Pt Alloy Electrocatalysts for the Oxygen Reduction Reaction: From

Jul 8, 2016 - Nevertheless, many of these alloys are far from being “model objects”; and their surface composition and structure are not stable un...
3 downloads 14 Views 1MB Size
Download PDF
Subscriber access provided by University of Sussex Library

Perspective

Pt-Alloy Electrocatalysts for the Oxygen Reduction Reaction: From Model Surfaces to Nanostructured Systems Viktor Colic, and Aliaksandr S Bandarenka ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00997 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

ACS Catalysis

Pt-Alloy Electrocatalysts for the Oxygen Reduction Reaction: From Model Surfaces to Nanostructured Systems

Viktor Čolić,a Aliaksandr S. Bandarenka a,b,*

a - Physics of Energy Conversion and Storage - ECS, Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany

b - Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

* Corresponding author. E-mail [email protected] (A.S. Bandarenka), Tel. +49 89 28912531

KEYWORDS: Electrocatalysis, Oxygen reduction reaction, Strain effect, Platinum alloy electrocatalysts, Nanostructured electrocatalysts.

1 ACS Paragon Plus Environment

ACS Catalysis

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

Page 2 of 28

Abstract

Polymer electrolyte membrane fuel cells are a promising alternative for future energy provision. However, their wider utilization is hindered by the slow rate of the oxygen reduction reaction (ORR) taking place at the cathode. In order to improve the ORR kinetics, alloys of Pt with late transition metals and lanthanides have been studied extensively, as they offer enhanced activity and in some cases acceptable stability. Nevertheless, many of these alloys are far from being “model objects”; and their surface composition and structure are not stable under operating conditions in PEMFCs. The solute metal can dissolve from the surface and near-surface layers. This process often results in a structure in which several Pt-enriched layers cover the bulk alloy and protect it from further dissolution. In this work, we analyze the literature results on the properties of these alloys, from single crystals and polycrystalline materials to nanoparticles, gathered in the recent decades. As a result of this analysis, we additionally propose a relatively simple method to overview the activities of de-alloyed PtnXtype alloys towards the ORR. Given that the Pt-overlayer is several atomic layers thick, the so-called strain effects should primarily determine the behavior of these catalysts. The strain in the system is the result of the differences between the lattice parameters of the alloy and Ptrich overlayers, causing dissimilar compressive strains in the lattice of the Pt-rich layer. This causes changes in the electronic structure, and, consequently, in the binding properties of the surface. We propose that the atomic radius of the solute metal can be used in some particularly complex systems (e.g. polycrystalline and nanostructured alloys) as a simple semi-empirical descriptor, statistically connected to the resulting lattice strain. The implications of this phenomenon can be used to qualitatively explain the behavior of e.g. some active Pt-alloy nanoparticles so far considered “anomalous”.

2 ACS Paragon Plus Environment

Page 3 of 28

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

ACS Catalysis

1. Introduction

Hydrogen is a promising candidate for future energy provision, as stated in the concept of “hydrogen economy”.1-5 Fuel cells offer the opportunity to close the proposed hydrogen cycle by using it as a fuel. Out of these, one of the most prominent candidates for widespread future applications are the polymer electrolyte membrane fuel cells (PEMFCs), which are appealing for automotive applications due to their low operating temperatures, relatively high power density, and short start-up times.6-8 However, wider application of these devices is impeded by the high loadings of the noble-metal catalysts needed to operate them at a satisfactory performance, which can be achieved by the efficient catalysis of the reactions taking place at the electrodes: the hydrogen oxidation reaction at the anode, and the oxygen reduction reaction (ORR) at the cathode. Most of the losses in fuel cells come from the ORR-side due to relatively slow kinetics of this cathodic reaction.9-12 For instance, there is ~0.4 V overpotential at the current density of 1 mA cm-2 on Pt13 – a widely used catalyst. The ORR is a multi-electron process with at least three intermediates *O, *OH, and *OOH

11,14,15

(“*” denotes adsorbed species),

which makes the optimization of the intermediates’ binding energies difficult due to their interdependence through the so-called scaling relations.16-18 Besides PEMFCs, the catalysis of ORR plays an important role in the development of other types of fuel cells,19 industrial processes,20 metal-air batteries,21 as well as in corrosion science.22 The number of materials capable of catalyzing the ORR in real-world applications is, however, fairly limited by the stability of materials under the harsh conditions under which the ORR takes place. In order to make PEMFCs economical and more competitive on the market, it is necessary to develop efficient (~2-10 times more active than Pt7,10,23) and more durable catalysts for the ORR.24 This requires a detailed understanding of the processes and effects taking place at the electrode/electrolyte interface. 3 ACS Paragon Plus Environment

