Chemically aggressive conditions prevail during the electrochemical splitting of water to produce hydrogen, wearing out the catalysts used. Further, engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a notorious challenge.
Now, chemists at the Centre for Electrochemical Sciences at Ruhr-Universität Bochum (RUB) have devised an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on in-situ self-assembly of catalyst particles during electrolysis. A team comprising Stefan Barwe, Prof Dr Wolfgang Schuhmann and Dr Edgar Ventosa from the Bochum Chair of Analytical Chemistry reports on this in the journal Angewandte Chemie International Edition. The work took place as part of the cluster of excellence Resolv.
Electrochemical water splitting by reduction and oxidation of water in separate compartments is promising for the conversion of electrical energy into hydrogen. The overpotentials necessary to drive the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, make the use of efficient electrocatalysts indispensable. Therefore, enormous efforts are currently devoted to the development of highly active catalysts for both reactions. Highly active materials inevitably undergo changes during electrochemical reactions. Hence, the intrinsic chemical stability of the catalyst as well as the stability of a catalyst film after attachment of the catalyst on a current collector are as important as catalytic activity itself for the envisaged application of catalysts, especially under harsh electrolysis conditions.
There are two general approaches of preparing catalyst films. The catalyst is directly grown on a current collector by electroless, electrochemical, or vapor deposition methods. Alternatively, a pre-synthesized catalyst nanopowder is immobilized on a current collector. The deposition methods may result in more stable catalyst films, however, immobilization allows for exploring an immense number of powders of tuneable properties, particle sizes and shapes. Unfortunately, it is still highly challenging to securely immobilize catalyst powders, e.g. nanoparticles, to form appreciably stable gas-evolving electrodes.
Here, we report an innovative approach for preparation of stable nanoparticle-based catalyst films.—Barwe et al.
In a feasibility study, the Bochum chemists demonstrated a new way of creating a highly stable catalyst film. They added catalyst nanoparticles in the form of a powder to the solution, which surrounds the electrodes. The particles pumped through the electrode chambers collide with the electrode surface; there, a particle film forms based on electrostatic attraction forces. Particles with a positively charged surface are deposited on the anode and particles with a negatively charged surface on the cathode. The catalyst film thus forms by itself.
Via the same mechanism, the catalyst surface regenerated during the reaction. New nanoparticles from the solution moved to the electrodes, where they freshened up the worn catalyst film. This self-healing effect lasted as long as catalyst particles were present in the solution.
The researchers worked with nickel foil as the current collector for both cathode and anode because of the use of Ni based electrodes in commercial alkaline water electrolyzers.
They evaluated the formation of self-assembling catalyst films for 4 highly active catalyst powders: NiFe layered double hydroxide (LDH) and CoMn LDH for the anode, and NixB and CoxP for the cathode. All the catalyst materials formed a film a few micrometers thick on the electrodes, as confirmed by electron-microscopic captures confirmed.
The catalyst films formed on Ni electrodes were stable for at least 22 days of continuous electrolysis at various current densities in the range of 50-100 mA cm-2.
This approach opens the door for the potential use of promising highly active catalyst nanopowders for which the stability of the film un- der OER or HER conditions is the main challenge.—Barwe et al.
In further studies, the chemists now want to investigate more closely the influence of particle shape and size as well as the influence of the electrolyte solution on the efficiency and stability of the catalysts.
The German Research Foundation supported the work as part of the cluster of excellence Resolv (EXC1069). Additional funding came from the German Federal Ministry of Education and Research as part of the “Mangan” project (FKZ 03EK3548).
Stefan Barwe, Justus Masa, Corina Andronescu, Bastian Mei, Wolfgang Schuhmann, Edgar Ventosa (2017) “Overcoming the instability of nanoparticle based catalyst films in alkaline electrolysers by self-assembling and self-healing films,” Angewandte Chemie International Edition doi: 10.1002/anie.201703963