Evaluation Method of Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media

Abstract

Introduction Alkaline water electrolysis (AWE) produces hydrogen without any carbon dioxide emission with renewable electric power supply. Anode of the AWE is usually used Ni based material which is stable under steady electrolysis. To improve the AWE performance, the anode overpotential has to be reduced under fluctuating electricity from renewable energy that enhances deterioration of the anode. So, durable effective electrocatalyst is needed to enhance the energy conversion efficiency. Currently, most of the transition metal-based OER composite catalysts reported are powders, which are subsequently coated onto conductive substrates with the aid of Nafion. 1, 2)The utilization of binder decreases interfaces of catalyst/electrolyte and catalyst/catalyst, and reduces observed electrocatalytic activity.3) In addition, they have rarely been investigated under practical current density and potential region, because generated babble destroys bonded catalyst layer structure. In this study, as a new powder electrocatalyst evaluation method, we have investigated the OER activity of LaNiO3 using an electrolyzer with the structure to fix the powder on a diaphragm membrane, and compared the OER activity with that of the electrode prepared by CIP and sintering method which could be evaluated even in the high current density region.4) Experimental Figure 1 shows the schematic drawing of 1cm2 electrolyzer with independent effective area contact control system.It makes setup to press the catalyst supported portion onto the membrane. Ag/AgCl references placed on the membrane of seal area contacted with electrolyte. Cell components are made of Ti with Pt plating for anode side, and PEFCs grade glassy carbon for cathode side, respectively. Anode backing was porous Ti or Ti rib with Pt plating to reduce contact resistance. Anode and cathode electrocatalyst were LaNiO3 and PtRu/C (TKK), respectively. The 0.8~1.2 mgcm-2-metal of anode catalyst was coated onto Zirfon membrane, and the 0.5 mgcm-2-metal of cathode catalyst was coated onto a carbon paper (35BC, SGL) as cathode electrode. 7M KOH was circulated as the electrolyte. After pretreatment, the OER performance was evaluated Chronoamperometry (CA) and slow scan voltammetry (SSV). The loading amount of LaNiO3 was analyzed by XRF before and after electrochemical measurements. Results and discussion Figure 2 shows the separation of the measured chronoamperogram using rib anode backing to charging the faradic currents.5) The blue curve in Figure 2 is the measured current from the CA at 1.6 V, and it can be separated as the charging current of green line, which numerical simulated from I charging = (ΔE/R s)exp(-t/R s C d) [eq. 1] and the faradic current of red line that was difference between measured current and charging current. For the measured current, the initial value of the measured current is dominated by the charging current. Therefore, the charging current was determined by extrapolation with the initial measured current and the eq. 1. After a few milliseconds, the current would be dominated by the oxygen evolution reaction and decreased with time. The decrease would be affected by mass transfer suppressed by oxygen babble. Figure 3 shows the faradic current as a function of the square root of time (red). The current increased in the initial period, and decreased. By extrapolation to time 0 for the linear region of current decrease (black line), the mass transfer free faradic current at time 0 could be found. The intercept is defined as the kinetic current, I k,. Figure 4 shows polarization curves of the I k from the CA and the SSV using the porous anode backing of this work with the CIP and sintering method and the ink coating method.6, 7) In this figure, the apparent OER activity of the I k of the CA method and the CIP and sintering method were higher than others and the current was stable in high current density. So, the I k of the CA method would be suitable method to determine OER activity of powder catalyst, because the CIP and sintering method is limited to the materials that allows sintering process. Acknowledgment This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference 1) A. Grimaud et al. Nat commun, 4, 2439 (2013) 2) M. S. Burke et al. Chem. Mater, 27, 7549 (2015) 3) J. Ji et al. ACS Nano, 7, 6237 (2013). 4) Y. Tsukada et al. of the 42th Meeting of Electrolytic Technology, 86, Japan (2018/11) 5) Y. Bao et al. Electrocatalysis, 10, 184 (2019) 6) W. Zhou et al. Mater. Horiz, 2, 495 (2015) 7) R. Liu et al. Nano Energy, 12, 115 (2015) Figure 1