A manganese–hydrogen battery with potential for grid-scale energy storage
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One kind of the aqueous battery with low cost, high energy density and long cycle life has been developed, which made the storage of intermittent energy sources at the grid scale possible.
The dendrite formation on the zinc anode of manganese–zinc batteries was avoided.
The Charge storage mechanism of manganese–zinc batteries was proposed and the scale up of the cell was performed.
    The renewable sources of energy such as solar and wind were intermittent, thus the electrochemical energy storage devices like batteries are necessary. The development of lithium-ion, lead–acid, redox-flow, sodium–sulfur and liquid-metal batteries shows promise for grid-scale energy storage. However, they have various issues of low energy density, poor rechargeability, high cost at the battery pack level and high operating temperature, which limited their application. 
Fig.1 Schematic and simulation of the Mn–H battery.
 
    In this paper, the cell was composed of a cathode-less porous carbon felt, a glass fibre separator, a Pt/C catalyst-coated carbon felt anode and a soluble Mn2+ aqueous electrolyte. When charging the Mn–H cell, soluble Mn2+ ions in the electrolyte diffuse to the cathode and deposit in the form of solid MnO2 on the carbon felt, while H2 gas evolution from H2O is driven by highly active platinum catalysts on the anode. During discharge of the battery, the uniform layer of as-deposited MnO2 on the cathode is dissolved back to soluble Mn2+ electrolyte and H2 is oxidized on the anode. The overall cell operation can be described in the following:
    The battery chemistry exhibits a discharge voltage of ~1.3 V, a rate capability of 100 mA cm−2 (36 s of discharge) and a lifetime of more than 10,000 cycles without decay. A gravimetric energy density of 139 Wh kg-1 with the theoretical gravimetric energy density of ~174 Wh kg−1 was achieved in a 4 M MnSO4 electrolyte.
Fig. 2 Electrochemical performance of the Swagelok-type Mn–H cell.

    Besides, theoretical calculation was performed to investigate the mechanism of battery charge/discharge, and the self-discharge of battery was also observed. Two kinds of battery structure were used to investigate the possibility of Mn-H battery in large-scale energy storage applications. It seemed that using thicker cathode carbon felts with a larger surface area can increase the cell capacity.
Fig.3 The setup of the Swagelok-type and cylindrical Mn-H cell.
Fig.4 Scale-up of the Mn–H cell.
 
    The Mn–H cell has at least seven unique features and advantages over the previously reported battery systems. 
  First, the cathode-less design avoided the complex preparation of the conventional MnO2 cathode and thus providing a cost-effective approach for the battery manufacturing. 
    Second, the manganese dissolution process was utilized for our benefit as the principal charge storage mechanism in the Mn–H cell, which was usually failure mechanism in conventional aqueous manganese batteries. 
    Third, the two-electron reaction of Mn2+/MnO2 gives rise to a high theoretical capacity of 616 mAh g−1 for the Mn–H cell based on the mass of solid MnO2, which doubles the value of most previous Mn batteries with a one electron reaction (308 mAh g−1). 
    Fourth, a highly reversible hydrogen electrode was adopted as the anode by utilizing Pt-catalyzed HER/HOR reactions to overcome the poor rechargeability of the conventional anodes. 
   Fifth, the fast kinetics of the Mn2+/MnO2 reactions at the cathode and the HER/HOR at the anode contribute to the high rate capability of the Mn–H cell. 
    Sixth, the high solubility of the Mn2+ ions in water gives rise to a high theoretical gravimetric energy density of ~174 Wh kg−1 and volumetric energy density of 263 Wh l−1 in the 4 M MnSO4 electrolyte for the Mn–H cell. 
   Seventh, the utilization of low-cost raw materials such as MnSO4 salt, carbon felt and glass fibre separators in the Mn–H cell could make it an inexpensive system.
Nevertheless, there are three problems need to be resolved before the practical application:
    First, the achieved capacity and energy density of the Mn–H cell is limited by the low efficiency of the carbon felt substrate.
    Second, highly efficient substances such as carbon nanofibers are essential to the improvement of the electrochemical performance of the Mn–H cell. 
    Third, platinum-free electrocatalysts are needed for the development of low-cost hydrogen batteries for large-scale energy storage.

Wei Chen, Guodong Li, Yi Cui et al. A manganese–hydrogen battery with potential for grid-scale energy storage. Nature Energy 2018.