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Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures Yawei Li, James L Hart, Mitra L. Taheri, and Joshua D. Snyder ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02398 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures Yawei Lia, James L. Hartb, Mitra L. Taherib, Joshua D. Snyder*,a a

Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States

b

Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States * Corresponding Author: [email protected] Keywords: Coarsening, Fuel Cells, Oxygen Reduction Reaction, Durability, Nanoporous Metals, Electrocatalysis Abstract Advances in electrocatalyst functionality have resulted from the evolution of complex nanostructured materials with increasing degrees of compositional and morphological complexity. Focused almost entirely on pushing the boundaries of intrinsic activity, electrocatalytic material development often overlooks stability. Operating in parallel to the typical mechanisms of electrochemical material degradation, three-dimensional nanomaterials are susceptible to an additional degradation process known as coarsening. Driven by the reduction of surface free energy, surface diffusion evolves the nanoporous morphology toward a solid spherical particle. Here, using nanoporous NiPt alloy nanoparticles (np-NiPt/C) as a representative three-dimensional electrocatalytic material, we demonstrate that coarsening is the dominant mechanism of degradation as observed during accelerated durability testing (ADT). The upper potential limit (UPL) of the ADT protocol is found to have a significant impact on coarsening, with the rate roughly scaling with the UPL. Here we demonstrate the viability of a methodology to limit the coarsening

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process by decoration of the surface with a foreign metal impurity, Ir, possessing a lower surface diffusivity than the catalytic species. Ir, present in a low coverage with negligible impact on the intrinsic activity, dramatically slows morphology evolution. This strategy is shown to result in significant improvements in the electrochemically active surface area and transition metal alloying component retention up to a UPL of 1.1 V versus the reversible hydrogen electrode (RHE). This proof-of-concept result demonstrates the utility of this strategy for improving the balance between activity and stability for threedimensional electrocatalytic nanomaterials with potential application to a broad range of nanoscale geometries and compositions. 1. Introduction Electrochemical energy storage and conversion technologies are central components to the carbon neutral renewable energy portfolio. The increase in their integration into consumer and industrial applications is predicated on technological advancements leading to improved efficiency and longevity. Polymer electrolyte membrane fuel cells (PEMFCs), which generate power through the extraction of electrons from chemical bonds, are a key electrochemical energy conversion component due to their superior efficiency, ~ 40 % well-to-wheel1, and ability to use a wide range of electrochemical fuels2–5. Superficially, the limiting factor for the wide-spread integration of PEMFC technologies is related to cost6–8, ignoring societal and political factors as well as the requisite need for equally efficient fuel generating technologies. To this end, significant effort has centered on the development of new, inexpensive material replacements for fuel cell components including the solid electrolyte membrane9,10, bipolar plates11,12, and anodic/cathodic electrocatalysts6,13,14. Electrocatalyst development has a high degree of 2 ACS Paragon Plus Environment

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visibility as both the commercial standard and state-of-the-art electrocatalysts are composed of Pt which is both expensive and has limited availability. The cathodic oxygen reduction reaction (ORR) is the limiting reaction in the fuel cell, necessitating high Pt contents in order to achieve desired power densities6,15,16. To date, significant advancements in ORR electrocatalyst development have yielded materials with improvements in both intrinsic and mass based activity through the evolution of nanoscale

morphology,

composition,

and

compositional

profile17–22.

These

electrocatalytic advancements have yielded a steady climb toward the critical activity necessary to lower the Pt content in the fuel cell to cost-competitive levels23–25. The emergence of a more complete fundamental understanding of the property-function relationships in ORR electrocatalysts has led to dramatic improvements in operational activities. The presence of a transition metal alloying component in a high concentration underneath a Pt-skin passivated surface7,17,18,22,26–28 facilitates manipulation of the surface lattice strain (geometric effects) and the d-band structure of the active surface Pt atoms (electronic effects)6,7,29. The overall result of these combined effects is the optimization of the free energy of adsorption of the reactants, intermediates, and products, improving turnover rate on a higher density of free active sites6,7,17,18,22,26–29. In a real device where cost is directly tied to precious metal utilization, an improvement in only the intrinsic/specific activity is not sufficient. To this end, significant improvements to Ptbased mass activities have been achieved through the development of unique, threedimensional, open, bicontinuous morphologies that increase the surface-to-volume ratio of nanoparticulate materials and improve precious metal utilization16,19,20,30–35. Nanoporous metals represent an important subset of morphologically complex, three3 ACS Paragon Plus Environment

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dimensional, porous materials, exhibiting unique mechanical36,37 and catalytic16,19,20,31,38– 41

properties that are directly tied to their length scale and high surface-to-volume ratios.

