Preparation of carbon-supported Pt–Ru core-shell nanoparticles using carbonized polydopamine and ozone for a CO tolerant electrocatalyst

https://doi.org/10.1016/j.ijhydene.2019.06.079Get rights and content

Highlights

  • PtRu/C catalysts were synthesized by protective coating method and ozone treatment.

  • Protective coating suppresses particle agglomeration during heat treatment process.

  • Ozone treatment effectively removes residual carbon layer on catalyst surface.

  • Heat, Ozone-treated PtRu/C exhibits enhanced CO tolerance and fuel cell performance.

Abstract

Carbon supported Ru@Pt/C catalysts with a Ru-rich core and Pt-rich shell structure are prepared by the solid-state diffusion of Pt and Ru via high-temperature heat treatment. In general, heat treatment at high temperatures causes a sintering effect that leads to the aggregation of nanoparticles into larger particles. Carbonized polydopamine is introduced as a protective coating to inhibit the movement of particles during the high-temperature heat treatment. This carbon layer, which inhibits the particle grain growth, should, however be removed after heat treatment because it blocks the active sites required for the hydrogen oxidation reaction. In this study, ozone treatment at room temperature for 15 min is used to effectively remove the carbon layer on the catalyst surface. Energy-dispersive X-ray spectroscopic line scan profile and X-ray photoelectron spectroscopic analysis confirm that the PtRu/C catalyst has a Ru-rich core and Pt-rich shell structure. From CO stripping voltammetry and polymer electrolyte membrane fuel cell tests using CO containing H2 gas, the core-shell structured PtRu/C alloy formed by high-temperature annealing is demonstrated to have higher tolerance to CO poisoning than PtRu/C catalysts synthesized by co-deposition via the polyol method.

Introduction

The proton-exchange membrane fuel cell (PEMFC) has been proposed as a sustainable clean energy device owing to its high power density, high energy efficiency, and zero emission of carbon dioxide [1], [2], [3], [4]. However, excessive use of Pt to overcome the low reaction rate is recognized as the main obstacle hindering the commercialization of PEMFCs. To resolve this issue, many studies have been conducted to maximize the catalytic activity and reduce the amount of Pt used [5], [6]. In general, the hydrogen oxidation reaction (HOR) occurring at the anode of PEMFC is much faster than the oxygen reduction reaction (ORR) taking place at the cathode. Hence, studies for increasing the reaction rate have been mainly focused on the ORR catalysts. Meanwhile, a typical issue of a HOR catalyst to be overcome is to enhance the tolerance of the catalyst to poisoning effects caused by impurities such as CO present in the hydrogen gas produced by the reforming of a hydrocarbon, because CO strongly adsorbs on the Pt surface and interferes with HOR, thereby leading to poor performance of fuel cells [7], [8].

For several decades, Pt-based alloy catalysts such as Pt–Ru, Pt–Sn, Pt–Pd, Pt–Rh, Pt–Mo and Pt–Ir have been reported as anode catalysts for PEMFCs with high CO tolerance [9], [10], [11], [12], [13], [14]. Among them, the PtRu alloy catalyst has received much attention owing to its high tolerance to CO poisoning. It is well known that Ru contributes to the ligand effect that weakens the bond strength between CO and Pt and a bifunctional mechanism for oxidizing CO to CO2 [15], [16], [17]. To achieve these effects, it is speculated that Pt and Ru atoms on the alloy catalyst surface should be distributed homogeneously with a well-developed boundary. However, recent studies have shown that a core-shell-structured catalyst such as Ru@Pt with a Ru-rich core and Pt-rich shell not only exhibits excellent HOR activity than those of homogeneous PtRu alloys and monometallic Pt, but also distinguishable CO tolerance [18], [19], [20], [21], [22], [23].

There are many methods reported to prepare Ru@Pt core-shell nanoparticles. An ethanol based synthesis method was introduced to prepare core-shell structured PtRu nanoparticles through a series of alcohol-aldehyde-carboxyl acid oxidation processes [24]. Core-shell nanoparticles were synthesized by the pulsed electrochemical deposition method [25] and also by the underpotential deposition method that utilizes the formation of a thin platinum atom layer by galvanic displacement of the sacrificial layer present on the seed metal [26]. A seed-mediated two-step method, in which Ru is prepared as a polyol to form a seed metal and Pt is reduced on the seed, has been mainly introduced for producing a core-shell PtRu/C catalyst [27], [28]. Sonochemistry was also applied to control the elemental distribution in the preparation of a core-shell structure with Pt-enriched shells [29], [30].

Most of these methods are electrochemical methods carried out at low temperatures. In contrast, high-temperature heat treatment using solid-phase diffusion between atoms is also known to be an effective way for fabricating core-shell-like nanoparticles [31], [32], [33], [34]. However, high-temperature heat treatment inevitably induces agglomeration of particles, thereby reducing the electrochemical surface area (ECSA). In recent studies, a protective coating layer is introduced to avoid this side effect of heat treatment. Polydopamine (PDA) or polypyrrole was applied as a protective coating layer to inhibit sintering of particles during the high-temperature heat treatment [35], [36].

In this study, Pt and Ru were simultaneously deposited on the carbon support by the polyol method and then a carbonized PDA coated on the entire PtRu/C catalyst acts as a protective coating during the high-temperature heat treatment. After forming a core-shell structure through the high-temperature heat treatment, the carbon protective layer was removed via ozone treatment developed in our previous study [37]. Since ozone treatment is carried out at a low temperature of 30 °C, agglomeration of the catalyst does not occur. The electrochemical and physical properties of the carbon-supported Ru@Pt catalyst were investigated and its CO tolerance was evaluated through CO-stripping and performance test of PEMFC.

Section snippets

Preparation of PtRu/C-polyol

The PtRu/C-polyol catalyst (45 wt %, a molar ratio of Pt to Ru of 1:1) was prepared in a manner similar to the preparation of Pt/C catalyst via a modified polyol process using 1-pyrenecarboxylic acid (PCA)-treated graphitized carbon as a catalyst support [38], [39]. To ensure stability against ozone, graphitized carbon was used as a support [37] and its surface was modified using a PCA for uniform loading of the catalyst on the graphitized carbon. First, 146 mg of graphitized carbon (RTX, Korea

Protective coating method using ozone treatment

Atomic rearrangement by high-temperature heat treatment is known to effectively yield a core-shell structure. However, the high-temperature heat treatment inevitably causes agglomeration of nanoparticles that leads to grain growth and reduced active surface area of the catalyst. In this study, carbon-supported PtRu core-shell catalysts were fabricated using carbonized polydopamine as a protective coating during heat treatment to suppress the agglomeration of particles. The crucial factor is

Conclusions

A synthesis process was developed to convert the PtRu/C-polyol alloy prepared by the polyol process into a core-shell PtRu/C-ozone catalyst using a protective coating method followed by high-temperature heat treatment and ozone treatment. The high-temperature heat treatment induces spontaneous segregation of Pt and Ru to form a structure with a Ru-rich core and Pt-rich shell; however, it causes sintering of nanoparticles resulting in decreased effective surface area of the catalyst. In this

Acknowledgments

This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (2019R1A6A1A11055660) and the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning, granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174010201640).

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