The role of interfacial species and nanostructure of Ni-based electrocatalysts for water splitting

• Given the high specific energy density and potentially clean production, hydrogen is a promising energy carrier to replace the traditional fossil fuel-based energy. Some major application bottlenecks so far are the low hydrogen conversion efficiency and the high cost. It is thus of high interest to design highly active and cost-effective electrocatalysts to increase the hydrogen fuel generation by water electrolysis. Here, we show that by modifying the structure and studying interfacial cause-effect-relationships in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in Ni-based nano-catalysts. It is noteworthy that the performance can be drastically improved after the modification. Based on different pH conditions during synthesis of NiO nanoflakes (NFs), the crystal growth direction and morphology of the NiO nanostructure can be altered according to different surface charges, which control the exposure of the surface active sites, resulting in much improved catalytic performance. Additionally, by incorporating a redox active dopant (Fe), NiO can be tuned into higher catalytic efficiency and becomes bi-functional with respect to the OER and the HER processes under alkaline conditions. Exploring the 3D synergistic NiFe nano-layered double hydroxide, we find improved catalytic performance after prolonged use, where the current density increases from 9.3 mA cm-2 to 12.7 mA cm-2 during 100 h running at 1.7 V without iR compensation in a 2-electrode system. In order to understand the function and precise mechanism of metal doping and the synergistic effect to improve the catalytic property after prolonged use, we use in situ Raman spectroscopic and electrochemical impedance spectroscopy (EIS) to monitor the interfacial redox species and reaction dynamics. The structural and physical characterization on 3D ultrathin NiFe nano-layered double hydroxide were also investigated with XRD, SEM, TEM and XPS before and after 100 h electrolysis in a two electrode configuration in 1 M KOH at room temperature. The results show that structure changes after the oxygen evolution reaction are minor, while, after the hydrogen evolution reaction, the catalyst undergoes recrystallization as observed by selected area electron diffraction (SAED), with markedly improved electrocatalytic activity. The successful identification of the underlying reason for the electrocatalytic improvement offers possibilities for a rational design of Ni-based nano-catalysts, and possibly also for other material systems for use as efficient electrocatalysts for practical alkaline HER and OER processes.

Publikations- och innehållstyp

ref (ämneskategori)

ovr (ämneskategori)