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Corrosion Protection of Platinum-Based Electrocatalyst by Ruthenium Surface Decoration Primož Jovanovi#, Marjan Bele, Martin Šala, Francisco Ruiz-Zepeda, Goran Draži#, Nataša Zabukovec Logar, Nejc Hodnik, and Miran Gaberscek ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00405 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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ACS Applied Energy Materials

Corrosion Protection of Platinum-Based Electrocatalyst by Ruthenium Surface Decoration Primož Jovanovič*a,b, Marjan Belea, Martin Šalab, Francisco Ruiz-Zepedaa, Goran Dražića, Nataša Zabukovec Logarc,d, Nejc Hodnik*e, Miran Gaberšček*a,f a

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

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

Department of Inorganic Chemistry and Technology, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia d University of Nova Gorica, Vipavska 13, SI-5000 Nova Gorica, Slovenia e

Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia f

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia

KEYWORDS: Oxygen reduction, Oxygen evolution, Corrosion, On-line dissolution, Degradation

ABSTRACT: A comprehensive insight into the electrochemical performance of PtCu3 electrocatalyst nanoparticles with and without Ru decoration is provided. The on-line dissolution investigation using the highly sensitive online analytical methodology of electrochemical flow cell coupled to inductively coupled plasma mass spectrometry reveals that the addition of Ru nanoparticles inhibits Pt dissolution due to presumably three effects: (i) suppression of Pt oxide formation and (ii) sacrificial corrosion of Ru and (iii) lowering of local surface pH. The Ru nanoparticles, however, also lead to a decrease of the amount of crystal structure ordering, which in turn is one of the reaons for the increase of the corrosion of Cu. By measuring the potential of total zero charge it is shown that Ru decoration does not alter the electrochemical properties of the native Pt surface. Finally, Ru decoration of the Pt-based electrocatalyst is shown to present a viable approach to enhance the platinum corrosion resistance, which is confirmed by thin-film rotating disc electrode accelerated degradation tests.

INTRODUCTION

Degradation of cathode catalyst layer in the corrosive acidic environment is the pivotal obstacle in widespread commercialization of low-temperature proton exchange membrane fuel cells (PEM-FCs).1–5 Among different corrosion events, nanoparticle dissolution is one of the important mechanisms.2,4–7 Different strategies have been employed in order to mitigate the necessary stabilization of nanoparticulate electrocatalysts. For example, dealloyed core-shell type particles in which the core is enriched in non-noble component - whereas the shell is

enriched in Pt – have been shown to be more active and stable in comparison to commercial Pt/C type catalysts.8– 10 Pt monolayer type11,12 and Pt skin type architectures13–15 have shown enhanced stability as well. Further on, nanoporous alloys had been proposed as potentially stable candidates as well16, however, were later on proven to lose surface area due to porosity coarsening even when using a mild degradation protocol.17 Further on, adding noble metals such as Au18–23, Pd, Ir to Pt-based catalysts has been reported to enhance the stability according to experimental findings24 and DFT predictions.24,25 Ruthenium on the other hand has rarely been considered as stability

