C Nanoparticles in an Interface with an

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Accelerated stress-test of Pt/C nanoparticles in interface with an anion-exchange membrane – an identicallocation transmission electron microscopy study Clemence Lafforgue, Marian Chatenet, Laetitia Dubau, and Dario R. Dekel ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04055 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Accelerated stress-test of Pt/C nanoparticles in interface with an anion-exchange membrane – an identical-location transmission electron microscopy study

Clémence Lafforgue 1, Marian Chatenet 1-2,*, Laetitia Dubau1, Dario R. Dekel 3-4

1

Univ. Grenoble Alpes, CNRS, Grenoble INP#, LEPMI, 38000 Grenoble, France

2

Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France

3

The Wolfson Department of Chemical Engineering, Technion – Israel Institute of Technology, Haifa 3200003, Israel

4

The Nancy & Stephan Grand Technion Energy Program (GTEP), Technion – Israel Institute of Technology, Haifa 3200003, Israel #

Institute of Engineering Univ. Grenoble Alpes

* Corresponding author: [email protected]

Abstract The durability of a state-of-the-art Pt/C electrocatalyst was assessed by accelerated stress test (AST) procedures conducted in liquid alkaline electrolyte (0.1 M NaOH) and in solid anion exchange polymer electrolyte using a “dry cell”, i.e. in absence of liquid electrolyte. In liquid environment, the positive and negative vertex potential values have a great influence on the extent and on the magnitude of the degradations: the loss of electrochemical surface area observed for a wide potential range (0.1 < E < 1.23 V vs. RHE) is ascribed to detachment of the Pt nanoparticles from their support. The Pt nanoparticles assist the local corrosion of the carbon support material, eventually yielding solid alkali-metal carbonates that mechanically expel them from their support. Such corrosion is linked to the propensity of the Pt nanoparticles to (i) accept carbon surface groups (COadlike species) when their surface is free of oxides (“reduced” metal state, for E < 0.6 V vs. RHE) and then to (ii) electrooxidize the COad species into CO2 in the well-known Langmuir-Hinshelwood COstripping reaction, possible if OHad species do form (“oxidized” metal state, for E > 0.6 V vs. RHE). As a result, when the AST is performed between 0.1 < E < 0.6 V vs. RHE or between 0.6 < E < 1.23 V vs. RHE, i.e. when the Pt nanoparticles are either mostly reduced or oxidized, respectively, the degradation processes at stake are less intense and different: Ostwald ripening proceeds in the former case and Pt nanoparticles agglomeration in the latter one. In contrast to the case of liquid electrolyte, when the most severe AST (0.1 < E < 1.23 V vs. RHE) is performed in the dry cell, the magnitude and main mechanisms of degradation significantly change. Because there is no excess water to dissolve the Ptz+ species formed by corrosion of the Pt nanoparticles, 3D Ostwald ripening and local redeposition on existing particles become more likely: the anion-exchange ionomer better traps the Ptz+ species and prevent their diffusion away from the active layer. In addition, the absence of free alkali metal cation avoids the precipitation of solid carbonates, and therefore the detachment of the Pt nanoparticles from their support is not favored. This shows that the degradation processes of a given electrocatalyst not only depend on its nature, but also on the vertex potential values scanned in the AST and importantly, on the nature of the electrolyte medium investigated. Finally, the very dramatic degradations experienced in liquid electrolyte for Pt/C nanoparticles are somewhat 1 ACS Paragon Plus Environment

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mitigated in solid alkaline electrolyte, which harbors hope to develop durable AEM-based fuel cells and electrolyzers.

Keywords: Alkaline fuel cells; Anion-exchange membrane; Durability; Carbon-supported platinum electrocatalyst (Pt/C); Carbon corrosion; Identical-location transmission electron microscopy (ILTEM).

