A general carbon monoxide-assisted strategy for synthesizing one-nanometer-thick Pt-based nanowires as effective electrocatalysts

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Highlights

  • A general CO-assisted strategy was developed for the synthesis of NWs.

  • The NWs show only one-nanometer thickness resulting in ultrahigh Pt utilization.

  • The NWs grow following an oriented growth mechanism.

  • The NWs exhibit larger ECSA than commercial Pt/C.

  • The NWs show great application potential in terms of both activity and durability.

Abstract

To balance the Pt utilization and the durability is the key issue for developing Pt-based oxygen reduction reaction (ORR) catalysts, and constructing ultrathin one-dimensional (1D) structure provides a practical solution. Here, a facile CO-assisted strategy has been proposed for synthesizing PtFe nanowires (NWs) with an ultrathin diameter of one-nanometer and high aspect ratio for the first time, which demonstrates great universality and can be extended to a ternary system. The NWs are found to grow following an oriented attachment mechanism facilitated by the preferential adsorption and reducibility of CO. Based on composition regulation, PtFe NWs and PtFeCo NWs exhibit superior catalytic performance, of which the electrochemical active surface areas are extremely high, achieving 1.5 folds of that of Pt/C catalyst. Benefiting from the synergistic effect endowed by alloying and the ultrathin anisotropic structure, PtFe NWs and PtFeCo NWs show remarkable mass activity of 0.57 and 0.58 A mg-1Pt, respectively, and the durability also meet the 2020 standard of DOE, holding great application potential.

Graphical abstract

CO-assisted synthesis of one-nanometer nanowires with ultrahigh Pt utilization as efficient oxygen reduction reaction catalysts.

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Introduction

Proton exchange membrane fuel cells (PEMFCs) have shown broad application prospects in portable electronic equipment and electric automobiles because of their rapid start-up at room temperature and high power density [1], [2]. Catalysts are the core components that determine the ultimate performance and cost of PEMFCs. To date, noble metal Pt remains the most effective in catalyzing the sluggish cathodic oxygen reduction reaction (ORR) [3], [4]. Whereas, the deficient Pt utilization and durability of commercial Pt/C catalyst make fuel cells less economically feasible [5], [6].

Designing Pt alloys represents an effective strategy to modify the electronic structure, modulate the adsorption character of Pt, and thus improve the electrochemical property [7], [8], [9]. Meanwhile, great advancement has been made to maximize the utilization of Pt within alloys via geometry and dimension engineering [10], [11], [12], [13]. Despite that decreasing the size of alloy nanoparticles (NPs) can increase the proportion of exposed Pt active sites, small particles are more prone to sintering, leading to declining durability [14], [15]. During the past decade, ultrathin nanowires (NWs) especially those with a few atomic layers are highly desired in balancing both Pt utilization and durability [16], [17], [18], [19]. The anisotropic structure and the multiple contact points with carbon strengthen the metal-support interaction and lower the mobility of NWs, which can effectively inhibit Ostwald ripening and coalescence [20], [21]. On this basis, as the electronic structure of surface Pt atoms is compositional dependent, the composition regulation of binary alloys or the preparation of multicomponent alloys plays a critical role in optimizing the activity of the NWs [22], [23]. However, previous strategies for the synthesis of ultrathin NWs usually involve multi steps or long reaction time, which are relatively complicated, and the preparation of NWs with only one-nanometer thickness is still challenging [24], [25]. Therefore, a robust general method that enables the preparation of one-nanometer Pt-based NWs with adjustable composition and can be extended to a ternary system is in great demand.

Carbon monoxide (CO) has been commonly accepted as reductant. Meanwhile benefiting from its strong adsorption on Pt, Pd and so forth, it has been used as capping agent as well. Therefore, CO has shown potential role in morphology control from the perspective of both kinetics and thermodynamics, such as nanocubes and nanosheets [26], [27], [28], [29], [30]. Several works have declared the successful synthesis of Au and AuAg NWs using a CO-mediated approach [31], [32]. But regarding the synthesis of ultrathin Pt-based NWs, it has been proved to be difficult solely under CO-surplus conditions and usually involves the employing of metal carbonyls, of which the zero-valent metals are claimed to be essential for the nucleation [33], [34], [35].

Herein, a CO-assisted approach has been firstly proposed for synthesizing PtFe NWs, which shows the universality in obtaining other binary or even ternary Pt-based NWs. Meanwhile, through this method, the composition of the NWs can be facilely regulated by precursors. The NWs exhibit an ultrathin diameter of only one-nanometer. By excluding the interference of zero-valent metals in metal carbonyls, the influence of CO was discussed in detail and an oriented attachment growth mechanism of the NWs was identified. Electrochemical tests reveal that all the NWs catalysts present a considerably high electrochemical active surface area (ECSA), which exceeds that of commercial Pt/C. Typically, benefiting from the optimized elemental composition, PtFe NWs and PtFeCo NWs exhibit superior mass activity (MA) and durability for catalyzing ORR, both exceeding the DOE 2020 standard.

Section snippets

Materials

All chemicals were used as received. Platinum (II) acetylacetonate (Pt(acac)2), Nickel (II) acetylacetonate dihydrate (Ni(acac)2), ferrous acetylacetonate (Fe(acac)2), Cetyl trimethylammonium bromide (CTAB), Cetyl trimethylammonium chloride (CTAC), and oleylamine were purchased from Energy Chemical. Glucose and cobalt (II) acetylacetonate (Co(acac)2) were purchased from Aladdin-Reagent Inc., and the 5 wt% Nafion was purchased from Sigma-Aldrich.

Synthesis of PtFe NWs

Pt(acac)2 (10 mg), Fe(acac)2 (6.4 mg), CTAB

Structural characterizations

The one-nanometer NWs were synthesized via a facile solvothermal method assisted by CO. Taking PtFe NWs for instance, TEM and STEM images demonstrate the high yield of well-defined ultrathin PtFe NWs with a mean diameter of 0.97 nm and aspect ratio around 98.7 nm, and the NWs terminates with enlarged ends (Fig. 1a–c). HRTEM images shows the well-resolved interplanar fringes. The d-spacings paralleled to the axial of the NWs are measured to be 2.22 and 1.92 Å, matching well with the {1 1 1} and

Conclusions

To summarize, a general CO-assisted method has been developed for synthesizing a series of Pt-based NWs with ultrathin one-nanometer thickness. CO was found to be critical for the formation of nanorod-like nuclei and promoting the subsequent oriented attachment growth. As the result of the ultrathin structure, the NWs display considerably large ECSA, exceeding that of Pt/C catalyst. Based on compositional regulation, PtFe NWs and PtFeCo NWs show excellent catalytic activity. Benefiting from the

CRediT authorship contribution statement

Na Cheng: Conceptualization, Methodology, Writing - original draft. Ling Zhang: Writing - review & editing, Visualization. Yingjie Zhou: Investigation, Data curation. Shengwei Yu: Validation. Liyuan Chen: Formal analysis. Haibo Jiang: Resources, Project administration. Chunzhong Li: Supervision, Funding acquisition.

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.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21838003, 91834301), the Shanghai Scientific and Technological Innovation Project (18JC1410600, 19JC1410400), the Social Development Program of Shanghai (17DZ1200900), the Innovation Program of Shanghai Municipal Education Commission, and the Fundamental Research Funds for the Central Universities (222201718002).

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