Elsevier

Electrochimica Acta

Volume 343, 20 May 2020, 136119
Electrochimica Acta

Platinum nanoparticles with TiO2–skin as a durable catalyst for photoelectrochemical methanol oxidation and electrochemical oxygen reduction reactions

https://doi.org/10.1016/j.electacta.2020.136119Get rights and content

Abstract

Performance degradation of Pt nanoparticles is considered to be among the most severe problems for electrochemical reactions. Pt–TiO2 catalysts are widely studied due to the optical effect of TiO2, which is beneficial for electron transfer during methanol oxidation. However, different configurations of these catalysts are insufficient in research. Here, we report rationally designed Pt nanoparticles with TiO2-skin as electrocatalysts for methanol oxidations and oxygen reduction reactions. Methanol oxidation current densities of the Pt–TiO2 nanoparticles are largely improved under irradiation. When the catalysts are tortured successive at ultrahigh current up to 1.0 mA cm−2, the potential-increase is as small as 90 mV. It shows good stability with a peak current fades of 4% after 250 cycles under illumination, compared with 13% in the dark. It also exhibits a much better oxygen reduction durability compared with Pt/C. By mechanism analysis, we believe the excellence in photoelectrochemistry is attributed to the TiO2-skin. The electrons generated in TiO2 spontaneously migrate to Pt nanoparticles, which provide a partially reducing atmosphere to prevent Pt nanoparticles from ionization and dissolution.

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 cm2, 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

References (66)

  • J. Lin et al.

    More efficiently enhancing photocatalytic activity by embedding Pt within anatase–rutile TiO2 heterophase junction than exposing Pt on the outside surface

    J. Catal.

    (2019)
  • N. Su et al.

    Plasma-induced synthesis of Pt nanoparticles supported on TiO2 nanotubes for enhanced methanol electro-oxidation

    Appl. Surf. Sci.

    (2017)
  • E. Antolini

    Photo-assisted methanol oxidation on Pt-TiO2 catalysts for direct methanol fuel cells: a short review

    Appl. Catal., B

    (2018)
  • J.A. Díaz-Real et al.

    Evaluation of transferable TiO2 nanotube membranes as electrocatalyst support for methanol photoelectrooxidation

    Appl. Catal., B

    (2018)
  • C. Odetola et al.

    Photo enhanced methanol electrooxidation: further insights into Pt and TiO2 nanoparticle contributions

    Appl. Catal., B

    (2017)
  • S. Kang et al.

    A resin-based methodology to synthesize N-doped graphene-like metal-free catalyst for oxygen reduction

    Electrochim. Acta

    (2014)
  • Z. Jusys et al.

    Composition and activity of high surface area PtRu catalysts towards adsorbed CO and methanol electrooxidation: a DEMS study

    Electrochim. Acta

    (2002)
  • S. Kang et al.

    Facial synthesis of porous hematite supported Pt catalyst and its photo enhanced electrocatalytic ethanol oxidation performance

    Electrochim. Acta

    (2015)
  • Q. Lv et al.

    Promotion effect of TiO2 on catalytic activity and stability of Pt catalyst for electrooxidation of methanol

    J. Power Sources

    (2012)
  • Y. Qu et al.

    Pt–rGO–TiO2 nanocomposite by UV-photoreduction method as promising electrocatalyst for methanol oxidation

    Int. J. Hydrogen Energy

    (2013)
  • Z.I. Bedolla-Valdez et al.

    Sonochemical synthesis and characterization of Pt/CNT, Pt/TiO2, and Pt/CNT/TiO2 electrocatalysts for methanol electro-oxidation

    Electrochim. Acta

    (2015)
  • H. Rostami et al.

    Poly (p-phenylendiamine/TiO2) nanocomposite promoted Pt/C catalyst for methanol and ethanol electrooxidation in alkaline medium

    Electrochim. Acta

    (2016)
  • Z. Hu et al.

    A wide-spectrum-responsive TiO2 photoanode for photoelectrochemical cells

    Appl. Catal., B

    (2015)
  • W. Chen et al.

    Surface engineering of nano-ceria facet dependent coupling effect on Pt nanocrystals for electro-catalysis of methanol oxidation reaction

    Chem. Eng. J.

    (2020)
  • H. Chiu et al.

    Fabrication and characterization of well-dispersed plasmonic Pt nanoparticles on Ga-doped ZnO nanopagodas array with enhanced photocatalytic activity

    Appl. Catal., B

    (2015)
  • U.A. Paulus et al.

    Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrode study

    J. Electroanal. Chem.

    (2001)
  • M. Shao et al.

    Recent advances in electrocatalysts for oxygen reduction reaction

    Chem. Rev.

    (2016)
  • Y.-J. Wang et al.

    Unlocking the door to highly active ORR catalysts for PEMFC applications: polyhedron-engineered Pt-based nanocrystals

    Energy Environ. Sci.

    (2018)
  • J. Fichtner et al.

    Top-down synthesis of nanostructured platinum–lanthanide alloy oxygen reduction reaction catalysts: PtxPr/C as an Example

    ACS Appl. Mater. Interfaces

    (2019)
  • Y. Liang et al.

    The nature of active centers catalyzing oxygen electro-reduction at platinum surfaces in alkaline media

    Energy Environ. Sci.

    (2019)
  • B. Garlyyev et al.

    Optimizing the size of platinum nanoparticles for enhanced mass activity in the electrochemical oxygen reduction reaction

    Angew. Chem. Int. Ed.

    (2019)
  • B. Zhang et al.

    Ordered platinum–bismuth intermetallic clusters with Pt-skin for a highly efficient electrochemical ethanol oxidation reaction

    J. Mater. Chem.

    (2019)
  • W. Zhao et al.

    Highly active and durable Pt72Ru28 porous nanoalloy assembled with sub-4.0 nm particles for methanol oxidation

    Adv. Energy Mater.

    (2016)
  • Cited by (0)

    1

    These authors contributed equally to this work.

    View full text