Electrochemical hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are important reactions for realization a renewable energy society. At the present, platinum group metals are known to be the bifunctional electrode catalysts in the acid medium. Unfortunately, these metals are rare, which leads to increase in the cost of the electrochemical reactors such as a water electrolysis and a fuel cell. Recently, we found that titanium oxiynitride catalyzed activated carbons (TiON/ACs) shows a catalytic activity for both HER and ORR in acid aqueous solution [1, 2]. In this presentation, we report the preparation of TiON/ACs (TiON/AC and TiON/ACF), and their activity for these reactions.

TiON/AC is successfully obtained through loading of titanium dioxide (TiO₂) on AC powder or ACF (activated carbon fiber) and subsequent NH₃ nitridation. At first, TiO₂ was deposited on the activated carbons by a modified liquid-phase deposition (LPD) method, in where H₃BO₃ and (NH₄)₂TiF₆ aqueous solutions were used. A desired amount of AC or ACF was added into H₃BO₃ aqueous solution and then, (NH₄)₂TiF₆ aqueous solution was added with stirring [1, 2].

NH₃ nitridation was carried out at over 700^oC with flowing NH₃ gas. The characterization was performed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS).

In order to evaluate the HER activity, the water electrolysis experiment was demonstrated in 0.5 mol dm^-3 of H₂SO₄ solution. The working electrode (cathode) was prepared by using TiON/AC and carbon paper as the substance. A carbon paper and Ag|AgCl were used as the counter and the reference electrode, respectively. Hydrogen was quantified by a gas chromatography during chronoamperometry.

We carried out a rotating ring electrode technique to evaluate the ORR activity. In this case, the electrolyte solution was 0.1 mol dm^-3 of HClO₄ solution saturated with O₂.

TiO₂ particles were confirmed on AC or ACF before NH₃ nitridation. After NH₃ nitridation, the diffraction peaks assigned to TiN and TiO and the broad peak of carbon were found in XRD patterns. The formation of Ti-O-N and O-Ti-N bonding were confirmed in XPS spectra. Therefore, we considered that TiO₂ was transformed to TiON during NH₃ nitridation. FE-SEM images show that TiON particles were found on AC or ACF substrates. These particles were considered the products composed of titanium, nitrogen and oxygen.

The hydrogen production rate was estimated from the results of gas chromatography. The dependency on the loading ratio of TiON was shown in Figure 1. Hydrogen production was confirmed at overpotential = 0.1 V. The TiON/AC with 19 wt% of TiON shows the highest hydrogen production rate.

The onset potential for ORR was estimated ca. 0.78 V vs. RHE and it is higher than that of AC. This result shows that TiON has also a catalytic activity for ORR. Thus, we consider that TiON/ACs have a potential to be the bifunctional catalyst for HER and ORR.

This work was partially supported by the Matching Planner Program (MP28116808401) from Japan Science and Technology Agency , JST and JSPS KAKENHI Grant Number 16K05940.

References

1. T. Kinumoto, et al., Journal of Power Sources, 273(1), 136 -141 (2015).
2. T. Furushiro, et al., The meeting abstract of AiMES2018, #Z01-2091 (2018).