ACS Catalysis

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

Page 4 of 28

The widely exercised, so-called surface science approach to this problem is to attempt to identify the active sites on the surface by elucidating the reaction in question on a number of well-defined single crystal surfaces, from which relations between the electrode surface status and catalytic activity can be deduced (see e.g.25-27). However, the study of well-understood model single crystal surfaces of metal alloys does not always give complete explanations for the behavior of “real-world” catalysts of similar compositions. For instance, to minimize the catalyst loading, the catalysts are usually implemented in the form of nanoparticles (NPs) or high surface-area thin films.28-31 The catalysts in these forms can display unique properties that multiple model metal-alloy surfaces probably cannot account for (the finite size effect, mass diffusion, substrate effect, cleanliness issues, etc).32-35 While model surfaces provide invaluable information for the elucidation of fundamental phenomena at the interface, the gap in the understanding between the well-defined model alloy surfaces and real-world catalysts should be surmounted. This requires the elucidation of easily assessable physical variables that can be logically linked to the electrocatalytic properties of the alloy systems. In heterogeneous catalysis, it has become an established practice to visualize activity trends by means of volcano plots, which quantify the Sabatier principle, one of the basic tenets of heterogeneous catalysis. In these plots, a variable that is connected to the catalytic activity (such as current density, so-called overpotential, “half-wave potential”, etc.) is plotted versus a “descriptor”, which is usually the calculated surface binding energy for a single intermediate or a variable logically linked to it. One example of such a plot for the ORR is shown in Figure 1. According to the current understanding, the optimal catalyst for the ORR should bind the reaction intermediates slightly weaker than Pt(111), namely ~0.2 eV11,12,36 weaker if one considers Oads, or ~0.1 eV, if the OHads binding energy is used.11

4 ACS Paragon Plus Environment

Page 5 of 28

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

ACS Catalysis

Figure 1. The volcano plot showing the theoretical activities of transition metals with fcc(111), hcp(0001), and bcc(110) surfaces towards the ORR, plotted against the calculated O-binding energy. Adapted with permission from ref 11. Copyright ACS Publications.

Alloys of platinum with 3d- and lanthanide elements are well-known and have been extensively studied for years for fuel cell applications (see e.g. references

12,29,37-42

) at least

since the 1970s.43 They show high activity as well as satisfactory stability under the ORR conditions,12,44,45 which makes them promising candidates for the improvement of the performance of the fuel cells. Conveniently, bimetallic catalysts also allow additional degrees of freedom in their design in comparison to pure metals - their composition. The surface electronic structure, and consequently the electrocatalytic activity of bimetallic catalysts is determined by a number of parameters, which are in general considered to be the result of so-called ligand and/or strain effects.29,46,47 The ligand effect exists due to the presence of a dissimilar neighboring atom or atoms in the close vicinity, which influence(s) the electronic structure of the surface atom(s). The strain effect occurs due to the difference in the lattice parameters between different phases which introduces strain (compressive or tensile) in the crystal lattice. In most materials these effects appear simultaneously. Therefore, it is often difficult to decouple and consider them independently. 5 ACS Paragon Plus Environment

ACS Catalysis

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

Page 6 of 28

In this work, we review the activity trends for PtnX alloys with lanthanide and 3d transitional elements. We propose relatively simple ways to summarize the effects of the nature of the alloying metals, which can (at least semi-quantitatively) give a rationale to some odd activity trends shown by some monocrystalline, polycrystalline or nanostructured Pt alloy electrocatalysts.

2. Oxygen reduction at model Pt alloy surfaces

Probably the simplest case of PtnX alloy systems is the low-index plane single crystals. When evaluating these electrodes one likely needs to take into account both ligand and strain effects, as their combined influence determines the surface electron structure. In order to estimate the surface binding energies based on experimental literature data from different groups, the adsorption isotherms of the ORR-intermediate reaction species can be constructed by identifying the electrode potentials necessary to achieve certain adsorbate coverage. The surface coverage of a particular species can be adequately estimated by the integration of the corresponding cyclic voltammograms (in O2-free standard HClO4 aqueous electrolytes), and correlating the said charge to the amount of the adsorbed species. If one determines the difference in the potentials necessary to reach a fractional coverage of θ=0.5θmax the shift of the potential should correspond to the shift at 0 K, which should, in turn, reflect the difference in the binding energy (illustrated in Figure 2). The latter is valid if one can assume that it is possible to neglect48 the heterogeneity of the adsorption sites, changes in the adsorbate-adsorbate interactions, and the effective change of the catalytically accessible surface area with step-density.

