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UNSW Sydney team develops inexpensive water-splitting catalyst using 2D MOF framework array

6 June 2017

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.

Ncomms15341-f1
Metal salts and substrate are firstly mixed together in an aqueous solution, and then introduces the organic ligand. Next, the MOF nanosheet array grows on the surface of substrate via a dissolution-crystallization mechanism. Duan et al. Click to enlarge.

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.

Resources

  • 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

June 6, 2017 in Catalysts, Hydrogen Production | Permalink | Comments (7)

Comments

No surprises, almost nobody at all is reading this website and almost no articles is easy to understand. It is only fake science and after more than a decade of researchs and many billions, there is absolutely no commercial water electrolisis done anywhere. You should tell that in the article, but they prefer lying and collecting billions instead.

Another demonstration that lower cost clean H2 could be produced in large quantity, with surplus/excess REs, for fixed and mobile FCs.

How soon can this process by mass reproduced together with FCEVs of all sizes?

We could see point of use hydrogen with renewable power contracts, FC buses and delivery trucks, cleaner air, less imported oil in the coming years.

How soon can this process by mass reproduced together with FCEVs of all sizes?

Just as soon as it makes sense economically. Fuel cells are getting better but they have been under development since 1838 (no that is not a typo). There are places where they make sense now. Spacecraft, maybe non-nuclear submarines, maybe drones, etc. As a fuel, hydrogen has a lot of problems. All this research did is make the production a very small amount more efficient. You still need to use the energy to split the water which is more than you will get back and need to either compress it or even more difficult, liquefy it. This is more energy gone. Do we have excess renewable energy? Where? For the most part cars, delivery trucks, and transit buses can run more economically on batteries. Maybe by the time fuel cells are ready for prime time, some one will have figured out how to make rechargeable lithium air batteries which will largely negate the need for fuel cells.

sd:

Your post that fuel cells have been under development since 1838 with the implication that they have not progressed very well forgot to mention that batteries have been under development since 1800! :-)

And electrified delivery trucks for anything but short routes are tending to make use of fuel cells, whether by La Poste and Michelin, UPS or the grocery delivery truck on this forum today, where the fuel cell will also provide cooling.

Economics is ONE way to look at an issue, it is NOT the only way. Many issues need to be resolved, but they are not the most profitable. Health, safety, national security, energy security and other consideration are taken into account.

Right now I think the best combination might be batteries and methanol fuel cells.

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