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1. Introduction As the Earth’s atmosphere and oceans accumulate increasing amounts of heat due to human activities (in particular the release of large amounts of greenhouse gases like CO2 in the atmosphere), the use of fossil energies must slow down 1. One solution to this tremendous issue is to harvest energy from renewable sources (e.g. sun, wind and water) and to focus on a “clean” vector like electricity. These renewable sources being by principle intermittent, one must be capable to store electricity upon production peaks to release it upon demand peaks. In that framework, electrochemical convertors can consist of a good technological solution, if these systems are efficient, durable and based on available materials. For example, it is no doubt that fuel cells and water electrolyzers will complement batteries to spread electrical energy to the automotive industry and to store efficiently and at large scale renewable electricity from sun and wind power 2-8. In the past few years, fuel cell and electrolyzer technologies based on acidic membranes (proton exchange membranes fuel cells, PEMFCs, and water electrolyzers, PEMWEs, the state-of-the-art) increasingly penetrated the market. However, the availability of their core materials and durability on the field are still unsatisfactory 7-11. Therefore, intense research presently concerns the development of alternative fuel cell and electrolyzer systems that would not be limited by the use of a PEM and of platinum group metals (PGMs). Alkaline fuel cells (AFCs) and alkaline water electrolyzers (AWEs) are such systems. Both have been practically used on the field (e.g. in the space conquest for the former and as a mean to produce clean hydrogen for the latter), in versions where the alkaline electrolyte was circulating alkali-solutions like aqueous KOH 7,8,12. Despite their interest, these systems cannot compete with PEM-based systems in terms of electrical performance and compactness, therefore impeding their wide deployment. Recent advances in anionic polymer (electro)chemistry made possible the development of anion-exchange membrane fuel cells (AEMFCs) and water electrolyzers (AEMWEs) 13 in which no liquid electrolyte is used, and one may now dream of equivalents to PEMFCs and PEMWEs that would not use strategic metals (like PGM) 14-24. One real interest of alkaline electrochemical systems is their ability to overcome some of the limitations of PEM-based systems. For example, AEMFC can be fed with fuels which are easier to store than hydrogen, without depreciating their performances 25-29, which represents a major asset for automotive and mobile applications. Besides, they can use non-platinum electrocatalysts at their electrodes 30-33, although 94% of the papers dealing with AEMFC performances include Pt-based electrocatalysts 29. The possibility of using non-Pt electrocatalysts is based on two properties of alkaline electrochemistry: (i) most complex reactions are faster in alkaline than in acidic conditions (and so, many electrocatalysts can be considered to promote them) 34, and (ii) many metals/oxides are more stable at high pH 35. This however does not mean that alkaline fuel cells and electrolyzers are stable in operation 29,36,37; their materials (e.g. anion-exchange membrane 14,38-41, carbon support 42-45 and metal/oxide electrocatalyst 46-49) and assembly 50 can degrade. Regarding the electrocatalyst, when a liquid alkaline electrolyte is used, Zadick et al. did demonstrate very harsh degradation phenomena for conventional materials based on carbon-supported platinum or (unsupported) palladium nanoparticles 51-56. In this case, neither the metal nanoparticles nor their carbon support do severely corrode, but instead the interface between the metal nanoparticles and the carbon is gradually and irreversibly destroyed by local and very superficial carbon corrosion catalyzed by the metal nanoparticles, resulting in local CO2 formation. In alkali-metal hydroxide liquid electrolytes, CO2 readily transforms into aqueous and then into solid carbonates 55, precipitating especially between the metal nanoparticles and their support. In that process, it is the propensity of the metal nanoparticles (i) to accept (adsorb) surface groups of carbon (COad-like species) at low potential, and then, (ii) to oxidize them into CO2 at higher potential, which determines the rate of solid carbonate formation; the latter process induces the detachment of metal nanoparticles and related loss of 3 ACS Paragon Plus Environment

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electrochemical surface area (ECSA) 55. When the metal nanoparticles do not promote CO2 formation, i.e. when the electrocatalysts is not capable to form CO2 from COad (or to adsorb COad), or when they are unsupported on carbon (avoiding any risk of COad groups formation), the degradations are much minored and proceed via different mechanisms 51,57. To go beyond these previous findings, the present study firstly aims to demonstrate that indeed, the loss of Pt/C nanoparticles in aqueous NaOH electrolyte (the benchmark electrolyte used in the studies of Zadick et al. 51-53,56, Pt/C being a clear benchmark for AEMFC studies, see above and Refs. 29,31 ) is linked to a “CO-stripping-like” process. To that goal, tests of Pt/C electrocatalysts are performed by varying the vertex potential values of the accelerated stress test (AST). In addition, the impact of the metal loading of the Pt/C nanoparticles is investigated by using 10 wt% Pt/C instead of 20 wt% Pt/C (as used in 52,53). Secondly, ASTs are also performed at the interface with an anionexchange membrane in a so-called dry-cell 58,59, to test whether the absence of excess water and the absence of possibility to form solid carbonates do affect the mechanisms and rate of the degradation of the Pt/C nanoparticles. Although Pt/C is surely not the best alkaline fuel cell or electrolyzer catalyst, this material was nevertheless used owing to its nature of benchmark and state-of-the-art material. We believe that results and insights obtained with Pt/C as a case study in this work will represent well other metal supported carbon that may be used as electrocatalysts in electrodes of both AEMFCs and AEMWEs.

