Pt nanowire growth induced by Pt nanoparticles in application of the cathodes for Polymer Electrolyte Membrane Fuel Cells (PEMFCs)

Highlights

• Pt nanoparticles in the carbon matrix improve uniformity and profile of Pt-NWs.

• The novel cathode with 0.205 mg_Pt cm⁻² is comparable to commercial one.

• A Pt-NW growth mechanism in the porous matrix is proposed.

• This work provides a strategy for tailoring the electrode architectures.

Abstract

Improving cathode performance at a lower Pt loading is critical in commercial PEMFC applications. A novel Pt nanowire (Pt-NW) cathode was developed by in-situ growth of Pt nanowires in carbon matrix consisting Pt nanoparticles (Pt-NPs). Characterization of TEM and XRD shows that the pre-existing Pt-NPs from Pt/C affect Pt-NW morphology and crystallinity and Pt profile crossing the matrix thickness. The cathode with Pt-NP loading of 0.005 mg_Pt-NP cm⁻² and total cathode Pt loading of 0.205 mg_Pt cm⁻² has the specific current density of 89.56 A g_Pt⁻¹ at 0.9 V, which is about 110% higher than that of 42.58 A g_Pt⁻¹ of the commercial gas diffusion layer (GDE) with Pt loading of 0.40 mg cm⁻². When cell voltage is below 0.48 V, the Pt-NW cathode has better performance than the commercial GDE. It is believed that the excellent performance of the Pt-NW cathode is attributed to Pt-NP induction, therefore producing unique Pt-NW structure and efficient Pt utilization. A Pt-NW growth mechanism was proposed that Pt precursor diffuses into the matrix consisting of pre-existent Pt-NPs by concentration driving, and Pt-NPs provide priority sites for platinum depositing at early stage and facilitate Pt-NW growth.

Introduction

Hydrogen fuel cells give a choice of ultimate energy solution and attract more and more attentions in recent years. Among all kinds of fuel cells, Polymer Electrolyte Membrane Fuel Cell (PEMFC) is in the overwhelming position that is well developed and applied in the fields of vehicles, combined heat and power (CHP) systems, backup powers and power plans, etc. However, the sluggish oxygen reduction reaction (ORR) at the cathode results in a high Pt loading (currently in the range of 0.3–0.4 mg _Pt cm⁻²) used which lead to high cost to the end users. To address this challenge, accelerating the ORR at a lower Pt loading without sacrificing performance is critical and has been pursued for decades [1], [2].

Until now, Pt-based electrocatalysts are practically the dominant choice in PEM fuel cells, and are mainly catalogued into the pure platinum, platinum alloys and core-shell structures [1], [2], [3], [4], [5], [6], [7], [8]. To reduce expensive platinum loading and improve electrocatalytic kinetics, the ability to tailor nanostructure of electrocatalysts is critical in order to tune their geometry and electronics state [1], [2], [3], [7]. Many fine structures, for example, Pt surface-enriched shell-core, single or multiple atom layers, multilayer alloy materials, Pt nanocage or Pt hollow, are synthesized or designed and investigated [2], [3], [4], [5], [6], [7], [8], [9], [10]. Huang et al. developed a Mo-Pt₃Ni/C alloy showed the best ORR performance, with a specific activity of 10.3 mA cm⁻² and mass activity of 6.98 A mg_Pt⁻¹, which are 81- and 73-fold enhancements respectively compared with the commercial Pt/C catalyst (0.127 mA cm⁻² and 0.096 A mg_Pt⁻¹) [2]. A polycrystalline Pt₅Pr alloy was prepared, which demonstrates ∼4-fold improvement over pure Pt, comparable to that of polycrystalline Pt₃Ni and many other polycrystalline Pt-alloys [4]. The issues for mass production arise due to the complicated processes and parameter sensibility and make them difficult in quality control in engineering, or practical applications have been limited by catalytic activity and durability [1], [2].

One- and two-dimensional nanomaterials with all the atoms exposed for modification act as ideal platforms for tailoring their properties and decreasing material costs [11], [12], [13], [14]. The prominent characteristics of Pt nanowires (Pt-NWs) include dominant (111) facets, less lattice boundaries, a lower number of surface defect sites, and easier electron and mass transport for better electrocatalytic activity and lower vulnerability to dissolution, Ostwald ripening, and aggregation than Pt nano particles (NPs) for enhanced stability [1], [12], [14]. High Pt content catalyst (such as 70% Pt/C) is favourable for improving fuel cell performance [15]. Comapring with Pt nanoparticle preparing, Pt NWs can be easily prepared by template method or template-free method. Meng et al. [16] reported factors Influencing the growth of Pt Nanowires on the template-free synthesis of Pt nanowires via the chemical reduction of Pt salt precursors with formic-acid. Liang et al. [17] used ultrathin Te@C nano cables with a very high aspect ratio as templates to form Pt@C nanocables by the galvanic replacement reaction. Kim et al. [18] developed a nanowire network catalyst that was made of highly-dispersed Pt nanoparticles into electrospun Pt nanowire network architecture.

