UNSW Sydney chemists have fabricated a new, inexpensive catalyst for water splitting based on an ultrathin nanosheet array of metal-organic frameworks (MOFs) on different substrates.
Their nickel-iron-based metal-organic framework array (NiFe-MOF) demonstrates superior electrocatalytic performance towards the oxygen evolution reaction (OER) with a small overpotential of 240 mV at 10 mA cm−2 and operates for 20,000 s with no detectable activity decay. The turnover frequency of the electrode is 3.8 s−1 at an overpotential of 400 mV. An open-access paper on their work is published in Nature Communications.
MOFs are generally considered to be poor electrocatalysts for electrochemical reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), the two core processes for electrochemical water splitting. Taking OER as an example, the state-of-the-art MOFs operate at a energy cost significantly above thermodynamic requirements, showing a high overpotential and small turnover frequency (TOF) during oxygen evolution.
The activity of an electrocatalyst is usually dependent on, among many other factors, accessible active centres, electrical conductivity and electrode geometry. Improvement in catalytic efficiency requires each of these parameters to be optimized, but increasing one of them without compromising the others is difficult. For example, MOFs have abundant intrinsic molecular metal sites, but few of them are utilized for electrocatalysis because of their poor electrical conductivity (usually ∼10−10 S m−1) and small pore size (usually within several nanometres). The recently reported strategies like calcinations at high temperature may sacrifice MOFs’ intrinsic molecular metal active sites, while hybridization with secondary conductive supports (polyaniline, graphene and so on) may block their intrinsic micropores, and the bulk conductive MOF has limited meso- and macro-porosity (tens of nanometres to several micrometres) for effective mass transport during electrocatalysis. Very recently, a few two-dimensional (2D) MOFs have been synthesized, but the majority of 2D MOFs reported to date have been prepared in powder form, and little effort has been devoted to increasing the macro-/meso-porosity, conductivity or number of catalytic centres.
In this work, we develop a strategy for the in situ growth of ultrathin nanosheet arrays of 2D MOFs on various supports. Unexpectedly, the integrated MOF electrodes demonstrate superior performances towards OER, HER and overall water splitting.—Duan et al.
The team prepared the material via a one-step chemical bath deposition method. The resulting crystal structure of the MOF consists of alternating organic hydrocarbon and inorganic metal-oxygen-layers.
By creating nanometer-thick arrays of metal-organic frameworks, the research team was able to expose the pores and increase the surface area for electrical contact with the water.
To test overall water splitting, the team constructed a two-electrode cell using NiFe-MOF as both the anode and the cathode. At the applied cell voltage of 1.6 V, a large amount of H2 gas bubbles evolve at the cathode and O2 gas bubbles evolve at the anode. The electrolytic cell demonstrated excellent catalytic activity and can deliver a current density of 10 mA cm−2 at a voltage only of 1.55 V, which is 70 mV smaller than using the benchmark precious metal-based Pt/C cathodes and IrO2 anodes.
… this work demonstrates a universal strategy to fabricate ultrathin nanosheet arrays of 2D MOFs, which can be easily adaptable to prepare many other metal-based 2D MOFs such as cobalt, manganese, titanium and molybdenum. The as-resultant material combines a number of remarkable features, and exhibited significantly enhanced catalytic performances with high catalytic activity, favourable kinetics and strong durability towards electrocatalysis such as OER, HER and overall water splitting. The performance of our electrode for water splitting challenges a common conception that MOFs themselves are an inert catalyst for electrochemical reactions.—Duan et al.
Jingjing Duan, Sheng Chen & Chuan Zhao (2017) “Ultrathin metal-organic framework array for efficient electrocatalytic water splitting” Nature Communications 8, Article number: 15341 doi: 10.1038/ncomms15341