed, modelled, and compared to the experimentally determined specific ORR activity of a polycrystalline platinum sample. Furthermore, the NATURE MATERIALS 1

2 existence of a causality between the variation of the potential drop within the compact layer and the enhancement of the ORR specific activity is discussed. The variation of the potential at the electrical double layer (EDL) compact layer due to the EDLs overlap effect was extensively investigated in the last century. Verwey and Overbeek 1 demonstrated the increase of the potential at the compact layer V c with decreasing the distance between identical interacting flat double layers. The authors stated that the increase of V c depends on the Debye length and the specific adsorption potential of counter-ions. Later, Usui 2 has extended the investigation to dissimilar interacting flat EDLs in which he reported similar observations. Only recently the concept of EDL overlap effect has been applied in electrocatalytic investigations. In studies concerning the structural effect of nanoporous electrodes on electrocatalytic reactions, it has been shown that an enhanced electrocatalytic activity observed on nanoporous Pt cannot simply be explained by a mere enhancement of the surface area 3. Other factors must play an additional role. This conclusion was verified by the fact that a significant enhancement in electrocatalytic activity remained even after normalizing the electrochemical redox behaviour of O 2 and H 2 O 2 to the surface area of the nanoporous Pt. In a more recent study, the authors highlighted the overlap of the EDL in the nano-confined space as additional contribution of the electrocatalytic performance 4. The importance of the EDL potential distribution inside the pores was discussed in terms of pore and ions size, volume exclusion effects, ionic concentration and dielectric permittivity inside the pores 4. Furthermore, a strong correlation between the adsorption of hydrogen and oxygen on platinum surface and the potential difference at the EDL was demonstrated by early works of Frumkin 5. Beside the existence of a well-defined connection between the adsorption fraction coverage and the electrode potential, i.e., the Frumkin isotherm, it was found that the adsorption of hydrogen and oxygen atoms contribute to setting up of a potential difference at the platinum/solution interface. This was explained by the fact that adsorbed hydrogen 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION and oxygen atoms form dipoles with the metal surface, thus influencing the potential difference at the compact layer 6. Therefore, due to the existing interrelation between the adsorption coverage and the potential at the reaction plane, we propose that a substantial alteration of the potential at the compact layer has a direct effect on the adsorbate coverage, and thus on the ORR activity. In our present communication, we related the enhancement of the oxygen reduction reaction (ORR) specific activity to the alteration of the EDL potential distribution due to the overlap effect. The experimental results show a clear enhancement of the ORR activity when the interparticle distance decreases. Considering that each nanoparticle is surrounded by an EDL, it is obvious that the EDLs overlap for separating distances below the EDL Debye length, and thus an alteration of the EDLs potential distribution occurs. The local change of the EDL potential, in the nano-confined region between the NCs, is not limited to the EDL diffuse part but also affects the potential at the compact layer V c, i.e., the EDL reaction plane. Consequently, the potential difference ΔV=E- V c (E: electrode potential) shifts within the EDL compact layer, which defines the EDL capacitance, specific adsorption and favours or hinders faradaic reactions occurring at the reaction plane. In particular, the coverage of adsorbed species and the potential drop ΔV are interrelated, as demonstrated e.g. by Frumkin (see above). As the adsorbate coverage defines the potential at the compact layer we hypothesized that a change of this potential, by means of the EDL overlap effect or by modifying the EDL structure (e.g. ionic strength), would affect the coverage, and thus the ORR activity. As mentioned earlier we investigated the EDL structure and its influence to the compact layer potential to emphasize the validity of the model. In order to compare experimental measurements with results of modelling the potential drop in the compact layer, we determined the ORR activity of polycrystalline Pt as a function of the electrolyte concentration of a perchloric acid electrolyte. These results are then compared with the modelled potential drop in the compact layer of a flat surface. NATURE MATERIALS 3