An unmitigated pursuit of higher and higher mass and specific activities, however, is often at the expense of material stability. Both the addition of more electrochemically active alloying species (Ni, Co, Cu, etc.) and complex morphologies characterized by small length-scale features and high radii of curvature often negatively impact durability. An inverse relationship between activity and stability is known to govern catalytic material behavior in an electrochemical environment26,42,43; active catalysts are not stable and stable catalysts are not active. It is critical, therefore, to develop a more detailed understanding of the impact of catalyst composition and morphology on the balance between activity and stability, particularly with regards to insight into the evolution of catalyst degradation

mechanisms with increasingly complex

three-dimensional

morphologies and compositional profiles. A significant fraction of the loss in maximum power density of PEMFCs following repetitive load cycling can be tied to the loss in intrinsic activity and electrochemically active surface area (ECSA) of the electrocatalyst, the rate of which is higher for the cathodic catalyst layer due to the operational potentials being in the oxidative regime for Pt44. The two common mechanisms invoked for the ECSA and overall activity loss of solid

nanoparticulate

electrocatalysts

are

agglomeration/sintering45–47

and

dissolution/Ostwald Ripening45–50. Additionally, with the increasing implementation of alloy nanoparticles, the gradual loss of the underlying transition metal alloying component, especially at the early stages of catalyst life8,24,25,51,52, through surface segregation26,53 or potential driven surface roughening54–56, results in a loss in intrinsic 4 ACS Paragon Plus Environment

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activity as well as an increase in the ionic resistance of the PEM57–59. For threedimensional, morphologically and topologically complex materials, there is an additional mechanism of material degradation. Porous materials are perpetually in a state of metastability. Due to their high surface-to-volume ratios, the thermodynamic desire to reduce this ratio drives a process known as coarsening. Coarsening, which may be made analogous to surface smoothening, is driven by surface diffusion currents proportional to gradients in the local chemical potential; these gradients are determined by the local curvature of the surface60–63. Erlebacher et al. investigated the morphology evolution of nanoporous metals, which are composed of areas of high positive and negative radius of curvature, during thermally driven coarsening using kinetic Monte Carlo (KMC) simulations64. It was illustrated that surface energy driven smoothening promotes diffusion of surface atoms to minimize the radius of curvature and lower the overall material surface area, driving evolution of the particle morphology from a tortuous porosity to the solid equilibrium Wulff shape64–67. While the electrochemical instability of nanoporous nanoparticles has previously been observed44,52,68, there have been no notable studies of the mechanism of, nor proposed strategies to mitigate, nanoporous metal coarsening under electrochemical conditions. Here, surface metal atoms are not only free to diffuse but also dissolve with both processes being influenced by the potential dependent interaction with water and supporting electrolyte ions. While interesting from a fundamental perspective, coarsening is undesirable as it results in the loss of the most compelling feature of nanoscale porous materials, namely their high surface-to-volume ratio. Additionally, through surface diffusion driven evolution,


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coarsening is also a mechanism for the exposure and loss of alloying components which greatly impacts the intrinsic electrocatalytic activity of the materials. Through an experimental study of the evolution of np-NiPt/C, better insight into morphological and compositional instability associated with these three-dimensional, bicontinuous, nanoporous structures is developed. Using a combination of in-situ and exsitu experimental techniques to assess the structural and compositional evolution of the nanoporous nanoparticles during accelerated durability testing (ADT), Department of Energy Fuel Cell Technologies Office protocol69, we have correlated the loss in ECSA and activity to the evolution of nanoporous morphology. We purport this morphology evolution to be the dominant mechanism of performance degradation in threedimensional, morphologically complex nanocatalysts. Additionally, we present a strategy to mitigate coarsening in open framework, porous nanocatalysts. The addition of a foreign metallic species, Ir, in sub-monolayer quantities on the surface impedes step edge movement, slowing the overall diffusive flux of material along the surface. This inhibits the mechanism by which the morphology evolves and imparts a dramatic improvement in durability as evidenced by the retained ECSA and ORR activity after ADT. 2. Methods 2.1. Nanoparticle and Catalyst Synthesis The precursor Ni80Pt20 alloy nanoparticles were synthesized through an organic solvothermal reduction method16. Ni(acac)2 (0.80 mmol), oleylamine (4 mL) and 1,2tetradecadeniol (TDD, 0.5 mmol) were initially introduced into 10 mL diphenyl ether (DPE) at 100 °C and then heated up to 180 °C in an Ar atmosphere. Following several 6 ACS Paragon Plus Environment

