Pt-Sandwiched Graphene Ultra-Durable and Highly Active Catalyst for Oxygen Reduction Reaction
Authors: Abdelhafiz Ali , Choi Ji Il , Jang Seung Soon , Alamgir Faisal M. , Liu Meilin 
Published: February 2020

Fuel cells are viable replacements as clean energy sources. Main obstacles towards technology-spread are low performance and high cost. Oxygen reduction (ORR) is a typical reaction at cathode side with sluggish kinetics, where state of the art catalyst is based on Pt supported on commercial carbon. Main objective is to overcome poor stability due to carbon corrosion and/or Pt agglomeration.2D materials (e.g. Graphene) possess tremendous chemical and mechanical stability. Sandwiching catalyst material between Graphene enables a robust support, providing a template dictating Pt growth as well. In addition, masking Pt with another graphene layer is plausible to enhance catalyst lifetime, while ORR activity is intact. Accordingly, the investigated structure is based on Pt nanoparticles sandwiched between two separate graphene sheets (GR/Pt/GR sandwich structure). The thickness of top graphene sheet varied between 1, 3 and 5 layers thick, where their respective effect on electrocatalytic activity and stability towards oxygen reduction reaction was evaluated.

Ultra-small Pt nanoparticles were sputtered on tri-layer thick graphene films. TEM analysis showed particles monodispersity with an average particle size of 2.1 nm. Particles grew on graphene following either (111) or (110) crystallographic orientation. STEM imaging showed cloud of single atomically dispersed Pt atoms around the peripheral of all Pt nanoparticles. Accelerated durability testing (ADT) was performed to track electrochemical active surface area, and catalyst stability in particular. Samples with graphene coverage (GR/Pt/GR) showed higher stability compared to state of the art commercial catalyst (Pt/carbon produced by JM company). GR/Pt/GR with 3-layers graphene thick as a top cover showed interesting phenomenon with an increase of ECSA up to 5K ADT cycles. Afterwards, ECSA was negatively affected by the presence of point defects introduced with graphene (as shown by Ex-situ Raman analysis). After 15K, samples with 3-layers thick graphene survived ~84% of ECSA. Samples with 5-layers graphene thick, showed no ECSA at the beginning, however ECSA signal was increased systematically with ADT cycling up to 30K cycles. Electrocatalytic activity was tested using rotating disk electrode (RDE) setup. Samples with 3-layers graphene coverage showed 7 folds enhancement of mass activity compared to state of the art commercial Pt/carbon catalyst (measured at 0.9V vs RHE).

Pt adatoms covalently bonded to C atoms of graphene enhancing adatom stability. In addition, absence of hydrogen waves during cyclic voltammetry scans of 5-layers thick graphene during ADT indicating that ORR reaction for less graphene-thick samples (i.e. 1 and 3 layers thick) occurred on top of graphene, where both graphene and Pt forming a hybrid catalyst. ADT introduces point defects within graphene mask. Single Pt atoms hop through defects between layers towards surface. DFT calculations showed Pt single-atom favors trapping at point defects, bonded to dangling carbon bonds. Pt lateral diffusion (release from defect site) activated with Pt clustering right underneath the defect site. Migrated Pt adatom travels between graphene layers till trapped in a defect site of an upper layer to hop above to the surface systematically.

Herein, we are reporting an optimization study of using graphene as a protective shield to suppress Pt catalyst dissolution. Results showed tremendous stability enhancement without any penalty of ORR activity, but boosting ORR electrocatalytic activity for GR/Pt/GR over state of the art commercial catalyst due to compressive strain induced on Pt metal adatoms by graphene. This shifts Pt d-band center position away from the fermi level. As a consequence, oxygen adsorption to Pt surface becomes weaker which facilitate the release of ORR reaction intermediates from Pt surface.