Challenging of Reducing Electrolysis Voltage by Superimposing Boiling on PEMWE–A Thermodynamic Coupling–

Thursday, 5 October 2017: 15:40
National Harbor 15 (Gaylord National Resort and Convention Center)
K. Ito (Department of Hydrogen Energy Systems, Kyushu University), K. Terabaru (Dept. of Hydrogen Energy Systems, Kyushu University,), H. Li (Dept. of Hydrogen Energy Systems, Kyushu University), A. Inada (Kyushu University), and H. Nakajima (Department of Hydrogen Energy Systems, Kyushu University)
We challenge to reduce electrolysis voltage by thermodynamic coupling. When PEMWE reaches 100°C under atmospheric pressure, boiling progresses spontaneously and contributes to entropy generation. In principle, the entropy generation accelerates water electrolysis reaction through the thermodynamic coupling, resulting in reducing the electrolysis voltage of PEMWE.

In this study, we experimentally clarify whether electrolysis voltage drop at boiling temperature. The electrolysis voltage of a PEMFC is continuously measured while raising operation temperature from 80°C to 120°C under constant operation pressure (0.1 MPa) and electrolysis current (0.002 A/cm2). If the electrolysis voltage drops at boiling temperature, this matter becomes an evidence for the thermodynamic coupling between boiling and electrolysis. PEMWE used in this study embeds a CCM made with spray and hot-press method. Polymer electrolyte membrane (PEM) is Aquivion membrane. Catalyst layer formed on the membrane consists of IrO2 (1.5mg/cm2) for anode and PT/C (0.5mg/cm2) for cathode. The area of catalyst layer is 4 cm2.

Figure shows the electrolysis voltage and HFR (high frequency resistance, 1 kHz) to the change of temperature. The HFR approximately corresponds to proton resistance through PEM. The electrolysis voltage begins to abruptly decrease just over 100°C, and turns to increase at 105°C, showing a valley on the temperature-voltage characteristics. Each slope on the characteristic (dV/dT) near 80°C and 120°C closes to temperature dependence of water-electrolysis Gibbs free energy change (ΔG(T)) in the case of liquid and gas, respectively.

The electrolysis voltage drop appeared is analyzed based on overvoltage component. HFR began to increase just over boiling point, because water in cell evaporated, PEM dried up and the proton resistance rose. However, ohmic overvoltage change attributed to the rise of proton resistance is neglectable to the electrolysis voltage change. Concentration overvoltage also can not explain the electrolysis voltage drop, because the current is controlled to be small as 0.002 A/cm2. Thus, the voltage drop appeared near boiling point attributes to the effect of boiling on activation overvoltage and/or Nernst voltage.

To summarize, the electrolysis voltage in PEMWE shows the abrupt drop near 100°C under atmospheric pressure and and small electrolysis current (0.002 A/cm2) when temperature successively rises from 80°C to 120°C. Boiling imposed in the cell is thought to impact the electrolysis voltage drop through the thermodynamic coupling. The boiling, specifically, is considered to impact on activation overvoltage and/or Nernst voltage, leading to the voltage drop.