Electrochemical Carbon Dioxide Reduction with Heteroatom Doped Ketjenblack Catalyst Using a Fuel Cell-Type Electrolytic Cell
Authors: Okuzaki Sho , Shinoku Kota , Shiroishi Hidenobu , Yoshida Lina , Yoshida Tsukasa , Morinaga Haruto 
Published: December 2024

Introduction
In recent years, non-metallic catalysts with high efficiency for electrochemical carbon dioxide reduction have been reported and have attracted much attention. We have synthesized Ketjenblack® catalysts co-doped with nitrogen and sulfur by hydrothermal treatment of Ketjenblack and demonstrated their capability for the electrochemical carbon dioxide reduction. In this study, sulfur mono-doped Ketjenblack (S-KB) catalyst and nitrogen mono-doped Ketjenblack (N-KB) catalysts were synthesized and the electrochemical carbon dioxide reduction activity for the catalysts was evaluated using a fuel cell-type electrolytic cell. The S-KB catalyst was obtained by the hydrothermal treatment of Ketjenblack with ammonium sulfide, while the N-KB catalyst was prepared by heat treatment of Ketjenblack in a tube furnace under an ammonia atmosphere.
Experimental

Catalyst synthesis: 300 mg of Ketjenblack® (ECP600JD, LION) (KB) was mixed with 10 mL of 0.01 M sodium hydroxide solution, 10 mL of 2-propanol, 10 mL of ammonium sulfide solution (5.0~6.0 wt% as S) and 15 mL of Milli-Q® water. The mixture was hydrothermally treated in an autoclave vessel (TAF-SR300-SV, Taiatsu Techno Corp.) at 170°C. The product was ground in an agate mortar for about 3 min to obtain the S-doped catalyst (abbreviated as S-KB_processed time). Ketjenblack was heat-treated in a tube furnace under ammonia atmosphere for 60 min, quenched, and ground in an agate mortar for 3 min to obtain N-doped catalyst (abbreviated as N-KB_processed Temp. (℃)). Characterization was performed using X-ray photoelectron spectroscopy (PHI X Tool, ULVAC-PHI).

Evaluation of CO2 reduction: To prepare catalyst inks, each prepared catalyst was mixed with 2-propanol, a 5 wt% Nafion® 2-propanol solution, Milli-Q water and zirconia balls, followed by shaking. The ink was then sprayed onto hydrophobic-treated carbon paper (TGP-H-060, Toray) at 1 mg cm−2. For the counter electrode, 38.1 wt% Pt/C (ISHIFUKU Metal Co., Ltd.) was sprayed onto carbon paper (without hydrophobic treatment) with 7 mm diameter holes (9.5 holes cm−2) at 1 mg cm−2. The carbon paper was hot-pressed onto a Nafion® membrane (NR-212, Dupont) at 135°C to form a membrane electrode assembly (MEA).
The MEA was integrated into a fuel cell-type electrolysis cell, as shown in Fig. 1(a), with the working electrode side filled with 1 atm CO2, while the counter electrode side compartment was filled with 0.1 M phosphate buffer solution (pH 6). A tube-type RHE was used as a reference electrode (1). The electrolysis was performed at a constant potential of -1.0 V vs. RHE, and the qualitative determination of the products was performed by gas chromatographs (GC-8A, SHIMADZU) equipped with TCD and FID detectors.
Results and discussion
Heat treatment in a tubular furnace under ammonia flow resulted in the highest percentage of pyrrole-type nitrogen in N-KB_600 and pyridine-type nitrogen in N-KB_700. In S-KB_8h, sulfide-type was the most abundant. The carbon monoxide production rates of both sulfur-doped and nitrogen-doped catalysts were faster than those of pristine KB, suggesting their enhanced carbon dioxide reduction ability (Fig. 1(b)).
References
(1) Kunimatsu, H. Qiao and T. Okada, J. Electrochem. Soc., 5, 152 (2005).

Figure 1