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vacuum/Ar purging cycles a solution of 0.20 mmol Pt(acac)2 and 3 mmol adamantanecarboxylic acid (ACA) in 3 mL dichlorobenzene (DCB) was injected into the stirring solution at 180°C and then heated to 225 °C. After holding the temperature for 1 hour at 225 °C, the solution was cooled to room temperature under flowing Ar. The formed nanoparticles were centrifuged at 8000 rpm, washed with hexane/ethanol and, finally, deposited onto carbon support (Vulcan XC-72) through a colloidal deposition process. After centrifugation and three washing cycles with hexane/ethanol, the as-made catalyst was annealed in a tube furnace at 180 °C in air for 1 hour followed by 400 °C in H2/Ar for 2 hours. The metal loading, determined through thermogravimetric analysis (TGA), was found to be 20 wt.% metal on carbon for the as-synthesized Ni80Pt20 alloy particles and 13 wt.% metal on carbon for the dealloyed np-NiPt/C catalyst. 2.2. Electrochemical Measurements The as-annealed nanoparticle catalysts were dealloyed, electrochemically characterized, aged, and assessed for ORR activity in a three-electrode cell using a rotating disk electrode (RDE) setup from Pine Instruments controlled by a Metrohm Autolab potentiostat (PGSTAT302N). The counter electrode was Pt mesh (99.9%, Alfa Aesar) bonded to the end of a Pt wire (99.9%, Alfa Aesar). The Ag/AgCl (BASi) reference electrode was calibrated against a hydrogen reference and found to have an offset of 0.27 V at 25 °C for 0.1 M HClO4. All potentials listed are referenced to the reversible hydrogen electrode (RHE). Prior to any electrochemical experiments, all glassware was cleaned by soaking in a solution of concentrated 1:1 H2SO4:HNO3 for at least 8 hours followed by rinsing and boiling in Millipore (Milli-Q Synthesis A10) water.


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The thin film catalyst layers, 15 µgPt cm-2 loading, were formed on glassy carbon (GC) disks (5mm diameter, 0.196 cm2) by drop casting from a catalyst ink and drying under a flow of Ar. The catalyst ink was composed of a 4:1 H2O:IPA volume ratio solvent solution with a solid catalyst concentration of 1 mgcatalyst ml-1. Additionally, 0.5 µL of Nafion 5 wt.% solution (Ion Power LQ-1105 1100 EW) per mg of catalyst was added to the ink (ionomer/carbon mass ratio ≈ 1: 37.2) to aid in dispersion and adhesion of the catalyst particles to the GC substrate. As a reference, commercial Pt/C (20 wt.% Pt, Fuel Cell Store) was also tested. Electrochemical dealloying of catalysts, to form np-NiPt/C, was accomplished in Arpurged 0.1 M HClO4, in a standard three-electrode electrochemical cell, by cycling the potential between 0.05 and 1.2 V vs. RHE at 250 mV s−1 until the cyclic voltammetry (CV) curve reached a steady state, at least 50 cycles. ADT consisted of 10,000 triangular wave potential cycles between two specified potential limits in Ar-purged 0.1 M HClO4 with a sweep rate of 50 mV s-1. The evolution of the catalyst ECSA was determined as a function of cycle number through the integration of the current in the hydrogen underpotential deposition (HUPD) region of the CVs. For the ORR activity measurement, the dealloyed catalyst, either before or after ADT, was transferred to a three-electrode electrochemical cell containing O2 saturated 0.1 M HClO4 at 25 °C. ORR polarization curves were recorded at 1600 rpm while running linear sweep voltammetry between 0.1 and 1.1 V vs. RHE at 20 mV s-1. All potentials are corrected for iR drop within the electrochemical cell. Partial monolayers (ML) of Ir were deposited on the surface of the np-NiPt/C nanoparticles through the galvanic displacement of Cu. First, a partial ML of Cu was 8 ACS Paragon Plus Environment

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deposited on the surface of the nanoporous nanoparticles through underpotential deposition in a 1 mM CuSO4 + 0.1 M H2SO4 solution at a constant potential of 0.30 V vs. RHE, see Figure S1(A) and S1(B). After formation of the partial Cu ML, the catalyst layer was immersed in a solution of 0.025 mM IrCl3 at open circuit potential to drive the galvanic displacement of Cu with Ir. After galvanic displacement of the Cu, the Ir-doped nanoporous nanoparticles were washed with DI water prior to further testing. 2.3. Morphological and Compositional Characterization Transmission electron microscopy (TEM) (JEOL JEM-2100) and scanning TEM (STEM) (JEOL JEM-2100F with a Shottky source) were performed at 200 keV to visually characterize the microstructure of the nanoparticles. STEM Energy-dispersive X-Ray spectroscopy (EDS) (Oxford EDS silicon drift detector) was used to measure Ni and Pt fractions and as well as generate elemental maps. STEM analysis was conducted with a probe size of approximately 1 nm and a high angle annular dark field (ADF) detector with inner and outer detection semi-angles of 27 and 54 mrad, respectively. The average compositions of np-NiPt/C particles in a thin film comprising the catalytic electrode, both before and after ADT, were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo Scientific iCAP 7000). Additionally, the amount of metal, Pt and Ni, transferred to the electrolyte during ADT through catalyst dissolution was quantified with post-mortem ICP-OES testing of the electrolyte. Identical location TEM (IL-TEM) was used to qualitatively track the change in nanoporous nanoparticle morphology as a function of cycle number during ADT. A Au TEM grid (Pacific Grid Tech) with a carbon supportive film was used as both the working electrode for ADT and