4 It is a well-known concept that increasing or decreasing the electrolyte concentration leads to a change of the counter-ion concentration at the compact layer, and thus to a shift of ΔV. Equivalently, the decrease of the electrolyte concentration shifts ΔV to lower values, and thus higher ORR activities would be expected. In order to experimentally determine these effects, it is of utmost importance to carefully design and perform the experiments in order to eliminate all disturbing effects, which can occur in low and high concentrated electrolytes. The ORR activity has been determined for polycrystalline platinum sample and all measurements in the different electrolyte concentrations were performed in a row using the same working electrode. The working electrode was a polycrystalline platinum disc (ø5mm, MaTecK) encapsulated in a PTFE cylinder. It was mirror polished prior to the experiments. The measurements were conducted with a careful analog compensation of the solution resistance 7. A saturated Calomel electrode and a platinum wire were used as reference and counter electrodes, respectively. The electrolyte concentrations used in the this study were 0.05M, 0.1M, 0.2M, 0.3M and 0.4M HClO 4. The specific activity of the ORR was measured as described previously 8. The reversible hydrogen potential of each electrolyte was carefully measured for each electrolyte prior to the measurement in order to employ exactly the same upper and lower potential limits in the measurements. To rule out an influence of the electrolyte conductivity on the reduction current, for each measurement the solution resistance was compensated to the same residual value of 2Ω. The results of the measurements were confirmed three times (on different days) in freshly prepared electrolyte solutions using two different perchloric acids (Merck, Suprapure and AnalR, Normapur ). 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Figure S1: Polarization curves and Tafel plot. A) Polarization curves recorded in oxygen saturated perchloric acid electrolyte of different concentrations; the scan speed was 100mVs-1, the rotation speed 1600rpm. B) Extracted kinetic currents of A) presented in a Tafel Plot. An increase in ORR activity towards lower concentrations is clearly visible. Figure S1 summarizes the experimental results for a representative measurement series in different perchloric acid solutions ranging from 0.05M to 0.4M in concentration. Figure S1 A) shows the capacitive background corrected anodic sweeps. With increasing electrolyte concentration, the half wave potential clearly shifts to lower values. The corresponding Tafel plots are presented in FigureS1 B). The Tafel plots are parallel to each other, exhibiting an identical shape from the kinetic region towards the beginning of the diffusion limited potential region (U~0.8V vs. RHE). This shows that in all measurements the electrolyte resistance is exactly the same (2Ω) and differences in conductivity could be completely eliminated by the positive feedback scheme of the home build potentiostat; thus demonstrating the high quality of the measurements. NATURE MATERIALS 5

6 Figure S2: Experimentally determined, normalized ORR specific activities at 0.9VRHE (red open triangles) and the modelled potential at the compact layer (blue open circles) as function of electrolyte concentration. Figure S2 compares the results from the experiment and computational modelling. It can be clearly seen that the ORR activity and the potential at the compact layer are following the same trend when changing the electrolyte concentration. The calculated potential at the compact layer V c increased by about 35mV when decreasing the electrolyte concentration, corresponding to an increase of the specific activity by a factor of 2. By comparison, the calculated V c in the NCs model shows an increase of about 100mV (at the same potential applied to the electrode), which corresponds to an enhancement of the specific activity by a factor of 5. The different magnitudes of V c follow from the way how the compact layer potential is varied in the experiments. According to our model, in the NCs experiment the potential at the compact layer is varied by the polarization effect due to the EDL overlap at different interparticle distance. On the other hand, for the flat Pt 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION electrode experiment, the potential at the compact layer is changed by variation of the electrolyte concentration. By using different procedures to vary the potential V c, we show in both experiments evidence of an interrelation between the EDL potential distribution and the ORR specific activity. However, other effects can also be involved in the observed enhancement of the ORR specific activity like electronic coupling, non-linearity and unusual structure of the solvent at the interface for small clusters and smaller separating distances. Our numerical simulation is based on a simple continuum approach. Various effects are simplified as the real shape of the NCs, the surface charge distribution and discreetness of charges at the interface, which would have an effect on the EDL structure. Nevertheless, the variation of the potential distribution at the compact layer with interparticle distance would remain even under a more accurate approach. We consider the model as a first step to a more refined model in future, which goes beyond the mean-field approach or electron transfer kinetics theory and would probably need an atomistic modelling that should include the EDL effect. This has never been done appropriately so far due to the complexity of the interface, but our experimental results indicate its implication for understanding electrocatalytic reactions. II: Finite element method The model presented in this study has been simulated using a commercial finite element method software package (COMSOL). The modified Poisson-Boltzmann (MPB) equation was implemented in the electrolyte domain above an electrode consisting of NCs distributed on a flat conducting support. A dense tetrahedral mesh was set at the boundaries of the flat electrode, the distributed NCs and the compact layers where a steeper potential gradient is expected. Moreover, we used the Booth equation, which is based on the Onsager and Kirkwood theories for polar dielectric solvent, to model the dependence of the dielectric permittivity on the electric field strength. This equation was implemented in the entire electrolyte domain including the NATURE MATERIALS 7

8 diffuse and the compact EDL regions. Due to the high non-linearity and the self-consistency of the equations, the solution with constant dielectric permittivity was used as a first guess. The water dipole interaction with the electric field in the EDL affects the potential distribution in the compact layer and hence the EDL potential profile. This effect is more pronounced when the EDLs are overlapping and their electric fields are interacting within a nano-gap separating the NCs. As the distance separating the NCs becomes smaller than the Debye length λ, the EDL electric field and the dielectric permittivity are affected. This dependence can be seen comparing Fig. S3a and b, which show the distribution of the electric field at the NC compact layer and the electric field lines in the nano-gap for two different separating distances. The EDLs overlap effect is reflected by the deviation of the electric field lines in the nano-gap emanating from opposite NCs as the edge to edge distance is decreased. As a consequence of the mutual interaction, the electric field at the compact layer is lowered in regions where the edge to edge is smaller. This leads to a local increase of the potential at the compact layer V c, thus decreasing ΔV. Moreover, the dielectric permittivity is significantly increased in regions at the compact layer edge where the electric field is lowered. Figure S3: Electric field distribution at the compact layer. Field lines are drawn in the region confined between two NCs for separating distance (a) d=2nm and (b) d=0.5nm. Note that for sake of clarity, the field stream lines are plotted only for the foreground NCs. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION III: Supplementary Experimental information Figure S4: Representative size distribution of Pt + clusters created by the laser ablation cluster source in the mass range of Pt 46 (8810 to 9150 amu). The integrated current during deposition is recorded while performing a slow mass scan with the QMS, thus revealing the current corresponding to a certain mass (black dots). Fitting tangents to the slopes of the peaks (orange), representing one single mass, the areas for absolute mass selectivity (green) can be determined by using the intersection with the x axis. For a selected deposition (here for Pt 46 ), the QMS is set to the mass value of the peak maximum (as indicated at 8970 amu) and consequently clusters with absolute control of size can be deposited. NATURE MATERIALS 9

