Improved Durability of Pt/C for Oxygen Reduction via Trapping at Graphene Defect Sites

Polymer electrolyte membrane fuel cells (PEMFCs) continue to be an exciting option for low-carbon transportation due to their high energy density and fast refilling capabilities. However, the sluggish oxygen reduction reaction (ORR) limits their efficiency and wide-spread adoption. Carbon-supported Pt is the leading catalyst for the ORR, but its high performance is plagued by the dissolution and agglomeration of Pt in oxidizing and acidic conditions. Efforts in improving Pt durability seek to minimize this degradation with the goal of maintaining near-peak activity over thousands of cycles. Here, we present a new “trapping” phenomenon supported by Density Functional Theory (DFT) for a Pt/C – graphene mixture, wherein defect sites on the graphene act as “traps” for Pt redeposition. Accordingly, we demonstrate improved activity and durability of the Pt/C carbon catalysts in fundamental <i>and</i> applied testing environments.<br/>We synthesized a mixture of commercial Pt/C and graphene flakes using the low-cost Pickering emulsion method. This mixture was made into a catalyst ink, dropcasted onto glassy carbon disks, and tested for ORR activity using a rotating disk electrode (RDE) setup in acidic media. Following a 30k-cycle Accelerated Stress Test (AST), the Pt/C-graphene sample exhibited a 36mV decrease in half-wave potential (E_1/2), whereas E_1/2for commercial Pt/C decreased by 47mV. Further, the Pt/C-graphene retained 75% of its initial electrochemically active surface area (ECSA, measured using the hydrogen underpotential deposition method), while the commercial sample only retained 64% of its ECSA. Both samples were also tested in a Membrane Electrode Assembly (MEA, loading 0.14 mg_Pt/cm²) to assess this durability under real-world commercial conditions. In line with the results from RDE, the Pt/C-graphene retained ORR activity and ECSA better than the control (36% of original ECSA for Pt/C-graphene vs. 25% for commercial) after 6k cycles at 80°C. Together, the RDE and MEA results confirm that the Pt/C-graphene mixture exhibits long-term activity and ECSA retention superior to commercial Pt/C in both fundamental and applied testing conditions.<br/>Comparison of scanning electron microscopy (SEM) images of the Pt/C-graphene mixture before and after the AST reveals the presence Pt nanoparticles on the graphene flakes after electrochemical cycling. The absence of such particles before electrochemical cycling suggests that these particles form as dissolved Pt redeposits onto the graphene. Furthermore, the Pt particles appear to grow on the edges of the graphene flakes, where graphene exhibits increased defect density. To investigate further, DFT calculations utilizing van der Waals-inclusive functionals were conducted to determine the impact of the added graphene by computing the relative binding energy of Pt to various graphene defect sites. Most notably, we find a favorable binding energy for Pt with edge sites of the graphene (-0.954 eV/Pt). This supports the evidence from SEM that the defect sites at graphene edges act as a trap for redeposition of dissolved Pt. Since the graphene is electrically conductive, the redeposited platinum continues to contribute to ORR activity. Further DFT calculations revealed that H₂O does not bind to the defect sites, allowing for the ORR product to be adequately removed from the surface after formation. In concert, these phenomena allow for the trapping effect at the defect sites to improve long-term durability of Pt/C catalysts by minimizing the loss of active Pt.<br/>In this work we present a proof-of-concept for the use of graphene defects as a Pt trap to improve the durability of Pt/C catalysts for the ORR. This trapping mechanism, achieved through low-cost means, helps to address the well-known shortfalls of Pt durability for use as an ORR catalyst and progresses PEMFC technology toward improved reliability.