Browsing by Author "Dutta, Dipak"

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Manipulation of Heteroatom Substitution on Nitrogen and Phosphorus Co-Doped Graphene as a High Active Catalyst for Hydrogen Evolution Reaction
(2019-09-12)
Hung, Yu-Han
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Dutta, Dipak
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Tseng, Chung-Jen
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Chang, Jeng-Kuei
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Bhattacharyya, Aninda J.
Graphene doped with heteroatoms is known to create a unique electronic structure with comparatively much higher active sites by the synergistic coupling effect. However, in the earlier attempts, the atomic structure of such co-doped graphene could not be altered; thus, there is a lack of reports discussing the influence of the atomic arrangement in the catalytic performance of the co-doped graphene. Here, by co-doping P and N atoms in graphene as a model system, we present a facile and two-step process wherein the sequence of doping helps in manipulating the heteroatom substitution, which is of great importance in defining better crystallinity and conductivity and favorable elemental functionalities and hence improving catalytic performance. The present method provides a clean, flexible, binder-free, and readily available electrocatalyst that avoids tedious conventional synthesis and device fabrication steps. The highest P-doping percentage (6 at. %) in the present work is superior to previous reports (3 at. %). By altering the sequence of N- and P-doping, the co-doped graphene electrode displayed excellent performance, with an increment of 148% in the sp(2) domain size and enormous lowering in overpotential and Tafel slope (78%). Further, the efficiency of the hydrogen evolution reaction catalyst sustains >98% for 20 h, which is significantly higher than the well-known MoSx (63%). Although here the P-N co-doped system was utilized as a proof of concept, this method could be adapted for other versatile co-doped graphene. This work may pave the way for the development of co-doped graphene-based devices where manipulation of atomic arrangement can result in a structure with properties desirable for catalytic or electronics applications.
Nanocatalyst-Assisted Fine Tailoring of Pore Structure in Holey-Graphene for Enhanced Performance in Energy Storage
(2019-10-09)
Dutta, Dipak
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Jiang, Jian-Yong
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Jamaluddin, Anif
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He, Shih-Ming
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Hung, Yu-Han
Nanoporous holey-graphene (HG) shows potential versatility in several technological fields, especially in biomedical, water filtration, and energy storage applications. Particularly, for ultrahigh electrochemical energy storage applications, HG has shown promise in addressing the issue of low gravimetric and volumetric energy densities by boosting of the ion-transport efficiency in a high-mass-loaded graphene electrode. However, there are no studies showing complete control over the entire pore architecture and density of HG and their effect on high-rate energy storage. Here, we report a unique and cost-effective method for obtaining well-controlled HG, where a copper nanocatalyst assists the predefined porosity tailoring of the HG and leads to an extraordinary high pore density that exceeds 1 x 10(3) mu m(-2). The pore architectures of the hierarchical and homogenous pores of HG were realized through a rationally designed nanocatalyst and the annealing procedure in this method. The HG electrode with a high mass loading results in improved supercapacitor performance that is at least 1 order of magnitude higher than conventional graphene flakes (reduced electrochemically exfoliated graphene (rECG)) in areal capacitance (similar to 100% retention of capacitance until 15 000 cycles), energy density, and power density. The diffusion coefficient of the HG electrode is 1.5-fold higher than that of rECG at a mass loading of 15 mg cm(-2), indicating excellent ion-transport efficiency. The excellent ion-transport efficiency of HG is further proved by nearly 4-fold magnitude lowering of its R-ion (the ionic resistance in the electrolyte-filled pores) value as compared with rECG when estimated for equivalent high-mass loaded electrodes. Furthermore, the HG exhibits a packing density that is 2 orders of magnitude higher than rECG, revealing the utility of the maximum electrode mass and possessing higher volumetric capacitance. The perfect tailoring of HG with optimized porosity allows the achievement of high areal capacitance and excellent cycling stability due to the facile ion-and charge-transport at high-mass-loaded electrodes, which could open a new avenue for addressing the long-existing issue of practical application of graphene-based energy storage devices.