2. Experimental The materials and procedures used herein have mostly been thoroughly described in past publications 30,51-54,56. In particular, the Pt/C electrocatalyst used in this study was 10 wt% Pt/Vulcan XC72 (E-Tek), used without any treatment (hereafter noted Pt/C). The anion-exchange membrane (AEM) used in this study was based on a poly(phenylene oxide) (PPO) backbone. Commercial PPO (Sigma-Aldrich) was dissolved, brominated and then aminated with trimethylamine. The resulting solution was then cast onto clean glass plate and dried, to produce the AEM 60. This membrane was either dissolved in N-methyl pyrolidone to make an electrocatalyst ink by blending with an appropriate amount of Pt/C in water (the ink contained ca. 8.3 wt% of AEM material with respect to the Pt/C material), or used as “solid” AEM in the dry cell, after ion-exchange into the OH- form by 24 h soaking in 0.5 M NaOH at room temperature. In that case, the AEM in the OH--form was thoroughly rinsed with ultrapure water (18.2 MΩ – < 3 ppb TOC, Millipore) and inserted in the dry cell in absence of liquid NaOH. The dry cell is the same than previously used and described in 58,59. It was flushed by inert (Ar) atmosphere in both the working electrode and counter-electrode compartments, the Ar gas being fully hydrated (100 % RH) to maintain sufficient membrane conductivity. The cell was operated at room temperature (T = 25 ± 2°C). In this cell, the reference electrode is a freshlyprepared reference hydrogen electrode (RHE) in contact with the AEM electrolyte via a Luggin capillary ended by a very fine capillary (preventing any consequent leak of liquid NaOH from the RHE into the AEM electrolyte of the dry cell); the position of the RHE was ca. 1 cm away from the position of the working electrode. The counter-electrode is a Pt mesh, positioned on the other side of the AEM with respect to the working electrode. For the experiments performed in 0.1 M NaOH at room temperature (T = 25 ± 1°C), Suprapur® NaOH (Merck) was used with 18.2 MΩ – < 3 ppb TOC ultrapure water to prepare the electrolyte solution. In that case, either thin-film rotating disk electrodes (RDE) or identical-location transmission electron microscopy gold grids were prepared with the catalytic ink described above (the binder being the AEM), as detailed in 55. The counter-electrode was a glassy-carbon plate, and the reference electrode 4 ACS Paragon Plus Environment

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either a freshly-prepared reference hydrogen electrode (RHE) or a mercury-mercury oxide electrode (but all the potential values are expressed on the RHE scale). The accelerated stress tests consisted of 150 (or 1000) voltamperometry cycles (CV) performed at v = 100 mV s-1 in potential ranges of 0.1 < E < 0.6 V vs. RHE, 0.6 < E < 1.23 V vs. RHE, or 0.1 < E < 1.23 V vs. RHE, depending on the experiment considered. These potential ranges were chosen to compare the present results with past studies of the authors 53,54,56, but also because of its relevance when the electrocatalyst is used to oxidize a liquid fuel like sodium borohydride or boranes 57. The electrochemical procedures also consisted of measuring the electrochemical surface area (ECSA) before and after the ASTs, using CO-stripping voltamperograms in supporting sulfuric acid electrolyte (see 53 for details). As in Ref. 53, the CO-stripping experiments were not made for the IL-TEM characterizations, but only for the RDE tests, the Pt/C thin-film RDE being hardly altered by these measurements 52-54. The identical-location transmission electron microscopy (ILTEM) experiments were performed using a Lacey-carbon TEM gold grid (300 mesh) as the working electrode (with a deposit of Pt/C + AEM ink). The TEM observations were done using a Jeol 2010 TEM equipped with a LaB6 filament (see e.g. 53 for details). The micrographs taken in selected representative regions were used to highlight the different processes of degradation, and to quantify the extent/nature of aging. To that goal, particle size distribution histograms were drawn by counting the Pt nanoparticles in similar regions before / after the ASTs, and by differentiating (when relevant) the isolated nanoparticles form the agglomerated ones. From the ILTEM data, typical nanoparticles diameters (number-averaged: dN, surface averaged: dS, and volume averaged: dV, see e.g. for details 61) and ECSA (TEM-based ECSA, ECSATEM calculated using the value of dS) were calculated and compared to the data extracted from electrochemical experiments.

