Preparation of Pt dendrites on Poly(diallyldimethylammonium chloride)-functionalized reduced graphene oxide as an enhanced electrocatalyst for the hydrogen evolution reaction in alkaline media
Graphical abstract
Introduction
Hydrogen is considered one of the cleanest, environmentally benign, and sustainable energy resources with high energy density to fulfill our ever-growing needs and as a potential alternative to conventional fossil fuels. The electrochemical hydrogen evolution reaction (HER) is an important and crucial step in hydrogen production from water electrolysis, in the chloro-alkali process, and in fuel cell technologies. Generally, the HER is a multistep electron transfer reaction whose mechanics vary depending upon whether the reaction is conducted in an acidic (reaction (1)) or alkaline (reaction (2)) medium.2H3O+ + 2e– → H2 + 2H2O2H2O + 2e– → H2 + 2OH−
Notably, the HER electron-transfer kinetics are significantly lower in alkaline medium (approximately two orders of magnitude in the case of Pt-catalyzed HER) than in acidic media because in the case of alkaline HER, hydrogen generation results from water dissociation [1], [2], whereas in acidic media, hydrogen is directly produced from the reduction of hydronium ions. Thus, methods of enhancing HER performance in alkaline electrolytes are highly sought after [3], [4], [5], [6]. Pt is currently recognized as the most efficient and suitable HER catalyst; despite its otherwise excellent performance, it has inherent disadvantages of high cost and low stability, necessitating the search for suitable alternatives, such as transition metal oxides, chalcogenides, and phosphides. Accordingly, several materials have been investigated, including relatively low-cost Ni–P [7], Co–P [8], WS2 [9], and MoS2 [10]. However, owing to the high overpotential of these materials, their efficiencies have remained significantly lower than that of Pt/C. The poor performance of these materials has inspired several studies to synthesize Pt-based catalysts with improved efficiency and durability, as well as reduced Pt unit mass, by controlling the morphology of Pt nanoparticles.
Thermodynamically, platinum tends to form cube-, tetrahedron-, and cuboctahedron-shaped particles [11], [12] owing to the preferred growth in the small-surface-energy (100) and (111) directions. The morphology of Pt particles is greatly influenced by the surface energies of the Pt crystal planes, which increase in the order of γ {111} < γ {100} < γ {110} < γ {hkl} [13], [14], implying that high-index-plane particles with large surface energies are very difficult to prepare. Hence, the synthesis of uniquely shaped Pt particles generally requires the use of surfactants to tune the surface energy or the utilization of advanced techniques. Xia et al. obtained highly concave Pt nanoframes enclosing the high-index {740} plane via a hydrothermal reaction at a high oleylamine concentration [15], in contrast to the usual formation of irregular spherical Pt nanoparticles. Additionally, Tian et al. succeeded in electrochemically growing tetrahexahedral platinum nanocrystals on glassy carbon by applying a square-wave potential in the range of EL (lower potential) = −0.1–0.2 V and EU (upper potential) = 1.2 V [16]. Pt nanoparticles of distinct shapes containing high-index planes with steps and terraces exhibit higher electrocatalytic activities than low-index-plane Pt particles owing to the presence of large amounts of unsaturated coordination atom sites, such as those at steps, edges, and kink sites, which act as electrochemically active sites [15], [16], [17], [18], [19]. Notably, the catalytic activity of the (110) plane is the highest among the low-index planes (e.g., (110), (111), and (100)) for the HER because the (111) and (100) planes feature flat terraces only, whereas the (110) plane contains (111)–(111) steps [20].
Herein, we describe a new route for synthesizing Pt dendrites with electrochemically active high-index planes on poly(diallyldimethylammonium chloride) (PDDA)-functionalized reduced graphene oxide (PFG) by a high-voltage electrochemical reduction (HVER) method. This method overcomes the procedural difficulty previously encountered in the preparation of two-dimensional (2D) Pt dendrites by using liposomes as templates [21] or oleic acid-in-water emulsions [22]. Subsequently, the HER activities of the prepared catalysts were investigated in alkaline solution. The experimental cell used for the HVER method was designed as an H-cell structure separated into a cathode side and an anode side by a membrane and Pt/PFG samples were prepared by transferring electrons from the Pt electrode in the cathode side to the metal precursor solution using a DC power supply. In the designed cell, negatively charged graphene oxide on the cathode side was attracted toward the anode side. However, appropriate modification of the surface charge of graphene oxide by functionalization of positively charged PDDA on its surface allowed electrons from the Pt electrode to be easily transferred to PFG and the Pt precursor. Although this technology is similar to that used for conventional electrodeposition, which is generally performed using DC pulse cycles or AC at low potentials, the technique utilized herein relies on the application of a high and constant DC potential of 6 kV. During electrodeposition, PtCl62− ions diffuse to the cathode at the upper potential owing to the application of DC pulse cycles or AC, whereas the reduction of these diffused PtCl62− ions at the lower potential results in nanocrystal deposition on the cathode [16]. Consequently, the conventional method deposits samples on a limited area of the target substrate, but the HVER method allows nanoparticles to be grown in solution rather than be deposited on a substrate. The HVER method utilized in this study afforded high yields of Pt/PFG in the presence of poly(vinylpyrrolidone) (PVP) within a short reaction time (Table S1). In addition, in the HVER method, an H-cell with a membrane was employed to prevent the diffusion of O2 generated at the anode to the cathode and thus increase the particle generation rate [23].
Section snippets
Materials
Graphene oxide (GO) solution was purchased from Graphene Supermarket, and other reagents were purchased from Sigma-Aldrich. Commercial 40 wt% Pt/C (Pt/C) was obtained from Premetek.
Preparation of PDDA-functionalized reduced graphene oxide (PFG)
Poly(diallyldimethylammonium chloride) (PDDA) solution (38 wt%, 2.5 mL) was diluted with deionized water (70 mL), sonicated for 10 min, and added dropwise to a dispersion of ultra-highly concentrated single-layer GO (30 mL, 6.2 mg/mL) under vigorous stirring. The resulting mixture was sonicated for 30 min to obtain a
Results and discussion
The HVER method was employed to prepare various Pt/PFG samples. Fig. 1 shows a schematic diagram of the HVER apparatus, which consists of an H-cell with a cationic membrane (Nafion 115) separating the anode and cathode compartments. The ballast resistance was set to 110 kΩ.
The zeta potentials of the GO, PFG, and PFG-Pt precursor solutions were determined to be −48.3, 28.9, and 9.5 mV, respectively (Fig. 2), showing that GO exhibited a high negative charge and was thus attached to the membrane
Conclusions
In this study, we successfully synthesized various high-index-plane Pt dendrites on PFG by the HVER method, revealing the importance of surface charge adjustment of graphene oxide. The prepared Pt/PFG catalysts featured a mixture of 2D and 3D dendrites. The samples synthesized with various amounts of PVP had different ECSAs depending on the ratio of 2D to 3D dendrites. The area fraction of 2D dendrites was optimal in the case of Pt/PFG13, which had a PVP-to-Pt weight ratio of 13 and exhibited
Acknowledgment
This work was financially supported through the Korea Institute of Science and Technology (KIST) and the Korea CCS R&D Center (2017M1A8A1072051) by the Ministry of Science, ICT & Future Planning.
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