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material support for TEM analysis. The standard three-electrode electrochemical setup with the catalyst loaded TEM grid as the working electrode is shown in Figure 1. 3. Results and Discussion The formation of nanoporosity through electrochemical dealloying of carbon supported Ni80Pt20 precursor nanoparticles is visualized in the before and after dealloying IL-TEM images in Figure 2. The process of dealloying and porosity evolution relies on the potential dependent rate of surface diffusion of the more noble component, exposing the underlying percolation of the less noble, more electrochemically active species, and propagating the etch front and porosity into the bulk of the precursor material. Nanoporosity evolution through dealloying of both bulk and nanoscale precursor materials has been well described previously16,61,70,71. The np-NiPt/C nanoparticles have a “beginning of life” Ni content of approximately 60 at. % residual composition after dealloying. This high residual Ni content, however, is not stable and at the early stages of ADT, the average Ni content quickly falls to approximately 25 – 30 at. % at which point the surface is sufficiently passivated in Pt and the overall composition changes at a much slower rate over the course of the ADT. The nanoporous nanoparticles possess a morphology comprised of an interconnected ligamentous structure where the nanoscale features are characterized by high radii of negative and positive curvature. It is this gradient in curvature that drives the evolution of the nanoporous morphology toward that of the lowest energy configuration, minimizing the surface-to-volume ratio. Previous assessment of nanoporous Pt-based nanoparticles highlighted their inadequate stability at ORR operating conditions due to their inherently meta-stable structure44,52,68. We also observe the morphological instability of these nanoporous nanoparticles at ORR 10 ACS Paragon Plus Environment

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conditions, Figures 1 and 4, and its impact on ORR performance metrics, Figure 3. As shown in Figures 1 and 4, if given sufficient driving force for the continuation of surface diffusion of metallic species post-dealloying, in this case through applied potential in an aqueous electrolyte, curvature accelerated smoothening driven by the desire to reduce the overall surface free energy causes these nanoporous structures to coarsen over time. Coarsening is characterized by the thickening of the ligaments, increase in pore size and an overall decrease in surface-to-volume ratio. In addition to the loss in ECSA, evolution of the nanostructure through movement of surface metal atoms leads to exposure of the underlying alloying component, Ni in this case, resulting in its dissolution upon exposure to the electrolyte. This gradual loss in transition metal alloying component occurring concomitantly with the loss in ECSA, results in a continual decay in the intrinsic and mass based activity for the electrocatalysts during ADT, Figure 3. The evolution of np-NiPt/C morphology and composition during ADT at a range of upper potential limits and its effect on electrocatalytic performance is characterized by a combination of IL-TEM, ECSA measurements, ICP-OES digestive and electrolyte analysis, and ORR activity metrics. From the qualitative and quantitative assessment metrics presented in Figures 3 – 7, it is shown that the rate and degree of material degradation, in the form of ECSA loss due to coarsening and transition metal loss due to the evolution of the surface and exposure of underlying Ni, is a function of the upper potential limit (UPL) applied during ADT. Figure 4 contains a series of IL-TEMs for representative np-NiPt/C nanoparticles as a function of both UPL and ADT cycle number. It is clear from this series of IL-TEMs that the UPL for the ADT in a pH = 1 electrolyte has a substantial effect on the morphological stability of these materials. With an UPL of 11 ACS Paragon Plus Environment

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0.9 and 1.0 V vs. RHE, negligible change in the ECSA, Figure 5, and nanoporous morphology, Figure 4, is observed. UPLs of 1.1 and 1.2 V vs. RHE, however, result in significant losses in ECSA with the percentage loss at 1.1 V for np-NiPt/C being in line with that for a traditional Pt/C electrocatalyst, Figure 5. We can qualitatively assess the link between ECSA loss and morphology evolution with IL-TEM, Figure 4. The degradation of the nanoporous morphology, especially at an UPL of 1.2 V, matches well the expected evolution of morphology with continually facilitated surface diffusion as shown through Kinetic Monte Carlo simulations of the thermal coarsening process64. In the thermal case, elevated temperatures help lower-coordinated atoms on the highlycurved surface to overcome the activation barrier for surface diffusion. Moving along chemical potential gradients established by the gradients in curvature, material moves from areas of positive curvature to negative curvature. The ultimate conclusion of the process is the evolution of the nanoporous structure into a solid particle with an equilibrium Wulff shape64. In contrast to thermal coarsening64–67,70,72, here structural evolution is driven at room temperature and the origin of enhanced surface diffusion rates, ability to surmount the activation barrier for surface diffusion, is no longer thermal in nature. Room temperature diffusivity/diffusion coefficients for metals such as Pt and Au are on the order of 10-19 cm2 s-1 as extrapolated from temperature dependent surface smoothening measurements in ultrahigh vacuum (UHV)73. The rate of surface diffusion governed by a diffusivity of this order of magnitude is not sufficient to evolve porosity during dealloying or effect any degree of coarsening at electrocatalytically relevant conditions. The influence of water and electrolyte, however, is found to dramatically enhance the rate of surface 12 ACS Paragon Plus Environment

