DFT (density functional theory) suggests that carbon nanotubes stabilize single platinum atoms and that hydrogen evolution reaction takes place more efficiently on their surfaces, compared to conventional platinum nanoparticles. Source: Aalto University. Click to enlarge.

Platinum-based catalysts currently have shown the most efficient electrocatalytic activity for the hydrogen evolution reaction (HER, 2H⁺ + 2e^- → H₂). However, the high price and scarcity of Pt oblige us to reduce the amount of Pt used in electrocatalytic materials. A promising strategy to reduce the use of Pt is the synthesis of single-atom or pseudo atomic-scale Pt. In this strategy, the size of Pt particles further decreases from nano- to sub- nanometer or atomic scale. Consequently, the catalytic properties of the atomic-scale metal can be also drastically changed, providing the possibility of selective tuning of the catalyst activity. Nevertheless, the atomic-scale catalysts are not stable because of their high surface energy and therefore, they should be decorated on the supports providing strong metal–support interactions to prevent the agglomeration of atoms.

… we have shown in this work that pseudo atomic-scale metallic Pt can be easily stabilized on the intact carbon network of the pristine single-walled carbon nanotubes (SWNTs). Furthermore, here, relatively large amount of Pt atoms is mainly located within the outermost surface layer of the electrode, resulting in the maximum noble-metal efficiency while the minimum number of platinum atoms are utilized.

—Tavakkoli et al.

The Aalto catalysts resemble pseudo atomic-scale Pt systems which mainly consist of a few to tens of Pt atoms dispersed on the sidewalls of the SWNTs. The Pt loading is only 0.19-0.75 at% (atomic percentage) at the electrode surface and characteristic peaks for Pt cyclic voltammograms are undetectable.

The atomic dispersion increases the portion of the surface active-atom sites and therefore, notably lower Pt loading is needed for attaining a high catalytic activity.

TEM (tunneling electron microscope) image of a carbon nanotube decorated with platinum clusters (bright color). Click to enlarge.

The SWNT/at-Pt electrodes showed extraordinary electrocatalytic activities for HER, close or even higher than that of Pt/C. DFT calculations revealed that the SWNT sidewall can efficiently immobilize Pt atoms, indicating the potential of SWNTs as a support catalyst for the atomic dispersion of Pt. The calculations also demonstrated that single Pt atom sites on the SWNT can electrocatalyze HER as efficiently as bulk Pt. Thi work opens up a new avenue to produce active electrochemical catalyst materials with ulta-low Pt loading but with ultra-high electrochemical activity.

—Tavakkoli et al.

Kallio emphasizes that so far the functionality of the electrocatalyst developed at Aalto University has only been proven in laboratory conditions.

In small-scale conditions and at room temperature, the electrocatalyst is stable and usable for a long time. The next step is to increase the scale of production and test the functionality of the electrocatalyst in practical applications, which are often carried out at a higher temperature.

—Tanja Kallio

Resources

• Mohammad Tavakkoli, Nico Holmberg, Rasmus Kronberg, Hua Jiang, Jani Sainio, Esko I Kauppinen, Tanja Kallio, and Kari Laasonen (2017) “Electrochemical activation of single-walled carbon nanotubes with pseudo atomic-scale platinum for hydrogen evolution reaction” ACS Catalysis doi: 10.1021/acscatal.7b00199