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promoter. However as shown by Atanasoski et al.26 ruthenium can increase corrosion stability indirectly due to its good catalytic oxygen evolution reaction (OER) activity, which protects the cathode potential from rising above the potential of OER onset (approximately 1.4 V vs RHE). This would potentially cause less degradation to the membrane electrode assembly (MEA), especially carbonbased support, the gas diffusion layer and bipolar plates. Here we highlight the beneficiary effect of ruthenium on the stability of Pt in nano-alloy electrocatalyst by directly measuring Pt corrosion which is significantly decreased in the presence of Ru. EXPERIMENTAL A detailed point-by-point description of the synthesis procedure is available in the Supporting Information (see section S1). Shortly, it consists of two vital steps, the first being annealing of a Cu metal precursor (e.g., a copper precursor) together with gelatine and carbon black to obtain embedded nanoparticles in a porous carbon matrix. In the second part, platinum precursor is added, and the mixture is freeze-dried and annealed again. In the case of PtCuRu/C a Ru precursor 15,27– is added after PtCu3/C nanoparticles have been obtained. 31 After subsequent annealing at 500 °C for 12 h under a reductive atmosphere (H2/Ar, 5 %), a PtRuCu/C composite is obtained (Fig. 1). Electrochemical flow cell coupled with ICP-MS setup was 18,32–36 already introduced in our previous publications. Shortly, a commercial BASi electrochemical flow cell (Cross-Flow Cell Kit MW-5052) with a teflon gasket of a 0.41 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 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 film procedure consisted of drop casting catalyst water suspension over a glassy carbon electrode and stabilized by a 5 µL of Nafion diluted by 2 isopropanol (1/50). The resulting Pt loading was 13 µg/cm geo. Thin film rotating disc electrode (TF-RDE) measurements were conducted in a two-compartment electrochemical cell in a 0.1 M HClO4 (purity > 99.99 %, Aldrich) electrolyte with a conventional three-electrode system controlled by a potentiostat (Compact stat, Ivium technologies). Ag/AgCl was used as a reference and a Pt wire as a counter electrode. The working electrode was a glassy carbon disk embedded in Teflon (Pine Instruments) with a 2 geometric surface area of 0.196 cm . Prior to each experiment, the electrode was polished to mirror finish with Al2O3 paste (particle size 0.05 µm, Buehler) on a polishing cloth (Buehler). After polishing, the electrodes were rinsed and -1 sonicated in a 18 MΩcm mili-Q water for 5 minutes. 20 µL of -1 1 mgmL water-based, well-dispersed catalyst ink was pipetted on the glassy carbon electrode completely covering it and dried. After drying, the electrode was mounted on the rotator (Pine Instruments). The electrode was placed in an Ar

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saturated electrolyte under potential control at 0.05 V vs RHE. The catalysts were electrochemically activated for 200 cycles between 0.05 V and 1.2 V (vs. RHE) with a scan rate of -1 300 mVs . ORR polarization curves were measured in an oxygen saturated electrolyte with rotation at 1600 RPM in the -1 same potential window with a scan rate of 20 mVs . After subtraction of background capacitive currents, the kinetic parameters were calculated at 0.9 V. Ohmic resistance of the electrolyte was determined and compensated for as reported 37 in ref. Electrochemical surface area (ECSA) was determined in a CO stripping experiment as described in the 38,39 literature. All potentials are given against the reversible hydrogen electrode (RHE). The values of RHE were determined by measuring open circuit potentials (OCP) in a H2 saturated electrolyte. In order to investigate the longterm performance of the two analogues TF-RDE degradation protocol consisted of 10 000 voltammetric cycles between 0.4-1.4 V vs. RHE (1 V/s) was performed. Degradation protocol was interrupted after 1000, 5000 and 10 000 cycles. Potential of total zero charge (PZTC) measurements were performed by TF-RDE setup as well. To determine the PZTC parameter, N2O bulk reduction was measured in the presence of N2O in the solution (Linde, class 1, 90% helium). The value of PZTC was obtained from the peak current of the N2O reduction under the cyclovoltammetric regime (20 mVs 1 40 ). Transmission electron microscopy (TEM) was carried out with a JEOL JEM-2010F and with a Cs-corrected microscope CF-ARM Jeol equipped with an SSD Jeol energy dispersive Xray (EDX) spectrometer; operation voltage was set to 200 kV and 80 kV, respectively. RESULTS AND DISCUSSION Structural characterization: According to XRD analysis of prepared samples, the two main differences in the Ru and non-Ru analogues are: a) Ordered (Pm-3m) PtCu3 crystal 27 phase diffraction is only clearly seen in the case of non-Ru analogue. This indicates that after the high-temperature annealing the added Ru directly affects the near surface PtCu3 Pm-3m ordered crystal structure (Fig. 1). The dissaperence of the ordered phase could be a result of the altered Pt/Cu ratio induced the process of Ru galvanic displacement of Cu or also due to narrower average particle sizes. A detailed description of the synthesis and annealing procedure is listed in Supporting Information (see Section S1). b) A pure Ru phase is detected in XRD spectra, which is in accordance with the low mixing tendency of Cu and Ru predicted by Ru-Cu phase diagram. A quantitative XRD analysis was performed, as described in the Supporting Information (see section S2). The results were also confirmed by TEM observation indicating that Ru nanoparticles were formed. c) As can be seen from the sharper XRD spectra we would expect a bigger particle size in the overall distribution of binary PtCu3 nanoparticles (Fig. 1 a). This as well, was later confirmed by TEM observation (Figure S4 a). This indicates that the addition of Ru also suppresses the particle size 41 growth/sintering during the high-temperature annealing. According to TEM analysis, in the as prepared sample Ru deposits in the form of small nanoparticles (ranging from 1 nm up to 10 nm) onto the surface of larger PtCu3 nanoparticles or separately on the carbon support (Fig. 1c and section


