생체 촉매를 모방한 탄소기반의 광전기화학 물분해 촉매 연구

Abstract

The development of efficient catalysts represents one of the most important and challenging issues for the electrochemical hydrogen production. Learning from the biomolecular catalysts such as an enzyme or photosystem in nature provides clues to resolve the related kinetic issues. For example, hydrogenase, which occurs in bacteria, archaea, and some eukarya, catalyzes a proton into a hydrogen evolution reaction (HER) with high activity very near the thermodynamic efficiency limit. The reaction takes place at a specialized metal active center. Functional protein assemblies surrounding a metal active site act as ligands for the metals, pockets for the catalytic reaction, and pathways for reactants and products. Inspired by biomolecular system, we have designed enzyme-mimetic carbon-based catalysts for HER and investigated the effect of each component with a systematic approach. As the simplest model carbon-based platform, 2D monolayer graphene was chosen as a HER catalyst. Graphene possesses excellent transmittance and superior intrinsic carrier mobility. For the first time, we have investigated new possibilities of monolayer graphene as the efficient HER catalyst. The catalytic activity can be further enhanced by generating more active sites. Treatment with N2 plasma also induces N doping and abundant defects. The catalyst exhibits a lower Tafel slope (45 mV/decade) and a higher exchange current density (7.1×10-5 mA/cm2) than those of other previously reported carbon-based HER catalysts, indicating performance comparable to that of Pt catalyst. Based on the electrochemical analysis, the active sites of the N-doped graphene have been identified and quantified. As an intermediate stage of extension from the single 2D platform to a complicate 3D structure, we have transferred graphene layer-by-layer as a well-defined model of the pseudo-3D system and investigated the layer dependence of catalytic activity. Comprehensive electrochemical analysis shows that there is an optimized structure of stacked graphene for the best catalytic activity and the highest charge transfer rate. Based on the understanding of the optimized carbon platform, metal active sites have been incorporated with high controllability and tunability. Moreover, another type of the biomimetic carbon-based nanosheets is addressed as a new HER catalyst. Our synthetic bioinspired HER catalysts are also highly transparent and are applicable to the co-catalyst for the Si photoelectrochemical (PEC) cell. The results indicate the applied bias photon-to-current efficiency of 2.29%, which is higher than that of any other carbon-based PEC catalysts reported to date. Controlling surface structure of the light-absorbing photoelectrode and the deposition of the co-catalyst represent a significant step toward enhancing the hydrogen production.