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diffusion73,74. This rate is potential dependent73,74. In the presence of halide anions, the surface diffusivity of Au is found to increase by at least 5 orders of magnitude as compared to that measured in UHV73,74. The source of this enhanced rate of surface diffusion is the formation of an [anion]-[metal] complex on the surface that cooperatively weakens the interaction between the surface metal atoms and their subsurface coordinated atoms73,74. The degree of interaction is dependent upon the identity of the anion, halides are known to dramatically enhance surface diffusion74, and the applied potential73,74. The higher the applied potential is above the point of zero charge (pzc), the greater the degree of interaction between anion and surface metal atoms, both in terms of strength of interaction and number of ions coordinated to the metal73,74. This is the origin of the potential dependent surface diffusion rates and one of the hypothesized origins of the apparent increased rate of coarsening observed for the higher UPLs, Figures 4 and 5. We concede that this description of the electrochemical mechanism of coarsening is over simplified. In addition to surface diffusion of low coordinated species, higher UPLs increase the probability of dissolution of material as well as the formation of a passive hydroxide/oxide film. Both the dissolution of material and reduction of an oxidized surface layer during the cathodic potential sweep result in the creation of low coordinated surface atoms. These adatoms and other low coordinated species will seek step edges to increase their coordination; the thermodynamically dictated equilibrium coverage of adatoms on a metallic surface is infeasibly low55, driving fast step edge incorporation following defect formation. The continuous formation of low coordinated defects and their subsequent incorporation into a neighboring step-edge gives the appearance of stepedge movement driven evolution of the morphology. An obvious coarsening mitigation


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strategy then becomes developing techniques or processes that can stop this step edge movement, pinning the step edges and preventing evolution of the morphology and structure through a surface diffusion driven mechanism. The observed evolution of morphology of the np-NiPt/C catalysts, Figures 1 and 4, can be correlated to the loss in electrocatalytic activity metrics including ECSA and ORR activity, Figures 5 and 6. While mass activity is directly correlated to the evolution of the morphology toward a decreased ECSA with a potential dependent rate, the reduction in specific activity is contributed to by additional factors. Post-ADT analysis of the “aged” electrolyte and digested catalyst materials through ICP-OES, Figure 7, shows the potential dependent dissolution of alloy components into the electrolyte as well as the change in catalyst composition. With an increase in the UPL from 1.0 V to 1.2 V vs. RHE, an increase in the concentration of both Pt and Ni in the electrolyte is observed. The absence of a trend in the Ni atomic fraction remaining in the catalyst after ADT, particle digestion analysis, with the UPL is likely due to the increased rate of Pt dissolution at 1.2 V as indicated by the increased amount of Pt found in solution, Figure 7(B). This lowers the observed decrease in Ni proportionate to Pt for the highest UPL. We argue that the bulk of the Ni loss occurs at early times in the ADT testing as the high Ni content in the as-dealloyed particles, upwards of 64 at.%, Figure 8, is quickly removed and the electrocatalyst reverts to a more chemically and thermodynamically stable composition, ~25 – 30 at.% residual Ni, as observed for other Pt alloy nanomaterials studied previously13,16,19–21,44. At this composition, the shell of Pt passivating the surface of the porous ligaments is sufficient to protect the underlying Ni, where without the evolution of the structure, Ni loss must occur through solid state 14 ACS Paragon Plus Environment

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diffusion of Ni to the surface, the rate of which is low at room temperature16. From the ICP-OES and EDS analysis, we find the Ni composition, after an early break-in period during ADT, to be between 20 and 30 at.%, which agrees with this assessment, Figures 7 and 8. Once the stable compositional configuration is reached through leaching of a majority percentage of the near surface Ni, the mechanism of further Ni loss is mainly due to the evolution of the nanoporous morphology/topology. Noble metal diffusion and overall movement of the surface exposes underlying Ni as the surface-to-volume ratio decreases. Regardless of the mechanism of Ni loss, the depletion of Ni near the Ptskinned surface diminishes both the geometric and electronic effect on the surface specific adsorption properties, decreasing the intrinsic ORR activity of the surface. Additionally, the shift from a generally high indexed surface to one that is defined by surface site orientations that have a higher degree of coordination as the highly porous particles convert to a more spherical shape, will result in a decrease in the density of more optimal ORR active site geometries75 and effectively decrease the intrinsic activity of the surface as the morphology evolves. These effects of material coarsening combine to result in a decrease in the intrinsic activity of the surface which is represented by the ECSA normalized specific activity and combine with the decrease in ECSA to result in a steady degradation in the mass activity. Taken together, the assessment of the stability of np-NiPt/C supports our hypothesis of a coarsening dominated mechanism for catalyst and ORR performance metric degradation. It stands then, that if coarsening can be slowed or even stopped, dramatic improvements in catalyst stability can be achieved. As mentioned previously, during coarsening, the potential dependent enhancement in surface diffusivity facilitates movement of material 15 ACS Paragon Plus Environment