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As prepared analogues in Supporting Information-section S3). EDX spot analysis made on different parts of the bigger particles confirms that: (i) some of the small nanoparticles are Ru, (ii) the core of the big ones is PtCu and (iii) the surface of PtCu nanoparticles contains small amounts of Ru nanoparticles (Fig. S3). This was further corroborated, as shown in Figure 1c, by EDX mapping of the Ru analogue after the electrochemical activation pretreatment (see Figures S5S9 in section S3 from Supporting Information). A more careful inspection confirms a platinum surface enrichment due to the Cu removal via dealloying. Moreover, at the surface of the PtCu3, a few core-shell Ru-Pt nanoparticles were found along with Ru particles, as shown on the EDX maps in Fig. S8 and S9. This could occur as a result of platinum dissolution, which has been confirmed to take place during the activation 35 cycling , and subsequent redeposition on to the Ru nanoparticle. The presence of Ru nanoparticles is also directly confirmed by CO stripping curves (see Fig.S15 a in Supporting Information). Specifically, the existence of Ru is seen as a shift of the onset of the CO-oxidation process to lower potentials (a well-known bi-functional electrocatalytic effect). In the case that Ru would not be in direct contact with Pt only a small CO stripping peak would be observed at low 42 potential due to the small amount of Ru in the sample . Additionally, a larger peak at higher potentials corresponding to PtCu nanoparticles would need to appear. Since only one large peak at low potentials is seen (Fig. S15 a), it is safe to assume that this peak is due to the direct contact between Ru and PtCu nanoparticles. It is also worth mentioning, that an additional feature observed is the less number of porous particles present in the Ru analogue sample when comparing both analogues after activation (Figure S4). An effect that has been previously related to corrosion resistance in gold 18,23 PtCu3:Au.

Figure 1: a) X-ray diffraction of PtCu3/C and PtCuRu/C. b) A representative image of PtCuRu/C obtained using Transmission Electron Microscopy. c) Energy dispersive X-ray mapping of the PtCuRu/C activated sample.

Potential of total zero charge (PZTC): In order to provide further details on the nature of Pt-surface, PZTC of the two samples were investigated. The coverage of Pt surface by water, spectator species and reactants can be effectively 15,43 investigated by measuring PZTC which is related to the phenomenon of adsorption, where the net electric charge on 44 a surface of the electrode equals zero. At this potential the platinum surface coverage is the lowest, and therefore the most accessible to the non-specific adsorbing molecules such as N2O. N2O reduction cyclic voltammogram can be used as a chemical probe sensitive to the local values of charge on 45,46 the interphase, namely PTZC. According to the study by

Climent et al., the rate of N2O reduction will be faster when the amount of adsorbed N2O is higher - if assumed that competitive adsorption between weakly adsorbing N2O and other adsorbed species (namely, hydrogen cations, anions and water) takes place on the electrode surface. When N2O gets in contact with a noble metal electrode surface, the reduction will proceed faster if the surface coverage is small, that is, when the number of available sites for N2O adsorption is big (see Equations 1-3). The important aspect of the application of this molecule as a probe of interfacial structure relies on the weakness of the N2O adsorption, which does not modify the adsorption isotherm of the other components at the interphase under ambient pressure. M N O → M

