Even minute amounts of oxygen can deactivate molecular catalyst that is added to fuel cells. Consequently, this problem has hindered the use of such a catalyst, based on copious metal; which imitates the active center of the natural biocatalyst, in technological-related applications.
Recently, researchers from the Ruhr-Universität Bochum (RUB), the Max-Planck-Institute for Energy Conversion in Mülheim and the from the Pacific Northwest National Laboratories in Washington, USA, have been able to equip such a catalyst with a self-defense mechanism against molecular oxygen. Their findings can be found in the February 28 issue of Nature Communications.
An Alternative for Scarce and Noble Catalyst
Hydrogen is said to be one of the most favorable energy vectors in the future. Usually, catalysts based on noble and scarce materials like platinum are used in very efficient H2/O2 driven fuel cells. A promising alternative for this costly and limiting catalyst materials are molecular catalysts based on plentiful metals like iron and/or nickel, which resemble a mimic of the active center of nature’s extremely active hydrogenases.
A remarkable class of such molecular catalyst is the DuBois type complexes. Their active center consists of a main nickel-atom that is coordinated by pendant beases. These catalysts reveal a high activity which is similar to those of the hydrogenases and their ligand structure can be changed to enable catalysis in aqueous systems and permit covalent attachment to electrode surfaces. “
The latter is of particular importance for technological applications since the immobilization enhances the performance of such fuel cell system” as it is explained by Prof Dr. Wolfgang Schuhmann, Analytical Chemistry, RUB, member of the cluster of excellence Ruhr explores Solvation (Resolv).
A downside of such catalyst is their high sensitivity to oxygen, which hindered the use of this material in technological applications in presently available fuel cell systems. However, in analogy to the hydrogenases that can be protected by integration of the biocatalyst into an oxygen reducing polymer matrix, the team was able to transpose this protection system also to a DuBois-catalyst.
A Polymer Induces Self-Protection
For the protection against oxygen, the research team added a hydrophobic and redox-inactive polymer as immobilization matrix for the nickel-complex based catalyst. The embedment of the catalyst into the polymer matrix guarantees the creation of two separated reaction layers: a catalytically active layer near the electrode surface and a protection layer at the polymer/electrolyte interface. The first layer allows for an effective conversion of hydrogen at the electrode surface and the second layer eliminates incoming oxygen at the interface and thereby protects the active layer from oxygen damage.
Electrically Disconnected Layers
According to Wolfgang Schuhmann, “the catalyst itself provides the protection against oxygen. For this, the catalyst uses electrons from the hydrogen oxidation in the outer polymer layer which are then used to reduce oxygen at the catalyst centers.”
This becomes conceivable because the developed polymer matrix electrically detaches the nickel-catalyst located in the outer polymer layer from the electrode surface. Hence, all electrons extracted from the hydrogen oxidation in the outer layer can be utilized for the reduction of destructive oxygen at the polymer/electrolyte interface.
In tandem, the polymer prevents the transfer of electrons from the active hydrogen oxidation layer at the electrode surface to the protection layer. Therefore, all electrons from the active layer are moved to the electrode and are not utilized for protection.
The polymer/catalyst altered electrodes displayed an outstanding long-term stability and extraordinary current densities which are both requirements for robust fuel cells. The proposed hydrogen oxidation electrodes are hence a favorable alternative for the development of sustainable and economical energy conversion systems.