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Even though “metal-free” carbonaceous electrocatalysts like heteroatom(s)-doped graphene exhibit several advantages, such as cost-effectiveness and high stability toward hydrogen evolution reaction (HER), and are expected to approach the HER performance of Pt, due to the lack of appropriate techniques for completely tailoring the allocation (proportion, content, and configuration) of the dopants and their dominant functionalities, their actual HER performance till date is worse than that of their metallic counterparts. Although it is extremely desirable, the effort to address this issue is scarce in the literature. In the present study, for the first time, we report a facile and simple methodology of the stepwise optimization of dopant allocation, taking P, N-co-doped graphene as a model system. In a two-step synthetic procedure, through the systematic variation of the precursor amount in the first step (optimization of composition) solely, we attempted to obtain a minimum N/P ratio (=2.04) at maximum N (=9.8%) and P (=4.8%) doping levels, while maintaining the optimized bonding configurations by controlling the NH3 pyrolysis parameters in the second step. The composition-optimized structure resulted in a unique configuration having an optimum pyN (pyridinic-N)/gN (graphitc-N) ratio (=5.75%) and relatively high PC (P–C bond)/PO (P–O bond) ratio (=3.29) at a nearly unaltered N/P ratio (=2.33). Further, we have found a protocol which demonstrates that for the coarse alteration of overpotential [@10 mA cm–2 (η10)], the N/P ratio should be tailored, while for a fine alteration, the pyN/gN ratio should be tailored, thus providing a “rule-of-thumb” for obtaining the best “metal-free” co-doped carbonaceous HER catalysts with the best dopant allocations. This, in our case, resulted in enhanced HER performance exhibiting a η10 of 338 mV and Tafel slope of 88 mV dec–1. This extremely low Tafel slope is comparable to or better than many of the “metal-free” heteroatom(s)-doped carbonaceous and state-of-the-art metallic HER catalysts. The stepwise dopant allocations, which also involves switching the dopant type and the established protocol, may be a step forward towards the design of highly efficient metal-free catalysts for the excellent water splitting performance.

Graphene has been applied to thermal technology including boiling and condensation heat transfer, from which the pool boiling enhancement relies on adjusting the surface morphology and wettability that is favorable to catalyze the vaporization on the fluid/graphene interface. However, previous works using graphene or reduced graphene oxide (RGO) flake coatings, where the morphology of graphene coating is nonuniform and most of the underlying structured cavities are sealed by graphene flakes. For a long time, this hampered the unraveling of the mechanism behind the enhanced boiling performance by graphene coatings. Moreover, the previous work relied on using water-based pool boiling, which limits the scope of its practical applications since the versatile nonpolar refrigerant has been widely used in boiling heat transfer. The pool boiling was carried out on a plain copper surface to study the effect of fluorinated graphene (F-graphene) coating using nonpolar refrigerant R-141b as the working fluid along with bubble dynamic visualization. It was found that the increase of contact angle leads to more active cavities and enhances heat transfer performance up to twice as much, by applying the F-graphene coating. Moreover, the mechanism of graphene-enhanced heat transfer performance was unraveled and mainly attributed to the hydrophobic surface and effective cavity structure. This research provides a practical and reliable route for enhancing the heat transfer through F-graphene-coatings, which paves the way for potential application in graphene-based thermal technologies.

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