Optimization of Annealing Catalyst Powder for High Temperature PEMWE

Abstract

1. IntroductionRenewable energy is an alternative choice to solve the universal problem of energy depletion and environmental pollution. Among various energy carriers, hydrogen is a most competitive one, because it is friendly to environment and can be produced from renewable energy through water electrolysis. Among types of water electrolysis, polymer electrolyte membrane water electrolysis (PEMWE) attracts most attention for its advantages because of simple structure and high energy conversion efficiency.Usually, water electrolysis needs higher voltage supply than theoretical value, due to high overvoltage in oxygen evolution reaction at anode. To minimize the anodic overvoltage, it is recently suggested that annealing catalyst powder at certain temperature and time can decrease both the HFR and activation overvoltage, and the annealing effects are proven in the conventional PEMWE, where operation temperature is around 80℃ [1-4]. The current mechanisms about how annealing impacts electrolysis performance are as followings: (i) annealing can largely decrease electrolysis voltage by reducing HFR [1]; (ii) annealing will enlarge the specific surface area [2]; (iii) annealing increases the catalyst activity by changing the structure of catalyst particle [3~4]. In order to examine the above three mechanisms, we evaluated I-V and I-HFR characteristics of IrO2 samples which have different specific surface area and were annealed under different temperatures and time. This study also challenges to find the optimum condition of annealing time and temperature, to minimize electrolysis voltage under rather high temperature operation of 100℃. 2. ExperimentPEMWE cell used in this study consists of a homemade CCM (with electro-catalyst area of 4cm2), porous current collectors, and separators with flow channels. For the CCM fabrication, anode and cathode catalytic electrodes are hot pressed to a membrane (Nafion117). Iridium oxide powder (IrO2 powder, type IV, Tokuriki Co., Japan) as for anode catalyst is annealed in air at different time and temperatures shown in table 1. Rasten et al [1] suggests that annealing at 490℃ and 8hrs results in the lowest electrolysis voltage. Siracusano et al [3~4] shows that the optimum annealing condition should be 350℃ and 1hr. Based on these, Annealing condition of the powder is at 350℃ and 490℃ from 1 to 8 hrs. To examine the effect of enlarging specific surface area, IrO2 samples with two different specific surface area were selected: sample 1 and sample 2 originally has the specific surface area of 42m2/g and 84m2/g respectively. A commercial 46% Pt/C (Tanaka Kikinzoku Japan) catalyst is used for cathode electrode. Ti mesh (Nikko Techno Co., Japan, fiber diameter 20μm) is used for anode current collector. As for cathode current corrector, carbon paper (hydrophobic, SGL Co., Germany, 34BC) is chosen for comparing with each other. Water at room temperature is fed into only anode channel at a flow rate of 1mL/min, which corresponds to the water utilization of 2% at 1A/cm2. All the experiments are conducted at atmospheric pressure and 100℃.3. Result and discussionFig. 1 (a) and (b) shows how specific surface area and annealing conditions impact on I-V characteristics and high frequency resistance at 10 kHz. The IV and I-HFR characteristics revealed that: (i) enlarging specific surface area to two times has little impact on electrolysis performance; (ii) annealing catalyst powder at 350℃ and 1hr introduced the best electrolysis performance and annealing at 490℃ and 8hrs introduced the highest electrolysis voltage. When annealing temperature is 350℃, long annealing time will raise the electrolysis voltage. But when annealing temperature is 490℃, annealing time makes a limited impact on electrolysis voltage. It also should be noticed that the OCV changes with annealing temperature, annealing catalyst annealed at 490℃ generates much higher OCV (about 80mV) than that annealed at 350℃. Such phenomenon means annealing at 490℃ will decrease the catalyst activity. Annealing catalyst at certain temperature can decrease the HFR, but the decreased ohmic overvoltage can’t explain the total electrolysis voltage difference. Therefore, among the above three main mechanisms, annealing method should impact the electrolysis performance by increasing the catalyst activity via changing the structure of catalyst particle.