3. Results and discussion 3.1. Aging of Pt/C 10 wt% in 0.1 M NaOH (liquid electrolyte) 3.1.1. Full potential range (0.1 < E < 1.23 V vs. RHE) Figure 1A shows the evolution of the base cyclic voltamperograms of Pt/C nanoparticles monitored in the course of a short-term accelerated stress test in 0.1 M NaOH at T = 25°C in a wide potential window (0.1 < E < 1.23 V vs. RHE). As already pointed out in previous studies 52,53, the extent of degradation in these mild conditions is extremely important. The features related to the hydrogen adsorption/desorption region (below E = 0.4 V vs. RHE) and the platinum oxide region (above ca. E = 0.6 V vs. RHE in the positive scan and down to E = 0.4 V vs. RHE in the negative scan respectively) clearly and gradually depreciate in the course of the AST. Overall, the active area, monitored from CO-stripping coulometry in 0.1 M H2SO4, decreases by 61% (Table 1), in perfect match with the results obtained for 20 wt% Pt/C nanoparticles 52,53. This demonstrates that the fate of the Pt/C nanoparticles are not linked with the initial loading/size of the Pt nanoparticles at the carbon substrate. A particle size effect had been put forth in a previous work dealing with Pd/C nanoparticles 54, but in that case, the different of sizes between small and large nanoparticles were much larger than in this study (in addition, the nature of the carbon support played little role, and the most-influencing parameter was the initial shape, texture and degree of agglomeration of the Pd nanoparticles 51,56).


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Figure 1: (A) Successive cyclic voltamperograms monitored in the course of a full-range (0.1 - 1.23 V vs. RHE) potential -1 cycling of a Pt/C electrode for 150 cycles at v = 100 mV s in 0.1 M NaOH at T = 25°C and (B) corresponding CO-stripping -1 voltamperogram monitored in 0.1 M H2SO4 at v = 20 mV s before (pristine) and after the 150 CV cycles of AST.

Table 1 : Mean nanoparticles diameter for the Pt/C electrocatalyst before / after 150 CV cycles performed at v = 100 mV s-1 -1 between 0.1 and 1.23 V vs. RHE in 0.1 M NaOH or before / after 150 and 1000 CV cycles performed at v = 100 mV s between 0.1 and 1.23 V vs. RHE in interface with an anion exchange membrane at T = 25°C. All the diameters and ECSAs are calculated by taking into account only the isolated nanoparticles, except for the aging in 0.1 M NaOH after 150 cycles, where the values marked with the * take into account both the isolated nanoparticles and the agglomerates, considered in first approximation as “spherical” objects (their outer average diameter was measured). The absolute error on the diameter values is ± 0.1 nm.

dN / nm dS / nm dV / nm Number of NPs / % loss of NPs / ECSATEM (dS) / m2 g-1Pt %loss ECSATEM (dS) / ECSA / m2 g-1Pt %loss ECSA / -

Aging in 0.1 M NaOH Pristine Post 150 CV 1.9 1.7 2.4 * 2.8 3.4 5.1 * 3.3 4.1 5.8 * 402 155 65 61* 101 83 55 * 18 45 * 93 36 61

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The same AST procedure was then performed on a TEM grid to obtain identical-location TEM micrographs. As expected, the representative images of Figure 2 show a very similar trend as in 53. Briefly, the degradation occurs mostly by detachment of the Pt nanoparticles from the carbon support (see the markers highlighting the degradations, in a non-comprehensive manner not to overload the images). The degradation mechanism at stake is as follows: (i) Pt nanoparticles assist carbon surface corrosion, (ii) thereby locally producing CO2, which (iii) reacts with OH- anions into CO32- anions, ultimately (iv) generating solid Na2CO3 by precipitation with Na+ in NaOH-based


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electrolyte, (v) the Na2CO3 crystals expelling the Pt nanoparticles from the carbon support; this mechanism was recently described in 55.

Figure 2 : Representative ILTEM micrographs of Pt/C nanoparticles before (Pristine) and after 150 CV cycles performed at v = 100 mV s-1 between 0.1 and 1.23 V vs. RHE in 0.1 M NaOH at T = 25°C. The markers are not comprehensive and just illustrate the main degradation mechanisms at stake during the potential cycling procedure.