6 ACS Paragon Plus Environment

Page 7 of 28

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

ACS Catalysis

Figure 2. Schematics illustrating the procedure of determination of the difference in the OHbinding energy, ∆∆EOH, from experimental voltammetric data. The isotherms can be constructed by the integration of the corresponding CV-curves. The differences between the isotherms at θ=0.5θmax reflect the difference in OH-binding energies.63

A volcano plot constructed using this approach is shown in Figure 3, where the activities of Pt-alloy (111) single crystal surfaces are plotted versus the binding energies of *OH, one of the key intermediates of ORR, are displayed. The binding energies for different surfaces were estimated using original data from.49-55

7 ACS Paragon Plus Environment

ACS Catalysis

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

Page 8 of 28

Figure 3. The relative activity of various Pt-alloy fcc(111) single crystal systems towards the ORR plotted at 0.9V vs, RHE vs OH-binding energies in 0.1M HClO4. Some designations in the Figure: full red circles - Cu/Pt(111) near surface alloys (NSA), full green circle – Pt3Ni(111) NSAs, full red square – bulk Pt3Co(111), full green square – bulk Pt3Ni(111), open green square - Pt3Ni(111) from a different work,53 full black square – 3 monolayer of Pt on Pd, open crooked black squares – monolayer of Pt on Pd(111), and Pt monolayer on an annealed Pd3Fe(111) electrode with a segregated Pd-layer. All the binding energies were estimated using original data from references.49-55

The uniformity of the low-index single crystal surfaces allows estimation of the binding energies for the reaction intermediates with a good accuracy using various experimental data available in the literature. In this sense, understanding of these systems is relatively good, i.e., the plot does show a volcano-like dependency, explaining the activities of Pt-alloy single crystal catalysts using both ab initio theoretical calculations and experimental approaches. The best catalyst from this group of materials (Figure 3) is Pt3Ni(111), and it shows a reported increase in activity of ~10 times in comparison to Pt(111). The surface of this alloy 8 ACS Paragon Plus Environment

Page 9 of 28

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

ACS Catalysis

binds *OH ~0.13 eV weaker than Pt(111), which is in good agreement with theoretical expectations. The activity of near-surface alloys exhibits a strong dependence on the fraction of the alloying metal in the sub-surface layer. Overall, one can see that alloying (bulk or subsurface) Pt with less noble metals decreases the binding energy of *OH, and it is therefore a feasible strategy to improve the activity of Pt(111) towards the ORR. However, in order to develop efficient “real-world” catalysts for the ORR, one must consider more complex surfaces. In order to further bridge the gap between the simplest model surfaces and applicable practical catalysts, one can consider stepped single crystal alloy electrodes. The surface of these is defined and contains quasi-periodic defects, i.e., steps and terraces of different lengths. Since under-coordinated sites are expected to bind adsorbates even more strongly than Pt(111) (which already binds too strong) one would expect that the stepped surfaces would “move away” from the tip of the volcano shown in Figure 1. However, Figure 4 (the binding energies in that Figure for different surfaces were estimated using original data from references51-53,56,57), which shows ORR-activity data for Pt, Pt-Co, and Pt-Ni (n(111)x(111)) and (n(111)x(100)) stepped single crystals, reveals that real trends are not as straightforward as one would expect at first glance. Since the number of different possible adsorption sites is greater in comparison to simple low-index single crystals, and the exact position of the active sites is still under debate, the theoretical calculation of the binding energies is more complicated. No theoretical binding energies describing the observed trend are reported in the literature up to date; only experimentally determined binding energies are currently available. Additionally, it should be noted that as a first approximation the dealloying effects in these systems can be neglected, as Pt(111) terraces of alloys are fairly stable.16,55

9 ACS Paragon Plus Environment

ACS Catalysis

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

Page 10 of 28

Figure 4. The relative activities of various Pt-alloy n(111)x(111) and n(111)x(100) stepped single crystal electrodes towards the ORR in 0.1M HClO4 at 0.9 V vs RHE relative to Pt(111), plotted as a function of the experimentally estimated OH-binding energies. Black squares – Pt-stepped crystals, red squares – Pt3Co-stepped crystals, green squares - Pt3Ni stepped crystals; n is the atomic width on the terraces. All the binding energies were estimated using original data from references.51-53,56,57

The first thing that needs to be noted is that the stepped crystals show an increased catalytic activity towards the ORR compared to basal low-index surfaces in general,56,58 e.g., it has been a well-known fact for more than two decades that the activity of Pt crystals towards the ORR in acidic media increases in the order Pt(100)