2D MOFs with Ni(II), Cu(II), and Co(II) as Efficient Oxygen Evolution Electrocatalysts: Rationalization of Catalytic Performance <i>vs</i> Structure of the MOFs and Potential of the Redox Couples

Abstract

Earth-abundant transition-metal-based metal-organic frameworks (MOFs) are of immense interest for the development of efficient and durable heterogeneous water splitting electrocatalysts. This repot explores the design of two-dimensional (2D) MOFs with redox-active metal centers (Ni(II), Co(II), and Cu(II)) containing two types of electron-rich linkers such as bis(5-azabenzimidazole), linear <b>L</b>_<b>1</b> and angular <b>L</b>_<b>2</b>, and aromatic dicarboxylates. The electron-rich linkers are considered to stabilize the higher oxidation state of the redox-active metal centers in the course of the electrocatalytic oxygen evolution reaction (OER) process. The 2D MOFs of <b>L</b>_<b>1</b> and <b>L</b>_<b>2</b> with Co(II) (<b>1</b> and <b>3</b>) and Ni(II) (<b>2</b> and <b>4</b>) have been produced <i>via</i> the conventional hydrothermal synthesis, while the MOFs of Cu(II) (<b>Cu@1</b> and <b>Cu@3</b>) are obtained by the postsynthetic transmetallation reaction of MOFs <b>1</b> and <b>3</b>. The electrocatalytic OER activities of the six MOFs have been studied to explore the influence of the redox potential of the transition-metal quasi-reversible couples and the coordination environment around the redox-active metal centers in the electrocatalytic activity. The lowest overpotential of 370 mV exhibited by MOF <b>2</b> with the highest current density and TOF value indicates the importance of the presence of coordinated water molecules and the lowest redox potential value of the most favorable quasi-reversible couple Ni⁺²/Ni⁺³. These catalysts exhibit a remarkable stability up to 1000 OER cycles. These studies pave the way for the design of MOF materials toward the development of a promising heterogeneous OER electrocatalyst.