A new type of bimetallic nanowires (PtCo, PtNi, PtFe, etc.) have been developed by wet chemical synthesis procedure and showed high electrocatalytic activity. A bimetallic PtCo-NW/C nanostructures possess the lowest Tafel slope, mass activity and near four-electron reduction kinetics for direct conversion of oxygen to water [19]. Xia et al. [13] reported an effective solvothermal method for the direct preparation of 3D PtCo nanowire assemblies (NWAs) with tuneable composition. The mass activity of Pt₅₉Ni₄₁ NWs is increased by a factor of 1.9 times in comparison with that of Pt NWs, and ∼3.7 times with that of commercial Pt (0.09 A mgPt⁻¹), and the higher catalytic activity and stability of Pt₅₉Ni₄₁ NWs for the ORR is attributed as a result of the composition dependent atomic-scale alloying and faceting properties [20]. Recently, a new class of Pt₃Fe zigzaglike nanowires (Pt-skin Pt₃Fe z-NWs) with stable high-index facets (HIFs) and nanosegregated Pt-skin structure is reported. Pt-skin Pt₃Fe z-NWs with a mass activity of 2.11 A mg_Pt⁻¹ and a specific activity of 4.34 mA cm⁻² for the oxygen reduction reaction (ORR) at 0.9 V versus reversible hydrogen electrode, which are the highest values in all reported PtFe-based ORR catalysts [21].

For many years, the process of the nucleation and growth of nanoparticles have been depicted by the LaMer burst nucleation and following Ostwald ripening to describe the change in the particles size. Watzky and Finke formulated an approach of constant slow nucleation followed by autocatalytic growth [22]. Gao et al. found that electrochemical deposition at a constant potential can overgrow Pt seeds, which are wet chemically synthesized Pt nanoparticles seeded homogeneously on diamond surface [23]. Simona et al. proposed an oriented attachment growth Mechanism for silver nanowire formation [24]. Whatever, the nucleation and growth mechanisms behind the simple chemistry are extremely complicated [25].

To boost electrocatalyst rule, optimal 3D architectures of the supports and electrodes are important to achieve efficient pt utilization and high performance in PEMFC environment as the current density of the catalyst layer is only 1/10th that if all of the transport rates are infinitely fast [26], [27]. For constructing 3D electrode architecture, a freeze-drying/reduction process was suggested and demonstrated ultra-high pt utilization [28]. An aqueous suspension of GO (graphene oxide) sheets, pt precursor and nafion ionomers was spread onto a GDL, then freeze-dried and reduced while the pt precursor and go sheets were reduced to metallic pt and graphene, respectively. Novel fuel cell nanofibrous electrodes (NFEs) based on self-standing electrospun carbon nanofibre webs covered by platinum ultrathin nanoislands deposited by high overpotential pulsed electrodeposition [29]. These structured electrocatalyst layers have high electrical conductivity for fast charge transport and sufficient macroporosity for efficient reactant mass transportation.

Our previous work designed firstly a porous carbon matrix and grew directly pt nanowires in the pore walls of the matrix, forming a so called “Pt nanowire electrode” where the Pt nanowire morphology and distribution in the catalyst layer can be adjusted by process parameters [3], [30], [31]. The “Pt nanowire electrode” realized truly a 3D architecture as Pt-NWs growing directly on the pore wall and hence almost 100% Pt exposed to oxidant. Our further studies on effects of the matrix materials shows that, comparing with the carbon matrix, the Pt-NWs growing in a Pt/C matrix display shorter and denser fluff on the carbon support [32]. This reminds us that the Pt nanoparticles supported on carbon are evolved into Pt nanowires and consequently can be favourable sites for Pt-NW growing. Following above idea, here we introduced small amount of Pt-NPs into the carbon matrix for controlling Pt-NW growth and profile, and demonstrated that the home-made electrode performance was greatly improved. Measurements of TEM, XRD, single fuel cell performance, electrochemical impedance spectrum (EIS) and cyclic voltammogram (CV) were used to characterize effects of the pre-existing Pt nanoparticles (Pt-NP) from Pt/C. Finally, a Pt-NW growing mechanism was proposed.