(1)

M e → MO

(2)

MO 2H 2e → M H O

(3)

Cyclic voltammograms for Ru and non-Ru analogues under N2O saturation are shown in Fig. 2. The potential at the maximum current observed (that is PZTC) is approximately the same for both catalysts. For the case of pure Ru, surface PZTC is expected to appear at more negative values. This is because Ru is more oxophilic, hence its coverage with oxo species is higher than in the case of Pt. Its PZTC could be shifted below hydrogen evolution potential (0 V). The similar values of PZTC for both analogues indicate that the exposed Pt surface has similar double layer structure and characteristics, i.e. the surfaces are similar or even entirely equal. Here we mostly refer to the coordination of Pt surface atoms that directly control the PZTC values. This has been shown for stepped surfaces of Pt single crystals where PZTC is shifting 40 more negative with increasing step density and for Pt-alloys where PTZC, as a parameter of surface coverage, is directly 15,43 proportional with ORR and MOR activity. The similar PZTC values, hence the same type of Pt surface, would suggest that the activity and stability of these two Pt surfaces should be similar. Since ORR activity and Pt dissolution profiles show the opposite, the effect of addition of Ru on the Pt surface must be considered in a different way, thus additional explanation is needed (see the section S5:Thin film rotating disc electrode (TF-RDE) measurements and section Pt dissolution below). In addition, we note that the slightly higher current maximum in the N2O reduction polarization curve for the Ru analogue (Fig. 2) is in line with the larger Pt surface area observed for this analogue (Fig. S10 a). Also, a careful observation of Fig. 2 reveals a very small potential shift of the N2O maxima to higher potentials, which is in line with a higher initial ORR activity due to the presumably 15,43 lower coverage of spectator species.



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Figure 2: Background corrected cyclovoltammetric response under N2O saturation for Ru and non-Ru analogue. A positive potential sweep is shown.

Online dissolution measurements: Pioneered by the 47 introduction of a microelectrochemical scanning flow cell , online analytics have been effectively employed for studying 18,33,34,48– noble and non-noble metal corrosion in recent years. 57 In the present work, time- and potential-resolved measurements by using electrochemical flow cell (EFC) on-line coupled to an ICP-MS device were performed in a two-step procedure. In the first step, the investigated catalysts were electrochemically activated by performing 200 cycles between 0.05-1.2 V vs. RHE to remove the sacrificial copper (dealloying-see Fig. S10 a) in order to obtain a stable platinum cyclic voltammogram (CV). Afterwards, a slow poten-1 tiodynamic experiment (5 mVs ) followed using potential cycling from 0.05 V vs. RHE to different upper potential limits (UPL). Namely, a slow potentiodynamic electrochemical treatment to different upper potential limits (UPLs) enables a better spectral resolution of the dissolution process. Cu dissolution: Dissolution profiles during the fast activation step are shown in Fig. S10 where a substantial difference between the analogues in the leaching of Cu (Fig. S10 a) is observed. This is more intense in the case of Ru analogue. Since Ru atoms are not incorporated into the crystal structure of Pt-Cu alloy (Fig. 1), no direct Ru-Cu interaction is expected. The more intense Cu dissolution in the case of Ru analogue should be ascribed to a lower amount of the ordered (Pm-3m) phase in comparison to the non-Ru analogue (Fig. 1a). The degree of structural order has been proven as a decisive parameter in the electrochemical stability of Cu, 58 exemplified in the case of Cu3Au. Similar conclusions have 27,35,59 recently been reported for the case of Cu3Pt and Co3Pt 60 alloys as well. The presence of ordered Cu3Pt structure in the same catalyst can substantially improve Cu stability in comparison with pure disordered PtCu3, namely up to 90% 35 more copper is retained at potentials below 1.2 V. The lack of the stabilizing effect of surface Ru in the case of Cu is rather surprising if compared to the surface decoration with 18,35 Au that increases Cu stability. However, after thermal annealing Au is incorporated into the PtCu structure as AuPt skin whereas Ru forms nanoparticles on the PtCu surface. Therefore it is not considered part of the PtCu structure. Nevertheless, it could also decrease the ability of Pt surface atoms diffusivity that is the main parameter slowing down 61 Cu dealloying. Further studies are needed to understand this phenomenon in more detail. Interestingly, the additions 62 63 of Rh and Ir were shown to hinder the Pt surface diffusion.

