Electrosynthesis of a Hybrid Film Consisting of Stacked Graphene and the Intercalated Manganese Oxide for Oxygen Evolution Reaction

Abstract

1. Introduction Considering the fact that the energy from most renewable energy systems can be converted into electricity, the electrochemical water splitting may be an ideal process for producing hydrogen, a clean fuel. However, the slow kinetics of the oxygen evolution reaction (OER, 2H2O → 4H+ + O2 + 4e-) that occurs at the counter electrode necessitates a considerable overpotential, thus lowering the overall efficiency of water splitting. This has motivated the development of efficient catalysts for OER. In nature, a highly efficient oxygen evolving complex is involved within photosystem II, with a core structure containing only of non-precious elements, calcium and manganese (Mn4CaO5 cluster). Inspired by the exceptional efficiency of this biological system, many research groups have focused on manganese oxides and derivatives. However, the bulk structures of manganese oxides do not show high catalytic activity toward OER because the poor electrical conductivity (10-5 to 10-6 S cm-1) severely restricts the activity of catalytic systems involving electron transfer. Thus, we expected to overcome this drawback with the use of graphene (GR), a two-dimensional sp2 nanocarbon with high surface area, excellent electrical conductivity (107 to 108 S cm-1 for isolated single sheet), mechanical strength, and chemical stability. We report herein, a new electrochemical strategy to synthesize a thin film consisting of stacked GR sheets and manganese oxide.1The process involved the anodic formation of negatively charged Mn oxide nuclei on the electrode substrate, which triggered the assembly of cationic GR colloids to the negatively charged oxide for charge compensation, followed by the deposition of more Mn oxide onto the already deposited GR surface with excellent conductivity. The resulting thin film of GR-supported Mn oxide exhibited superior OER activity compared to pristine Mn oxide. 2. Experimental Graphene oxide (GO) was prepared from commercial graphite powder by a modified Hummers method. Cationic GR colloids were synthesized as described in the literature.2 In brief, an aqueous dispersion of GO was add to an aqueous solution of poly(diallydimethylammonium) (PDDA+). Then, NaBH4 was added, and the solution was refluxed for 24h at 100°C. The obtained GR colloids whose surfaces were modified with PDDA+ were washed thoroughly in water and centrifuged repeatedly to completely remove any other cationic species. MnOx/PDDA-GR+ film was prepared on an ITO electrode by anode electrolysis of a MnSO4 solution in the presence of PDDA+-modified GR colloids. The PDDA that had been attached to the GR surface was extracted by immersing the as-deposited film in an aqueous solution containing sodium polystyrene sulfonate (NaPSS). Characterization of the resulting films was made using XRD, TEM, and XPS. On the other hand, OER activity was studied in 0.1 M KOH. 3. Result and discussion Cross-sectional TEM images and XRD measurements of the as-deposited film suggested that the laminate structure consisting of GR sheets and MnOx was constructed (MnOx/PDDA-GR+ film) on the substrate. XPS spectra of the film immersed in a NaPSS-containing aqueous solution showed disappearance of the N 1s peak due to PDDA, while a new peak occurred in Na 1s region. This suggests that the PDDA cations attached to the GR surface was extracted into solution by PSS, and the resulting GR surface was neutralized by Na+ ions in solution. The thus-formed product corresponds to MnOx/GR. The catalytic activity of Mn oxide for OER in alkaline electrolyte was dramatically enhanced by hybridizing it with GR. Namely, the MnOx/GR film exhibited a onset potential (determined on the basis of the beginning of the linear portion in the corresponding Tafel plots) of 1.56 V vs. RHE, which was about 200 mV less positive than that of the counterpart without GR. In addition, the MnOx/GR film provided a Tafel slope of 58 mV per decade, being significantly smaller than most values reported for pure Mn oxides. The stability of the MnOx/GR catalyst was evaluated by chronopotentiometry with an applied anodic current density of 2.5 mA cm-2for 90,000 s. The potential increase remained within 0.11 V (1.748–1.856 V). Since GR has no catalytic activity for OER, the above-obtained results can be attributed to a synergistic effect of the catalytic activity of nano-sized Mn oxide deposits and the ability of GR to transfer electrons from the catalyst to the external circuit. References (1) M. Nakayama, Y. Fujii, K. Fujimoto, M. Yoshimoto, A. Kaide, T. Saeki, and H. Asada, RSC Adv., 6, 23377 (2016). (2) S. Wang, D. Yu, L. Dai, D.-W. Chang, and J.-B. Beak, ACS Nano, 5, 6202 (2011).