Abstract:

With the increasing energy demands for electronic equipment, numerous studies have been conducted to achieve higher energy conversion and develop storage devices such as metal-air batteries, water splitting devices, and fuel cells. All these devices are related to the oxygen evolution reaction (OER) and/or oxygen reduction reaction (ORR). Currently, platinum group metals (PGMs) or their oxides are the most active electrocatalysts for OER and ORR. However, the high cost and scarcity of these noble metals hinder their widespread application. Therefore, the development of a low-cost electrocatalyst that exhibits catalytic performance comparable to or better than that of PGMs is essential.

Metal-organic frameworks (MOFs) are a new class of porous materials constructed from metal ions and organic linkers. MOF materials have diverse metal centers. In addition, organic ligands containing various heteroatoms can change the microenvironment of these metal centers. Moreover, the size, morphology, and porosity of MOF materials can be precisely tuned. These advantages of MOF are beneficial for electrocatalytic reactions. However, MOF is generally considered to be a poor electrocatalyst and is rarely used in the field of electrocatalysis because of its low electrical conductivity. To increase the electrical conductivity of MOF, high-temperature calcination or hybridization with conductive supports is necessary. However, high-temperature calcination may sacrifice the intrinsic molecular metal active sites of MOFs, whereas hybridization with conductive supports may block their inherent micropores. The development of MOF materials with high electrical conductivity is vital for electrocatalysis.

Herein, we report a two-dimensional conductive MOF based on copper foam growth (Cu₃HITP₂/CF, where HITP = 2, 3, 6, 7, 10, 11-hexaaminotriphenylene hexahydrochloride, CF = copper foam), which has high electrical conductivity and excellent catalytic stability and can be used as a bi-functional electrocatalyst in OER and ORR. In addition, this catalyst does not require heat treatment or the addition of a conductive agent. We first electroplated needle-shaped Cu(OH)₂ nanowires onto the surface of a blank copper foam, and then immersed it in a solution of HITP to convert it into Cu₃HITP₂ at 65 ℃. To confirm its physicochemical properties, the as-synthesized Cu₃HITP₂/CF was characterized and analyzed by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The morphology was characterized by scanning and transmission electron microscopy. The as-synthesized Cu₃HITP₂/CF maintained a two-dimensional needle-like morphology during the reaction and could be stably operated in an alkaline solution. The overpotential at 10 mA·cm^-2 in the OER was only 1.53 V, and the current density did not decrease significantly after 24 h. The Faraday efficiency was as high as 96.84%, and only 1.57% of the by-product H₂O₂ was produced. In addition, during the ORR, the half-wave potential of Cu₃HITP₂/CF reached 0.75 V and its activity did not decrease significantly after 2000 cycles of voltammetric scanning. Moreover, its electron transfer number was 3.85, with 5.7% H₂O₂ generation. Comparative experiments with powder Cu₃HITP₂ showed that Cu₃HITP₂ grown on copper foam had a larger electrochemical specific surface area and exhibited superior OER and ORR properties, which was due to its two-dimensional needle-like morphology. In general, this study not only provides a method for in-situ growth of MOF materials on copper foam but also provides new ideas for developing two-dimensional conductive MOF materials in the field of electrocatalysis.

Key words: 2D conductive MOF, Oxygen evolution reaction, Oxygen reduction reaction, In-situ conversion