Production of hydrogen by electrochemical water splitting has been hindered by the high cost of precious metal catalysts, such as Pt, for the hydrogen evolution reaction (HER). In this work, novel hierarchical β-Mo2 C nanotubes constructed from porous nanosheets have been fabricated and investigated as a high-performance and low-cost electrocatalyst for HER. An unusual template-engaged strategy has been utilized to controllably synthesize Mo-polydopamine nanotubes, which are further converted into hierarchical β-Mo2 C nanotubes by direct carburization at high temperature. Benefitting from several structural advantages including ultrafine primary nanocrystallites, large exposed surface, fast charge transfer, and unique tubular structure, the as-prepared hierarchical β-Mo2 C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions, as well as remarkable stability.

environments, [7-12] where Zn deposition even during shelf time is continuously interfered by competitive hydrogen evolution through H 2 O decomposition (Zn + 2H 2 O → Zn(OH) 2 + H 2) and by consuming both the electrolyte and active Zn metal. This long-neglected problem will remarkably affect calendar life of batteries, which is equivalently important with the intensively studied cycling life of batteries. As revealed in our preliminary quantitative study shown in Figure 1a, immersing Zn in 2 m ZnSO 4 electrolyte will induce an ≈0.25 mmol h-1 cm-2 hydrogen flux. The continuous hydrogen evolution will cause local pH changes, which further induce the formation of loose and brittle Zn 4 SO 4 (OH) 6 •xH 2 O (3Zn(OH) 2 + ZnSO 4 •xH 2 O → Zn 4 SO 4 (OH) 6 •xH 2 O), as revealed in Figure 1b. [13-18] It is generally assumed that these by-products augment the tortuosity and irregularity at the electrode/electrolyte interface with physical contact surface being increased (Figure 1c-h), which will further accelerate hydrogen evolution reaction. The above issues necessitate an effective method to detect hydrogen evolution. Nevertheless, for a Zn-based battery, these assumption and hypotheses are only based on oversimplified observations of battery swelling, gas bubbles, or conducting polarization curve in the absence of Zn 2+ condition, not reflecting physical truth of battery. [19-21] Hydrogen production during electrochemical procedure and even shelf time has not been precisely quantified. Consequently, efforts to make Zn metal a valid anode material may be misdirected. Quantifying the hydrogen production on the electrode during Zn deposition is key to understanding the mechanisms leading to capacity loss and battery failure. On the other hand, these Zn protrusions caused by hydrogen evolution reaction will attract more Zn 2+ flux ("tip" effect) [22] under concentrated electric field during electrochemical cycling, thus accelerating the vertical growth of Zn dendrites instead of planar growth and hydrogen production further flourish (Figure 1g). Up to now, various strategies have been evolved to prohibit the Zn dendrite growth, such as electrolyte optimization, [8,23-25] Zn surface coating, [9,21,26-29] and alloying. [30] To some extent, these strategies stabilize Zn metal, but they do The hydrogen evolution in Zn metal battery is accurately quantified by in situ battery-gas chromatography-mass analysis. The hydrogen fluxes reach 3.76 mmol h −1 cm −2 in a Zn//Zn symmetric cell in each segment, and 7.70 mmol h −1 cm −2 in a Zn//MnO 2 full cell. Then, a highly electronically insulating (0.11 mS cm −1) but highly Zn 2+ ion conductive (80.2 mS cm −1) ZnF 2 solid ion conductor with high Zn 2+ transfer number (0.65) is constructed to isolate Zn metal from liquid electrolyte, which not only prohibits over 99.2% parasitic hydrogen evolution but also guides uniform Zn electrodeposition. Precisely quantitated, the Zn@ZnF 2 //Zn@ZnF 2 cell only produces 0.02 mmol h −1 cm −2 of hydrogen (0.53% of the Zn//Zn cell). Encouragingly, a hig...

Hierarchical Fe3 O4 hollow spheres constructed by nanosheets are obtained from solvothermally synthesized Fe-glycerate hollow spheres. With the unique structural features, these hierarchical Fe3 O4 hollow spheres exhibit excellent electrochemical lithium-storage performance.