The representative ILTEM micrographs of Figure 2 and Figure S1 enable to calculate that ca. 65% of the initial isolated nanoparticles are lost after this AST (Table 1). This is in perfect agreement with the electrochemical data presented in Table 1 - when agglomerates are taken into account, the loss of nanoparticles slightly “decreases” to 61%. One can therefore confirm that for 10 wt% Pt/C as well, the main degradation mechanism in these conditions is the simple detachment of the Pt nanoparticles from the carbon support (and to non-negligible agglomeration in some areas). Dissolution and/or redeposition mechanism is also possible in these conditions, but was not demonstrated prevalent. From this scenario of degradation, one understands that Pt will only be capable to assist the corrosion of the carbon support provided the potential is scanned back and forth from a state where the Pt nanoparticles are reduced to be capable to adsorb COad-like groups from the carbon support (which is the case at E = 0.1 V vs. RHE) and another state where they can strip these COad-like species (which is the case at E = 1.23 V vs. RHE), as fully described in 55. It is anticipated that if the scanned potential window in the AST is reduced so that the Pt nanoparticle are either never completely oxidized or never completely reduced, the aging would be less dramatic. This hypothesis was verified and discussed below.

3.1.2. Low (0.1 < E < 0.6 V vs. RHE) or high (0.6 < E < 1.23 V vs. RHE) potential range Figure 3 shows that the degradation of the Pt/C electrocatalysts submitted to an AST in a reduced potential window are much lower than those monitored for 150 CV cycles of AST in the full potential 7 ACS Paragon Plus Environment

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window. The loss of electrochemical active area is ca. -24% for an AST conducted between E = 0.1 and 0.6 V vs. RHE and of ca. -30% for an AST conducted between E = 0.6 and 1.23 V vs. RHE. Besides, only in the latter case does the agglomeration of Pt nanoparticles start to be visible (via the pre-peak of CO-stripping monitored at ca. E = 0.7 V vs. RHE 62).

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Figure 3: CO-stripping voltamperogram monitored for a Pt/C electrode in 0.1 M H2SO4 before (pristine) and after 150 CV -1 cycles of AST at v = 100 mV s in 0.1 M NaOH at T = 25°C in the potential range (A) 0.1 – 0.6 V vs. RHE and (B) 0.6 – 1.23 V vs. RHE.

Representative ILTEM micrographs of Pt/C nanoparticles before and after 150 CV cycles performed at v = 100 mV s-1 between 0.1 and 0.6 V vs. RHE in 0.1 M NaOH (Figure 4, see also Figure S2) confirm the more moderate extent of degradation of the 10 wt% Pt/C electrocatalyst in these conditions. The markers illustrate the main degradation mechanisms at stake during the potential cycling procedure in the low-potential range: Ostwald ripening and loss of Pt nanoparticles do proceed, but in a much weaker manner than for the full potential range.


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Figure 4 : Representative ILTEM micrographs of Pt/C nanoparticles before (Pristine) and after 150 CV cycles performed at v = -1 100 mV s between 0.1 and 0.6 V vs. RHE in 0.1 M NaOH at T = 25°C. The markers are not comprehensive and just illustrate the main degradation mechanisms at stake during the potential cycling procedure.

In the high-potential range, the loss of Pt nanoparticles is nearly absent (Figure 5 and Figure S3). The shape of some neighboring Pt nanoparticles is nevertheless changing, but unlike for the case of the low-potential range (see Figure 4), this is not following an “agglomeration-coalescence” mechanism (local Ostwald ripening); it rather consists of the separation of initially-agglomerated Pt nanoparticles (or the opposite), which signs the surface mobility of the Pt nanoparticles on a carbon substrate which is heavily functionalized (large presence of carbon-oxide groups) in this high potential range. In that case, these differences between the low- and high-potential ranges of cycling can be understood in terms of difference of “oxidizing” conditions. In the low-potential range (“reducing” conditions), neighboring Pt particles are more prone to coalescence, in a process that can be compared to that occurring in the gas phase under H2: Coutanceau et al. 63-65 demonstrated the growth of Pt/C nanoparticles in these conditions (recrystallization), together with a “reforming” of the carbon (formation of hydrocarbon species), which enables to explain why some nanoparticles detach here. On the contrary, in the high-potential domain (“oxidizing” conditions), recrystallization of Pt is not likely, but instead, Pt and Pt-oxides can assist carbon corrosion, in a process that resembles the thermal oxidation process of Pt/C in neutral or oxidizing atmosphere 65: in this scenario Pt nanoparticles would migrate at the functionalized carbon surface until they “self-stabilize” by forming agglomerate structures.

Figure 5 : Representative ILTEM micrographs of Pt/C nanoparticles before (Pristine) and after 150 CV cycles performed at v = 100 mV s-1 between 0.6 and 1.23 V vs. RHE in 0.1 M NaOH at T = 25°C. The markers are not comprehensive and just illustrate the main degradation mechanisms at stake during the potential cycling procedure.