10 Figure S5: Electrochemical roughness factor calculated from the integrated CO stripping charge vs. geometrical roughness. The geometrical roughness is calculated by adding up surface of clusters per cm² measured during deposition taking into account the contact face to the support. The data clearly show a linear relationship between the deposition measured and the electrochemically measured Pt surface area of the NCs. 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Figure S6: Determination of the number of absorbed hydrogen (H ads ) and carbon monoxide (CO ads ) atoms, respectively, per Pt 20 NC. The H ads is determined from the H upd charge in the CV, CO ads from CO stripping curves. Fitting a linear relation between the total number of adsorbed species and the cluster density we calculated N = 9 ± 1 for the number of adsorbed species per Pt 20 NC. This value corresponds to an ECSA of ~80 m 2 g Pt -1. For the Pt 46 and Pt >46 samples average values of ~110 m 2 g Pt -1 and ~95 m 2 g Pt -1, respectively, were obtained. For the Pt >46 samples the individual ECSA values varied more, as the deposition was done in the RF-only (ion guide) mode of the QMS, which acts as high-pass rather than a band-pass filter. NATURE MATERIALS 11

12 Figure S7: RDE polarization curves (1600 rpm) of the ORR. Different Pt NC samples are shown from which the activities of Figure 1 were calculated. Different diffusion limited currents are detected due to the different (geometric) coverage of the Pt NCs. As reference we included Pt poly. For the establishing the correct normalized activities Tafel plots were established. The ORR activity of the samples is determined with two normalizations. Normalized to surface area, i.e. SA, and normalized to Pt mass, i.e. MA. To obtain the SA the kinetic ORR current obtained in the RDE measurements is divided by the CO oxidation charge determined in CO stripping experiments after the ORR measurements assuming a charge CO oxidation charge of 400 μc cm Pt -2 for Pt. To obtain the MA, first the ECSA is calculated from the CO oxidation charge and the cluster deposition current assuming single charged clusters. Then the MA is calculated by the simple relation MA = SA * ECSA. Alternatively we calculated the MA from the absolute kinetic currents of the RDE measurements and the cluster deposition current, thus avoiding any influence of the surface area determination on the results. Both calculations let to the same (within the experimental error) values. 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION Figure S8: Tafel plots of the ORR SA for the size selected Pt 20 NC series. It is seen that due to the small Pt surface area at low NC coverage an activity determination at 0.9V RHE is less reliable. Therefore for all samples the ORR activity was determined at 0.85V RHE. References for Supplementary Information 1 Verwey, E. J. W. & Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids. (Elsevier, Amsterdam,, 1948). 2 Usui, S. VARIATION OF THE POTENTIAL OF THE STERN PLANE WITH INTERACTION OF DISSIMILAR FLAT ELECTRICAL DOUBLE-LAYERS. J. Colloid Interface Sci. 97, , doi: / (84)90290-x (1984). 3 Han, J. H., Lee, E., Park, S., Chang, R. & Chung, T. D. Effect of Nanoporous Structure on Enhanced Electrochemical Reaction. J. Phys. Chem. C 114, , doi: /jp909382b (2010). 4 Bae, J. H., Han, J.-H. & Chung, T. D. Electrochemistry at nanoporous interfaces: new opportunity for electrocatalysis. Physical Chemistry Chemical Physics 14, (2012). 5 Frumkin, A. THE DEPENDENCE OF THE DOUBLE LAYER STRUCTURE ON THE NATURE OF THE METAL SURFACE. J. Res. Inst. Catalysis 15, (1967). 6 Petry, O., Frumkin, A. & Kotlov, Y. THE DEPENDENCE OF THE SURFACE CHARGE OF PLATINUM ELECTRODES ON THE POTENTIAL IN ALKALINE SOLUTIONS. J. Res. Inst. Catalysis 16, (1968). 7 Wiberg, G. K. H. The development of a state-of-the-art experimental setup demonstrated by the investigation of fuel cell reactions in alkaline electrolyte, Technische Universität München, (2010). 8 Nesselberger, M. et al. The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts: Influence of Electrolyte and Relation to Single Crystal Models. Journal of the American Chemical Society 133, , doi: /ja207016u (2011). NATURE MATERIALS 13