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from areas of positive curvature to negative curvature. This is manifested as a general movement of step edges along the curved surface. We propose to stop this motion and hence slow or even stop coarsening by doping the surface with a low coverage of an impurity species that possesses a reduced diffusivity in comparison to Pt. In addition to the diffusivity criteria, for this impurity species to be effective, it must have an acceptable degree of electrochemical stability to avoid being dissolved at anodic potentials. Diffusivity of zero valent metals roughly scales with melting point; diffusivity values are found to depend on the ratio ܶ/ܶ݉ where ܶ݉ is the melting point of the metal76,77. Iridium is a relatively passive metal that is slightly more oxophilic than Pt. Iridium has a higher melting point by ~680 °C78 suggesting the rate of diffusion at anodic potentials is lower than that for Pt. Measured values of the diffusion coefficient for surface self-diffusion under high vacuum conditions indicate an approximately two order of magnitude difference between Ir and Pt77,79,80. We hypothesize that the presence of Ir, in a partial ML coverage, will act to pin receding step-edges, driven by curvature gradients, due to its low rate of diffusion. By limiting the degree of step-edge movement, Ir will effectively stop the evolution of the nanoporous morphology toward that of a solid spherical particle and prevent the concomitant decrease in the active surface-to-volume ratio. There is previous precedence pointing to the potential viability of the proposed strategy. The addition of a small fraction of Pt, ~5 at.%, to a Ag-rich AgAu alloy resulted in an increase in the critical potential for the evolution of porosity through dealloying and a decrease in the average pore/ligament diameter, from 25 nm to ~2 nm with the addition of Pt81, as comparted to the traditional dealloying of AgxAu1-x for the formation of nanoporous gold (NPG)56,60,70,71. The shift in dealloying behavior and final material


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morphology is due to the presence of the impurity Pt with a surface diffusivity orders of magnitude lower than that of Au81. The increase in the critical potential is due to the lower rate at which step edges move at the etch front to expose the underlying less-noble Ag. Higher potentials are then required to initiate movement of the Pt passivated steps and propagate the etch front further into the bulk of the alloy. As the porosity evolves, slow moving Pt enriches on the surface. Homogeneous decoration of the porous structure is found to impart a dramatic improvement in morphological stability, both thermally and electrochemically81. What is particularly striking about this result is that significant morphology stabilization can be obtained through the addition of a very small fraction of impurity species. A partial ML of Ir is evenly distributed on the surface of np-NiPt/C, Figure 9, through the surface limited redox replacement (SLRR)82,83 method involving the galvanic displacement (GD) of underpotentially deposited (UPD) Cu on the surface of the nanoporous nanoparticles. SLRR is a routine procedure for the formation of precious metal skins on metallic core nanoparticles28,82–84. The replacement of Cu2+ with Ir3+ results in only a partial ML coverage with Ir located at lower-coordinated defects on the surface, passivating sites on the step edges. EDS and ICP-OES analysis of a sample of particles indicates an Ir fraction of approximately 5 at.%. Elemental EDS mapping of an Ir decorated np-NiPt/C nanoparticle shows an even distribution of a small fraction of Ir over the surface of the particle, Figure 9. Ir, being more oxophilic than Pt, is known to have a significantly lower specific activity for the ORR in acidic electrolytes compared to Pt85,86. Comparing the ORR polarization curves for as-dealloyed np-NiPt/C and npNiPt/C + Ir, Figure 10, a small decrease in ORR activity is observed; however, for the 17 ACS Paragon Plus Environment

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purposes of this proof-of-concept test, this degree of deactivation is not considered to be significant. Figures 11 and S2 contain a series of IL-TEMs taken during ADT of the Ir-decorated npNiPt/C nanoparticles. It is clearly evident from this series of images that the presence of a partial ML of Ir on the surface of the nanoporous nanoparticles, in contrast to the absence of Ir, results in a negligible change in morphology after 10,000 ADT cycles with an UPL of 1.1 V vs. RHE. In Figures 6 and 12, the enhanced stability of the Ir doped nanoporous nanoparticles is quantified by a significant enhancement in the ECSA retention at 1.1 V UPL and a negative percent reduction in ORR activity. This negative value indicates that with the presence of a partial ML of Ir, the activity improves after ADT in comparison to the initial np-NiPt/C + Ir catalyst. We propose that this improvement is not due to some optimization of the catalyst surface or compositional profile during ADT, rather it is due to a slow loss of Ir. According to the Pourbaix diagram87 for Ir, 1.1 V vs. RHE and pH = 1 is right at the transition of immunity and passivation. Dissolution may then be possible during repetitive reduction/oxidation cycles, however likely occurring at a slow rate. Post-ADT analysis of the solution through ICP-OES did not indicate any presence of Ir above the detection limit; however, this does not mean that Ir could not have been dissolved into solution, rather, the total amount of Ir present on the catalyst film was not sufficient to surpass the detection limit of the ICP-OES. Ir being a poor ORR catalyst, its slow loss over the course of the ADT brought the ORR specific activity back to that of the unmodified np-NiPt/C catalyst at pre-ADT testing levels. Post-ADT ICP-OES digestion analysis of the np-NiPt/C and np-NiPt/C + Ir catalyst films, Figure 13, shows that the presence of Ir helps to retain a higher fraction of Ni. We argue that this is due to 18 ACS Paragon Plus Environment