However, in these two cases, the extent of Pt/C nanoparticles degradation is by no means comparable that the one occurring in the full potential range (0.1 < E < 1.23 V vs. RHE) in similar experimental conditions 52,53 (Figure 1, Figure 2, Figure S1). In that case, huge loss of Pt nanoparticles 9 ACS Paragon Plus Environment

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by detachment from the carbon support is witnessed, Ostwald ripening occurring in a very minor extent. This major difference of rate and type of degradation is accounted for by the mechanism of corrosion at stake when the scanned potential domain is wide, where Pt assists the corrosion of its carbon substrate 53,55. At that stage, it can be concluded that carbon-supported Pt nanoparticles are not stable when operated in a wide potential window in liquid NaOH electrolytes (or in any other alkali-metal cation hydroxides 55, provided the pH is high enough to enable solid carbonates formation), i.e. the conditions of characterization of electrocatalytic materials for the alkaline oxidation of liquid fuels, see e.g. hydrazine 66-68, borohydride 26,69-71 or alcohols 72-74. Now, in hydrogen-fed alkaline fuel cells, no liquid alkaline anolyte solution is required and the electrolyte is a solid polymer anion exchange membrane. In these solid electrolyte conditions, one can wonder whether the degradation witnessed herein would also be at stake. To answer this question, the same AST was reproduced for the same 10 wt% Pt/C electrocatalyst operated in a “dry cell”, where the electrolyte is only a solid AEM (with no liquid NaOH solution present in the cell). The results are presented and discussed in the next section.

3.2. Aging of Pt/C 10 wt% in interface with an anion-exchange membrane (no liquid electrolyte) Figure S4A shows an example of voltamperometric trace of the Pt/C electrocatalyst in interface with an AEM, as characterized in the dry cell using an ultra-microelectrode with cavity loaded with 10wt% Pt/C as the working electrode. Figure S4B shows similar data for a gold TEM grid loaded with the Pt/C electrocatalyst during the AST performed in interface with an AEM, as performed in the dry cell. Figure 6 shows a set of representative ILTEM micrographs acquired on the same region for increasing magnifications (x 250, x 1000, x 10 000, x 50 000 and x 200 000 for two different regions in the case of the higher magnification) in the pristine state (before any electrochemistry had been performed, top pictures) and after 150 (middle pictures) or 1000 (bottom pictures) cycles of AST. The accelerated stress test consisted of repetitive voltamperometric cycles performed at v = 100 mV s-1 in the range 0.1 < E < 1.23 V vs. RHE, the working electrode being the TEM grid in interface with an anionexchange membrane in the dry cell at T = 25°C, onto which the 10 wt% Pt/C electrocatalyst was immobilized. Comparing Figure 6 with similar micrographs obtained for the same electrocatalyst in the same AST performed in 0.1 M NaOH electrolyte (Figure 2) clearly demonstrates that the extent of degradation is significantly smaller when the AST is performed in AEM-based dry cell environment than in the case of liquid NaOH electrolyte.


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Figure 6: Representative ILTEM micrographs of Pt/C nanoparticles before (Pristine) and after 150 or 1000 CV cycles -1 performed at v = 100 mV s between 0.1 and 1.23 V vs. RHE in interface with an anion-exchange membrane in the dry cell at T = 25°C. The three sequences of micrographs detail the way similar regions are located on the TEM grid, for increasing magnifications (x 250, x 1000, x 10 000, x 50 000 and x 200 000 for two different regions). The red squares are guides for the eyes to visualize in which region of the lower-magnification image, the next higher-magnification image was acquired.

