Hydrogen production by water electrolysis is one of the techniques for establishment of the economy using hydrogen as an energy source. At the present, platinum group elements are used as the electrode catalyst for both anode and cathode in the acid medium, leading to be high initial cost of the water electrolysis system. We recently found that titanium oxiynitride catalyzed activated carbon (TiON/AC) has a low overpotential for hydrogen production in the acidic electrolyte. The overpotential is around 40 mV, which is very low among the catalyst based on the non-noble metals. In this presentation, we report the preparation of TiON/AC and its hydrogen production ability.

TiON/AC is successfully obtained through two-steps as loading of titanium dioxide (TiO₂) on activated carbon powder and then the nitridation. At first, deposition of TiO₂ on the activated carbon was carried out by a modified liquid-phase deposition (LPD) method, in where H₃BO₃ and (NH₄)₂TiF₆ aqueous solutions were used. A desired amount of activated carbon powder [specific surface area is 2343 m²/g purchased from Kuraray. Co., Ltd.] was added into H₃BO₃ aqueous solution. Then, (NH₄)₂TiF₆ aqueous solution was added under stirring.

The product was mounted in an alumina boat. Subsequent NH₃ nitridation was carried out at 700, 750, 800 and 900°C. The characterization was performed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS).

To evaluate hydrogen production ability of TiON/AC, the water electrolysis experiment was demonstrated by using 0.5 mol dm^-3 of sulfuric acid solution as the electrolyte. The working electrode (cathode) was prepared by TiON/AC carbon paper. 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.

Figure 1 shows the XRD patterns before and after NH₃ nitridation at 700, 750, 800 and 900^oC. Before the nitridation, the diffraction peaks assigned to both the anatase phase of TiO₂ and carbon were recognized. After the nitridation at 700^oC, the peaks assigned to TiO₂, TiN and TiO were appeared. Furthermore, the peaks assigned to TiN and TiO was obvious and the broad peak of carbon was also recognized when the nitridation temperature was over 750^oC. In addition, from the XPS results, the formation of Ti-O-N and O-Ti-N bonding were confirmed. Therefore, we considered that the nitridation of TiO₂ to TiON was surely happened over 750^oC.

Figure 2 shows FE-SEM images of the product after the nitridation of 800^oC. The tiny particles as 30 nm were found on AC substrate. These particles were considered the products composed of titanium, nitrogen and oxygen.

The hydrogen production rate was estimated from the results of gas chromatography. And, its dependency on the overpotential was shown in Figure 3. Hydrogen production was confirmed at overpotential = 0.1 V for the products after the nitridation of 800 and 900^oC. The products obtained at 900^oC of the nitridation shows the highest hydrogen production rate.

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