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the limitation of surface movement and further exposure of underlying Ni. By preventing the movement of surface Pt atoms and the evolution of the nanoporous structure, we retain a fraction of Ni near the stable Pt3Ni composition13,16,19–21,44 where a sufficient percolation to initiate Ni loss beyond the first few atomic layers is not present. In Figure S3, we report the concentration of Pt in electrolyte after ADT with an UPL of 1.1 V vs. RHE for both np-NiPt/C and np-NiPt/C + Ir. The Ir protected catalysts displayed a 20% drop in the amount of Pt that was dissolved into solution. While this drop is significant, it is an indication that Pt dissolution still occurs in the absence of morphology evolution. And this result is intuitively satisfying; Ir is more oxophilic than Pt, so it is not expected to impart any degree of enhanced electrochemical stability to the exposed surface Pt, in contrast to what is observed with the addition of more noble metals to the surface13,88–91. The addition of Au to the surface and subsurface of Pt particles is found to change the electronic and electrochemical properties of neighboring Pt, increasing their nobility and resistance to dissolution at anodic conditions13,88–91. However, we do observe a slight decrease in the degree of Pt loss due to electrochemical dissolution as by limiting the degree of surface movement of Pt, we are limiting the continuous formation and annihilation of defected, low coordinated, Pt atoms which are more susceptible to dissolution due to their lower atomic coordination. Briefly, we discuss the observed behavior for np-NiPt/C + Ir at an ADT UPL of 1.2 V vs. RHE. At this UPL, the driving force for Pt dissolution is much greater than at 1.1 V. Therefore, Pt dissolution plays a greater role in the degradation of the catalytic material. With an UPL of 1.2 V, we find that Ir does little to limit the degradation of the catalyst whereby coarsening and Ni/Pt loss are significant and the ORR activity degradation after 19 ACS Paragon Plus Environment

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ADT is the same regardless of the presence of Ir. At 1.2 V vs. RHE and pH = 1, Ir is expected to form a stable insoluble oxide, IrO2, according to the Ir Pourbaix diagram87,92 and supported by surface sensitive measurements of Ir during the oxygen evolution reaction93–95. However, assessment of the UPL dependence of Ir dissolution under potentiodynamic conditions indicates a jump in the degree of Ir loss during the cathodic sweep when going from a UPL of 1.1 V to 1.2 V

96

. Therefore, the rate of Ir loss,

especially at partial ML coverages, is expected to increase with the increase in ADT UPL. This results in the quick loss of Ir and the negligible impact of Ir on nanoporous nanoparticle stability at a UPL of 1.2 V. As the UPL increases, the dominant mechanism of material degradation for these nanoporous nanostructures likely shifts from coarsening to dissolution as we move further and further above the equilibrium potential for Pt. Focusing on the observed stabilization at an UPL of 1.1 V vs. RHE, which is already above the suggested UPL for metallic catalyst degradation ADT protocols from the Department of Energy69, the presented results demonstrate that for these topologically complex nanostructures, morphological evolution is the dominant mechanism of ECSA and transition metal loss and consequently mass/specific activity loss. This is a significant result as it suggests that rather than adding more protective layers of the more noble component (Pt) to the surface, which would decrease both mass and specific activity, we can improve transition metal alloy component retention and overall catalytic material integrity for complex, three-dimensional electrocatalysts by slowing the evolution of the material morphology. While the stabilizing capability of Ir is limited in terms of time and UPL, these results are a convincing proof-of-concept, indicating that dramatic improvements in electrocatalytic stability can be achieved by focusing on 20 ACS Paragon Plus Environment

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strategies that limit the evolution of nanostructured morphology and can do so with minimal impact to activity, breaking away from the traditional inverse relationship between activity and stability. 4. Conclusions In conclusion, through in-situ and ex-situ assessment of morphological and compositional evolution of nanoporous nanoparticle electrocatalysts during ADT and its effect on ORR performance metrics, specific/mass activity and ECSA, we investigate the convolution of electrochemical dissolution and surface energy driven coarsening mechanisms during electrocatalyst degradation for complex, three-dimensional nanoporous nanostructures. By addressing the cause and effect of electrochemically enhanced surface diffusion that drives the coarsening process, we developed a strategy to minimize this morphology evolution by impeding step edge movement using foreign metal adsorbates on the surface. With a lower intrinsic rate of surface diffusion, decoration of the step edges of np-NiPt/C particles with Ir dramatically decreases morphological evolution and transition metal loss. The enhancement in durability and retention of the three-dimensional morphology can be attributed to the lower rate of surface diffusion of the Ir surface dopant, surface diffusion rates of metals roughly scale with melting point. The slower moving dopant species acts to pin step edges, preventing their movement and limiting the rate and degree of coarsening. In this manuscript, we have shown that surface evolution and coarsening of the three-dimensional morphology is the dominant mechanism of ECSA loss during electrochemical accelerated durability testing. The results presented here emphasize a different approach to address the morphological and compositional stability of complex, three-dimensional, nanoscale electrocatalytic materials where prevention of surface 21 ACS Paragon Plus Environment

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diffusion and coalescence of the active species can dramatically improve ECSA retention and general durability of the nanostructure. Supporting Information Figure S1: Cu UPD on np-NiPt/C Figure S2: IL-TEM series of np-NiPt/C + Ir during ADT Figure S3: ICP-OES measurement of Pt in electrolyte after ADT Table S1: Table of ORR activity metrics before and after ADT Acknowledgements The authors would like to acknowledge Samantha L. Shumlas and Daniel R. Strongin for assistance with the ICP-OES measurements. Authors MLT and JLH gratefully acknowledge funding from the National Science Foundation’s Major Research Instrumentation Program under award #1429661.