Under a slow potentiodynamic regime (5 mVs ), well resolved dissolution profiles are obtained (Fig. 3). Cu dissolution shows a similar trend as in the case of fast activation cycles. Namely, in the case of Ru analogue Cu dissolution is more intense. Furthermore, Cu dissolution onsets already in the low potential region, namely when cycling till UPL 0.5 V vs RHE (Fig. 3a). In the case of non-Ru analogue virtually no Cu dissolution is detected under this potential sequence (Fig. 3a). This raises a question of overall lower stability of Cu atoms in the case of Ru analogue. Since Ru is not structurally incorporated in the Pt-Cu alloy (Fig. 1b), its effect on Cu dissolution is indirect. Regarding this, one should take into consideration the following facts: (i) Ru goes through many oxidation states, when cycled, (ii) Ru is a very active catalyst for water oxidation, (iii) the degree of structural order (Pm3m phase) is substantially lower in the Ru analogue (Fig. 1a) and (iv) Pt surface diffusion retardation by the presence of Ru nanoparticles. During processes under (i) and (ii) protons are formed, hence pH decreases. Referring to Fig. 3, we note that at low potentials reactions under (i) and (ii) are unlikely to decrease the surface pH due to the large buffer effect of 0.1 M perchloric acid and insufficiently high current densities of 64 reactions under (i) and (ii) at low potentials. Therefore, the predominant Ru effect is ascribed to the characteristics mentioned under (iii) and (iv), namely a lower amount of the ordered phase and a lower ability of surface Pt to protect Cu in the PtCu structure. In our previous publications, we have shown that Cu dissolution is strongly affected by the degree 27,35,59 of structural ordering which enhances Cu stability. This effect alone could explain the difference in Cu dissolution. At this point we are unable to prove which of these effects is the dominant one.

Figure 3: Copper dissolution profiles of Ru and non-Ru analogue during slow (5 mVs-1) potential cycling from 0.05 V to gradually increased upper potential limits: a) 0.5-0.7 V, b) 0.9-1.1 V.

Cu dissolution trend remains the same when cycling is carried out till higher potential regions where the dissolution profile shows a typical characteristic shape consisting of 35 three peaks , namely two anodic and one cathodic, labeled in Fig. 3b as 1, 2 and 3, respectively. For the case of Pt-Cu alloys, similar three Cu dissolution peaks have already been 35 extensively discussed in our recent work. Briefly, the first anodic peak (peak 1 in Fig. 3) with the onset of approximately 0.6 V is related to the dissolution of underpotentially deposited Cu atoms (originating from Pt-Cu coordination parts). The second anodic peak (peak 2 in Fig. 3) is related to Cu removal from the Pt-Cu subsurface (Cu dealloying and/or leaching). The third and the least intense cathodic peak (peak 3 in Fig. 3) occurs during the cathodic scan and is 34,52,53 ascribed to the reduction of Pt oxide which uncovers additional subsurface Cu atoms. A comparison of the Cu dissolution profiles shows that anodic peaks are always more intense in the case of Ru analogue (Fig. 3). This is again in


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complete agreement with the higher stability of Cu in the 35,58 ordered (Pm-3m) crystal structure which is more abundant in the non-Ru analogue (Fig. 1a) and higher surface area of the Ru sample. The total amounts of dissolved Cu during electrochemical treatment are presented in Fig. S11 a. Pt dissolution: During recent years platinum dissolution has 17,18,34,36,52,53,57,65–69 been receiving increasing interest. It has been shown that this dissolution is predominantly a transient process occurring due to formation/reduction of the oxide. This can be manipulated by changing the electrochemical treatment (scan rate, anodic and cathodic potential window), gas atmosphere, temperature, particle size and 34,36,52,65,70–72 electrolyte. Under potentiodynamic conditions, the dissolution process has been shown to consist of two non-equivalent branches, an anodic and a more intense 53 cathodic counterpart. We again note that both analogues presented here have been treated under the same electrochemical conditions.

