Ultrahigh electrocatalytic activity with trace amounts of platinum loadings on free-standing mesoporous titanium nitride nanotube arrays for hydrogen evolution reactions.

Minimizing Pt loadings on electrocatalysts for hydrogen evolution reactions (HERs) is essential for their commercial applications. Herein, free-standing mesoporous titanium nitride nanotube arrays (TiN NTAs) were fabricated to serve as a substrate for Pt loadings in trace amounts. TiN NTAs were prepared by thermal treatment of anodic TiO2 NTAs at 750 °C for 3 h in a NH3 atmosphere. The uniform TiN NTAs showed an inner diameter of ∼80 nm and a length of ∼7 μm, with many mesoporous holes ranging from 5 to 10 nm in diameter on the nanotube walls. Pt species dissolved from the Pt counter electrode in electrochemical cycling were redeposited on the mesoporous TiN NTAs to produce Pt-TiN NTAs with an ultra-low Pt loading of 8.3 μg cm-2. Pt-TiN NTAs exhibited 15-fold higher mass activity towards HER than the benchmark 20 wt% Pt/C in acidic media, with an overpotential of 71 mV vs. RHE at a current density of 10 mA cm-2, a Tafel slope value of 46.4 mV dec-1 and excellent stability. The performance of Pt-TiN NTAs is also much better than that of Pt species deposited on non-mesoporous nanotube arrays due to the shortcuts originating from the mesoporous holes on the nanotube walls for electron and mass transfer.