The effect of thermal treatment on structure and surface composition of PtCo electro-catalysts for application in PEMFCs operating under automotive conditions

Abstract

Structure and surface characteristics of carbon-supported PtCo cathode electro-catalysts were investigated to evaluate their performance and resistance to degradation under high temperature (∼110 °C) operation in a polymer electrolyte membrane fuel cell (PEMFC). Two different thermal treatments were investigated, i.e. 600 °C and 800 °C causing the occurrence of a disordered face-centered cubic (fcc) structure and a primitive cubic ordered (L1₂) phase. A specific colloidal preparation route and a carbothermal reduction allowed to obtain a similar mean crystallite size, i.e. 2.9 and 3.3 nm for the catalysts after the treatment at 600 °C and 800 °C, as well as a suitable degree of alloying. Both electrocatalysts were subjected to the same pre-leaching procedure to modulate the surface characteristics. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He⁺ at 1 kV). A Pt segregation in the outermost surface layers and similar electronic properties for the materials were observed. Both catalysts showed good performance under PEMFC operation; however, the catalyst characterised by the disordered fcc structure performed slightly better at low temperature (80 °C) and full humidification; whereas, the primitive cubic ordered structure catalyst showed superior characteristics both in terms of performance and stability at high temperature (110 °C) and low R.H. These operating conditions are more relevant for automotive applications. The enhanced stability of the catalyst characterised by primitive cubic ordered structure was attributed to the growth of a stable Pt-oxide layer during operation at high temperature and low R.H. hindering sintering and dissolution processes at the catalyst surface.

Introduction

One of the main limitations of polymer electrolyte membrane fuel cells is the slow oxygen reduction reaction (ORR) in the potential region close to the reversible potential [1], [2]. In general, the PEMFC electrochemical processes need of Pt-based electro-catalysts to occur at significant rates [3], [4]. Several methods to improve the electrocatalytic activity of Pt-based catalysts are actively investigated by either tailoring the particle size or alloying Pt with transition metals [5], [6], [7]. Pt utilisation can be enhanced by increasing either its dispersion on the support or the interfacial region with the electrolyte. Under practical conditions, mass activity (MA), i.e. the current normalised by the Pt loading, reaches a maximum for Pt and Pt alloy catalysts with a mean particle size of approximately 3 nm [5], [8]. On the other hand, specific activity (SA), i.e. the current normalised by the electrochemical active surface area (ECSA), increases with an increase in Pt particle size [5].

The enhancement in electrocatalytic activity for oxygen reduction by alloying Pt with transition metals has been interpreted differently; several studies have addressed an analysis of bulk and surface properties for specific alloy combinations. Electrocatalytic effects have been ascribed to factors such as interatomic spacing, preferred orientation, and electronic interactions [7], [8], [9], [10], [11]. A better comprehension of the factors determining electrocatalytic activity has been acquired recently with the investigation of the surface characteristics of extended Pt-alloy model surfaces in UHV and through an analysis of oxygen adsorption features by density functional theory studies (DFT) [12], [13], [14], [15].

It was observed that extended alloys which were surface enriched in Pt showed enhanced ORR and were characterised by increased stability [12], [13], [14], [16]. The occurrence of transition metals such as Co, Ni, Fe on the top-most catalyst surface layers generally reduces the number of active sites for the ORR and may cause membrane contamination by releasing the corresponding cations upon dissolution [17], [18], [19], [20], [21], [22], [23]. Dissolution and re-precipitation on larger particles (Ostwald ripening) is often considered one of the most common degradation phenomena occurring for PEMFC cathodes [18], [19]. On the other hand, the activity enhancement in the presence of Pt segregation has been initially attributed by Watanabe et al. [16] to an increase of d-electron vacancies in the Pt-enriched surface layer caused by the underlying transition metal. This should result in weakened O–O bonds. More recently, Markovic et al. [14] have suggested the occurrence of a lowered Pt valence band center relative to the Fermi level in Pt-skin layer based alloys caused by the underlying transition metal. This appears determined both by an electronic ligand effect and the decreased Pt–Pt distance in the surface atomic structure. Such effects should cause a weakening of the metal–oxygen strength which may favour an easier desorption of reaction intermediates.

