Abstract
Catalysts typically lose effectiveness during operation, with much effort invested in stabilising active metal centres to prolong their functional lifetime for as long as possible. In this study palladium nanoparticles (PdNP) supported inside hollow graphitised carbon nanofibers (GNF), designated as PdNP@GNF, opposed this trend. PdNP@GNF exhibited continuously increasing activity over 30000 reaction cycles when used as an electrocatalyst in the hydrogen evolution reaction (HER). The activity of PdNP@GNF, expressed as the exchange current density, was always higher than activated carbon (Pd/C), and after 10000 cycles PdNP@GNF surpassed the activity of platinum on carbon (Pt/C). The extraordinary durability and self‐improving behaviour of PdNP@GNF was solely related the unique nature of the location of the palladium nanoparticles, that is, at the graphitic step‐edges within the GNF. Transmission electron microscopy imaging combined with spectroscopic analysis revealed an orchestrated series of reactions occurring at the graphitic step‐edges during electrocatalytic cycling, in which some of the curved graphitic surfaces opened up to form a stack of graphene layers bonding directly with Pd atoms through Pd−C bonds. This resulted in the active metal centres becoming effectively hardwired into the electrically conducting nanoreactors (GNF), enabling facile charge transport to/from the catalytic centres resulting in the dramatic self‐improving characteristics of the electrocatalyst.
Highlights
HRTEM confirmed the metallic nature of the palladium nanoparticles (PdNP) in PdNP@graphitised carbon nanofibers (GNF) with the spacing of the lattice fringes (0.22 nm) corresponding to the (111) planes of the face-centred cubic packing of Pd metal (Supplementary Figure 1b), in agreement with the diffraction peak centred at 39.5° (2q) in the powder X-ray diffraction (XRD) measurements (Figure 2c)
thermogravimetric analysis (TGA) studies of PdNP@GNF showed that there was a small weight loss (~ 2%) at ~ 200 °C, which could be due to the presence of a small amount of residual dibenzylideneacetone ligand left adsorbed in the sample
Over 30000 cycles of the hydrogen evolution reaction, the activity of palladium nanoparticles confined within the carbon nanoreactor continually increases, surpassing other palladium catalysts and matching that of platinum
Summary
In the last two decades there has been an increasing demand for clean and renewable energy sources as alternatives to fossil fuels.[1,2,3,4] For instance, the water-splitting reaction, which consists of the hydrogen and oxygen evolution half-reactions (HER and OER, respectively), has attracted a great deal of attention as a sustainable source of hydrogen (H2).[5,6,7] Hydrogen gas has been identified as a key energy carrier that can be used to produce clean electricity in fuel cells, where the hydrogen oxidation and oxygen reduction reactions (HOR and ORR, respectively) convert chemical energy into electrical energy.[7,8] driving the HER with renewable sources of energy can lead to a sustainable source of hydrogen fuel that can be used in a zeroemission fuel cell. The hydrogen evolution reaction (2H+ + 2e− → H2) - the cathodic reaction in electrochemical water splitting, is a classic example of a two-electron transfer reaction with one catalytic intermediate, H* (where * indicates a site on the electrode surface) and may occur through either the Volmer-Heyrovsky or the Volmer-Tafel mechanism: Volmer step: H+ + e– + * → H* Equation (1). The electron transfer increased after supporting Pd-MoS2 on multiwalled carbon nanotubes (MWNT) These studies demonstrate an excellent HER activity for Pd-MoS2/MWNT compared to that observed for free standing Pd-MoS2 and MoS2/MWNT, as well as notable stability in acid media after 500 potential cycles.[18]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