We assume that the substantial difference in Pt dissolution profile between Ru and non-Ru analogue cannot be only due to the lower amount of Cu in the structure, but has to have another origin, i.e. inhibited formation of Pt oxide in the presence of Ru. We note again that under slow potentiodynamic conditions a Pt dissolution is attributed to an inter34,52 play between the oxide formation and oxide reduction. Thus if the amount of Pt oxide formed/reduced is different between the two analogues, this should be reflected in the dissolution profile. Indeed, the Pt dissolution trend is in excellent agreement with Pt oxide reduction peak for the two analogues where less Pt oxide is formed in the Ru analogue (Fig. S14). In short, the presence of Ru particles leads to a noticeably lower extent of oxidation of Pt surface, hence a much lower Pt dissolution. The lower Cu content alone could 35 only account for approximately 10 % lower Pt dissolution , which is clearly not the case in the present paper.

The present results indicate that during fast activation cycles there are no significant differences in the Pt dissolution trends for both analogues (Fig. S10 b). This can be due to a similar anodic dissolution pathway that is the dominating degradation mechanism under fast scan rates where only low 67,73 amounts of Pt-oxide are being formed. Under fast scan 74 rates, the formation of oxides is kinetically hindered. Furthermore, corrosion of Pt under high scan rates is attributed to the dissolution of the less stable low coordinated Pt sur75 face atoms. Since dissolution profiles of both analogues are rather similar, the surface concentration of low coordinated Pt atoms is most likely the same as predicted by similar PZTC. However, a detailed inspection of Pt dissolution is possible in -1 the subsequent slow potentiodynamic (5 mVs ) experiment (Fig. 4). In general, in contrast to Cu dissolution, Pt dissolution is much more enhanced in the case of non-Ru analogue under all UPLs (see the overall integrated dissolved amounts in Fig. S11). The anodic dissolution trend is in agreement with 35 previous work on Pt alloys. As already shown, Pt dissolution is dependent on the residual Cu content in nanoparticles. With the increase of the amount of residual Cu, Pt anodic 35 dissolution is accelerated. This has been ascribed to a less stable Cu-Pt bond in comparison to Pt-Pt. A similar phenomenon is noticed here, namely, in the case of higher Cu retention (non-Ru analogue) the anodic Pt dissolution is enhanced (Fig. 4). In the case of cathodic Pt dissolution, it has been shown that it can be inhibited by redeposition in the nanopores of metal particles, due to the suitable envi35 ronment provided by the nano-confined space. Since the particle size is smaller in the case of Ru analogue (Fig. S4 a), an increased Pt cathodic dissolution is expected due to i) lower percentage of porous particles and ii) particle size 4,5,34 effect. However, the particle size was shown to be deci34,76 sive only in the case of smaller particles (below 5 nm) . Nevertheless both i) and ii), are in disagreement with the inhibited Pt dissolution observed in the case of Ru analogue. Thus, an additional explanation is necessary, especially since the differences between the analogues are much higher than in the case of the recently published work on Pt-Cu alloy nanoparticles where analogues with different porosity and 35 degree of structural ordering were comparatively studied.

Figure 4: Platinum dissolution profiles of Ru and non-Ru analogue during slow (5 mVs-1) potential cycling from 0.05 V to gradually increased upper potential limits: a) 1.1 V b) 1.2 V, c) 1.3 V and d) 1.4 V.

