Innovative electrolytic cell of sulfur-doped MnO2 nanorods: Synergistic hydrogen production and formaldehyde degradation at an ultra-low electric energy consumption

Electrocatalytic hydrogen production by water splitting is highly energy-intensive due to poor electrocatalysts and sluggish four-electron oxygen evolution reaction (OER) kinetics. Herein, MnO2 and sulfur-doped MnO2 (S-MnO2) are prepared by a simple hydrothermal method. The results show that S-MnO2 exhibits a higher OER activity than MnO2 because the sulfur doping gives rise to more Mn3+ active sites, oxygen vacancies (VO) and electrochemically active surface areas. Density functional theory (DFT) calculation further confirms that the abundant VO leads to a higher surface energy of S-MnO2, which is conducive to the adsorptions of H2O and OH- on Mn3+ sites. Moreover, formaldehyde oxidation reaction (FOR) is employed to substitute for sluggish OER to improve hydrogen evolution reaction (HER). Compared to OER-based electrolyzer (3.354 V), the cell voltage of FOR-based electrolyzer (2.778 V) at 100 mA cm−2 has decreased by 17.17 %, and the Faradic efficiency of hydrogen production increases from 89.6 % to 98.6 %. The results indicates that to produce the same amount of hydrogen, 17.17 % of electric energy can be economized. Thus the cost of hydrogen production decreases greatly. The HER efficiency is greatly improved because FOR has faster kinetics than OER. Meanwhile, after running for 2 h at 1.75 V, 52 % of formaldehyde has been degraded. The results demonstrate that the innovative electrolyzer can not only greatly improve the HER efficiency, but also efficiently degrade formaldehyde pollutants.