Development of Electrochemical Cell Combined with Gas Chromatograph for in-Situ Detection of Hydrogen Evolution on Mg

Abstract

Magnesium (Mg) and its alloys are used for the negative electrode of the Mg air batteries because of its high reactivity in the aqueous solution. The hydrogen evolution on Mg in operation of Mg air batteries is one of the key issue because it may cause the self-discharge and loss of discharge capacity of Mg air batteries. The hydrogen evolution rate on Mg is increased when the anodic current is imposed to the Mg or Mg is anodically polarized above its corrosion potential. This phenomenon is called the negative difference effect, NDE. In order to clarify the NDE phenomenon, the volumetric hydrogen collection method was applied to the investigation of the hydrogen evolution on Mg during anodic dissolution1-3). We developed an electrochemical cell combined with a gas chromatograph 4). It allows the qualitative and quantitative analysis of the gas generated on working electrode because the gas is directly delivered to a gas chromatograph by a stream of carrier gas 4). The details of the developed system and measurement conditions are described as follows. The working electrode and counter electrode were an Mg and Pt respectively. In order to avoid the mixing of gases evolved on each electrode surface, the working electrode and counter electrode were placed in compartments separated by a glass filter and polymer membrane. The reference electrode was a KCl-saturated Ag/AgCl electrode (SSE). The electrolyte solution was 0.5 M NaCl of 10 ml, which was degassed by a nitrogen stream for 1 h before the measurement. The potentiostatic polarization was performed at -1.3 V vs. SSE for 2000 s. The gas-chromatographic measurements were performed starting 2 min after starting potentiostatic polarization. The hydrogen evolution rate was estimated by the results of the gas-chromatographic analysis. The volume of hydrogen gas V H2 (mL) and the hydrogen evolution rate v (mol s−1) are derived by the following equations. VH2 =Pc ×Cs ×A×Vs /(Pa ×As ×100) (1) ν=VH2 ×Ψ/(Vs ×22400×S) (2) In these equations, P c is the pressure of carrier gas (kPa), C s is the concentration of standard gas (%), A is the peak area of evolved hydrogen gas in the chromatogram (cm2), V s is the volume of the sample loop (ml), P a is the atmospheric pressure (kPa), A s is the peak area of standard hydrogen gas in the chromatogram (cm2), y is the flow rate of carrier gas (mL s−1), and S is the effective electrode surface area (cm2). In the present study, we determined the hydrogen evolution rate on Mg during the anodic dissolution by the developed system4). In addition, a nitrogen gas was investigated by the gas-chromatographic analysis during the anodic dissolution of Mg. The effect of solubility of hydrogen in the electrolyte solution on the amount of hydrogen gas detected by the present method was discussed. Reference: [1] G. S. Frankel, A. Samaniego and N. Birbilis, Corros. Sci., 70, 104 (2013). [2] S. Lebouil, A. Duboin, F. Monti, P. Tabeling, P. Volovitch, K. Ogle, Electrochim.Acta., 128, 284 (2013). [3] S. Fajardo and G. S. Frankel, J. Electrochem. Soc., 162, C693 (2015). [4] Y. Hoshi, R. Takemiya, I. Shitanda, and M. Itagaki, J. Electrochem. Soc., 163, C303 (2016).