Upon cycling till 1.5 and 1.6 V vs. RHE the Pt anodic dissolution trend is reversed, i.e., the non-Ru analogue now shows a higher stability (Fig. 5). We note here that when cycling till 1.5 V a substantial dissolution of Ru is taking place (see section S4.3 in the Supporting Information). Therefore, the stabilizing effect of Ru on Pt dissolution is attenuated. A closer inspection reveals a multiplicity of the anodic dissolution profile (Fig. 5). Specifically, dissolution consists of two maxima labeled as peak A1 and A2. The latter is more intense, whereas the former is suppressed in the Ru analogue. Peak A1 is ascribed to the transient nature of Pt dissolution due to Pt oxide formation and has been the only 53,66 anodic dissolution peak at UPLs ≤ 1.4 V vs. RHE (Fig. 4). The reasons for its lower intensity in the case of Ru analogue when cycling till 1.5 V are the same as described above, i.e., less Pt oxide is formed. Peak A2, on the other hand, is positioned in the OER region and was reported to be related to 66,77,78 the dynamics of this reaction. The fact that peak A2 is more intense in the case of Ru analogue (Fig. 5a) could be related to Ru dissolution. Namely, Ru surface oxides are 32,77 participating in the OER, which leads to Ru dissolution. This removal of Ru from the electrocatalyst surface exposes new Pt atoms, which are then prone to dissolution. An additional factor to be considered is the effect of pH, due to higher OER rates on Ru surfaces (see Fig. S13) where the surface pH is lowered. This was shown to also increase the 66 dissolution of Pt under peak A2. Pt dissolution trend ob-



served upon cycling till 1.5 V is continued when the UPL is increased to 1.6 V vs. RHE (Fig. 5b).

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ASSOCIATED CONTENT Supporting Information Supporting Information is available: We provide additional information on the synthesis, quantitative XRD analysis, complementary information about the samples with TEM images, additional potential and time dissolution profiles obtained with electrochemical flow and rotating disc electrode characterization results.

Figure 5: Magnification of platinum anodic dissolution profile during potential cycling from 0.05 V to a) 1.5 V and b) 1.6 V.

Corresponding Author

In order to estimate the long-term performance of the two analogues, a TF-RDE accelerated degradation test simulating the start/stop conditions was performed (see section S5 in the Supporting Information). Importantly, the trend in the electrocatalytic performance is in line with on-line dissolution results. Namely, Pt surface area (ECSA) of the Ru analogue is surpassing that of the non-Ru analogue throughout the entire degradation protocol (Fig. S15 b). In the case of specific activities, the trend is reversed, the non-Ru analogue initially maintains a higher activity, however eventually reaches the same plateau after 5000 cycles) (Fig. S15 c). Nevertheless, if mass activities of the two analogues are considered (Fig. S15 d), the trend is shifted in favor of the Ru analogue relatively soon (after 1000 cycles).

Highly sensitive online analytics (EFC-ICP-MS) enabled a new insight into dissolution dynamics of multicomponent electrocatalysts and the mutual corrosion dependence of individual components. By decorating a high-performance PtCu electrocatalyst with Ru nanoparticles, we were able to further increase its stability – via lowering the Pt corrosion. EFC-ICP-MS measurements directly proved the increased platinum corrosion resistance of PtCuRu electrocatalyst. The main conclusions can be summarised as follows:

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*[email protected] *[email protected] *[email protected]

ACKNOWLEDGMENT This study was supported by the Slovenian Research Agency for the research programme P2-0152, P2-0082, P2-0393 and project Z2-8161.

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CONCLUSION

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AUTHOR INFORMATION

The Pm-3m phase of Pt-Cu alloy and the average particle size decreased when incorporating Ru into the system. When cycling till high potentials Pt dissolution is noticeably decreased due to the inhibited formation of Pt oxides caused by Ru nanoparticles sacrifitial corrosion. In the OER potential region (at 1.5 and 1.6 V vs. RHE) Pt anodic dissolution out of PtCuRu/C is accelerated by oxygen evolution reaction due to smaller passivation of Pt surface and presumabely lowering of local surface pH. For the same reason, the usually much more dominant and thus damaging Pt cathodic dissolution is strongly inhibited.

Besides providing new fundamental insights into Pt corrosion, the results of this study are also of high practical importance for design of multicomponent electrocatalysts for oxygen reduction and oxygen evolution reactions.

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