Sequential Generation of Hydrogen and Oxygen By Water Electrolysis Using Iodine Redox
Authors: Kai Asuna , Tomisaki Mai , Matsumoto Hiroshige 
Published: December 2024

Introduction

In a hydrogen-centric society, hydrogen production via water electrolysis plays a crucial role1. Conventional water electrolysis generates hydrogen and oxygen simultaneously, necessitating a separation mechanism to prevent purity reduction and explosion risks induced by gas mixing. This study aims to selectively generate hydrogen and oxygen sequentially in two steps by utilizing the redox reaction between iodide and iodate ions. Specifically, exploiting the lower oxidation potential of iodide ions compared to the oxygen evolution potential, hydrogen is generated during the first step alongside the oxidation of iodide ions. Utilizing the lower reduction potential of iodate ions compared to the hydrogen evolution potential, oxygen is generated during the second step alongside the reduction of iodate ions. This presentation reports the results of investigating the feasibility of generating hydrogen and oxygen based on this principle.

Experimental

A two-compartment cell separated by a Nafion membrane was employed as the electrolysis cell. Platinum electrodes were used as the working and counter electrodes, and Ag/AgCl served as the reference electrode. Potentials were expressed relative to the reversible hydrogen electrode (RHE) which was converted from the measured potential (vs. Ag/AgCl) by the following equation: E
RHE = E
Ag/AgCl + 0.198 + 0.059 pH (V)2. Linear sweep voltammetry (LSV) measurements were conducted at each step to observe the current-potential characteristics and to determine at which potential only iodide ions and iodate ions react. In the first step, the anode was filled with 10 mM, 50 mM, and 100 mM KI aqueous solutions (in 100 mM KOH), while the cathode was filled with 100 mM KOH aqueous solution. In the second step, the cathode was filled with 10 mM, 50 mM, and 100 mM KIO3 aqueous solutions (in 100 mM KOH), while the anode was filled with 100 mM KOH aqueous solution. Control experiments using 100 mM KOH aqueous solution in both compartments were conducted for comparison with two-step water electrolysis. Constant potential electrolysis was performed at the potentials determined from LSV measurements, and the reactants and products were analyzed using a gas chromatography and a high-performance liquid chromatography to examine current efficiency. All electrochemical experiments were conducted at room temperature.

Results and Discussion

LSV measurements in the first step revealed a peak of oxidation current between +1.4 V and +2.0 V, increasing with the initial concentration of iodide ions. Comparison with control experiments indicated that this oxidation potential was lower than that of oxygen evolution reaction. Hence, under conditions where iodide ions are contained in the anolyte, oxidation of iodide ions occur at a lower potential than oxygen evolution reaction. In the second step, a peak of reduction current appeared between +0.2 V and –0.6 V, increasing with the initial concentration of iodate ions. Comparison with control experiments suggested that the reduction reaction of iodate ions occurred at a lower potential than hydrogen evolution reaction. Utilizing the identified potentials which iodine redox reactions selectively occur, constant potential electrolysis revealed iodate ions being generated at the anode in the first step and iodide ions being generated at the cathode in the second step. It was found that the maximum current efficiency for the oxidation reaction of iodide ions was 98% at +1.9 V in the first step, while the maximum current efficiency for the reduction reaction of iodate ions was almost 100% at +0.1 V in the second step.
These results confirmed the feasibility of water electrolysis using iodine redox reactions, with desired reactions occurring efficiently.

Acknowledgment

This research has been supported by Science and Technology Research Partnership for Sustainable Development (SATREPS), Japan Science and Technology Agency (JST) / Japan International Cooperation Agency (JICA) JPMJSA2306.

References

1) D. Agrawal, N. Mahajan, S. A. Singh, I. Sreedhar, Fuel
2024, 359, 130131.
2) Y. Liu et al., J. Am. Chem. Soc.
2015, 137, 11631-11636.

Figure 1