This statement is even clearer on high-magnification micrographs (Figure 7 and Figure S5). Although some loss of Pt nanoparticles from the carbon surface can be detected after 150 cycles of AST in AEM interface, the amount of loss is considerable much lower than the one witnessed after AST in 0.1 M NaOH. Even after 1000 cycles of AST, the extent of nanoparticles detachment remains smaller than after a mere 150 cycles in 0.1 M NaOH, and this is clearly quantified in Table 1: after 150 cycles in the dry cell, only 13% of the Pt nanoparticles had disappeared (this number rises to 47% after 1000 cycles) versus 65% for the tests in 0.1 M NaOH (or 61% if agglomerates are taken into account in the aged electrocatalyst). Moreover, the contrast of the nanoparticles changes from one case to the other. Whereas upon AST in 0.1 M NaOH, the micrographs reveal a rather unchanged contrast of the remaining individual Pt nanoparticles, those corresponding to the Pt/C electrocatalyst aged in AEM interface are clearly “darker” and “rounder” after AST than before. This observation suggests that in the latter case, Pt dissolution and redeposition occurred in a larger extent than in the former case. This can be rationalized as follows: in liquid electrolyte, if Ptz+ ions are formed by oxidation of the Pt nanoparticles, the chance to witness redeposition of these cations on remaining Pt nanoparticles is small, because there is a large excess of liquid electrolyte and because mass-transport by diffusion of these cations is fast in a liquid, overall enabling these species to fast diffuse away from the active layer after their formation. On the contrary, the AEM likely traps the Ptz+ species in the vicinity of their formation, enabling easier redeposition when the electrode potential is appropriate for PtZ+ reduction. As a consequence, signs of redeposition (appearance of new round-shaped nanoparticles) are evident after aging in AEM interface (Figure 7 and Figure S5), those having never been encountered after an AST in liquid electrolyte (Figure 2 and Figure S1) 52,53.


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Figure 7 : Representative ILTEM micrographs of Pt/C nanoparticles before (Pristine) and after 150 or 1000 CV cycles -1 performed at v = 100 mV s between 0.1 and 1.23 V vs. RHE in interface with an anion-exchange membrane in the dry cell at T = 25°C. The markers are not comprehensive and just illustrate the main degradation mechanisms at stake during the potential cycling procedure.

The particle size distribution histograms of Figure 8 confirm the understanding of the processes at stake during the AST. On the one hand, upon AST in liquid electrolyte (Figure 8-A), the number of isolated nanoparticles does present a net decrease for all sizes (no growth at all is noticed). As a matter of fact, when the agglomerated nanoparticles (agglomerates) are taken into account (Figure S6), only the population of very large nanoparticles (diameter above 5 nm) grows, due to the severe agglomeration of individual nanoparticles that have lost their anchoring points to the carbon surface. On the other hand, in the case of an aging in AEM interface, the fraction of (initially round-shape and isolated) nanoparticles smaller than 3.6 nm gradually decreases at the expense of (round-shaped and isolated also) larger nanoparticles of diameter comprised between 3.6 and 6 nm. In addition, the fraction of very large nanoparticles (agglomerates) does barely change in that latter case. B A

200

200

NaOH Interface Pristine: 402 nanoparticles 150 CV: 140 nanoparticles (-65%)

150

Number of particles / -

Number of particles / -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

50

AEM Interface Pristine: 418 nanoparticles 150 CV: 362 nanoparticles (-13%) 1000 CV: 220 nanoparticles (-47%)

150

100

50

0

0 .2 6.

01

Nanoparticles diameter / nm

Figure 8 : Particle size distribution histograms constructed by counting isolated (not agglomerated) nanoparticles from representative ILTEM micrographs of Pt/C before (Pristine) and after (A) 150 CV cycles performed in 0.1 M NaOH or (B) 150 or 1000 CV cycles performed in interface with an anion-exchange membrane in the dry cell; in all case, the potential was -1 varied at v = 100 mV s between 0.1 and 1.23 V vs. RHE and the cell temperature was T = 25°C.

As a result, in absence of liquid electrolyte (in the dry cell), not only the extent of degradation is significantly reduced than in the case of liquid electrolyte, but also the mechanisms of degradation 12 ACS Paragon Plus Environment

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are different. This has already been put forth for the aging of Pt/C and PtCo/Vulcan XC 72 electrocatalysts in sulfuric acid or Nafion® electrolytes 58,59,75. In a similar manner than for acidic electrolytes, these marked differences can be accounted for by the absence of excess liquid water when a solid polymer electrolyte is used. As a result, for a test performed in the dry cell in interface with an AEM, the Ptz+ cations that may be formed upon sweeping the potential back and forth around the onset of Pt-oxide formation have less propensity to be diluted in the water-based liquid electrolyte and to diffuse away from the electrocatalyst surface. Therefore, re-deposition and 3DOstwald ripening become favored processes when the Pt/C electrocatalyst is aged in interface with an AEM. On the contrary, platinum re-deposition and 3D-Ostwald ripening is severely depreciated when the aging is performed in liquid electrolyte, resulting overall in a decrease of the average isolated nanoparticles sizes. In addition, one must recall here that liquid alkaline electrolytes (via the presence of OH- and alkali cations, e.g. Na+) have a peculiarity compared to their acidic counterparts: these species orient the “conventional” carbon support oxidation into CO2 towards the formation of solid carbonate species 55. This pathway, which is likely at the origin of the major detachment of Pt (and Pd) nanoparticles in liquid alkaline environments 52-54 is not possible when the electrocatalyst is operated in interface with an AEM in the dry cell: the counter-cation is immobilized at the polymer backbone and can therefore not participate to the precipitation of carbonate ions, CO32- 7677, which explains the results presented herein. In particular, severe loss of isolated nanoparticles is detected upon AST in liquid alkaline electrolyte, but also severe formation of agglomerates, because the nanoparticles lose their anchoring points on the carbon and can be ejected from their original position by solid carbonates and then collected elsewhere, which is not the case after an aging in AEM interface in absence of liquid NaOH electrolyte.