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Cherevko, S.; Reier, T.; Zeradjanin, A. R.; Pawolek, Z.; Stresser, P.; Mayrhofer, K. J. Electrochem. commun. 2014, 48, 81–85.


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Figures

Figure 1: IL-TEM electrochemical configuration (left), IL-TEM electrochemical coarsening image sequence (middle) and CVs (right) for accelerated durability test of npNiPt/C in 0.1 M HClO4 with cycling limits of 0.6 and 1.2 V vs. RHE at 50 mV s-1. TEM images from top to bottom represent as-dealloyed, 6,000, and 10,000 potential cycles. Scale bar is 10 nm.


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Figure 2: IL-TEM results for electrochemical dealloying of Ni80Pt20/C. Representative particles before (left) and after (right) 50 potential cycles between 0.05 and 1.2 V vs RHE at 250 mV s-1 in Ar purged 0.1 M HClO4.


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Figure 3: CVs (A), ORR curves (B), 20 mV s-1 sweep rate, and Tafel plots (C) for npNiPt/C recorded before, during, and after ADT cycling between 0.6 - 1.1 V vs. RHE at 50 mV s-1 in 0.1 M HClO4. (D) Percent decrease in ECSA, specific activity and mass activity of np-NiPt/C after ADT.


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Figure 4: IL-TEM series of np-NiPt/C during ADT in 0.1 M HClO4 at room temperature with the indicated UPLs and a sweep rate of 50 mV s-1. TEMs from left to right represent as-dealloyed, 3,000, 6,000, and 10,000 potential cycles. Scale bar is 10 nm.


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Figure 5: Percent ECSA retained as a function of ADT cycle number for np-NiPt/C with UPLs of 0.9 (pink), 1.0 (green), 1.1 (red), and to 1.2 (blue) V vs. RHE. ECSA aging curve for commercial Pt/C (black) with a UPL of 1.1 V vs. RHE is also included. Durability was assessed in Ar purged 0.1 M HClO4 at room temperature with sweep rate of 50 mV s-1.


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Figure 6: Percent change in ECSA (blue), ORR specific activity (red) and ORR mass activity (green) for np-NiPt/C in O2 saturated 0.1 M HClO4 after ADT 10,000 potential cycles to the indicated UPL. Comparison of bare np-NiPt/C to np-NiPt/C + Ir with an upper potent limit of 1.1 V vs. RHE.


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Figure 7: ICP-OES measurement of residual composition of digested catalyst materials (A) and concentration of dissolved Pt (blue) and Ni (green) in “aged” electrolyte (B) for bare np-NiPt/C after 10,000 ADT potential cycles to the indicated UPL.


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Figure 8: TEM EDS measurement of the residual Ni atomic composition of np-NiPt/C during ADT with UPL of 1.1 V vs. RHE.


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Figure 9: EDS elemental maps of a sampling of np-NiPt/C particles decorated with a partial ML of Ir. Quantification indicates an average Ir fraction of approximately 5 atomic %. Scale bars are 10 nm.


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Figure 10: ORR polarization curves in O2 saturated 0.1 M HClO4 at a sweep rate of 20 mV s-1 for bare np-NiPt/C (black) and np-NiPt/C + Ir before (red) and after (blue) 10,000 ADT potential cycles between 0.6 and 1.1 V vs RHE at 50 mV s-1.


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Figure 11: IL-TEM series for ADT of Ir surface decorated np-NiPt/C. Moving from left to right: as-dealloyed, 3,000, 6,000, and 10,000 ADT potential cycles at 50 mV s-1 between 0.6 and 1.1 V vs. RHE in Ar purged 0.1 M HClO4. Scale bars are 10 nm.


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100 90 ECSA Retained (%)

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Figure 12: Percent ECSA retained as a function of ADT cycle number for np-NiPt/C (red), np-NiPt/C + Ir (green) and commercial Pt/C (blue) between of 0.6 and 1.1 V vs. RHE at a sweep rate of 50 mV s-1. Durability is assessed in Ar purged 0.1 M HClO4 at room temperature.


ACS Catalysis

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Figure 13: Residual composition of np-NiPt/C (left) and np-NiPt/C + Ir (right) after 10,000 ADT potential cycles in Ar purged 0.1 M HClO4 at 50 mV s-1 with an UPL of 1.1 V vs. RHE. Composition measured by post-ADT digestion analysis with ICP-OES.


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