Firstly, it is still unsatisfactory on the relatively high cell voltage in two-electrode system, especially for high current densities, which makes it desired to develop new materials with excellent catalytic activity for both HER and HzOR. Despite these progress, there are still several remained challenges in this area. Non-precious Co 3Ta intermetallic nanoparticles prepared by Xia’s group shows an onset potential of −86 mV and two times higher specific activity than commercial Pt/C 24. Xia and co-workers reported that the tubular CoSe 2 nanosheets grown on Ni foam could act as bifunctional HER and HzOR electrocatalysts, which required a cell voltage of 0.164 V to achieve a current density of 10 mA cm −2 in the two-electrode system 21. presented that the Ni 2P nanoarrays grown on Ni foam exhibited superior catalytic activity towards HzOR and could output 500 mA cm −2 at a cell voltage of 1.0 V in the two-electrode system 16. Some pioneering works have achieved inspiring progress regarding the HzOR assisted H 2 production 16, 21, 22, 23. More importantly, the HzOR coupled H 2 production (i.e., overall hydrazine splitting, denoted as OHzS) generates N 2 as the only byproduct, which is much safer compared to water splitting producing the mixure of H 2 and O 2 19, 20, as well as enabling the utilization of separator-free electrolyzer. Among these, hydrazine oxidation reaction (HzOR, N 2H 4 + 4OH −→N 2 + 4H 2O + 4e −) possesses unique feature of tremendously lower theoretical potential of −0.33 V (vs. Recently, it has been demonstrated to possibly overcome this obstacle by replacing anodic OER with thermodynamically more favorable electrocatalytic oxidation of small molecules, including tetrahydroisoquinoline 9, benzyl alcohol 10, urea 11, 12, 13, 14 and hydrazine 15, 16, 17, which can hugely decease the cell voltage for H 2 production. Hence, it is greatly desired to develop alternative strategy different from fabricating high performance OER electrocatalysts in order to avoid the high energy consumption at the anode. RHE) involving four-electron-transfer process 7, which not only lead to the energy wastage, but also further increase the cost due to the utilization of noble metal based electrocatalysts, thus highly limits the large-scale application 8. However, the key challenge originates from the intrinsically sluggish kinetics of anodic oxygen evolution reaction (OER, 4OH −→O 2 + 2H 2O + 4e −, 1.23 V vs. Thus, it is one of the central task to exploit green and efficient approaches to produce H 2, among which electrocatalytic water splitting is deemed as the suitable technique 5, 6. Hydrogen (H 2), as the energy carrier with the highest energy density and zero carbon emission, has been placed much expectation as one of the most fascinating candidates to change the current fossil fuel dominated energy structure 1, 2, 3, 4. The rapidly increasing energy consumption and the deteriorating global environmental concerns have compelled the stringent demand on clean and sustainable energy sources. Importantly, a self-powered H 2 production system by integrating a direct hydrazine fuel cell with a hydrazine splitting electrolyzer can achieve a decent rate of 1.25 mmol h −1 at room temperature.
DFT calculations decipher that the doping optimized H* adsorption/desorption and dehydrogenation kinetics could be the underlying mechanism. Inspiringly, a record low cell voltage of 28 mV is required to achieve 10 mA cm −2 in two-electrode system. Herein, we report a bifunctional P, W co-doped Co 3N nanowire array electrode with remarkable catalytic activity towards both HzOR (−55 mV at 10 mA cm −2) and hydrogen evolution reaction (HER, −41 mV at 10 mA cm −2). However, the relatively high cell voltage in two-electrode system and the required external electric power hinder its scalable applications, especially in mobile devices. Replacing sluggish oxygen evolution reaction (OER) with hydrazine oxidation reaction (HzOR) to produce hydrogen has been considered as a more energy-efficient strategy than water splitting.
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