From a practical point of view, the procedures that can produce an enrichment of Pt in the outermost layers of both extended alloy surfaces and supported nanoparticles are an induced surface segregation by high-temperature annealing and a removal of the less noble transition metal from the alloy surface by pre-leaching in an appropriate acid. As reported in the literature [24], [25], [26], [27], [28], pre-leaching usually results in a better electrochemical activity. After these treatments, different surface structures may occur such as a Pt skin layer which is a compact top surface layer of pure Pt, a skeleton structure that is the occurrence of a strong Pt enrichment on a corrugated surface, the percolated or sandwich-segregation structure where the Pt enrichment on the surface is accompanied by a depletion of Pt in the second layer and composition oscillation in the core layers. Most of these evidences derives from the UHV analysis of bulk alloys. However, the surface characteristics of carbon supported Pt-alloy nanoparticles subjected to acid leaching may considerable deviate from those envisaged by UHV studies of extended Pt alloy model surfaces. Accordingly, several attempts have recently been made to use analytical tools directly on practical carbon supported Pt-alloy nanoparticles to investigate the near surface composition [24], [29]. Shao-Horn et al. have used aberration-corrected high-angle annular dark-field (HAADF) scanning transmission electron microscopy to investigate the composition of Pt₃Co nanoparticles on an atomic scale [24]. Percolated and core–shell structures as well as surface Pt segregation have been revealed for the supported nanoparticles. Although this approach provides excellent information on the single nanocrystal, one of the limitations appears to be related to the fact that it provides a local information concerning with a few selected nanoparticles that may be not representative of the overall sample. In other words, the sample volume that is investigated at the atomic level by electron microscopy is quite small and thousands of nanoparticles in different regions should be examined to provide an information that may be considered statistically representative. In a recent study of our group [29], we have used low-energy ion (3He⁺) scattering spectroscopy (LE-ISS) to get information about the chemical composition of the outermost atomic layers in practical carbon supported Pt-alloy nanoparticle catalysts. The analysis region with this technique is typically several millimetres wide and several analyses in different sample regions may provide a statistically relevant information about the top-most surface composition of the sample.

Another aspect worth mentioning is that most of the studies, which address the correlation of structural and surface characteristics with the ORR activity, report electrochemical results obtained by rotating disc electrode (RDE) techniques in acidic liquid electrolytes. By using this technique, several conjectures are thus made regarding the effective perspectives of the advanced electrocatalysts under real fuel cell operation. Whereas it is of relevant practical interest to correlate the structural and surface properties to the mass activity and performance directly measured under fuel cell conditions to clear understand the effective level of enhancement that may be achieved with the novel nanoparticle structures [29]. For a practical application, the electrocatalytic activity is not the only parameter determining the successful utilization of an electrocatalyst; aspects related to mass transport characteristics and electrochemical stability are of similar importance. Moreover, since the electrochemical reactions occur at the catalyst surface–electrolyte interface, the ORR behaviour is not only determined by intrinsic catalyst characteristics but also by a proper matching of electrocatalyst and electrolyte properties. Thus, the role of surface composition and structure (these properties are recognised to govern the activity and stability) needs to be related to the specific electrolyte and operating conditions which are relevant for fuel cell applications.

Recent advances in PEM fuel cells technology demand operation at high working temperatures to improve efficiency, tolerance of contaminants and for an easy water management [2]. The limited availability of polymer electrolyte membranes that can operate efficiently under conditions relevant to automotive applications (e.g. 110–130 °C, R.H. < 33% [2]) has also restricted the number of electrocatalytic investigations under such conditions [17], [29]. Since PEMFCs for automotive applications are essentially based on perfluorosulphonic acid membranes (PFSA), it appears important to determine the catalytic activity and stability in the presence of such benchmark electrolytes. Since a wide number of research groups is actively involved in improving and modifying these polymer electrolytes for operation under conditions useful for automotive applications [30], [31], [32], it appears appropriate to analyse the behaviour of electro-catalysts under conditions similar to the target application for electro-traction [17]. We have overcome the constraints related to the dehydration behaviour at high temperature of benchmark Nafion membranes by carrying experiments under pressurised conditions. Although, the operating conditions may not exactly reproduce those aimed by the automakers, i.e. almost ambient pressure [33], the present approach may provide a basis to compare catalytic activity and stability under conditions which are as close as possible to the practical automotive application.

With regard to the catalyst preparation we have used a colloidal deposition method, carbothermal reduction at different temperatures and a pre-leaching procedure to enrich Pt in the outermost catalyst layers of PtCo alloys. As observed in the literature, for extended Pt alloy surfaces [12], [13], [14], the segregation of Pt on the surface has the role of maintaining the electronic properties of the alloy while avoiding any occurrence of the electropositive element on the surface that could dissolve into the electrolyte. By using proper catalyst preparation procedures, it may be possible to obtain different crystallographic phases for the same alloy formulation. The relative electrochemical activity of ordered and disordered Pt–Co alloy phases coexisting in multi-phase catalyst materials was recently investigated by Strasser et al. [34]. It was observed that Co-rich disordered phases were characterised by high catalytic activity.

In this work, we have specifically addressed our efforts to examine the role of the thermal treatment in determining the occurrence of possible different surface compositions and structures and we have analysed the resulting effects on catalytic activity and stability in PEMFCs operating in a wide range of conditions including those relevant for automotive applications.