Study on electrochemical water splitting : designing efficient oxygen evolution reaction catalysts and Exploring Scientific Issues in activity evaluation of hydrogen evolution reaction
Date of Issue2017-12-04
School of Chemical and Biomedical Engineering
Developing sustainable and environmental friendly energy and energy storage devices is highly desirable, owing to the increasing demand of energy, limiting storage of fossil energy and rising concerns of environment. Hydrogen is the clean energy carrier with highest energy density and the only product of the conversion is water. Hydrogen produced from electrochemical water splitting has been under intense investigation for several decades. Water splitting involves two half reactions: hydrogen evolution reaction (H2O+2e→H2+2OH-) and oxygen evolution reaction (4OH-→H2O+4e+O2). OER is kinetically sluggish and developing highly efficient OER electrocatalysts is essential for sustainable energy. Three-dimensional NiCo2O4 core-shell nanowires made up of NiCo2O4 nanowire core and NiCo2O4 nanoflake shell have been fabricated by a simple two-step wet chemical method on flexible conductive carbon cloth substrate for oxygen evolution reaction. The combination of high surface area, enhanced mass and charge transport as well as three-dimensional conducting pathway enables superior oxygen evolution reaction. Notably, the NiCo2O4 core-shell nanowire electrode exhibits large anodic current and low onset overpotential for OER with an overpotential of ~ 320 mV at a current density of 10 mA/cm2. Furthermore, the NiCo2O4 core-shell nanowire electrode possesses excellent electrocatalytic stability with long hour electrolysis showing no visible degradation, which is highly desirable for a promising OER electrocatalyst. In spite of the three-dimensional structure and high surface area of the NiCo2O4 core-shell nanowire electrode, more than 300 mV overpotential is still required to drive a current density of 10 mA/cm2. Furthermore, we engaged in designing efficient OER electrocatalysts with higher intrinsic activity. We further prepared the stable colloidal NiFe-LDH nanosheets at room temperature with the facile and scalable co-precipitation method. To overcome the poor conductivity of LDH and weak connection between LDH and conductive support in practical OER applications, we scrolled NiFe-LDH nanosheets into well-aligned multi-walled carbon nanotube (MWCNT) sheets to form binder-free hybrid microfiber electrodes which showed excellent OER activity, reaching 180 mA/cm2 at a small overpotential of 255 mV with outstanding durability. For the water electrolyzer to be commercially viable, the other half reaction of water splitting, the hydrogen evolution reaction (HER) is under intense investigation as well, in order to developing catalysts with earth abundant elements and similar activity to Pt. For this reason, sustainable hydrogen generation from water electrolysis has been extensively studied. Unfortunately, in the past studies, much attention has been paid only to the catalyst electrode while neglecting the proper selection of the counter electrode during the measurement of the electrocatalytic activity. We show that the use of widely applied Pt counter electrode without an ion exchange membrane during the HER measurement can cause significant false evaluation on the electrocatalytic activity of non-precious metal catalysts due to the chemical and electrochemical dissolution of platinum from platinum counter electrode and re-deposition of platinum on the hydrogen catalytic electrode. Overall, we have successfully designed efficient OER electrocatalysts and have addressed the issue easily ignored during evaluation of HER electrocatalysts.