Full Length ArticleEffective oxygen reduction reaction and suppression of CO poisoning on Pt3Ni1/N-rGO electrocatalyst
Graphical abstract
Introduction
High-performance catalysts for low-temperature oxygen reduction reactions (ORRs) in proton-exchange membrane fuel cells (PEMFCs) can be developed by alloying two or more metals. Alloy nanoparticles composed of only non-noble metal are not suitable ORR catalysts because their ORR activities and durabilities are considerably inferior to those of Pt-based alloys [1]. In addition, alloys containing precious metals, such as Rh, Pd, and Ru, have been commercialized, a result of their excellent ORR performance and high durabilities; however, they are as expensive as Pt, which hinders their applications [2], [3], [4]. To address these issues, transition metals such as Fe [5], Co [6], Ni [7], and Cu [8] can be employed owing to their low costs, as alloying with them can significantly reduce the costs of PEMFCs by minimizing the amount of Pt used. Moreover, alloying Pt with a transition metal can optimize the atomic and electronic structures of pure Pt, thereby improving ORR performance. Additionally, in the volcano-type plot of PtM alloys (M = transition metal), Pt3Fe, Pt3Co, and Pt3Ni alloys dominate the upper part of the plot, which is indicative of their higher ORR activities than those of other PtM alloys and pure Pt [9]. For instance, a Pt3Ni octahedral nanocatalyst with an exposed [1 1 1] facet exhibited four- and 1.8-fold higher ORR activities than those of commercial Pt/C and pristine Pt3Ni particles, respectively [10]. Furthermore, the superior ORR performance of this catalyst was found to be due to the improved surface properties of the Pt3Ni icosahedron compared to those of the Pt3Ni octahedron [11].
Although ORR performance has been improved by minimizing the amount of Pt through use of alloys, the straightforward compositional separation remains an issue for these alloys [12]. This problem occurs when elements easily leach from the crystal structure (de-alloying) during repeated ORR reactions, and each leached element undergoes anisotropic growth (morphological evolution), ultimately leading to catalyst degradation [13]. Therefore, a more complex strategy is required to suppress catalyst degradation. In general, the metal components in alloys that are likely to be oxidized in acidic media must be protected due to de-alloying. For instance, a catalyst with CNT-encapsulated Fe nanoparticles exhibited superior durability in long-term PEMFC testing because of its carbon shell, which was presumed to protect the Fe clusters from acid corrosion [14]. Moreover, mesoporous metal–N-doped carbon electrocatalysts exhibit highly efficient ORR performance and are highly durable owing to their large surface areas and uniform Co-Nx distributions [15].
In this study, a Pt3M1/N-doped-reduced-graphene-oxide (rGO) electrode was fabricated to develop a low-cost, efficient, and durable ORR catalyst, whose commercial viability was confirmed over 5,000 cycles of durability testing. First, a uniform Pt3M1 alloy nanocatalyst (M = Fe, Co, Ni, and Cu, that is, transition metals with 3d5 or more configurations; ≤5.0 nm in size) was prepared. Second, the synthesized Pt3M1 alloy nanocatalyst (10 wt%) was dispersed and loaded onto rGO to increase the surface area by enclosing carbon atoms around the Pt3M1 alloy and preventing the corrosion of M in acidic media. Third, N was doped into rGO; the CN group, with additional electrons compared to C, was directly linked to the Pt3M1 catalyst to create an electron-rich Pt–M–(NC) three-phase interface that increased oxygen gas adsorption and the number of electron-donating active sites. Finally, the prepared Pt3M1/N-rGO electrode was characterized using various physicochemical and electrochemical techniques to determine its commercial potential as an ORR electrode, and the underlying mechanism was elucidated to examine the abovementioned problems and explore its prospects.
Section snippets
Preparing the Pt3M1/N-rGO electrocatalyst
As shown in Scheme S1, rGO and GO were synthesized using a modified Hummers method [16]. The synthesis of N-doped rGO (N-rGO) is described in this section. Graphite (3.0 g; 99.9995%, ∼200 mesh, Arcos Organics, Germany) was added to concentrated sulfuric acid (300 mL; H2SO4, 95%, Junsei Co. Japan) and the temperature was adjusted to 0 °C. The solution was sonicated for 3 h to ensure its homogeneity. NaNO3 (4.0 g; 99%, Junsei Co. Japan) was dissolved in the preceding solution and stirred for 1 h
Effects of alloyed metals: Physical properties and electrochemical performance of the Pt3M1/C electrodes
The XRD patterns of the Pt/C and Pt3M1/C catalysts in Fig. 1a show broad diffraction peaks for the (0 0 2) plane at 2θ = 26.35°, which correspond to the hexagonal graphitic carbon of the carbon black support (JCPDS No. = 08-8812) [20]. Peaks corresponding to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) diffraction plane of cubic Pt metal were observed at 2θ = 41.1°, 47.7°, 69.2°, and 83.4°, respectively, in the spectrum of the Pt/C catalyst [21]; these peaks appear higher angles in the spectra of the Pt3M1
Conclusions
ORR performance was improved by reducing the amount of Pt and increasing the electron density of Pt, thereby lowering the binding strength to OH and increasing the adsorption strength for O2. N-rGO was used as the support instead of carbon black, and a Pt3Ni1 alloy was loaded thereon. The resulting ORR performance was considerably improved, and CO poisoning was remarkably reduced, highlighting the possible commercializability of Pt3Ni1/N-rGO. This investigation was facilitated by O2-TPO,
CRediT authorship contribution statement
Junhee Lee: Conceptualization, Investigation, Visualization, Writing – review & editing. Namgyu Son: Conceptualization, Investigation, Visualization, Writing – review & editing. Byung Hyun Park: Methodology, Formal analysis. Sujeong Kim: Methodology, Formal analysis. Dasol Bae: Software, Data curation. Minkyu Kim: Software, Data curation. Sang Woo Joo: Methodology, Formal analysis. Misook Kang: Writing – review & editing, Supervision, Funding acquisition, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019R1A5A8080290, No. 2022R1A2C2008313).
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These authors contributed equally to this work.