Proposal and Fundamental Investigation of Energy Storage and Hydrogen Supply System Using the Lithium
Authors: Uemura Suguru , Ikegami Shuntaro , Tabe Yutaka 
Published: February 2024

In recent years, variable renewable energy such as solar power and wind power spreads rapidly. At the same time, taking measures against output fluctuations and utilization of surplus electricity become important issues. Conversion of electricity to hydrogen is considered as one of the utilizations of surplus electricity [1]. The hydrogen is stored and can be supplied to the fuel cell vehicles according to demand, regardless of weather or time. However, the disadvantage of hydrogen is its low energy density. Although several methods have been investigated for high-density storage of hydrogen, there are still issues about safety, energy loss, cost, and durability [2].
In this study, we propose a new energy storage and hydrogen supply method using lithium (Figure 1). The surplus electricity can be converted by the following reaction, and the energy is stored as the lithium.
Li+ + e- → Li (-3.04 V vs. SHE) (1)
4LiOH → 4Li+ + O2 + 2H2O + 4e- (0.41 V vs. SHE) (2)
The lithium can be stored at room temperature and pressure, and its volumetric energy is higher than that of conventional hydrogen storage methods. When hydrogen is needed, the system can be discharged to produce hydrogen by the following reaction.
Li → Li+ + e- (-3.04 V vs. SHE) (3)
2Li+ + 2H2O + 2e-→ 2LiOH + H2 (-0.82 V vs. SHE) (4)
Since the lithium is highly reactive, discharge reaction with water occurs, and hydrogen can be produced without energy input from the outside. As shown in Equation (2) and Equation (4), charge and discharge reactions at the cathode are different, and this system combines the characteristics of lithium-air cell [3] and lithium-water cell [4]. This system has a potential to utilize the surplus electricity; but it has not been fully evaluated from the viewpoint of hydrogen production [5]. Therefore, present study investigated the characteristics of the discharge reaction and hydrogen production behavior.
Figure 2 shows experimental cell. The composite anode was water-stable and had a multilayer structure [6]. LATP (Li1+x+yAlxTi2-xSiyP3-y, Ohara Co., Kanagawa, Japan), which is a water-stable lithium-ion-conducting glass ceramic, was used, along with PEO18LiTFSI as the organic electrolyte. The lithium foil and polyethylene oxide (PEO) membrane were sealed inside the cell to prevent direct contact with water vapor and oxygen. Since the cell resistance depends on the temperature, the composite anode was kept at 70°C. For the cathode, glassy carbon rod was used.
Figure 3 shows current-voltage (I-V) characteristics. The cathode potential at low-current discharge is higher than theoretical value (-0.82 V). It was caused by reaction with dissolved oxygen in the electrolyte. As the current value is increased, the cathode potential drops sharply, and many bubbles were observed on the cathode end face. It is thought that oxygen concentration overvoltage increased significantly and the discharge reaction with water (hydrogen production as shown in Equation (4)) was occurred.
The volume of the produced bubble was measured with using a sealed cathode and adding a liquid level gauge (not shown in Figure 2). As a result, the measured volume showed good agreement with the theoretical hydrogen volume produced by the reaction of Equation (4). It is considered that the reaction shifts to hydrogen production and the effect of dissolved oxygen can be negligible at sufficiently low cathode potential (i.e. high current discharge).
The efficiency of this system was estimated by using the obtained experimental results. Generated electricity and enthalpy of produced hydrogen was calculated. When a 20 μA discharge test was conducted by using experimental cell, the discharge efficiency and conversion rate to hydrogen were 30% and 39%, respectively. For comparison, the experimental cell was converted to the lithium-air battery mode and discharged at 20 μA. The discharge efficiency was 56%. Considering the conversion ratio of lithium to electricity and hydrogen, the proposed hydrogen supply system using the lithium can utilize the variable renewable energy.

References:

[1] Y. Meiling et al., Renew. Sustain. Energy Rev., 46, 111180 (2021)
[2] I.A. Hassan et al., Renewable Sustainable Energy Rev., 149, 111311 (2021)
[3] S. Uemura et al., ECS Trans., 58-12, 15-20 (2019)
[4] N. Shuster, Proceedings of the 34th International Power Sources Symposium, 118-123 (1990)
[5] Y. Wang et al., ChemSusChem, 3, 571-574 (2010)
[6] T. Zhang et al., J. Electrochem. Soc., 155, A965 (2008)

Figure 1