ZIF-67 Grown in Situ over Carbon Fiber Felt (CFF) for Alkaline Water Splitting

Abstract

Aiming for clean and affordable energy through green chemistry, water splitting emerges as one of the most effective and promising methods of producing high-purity hydrogen and oxygen without emitting harmful greenhouse gases into the atmosphere. As a result, the design and construction of low-cost, highly active, and long-term stable electrocatalysts is a challenge that must be highlighted. Considering their crystalline structure, morphological feature with high surface area, well defined pores, and tunable functionality, Metal-Organic Frameworks (MOFs) have attracted attention as materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. The MOF used in this case is known as ZIF-67, which is a 3D self-assembled structure with Co(II) ions interconnected with 2-methylimidazole (Hmim) and has the same ligation angle as sodalite zeolites (145°). The ZIF-67 was anchored in situ on a carbon fiber felt (CFF; a great current collector), promoting the construction of a bottom-up electrode with MOF nanoparticles, which allows the electrode surface to have greater exposure of active sites due to interaction with the polar surface of the substrate in question. Nanoparticles also allow for greater electrocatalytic impacts by having surface effects that are enhanced when compared to bulk effects. Such prepared electrode was tested for water splitting in alkaline (1 M KOH) media, i.e., for hydrogen evolution as anodic reaction and oxygen evolution as cathodic reaction. Low onset potential accompanied by high current densities was observed under anodic polarization conditions. Thus, current density of 10 mA cm-2 was reached at potential of ca. 1.6 V vs RHE and current density as high as ca. 140 mA cm-2 at 2 V vs RHE. Such high performance could be attributed to the low charge-transfer resistance observed during electrochemical impedance measurements. The charge-transfer resistance further decreased with shift of potential to more positive values. Current density of 10 mA cm-2 was recorded at -0.2 V and current density of ca. 40 mA cm-2 at -0.4 V vs RHE under the cathodic polarization conditions. High performance suggests application of the herein prepared CFF/ZIF-67 as bifunctional electrode material for alkaline water electrolyzers. References Zhong, D. Liu, J. Zhang, J. Mater. Chem. A 2018, 6, 1887M. Birkholtz, DJ. Liu, Mater. Horiz. 2017, 4, 20Li, C.; Ma, D.; Zhu, Q, Angew. Chem. 2022, 12, 848. Figure 1