Platinum nanoparticles with TiO2–skin as a durable catalyst for photoelectrochemical methanol oxidation and electrochemical oxygen reduction reactions
Introduction
Platinum (Pt) nanoparticles dispersed on carbon support (Pt/C) is the most prevalent catalyst system for electrochemical reactions such as oxygen reduction [[1], [2], [3], [4], [5], [6], [7]], methanol oxidation [[8], [9], [10], [11]], hydrogen evolution [12,13] and many others [14,15]. However, long-term durability of Pt nanoparticles is considered to be among the most severe problems to guarantee performance continuity over an extended lifetime [16]. Thus, stabilizing Pt nanoparticles during harsh electrochemical processes is of great significance for boosting the clean and sustainable energy technologies including direct methanol fuel cells (DMFC), polymer electrolyte membrane fuel cells (PEMFC) and water splitting [17,18]. Three major mechanisms are responsible for the degradation of Pt catalysts [19]. (1) Dissolution: despite that Pt is thermodynamically stable towards dissolution in a large pH and potential window, Pt is susceptible to dissolution in the acid electrolyte [20]. Whereafter, dissolved platinum can be found in the electrolyte or redeposited in the ionomer as a result of reduction [21]. (2) Growth and aggregation: growth is strongly linked to dissolution. The dissolved Pt is redeposited on larger Pt particles so that the overall particle size increases under a driving force of the diminishing surface energy, which is also referred to as Ostwald ripening [22]. Migration of Pt and successive agglomeration further result in coarsening [23]. (3) Detachment: a weakening of the interaction between Pt particles and the carbon support leads to detachment as well as agglomeration [24]. Dissolution of Pt is an oxidation process, which mainly due to electron-loss. It is the key reason for the agglomeration and detachment of Pt nanoparticles. We realize that a photoelectric strategy is able to generate photoexcited electrons, thus providing a partially reducing atmosphere for Pt. These electrons surround on Pt nanoparticles and prevent them from being oxidized, ionized and dissolved in the electrolyte. TiO2, a semiconductor photocatalyst, is widely used as a support or an additive to improve the methanol oxidation reaction (MOR) activity of Pt and Pt-based catalysts [25]. Although many articles have been reported about the solar-inducing enhancement of electrochemical properties [26,27], studies about the photo-enhanced stability and the corresponding mechanisms are few [28,29].
Introduction of a physically protective layer is a generally approbatory strategy to overcome the aggregation of nanoparticles. A target particle encapsulated by a spatial, robust membrane such as carbon or titanium dioxides, has recently been horned widely in advanced nanomaterials, for example, Si/C [30] and Sn/TiO2 [31] for lithium-ion batteries, sulphur-TiO2 for lithium-sulphur batteries [32], Pt encapsulated in nitrogen containing carbon for oxygen reduction reaction (ORR) [33], Pt-carbon for MOR [34], Pt-mesoporous silica for ethylene hydrogenation and CO oxidation [35], Pt decorated Pd–Fe@Pd/C nanoparticles for ORR [36], etc. More rationally, this protective shell should be a porous layer for reactants (such as H+, OH−, water molecules, methanol molecules) passing through.
Pt–TiO2 catalysts are widely studied for electrochemical reactions [[37], [38], [39], [40]], however, durability of these electrocatalysts is rarely investigated [41,42]. Recently, embedding Pt within TiO2 has been reported to be more efficient for photocatalytic activity than exposing Pt on the outside surface [37]. In this article, we, for the first time, synthesize a thin TiO2 skin around Pt kernel. This TiO2 layer not only physically protect Pt particles from aggregation, but also generates electrons under illumination, thus stabilize Pt during electrochemical processes. The Pt–TiO2 nanoparticles deliver high durability upon methanol oxidation and oxygen reduction reactions.
Section snippets
Experimental
Synthesis of Pt nanoparticles: Pt nanoparticles were synthesized via an easy and manageable chemical reduction method. In a typical experiment, 14.7 mg of H2PtCl6·6H2O (99.9%, Adamas Reagent) was dissolved in 2.5 mL of deionized (DI) water, marked as solution A. Solution B was obtained by dissolving 0.2 g of NaBH4 (98%, Adamas Reagent) in 50 mL of DI water. Then, solution A was added into solution B and stirred continuously at room temperature using a magnetic hot plate (MS-H-S10, DLAB
Results and discussion
Configuration of single Pt nanoparticle (3 nm) wrapping by a porous thin TiO2 layer (∼0.5 nm, as shown in Fig. 1) has multiple advantages: (1) nanosized primary particle ensures large electrochemically active surface area and high utilization of Pt, which is a necessity for electrocatalytic systems [43,44]; (2) TiO2 layer is thin enough to be an electron conductor [45], therefore, TiO2 framework works as an electrical highway so that all nanoparticles are electrochemically active; (3) pores in
Conclusions
In summary, we developed a well-designed architecture comprising of Pt kernel (∼3 nm) wrapping by a thin TiO2 layer. Methanol oxidation activities of the Pt–TiO2 nanoparticles are largely improved under irradiation. When the catalyst was tortured successive at ultrahigh current up to 1.0 mA cm−2, the potential-increase was as small as 90 mV, which indicates fast kinetics. The peak current of Pt–TiO2 fades to 96% after 250 cycles under illumination, compared with 87% in the dark, which
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
Sample synthesis, regular sample characterizations and electrochemical performance tests were carried out at the Chongqing Institute of Green and Intelligent Technology (CIGIT). STEM images and EDS mappings were collected in the Analytical and Testing Center of Chongqing University. All authors acknowledge the support from the CIGIT Young Innovators Awards (Y82A240H10), the Chongqing Innovators Program for Returned Overseas Scholars (cx2018152), the Program for Basic Science and Frontier
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These authors contributed equally to this work.