M. Nath, J. Masud, W.P.R. Liyanage, U. DeSilva, A. Swesi
Missouri University of Science & Technology,
Keywords: OER electrocatalysts, PGM-free catalysts, water electrolysis
Summary:In this presentation we will discuss about the highly efficient electrocatalytic activity of transition metal chalcogenides towards oxygen evolution reaction (OER) in alkaline medium. Extensive studies has been performed to understand the enhanced catalytic activity and it was realized that one of the primary cause is increased covalency of the metal-chalcogen bond as a function of decreased electronegativity from oxide to telluride. Comparison of the catalytic activity across a series of Ni-chalcogenides, Ni2S3, Ni2Se3, and Ni2Te3, which showed that the tellurides and selenides were indeed better catalysts than oxide further confirmed this hypothesis. Through materials innovation studies we have successfully identified plethora of transition metal selenides and tellurides including those based on Ni, Ni-Fe, Ni-Co, Cu, Co, Cu-Co, and Ni-Co, which show high catalytic efficiency characterized by low onset potential and overpotential at 10 mA/cm2 [Ni3Se2 - 290 mV; NiSe2; Ni3Te2 – 180 mV; Co7Se8 - 260 mV; FeNi2Se4-NrGO - 170 mV (NrGO - N-doped reduced graphene oxide); NiFe2Se4 - 210 mV; CoNi2Se4 - 190 mV; NixFeyCozSe4 – 230 mV; Cu2Se – 210 mV]. These are some of the lowest overpotentials that has been reported for OER in alkaline medium, and are significantly better than state-of-the-art PGM-based catalysts, IrOx and RuO2. A primary reason for the enhanced OER catalytic activity was proposed to be lowering of the redox potential at the catalytically active transition metal site, which was based on experimental evidence from Ni-based electrocatalysts as well as comparison of the catalytic activity in series of Ni2E3 (E = S, Se, Te). Most importantly through extensive studies of the catalyst surface through surface analytical techniques as well as in situ electrochemical measurements we confirmed that the chalcogenide catalyst surfaces were exceptionally robust in alkaline medium and produced reproducible catalytic activity over hundreds of OER cycles. The catalytic activity of these highly efficient OER electrocatalysts could be further enhanced through nanostructuring. The implications of these observations could be far-reaching and can provide significant technological advance in the field of water electrolysis and fuel cells, by providing opportunities to replace PGM-based electrocatalysts with highly efficient, robust, and affordable NPGM catalysts. Along with materials innovation, we have also focused on technological innovation and have developed a protocol to prepare catalyst ink from these transition metal based new electrocatalysts discovered in the Nath lab, by dispersing the catalyst in a butadiene based polymeric resin along with conductive carbon powder. By controlling the viscosity of the dispersion, we could produce stable catalytic inks that could be painted on any surface rendering them catalytic active towards water electrolysis. Free standing catalyst-modified flexible electrodes could be also synthesized by controlled drying of the catalyst ink. The protocol developed in the Nath lab has been filed for provisional patent and relevant details will be discussed in the presentation.