4. Conclusions The durability of a 10 wt% Pt/C electrocatalyst was evaluated in a very mild accelerated stress test (AST) procedure; two media were compared: liquid alkaline electrolyte (0.1 M NaOH) and solid polymer electrolyte (using a “dry cell”, in absence of excess liquid NaOH), both considered at T = 25°C. In the former case, the results show that the positive and negative vertex potential values do greatly influence the extent and magnitude of the degradations. When the AST is performed in wide potential domain (0.1 < E < 1.23 V vs. RHE), major detachment of the Pt nanoparticles from their support is promoted by the local corrosion of the carbon support material by its host Pt nanoparticles. This carbon corrosion into CO2 leads to the formation of firstly soluble carbonate ions (CO32-) and then solid alkali-metal carbonates (Na2CO3), the latter species forming preferentially at the interface between the carbon support and the Pt nanoparticles, and therefore mechanically expelling the Pt nanoparticles from their support. Of course, this corrosion process depends firstly on the ability of the metal nanoparticles to adsorb carbon surface groups (COad) originating from the early stages of carbon corrosion/functionalization (which happens for “reduced” metal states and oxidized carbon states, for 0.2 < E < 0.6 V vs. RHE) and secondly to the fact they can electrooxidize COad into CO2 in a Langmuir-Hinshelwood CO-stripping process (which is possible for “oxidized” metal states, for E > 0.6 V vs. RHE). Therefore, AST performed by reducing the amplitude of potential sweeping (0.1 < E < 0.6 V vs. RHE or 0.6 < E < 1.23 V vs. RHE) yield different degradation processes and rates; Ostwald ripening essentially proceeds in the former case (the Pt nanoparticles remain essentially reduced) and Pt nanoparticles agglomeration in the latter one (the Pt nanoparticles remain essentially oxidized and carbon is more subjected to corrosion). When the most severe AST (0.1 < E < 1.23 V vs. RHE) is performed in the dry cell, the magnitude and main mechanisms of degradation change compared to the liquid electrolyte environment. Because 13 ACS Paragon Plus Environment

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there is no excess water to dissolve the Ptz+ species formed by corrosion of the Pt nanoparticles, 3D Ostwald ripening and local re-deposition of PtZ+ on existing Pt nanoparticles is favored, as a result of the propensity of the anion-exchange ionomer to trap the Ptz+ species and prevent their diffusion away from the Pt/C active layer. In addition, and importantly, because there is no alkali metal cation to precipitate the carbonate ions formed by corrosion of the carbon support, the detachment of the Pt nanoparticles from their support is not favored (the detachment being promoted by the formation of solid carbonates at the interface between the carbon support and the Pt nanoparticles). Overall, Pt/C is much more stable when operated in interface with an AEM than in liquid alkaline electrolyte. This study demonstrates that the degradation processes not only depend on the nature of the electrocatalyst, but also on the vertex potential values scanned in the AST and finally (and very importantly) on the nature of the electrolyte medium investigated. Thankfully, these results show that the very dramatic degradations experienced in liquid electrolyte for carbon-supported Pt nanoparticles are somewhat mitigated in solid alkaline electrolyte, which harbor the hope of developing durable anion-exchange membrane fuel cell and electrolyzers using (noble) electrocatalyst materials.

5. Acknowledgements The authors thank the US office of naval research global (ONRG) for funding the PhD thesis of CL (grant number N62909-16-1-2137). This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” n◦ AN-10-LABX-44-01. MC thanks the French IUF for its support. DRD thanks the kind financial support of the 2nd Israel National Research Center for Electrochemical Propulsion (INREP2-ISF).

6. Supporting information Representative ILTEM micrographs in conventional 3-electrode cell and in the dry cell; representative voltamperograms monitored in the dry cell; particle size distributions histograms determined from the ILTEM micrographs.

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C Nanoparticles in an Interface with an

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