Investigation of Ni/C and Pt/C Catalyst Stability in an Anion Exchange Membrane (AEM) Fuel Cell

Abstract

Anion exchange membrane fuel cells (AEMFC) have gained increasing research interest because its alkaline chemistry enables non-platinum group metal (non-PGM) catalysts to be used. Despite this opportunity, many researchers still focus on PGM material due to their performance. Often the approach is to minimize PGM content by using non-PGM supporting substrates, such as Pt-Ni foam and Pt-Ni/Ni-B catalysts in order to mitigate cost [1] [2]. Although the performances of both PGM and non-PGM catalysts have been thoroughly researched, their stability under an AEMFC environment is seldom questioned. This is because PGM materials, typically Pt, are noble metals and many non-PGM materials are considered stable under alkaline conditions, especially nickel. However, the Pt stability in PEMFCs has been severely challenged during fuel cell operations. Ferreira et al. reported the Pt catalyst particle aggregation with decreased PEMFC performance [3]. Helmly et al. reported Pt dissolution in a PEM electrolyte. In alkaline fuel cell operation, the amount of dissolved Pt was reported to be even higher than from PEM FC operation [4]. Hence, the Pt stability is of concern under AEMFC conditions. On the other hand, Ni stability does not appear to have been reported. Therefore, the focus of this study is to examine catalyst stability of Pt and Ni under identical AEMFC operating conditions. This study focuses on pure Pt and Ni carbon supported catalysts, which have been conventionally used to operate AEMFCs. To study and compare the two materials under the same test condition, both Ni/C and Pt/C catalysts were applied on the same AEMFC in separate locations as shown in Fig.1. A sandwiched AEM was used so that catalyst migration could be studied without interference from direct catalyst contact. The AEMFC was held at 0.9V by a potentiostat with H2 and O2 gas flowing at each specific electrode. Once per day, the cell open circuit potential (OCP) was measured without the imposed voltage. To identify any catalyst morphology change after 30 days (~500hrs) operation, scanning electron microscopy (SEM) was conducted on a cross section of each catalyst layer; transmission electron microscopy (TEM) was used to observe catalyst particles from each layer; inductively coupled plasma (ICP) analysis was used to determine if any catalyst could be detected in the middle AEM. The OCP was found to have dropped by 0.2V during the 30 day operation. Significant catalyst morphology changes were found at the cathode for both Ni/C and Pt/C catalysts. SEM analysis showed that the Ni material was found to develop a band like structure at the AEM-catalyst layer interface. The TEM analysis confirmed that the Ni nanoparticles developed a smeared shape on the carbon substrate, which was verified as a Ni oxide species using energy-dispersive X-ray (EDX) mapping. The SEM images showed that the Pt signal from the cathode catalyst layer was much weaker than the one from the anode. TEM analysis showed that the Pt remained as nanoparticles on the carbon substrate, but that the nanoparticle density appeared lower than on the anode, consistent with Pt dissolution. The ICP analysis showed both Ni and Pt in the middle AEM. The results indicate, substantial morphology changes occurred on both Ni/C and Pt/C catalysts during AEMFC operation. Additional research is needed to investigate the change of morphology mechanisms, which is key to developing enhanced stability of AEMFC catalysts. [1] Daping He, Libo Zhang, Dongsheng He, Gang Zhou, Yue Lin, Zhaoxiang Deng, Xun Hong, Yuen Wu, Chen Chen & Yadong Li, Amorphous nickel boride membrane on a platinum–nickel alloy surface for enhanced oxygen reduction reaction, Nature Communications volume 7, Article number: 12362 (2016) [2] Julia van Drunen, Brandy K. Pilapil, Yoseif Makonnen, Diane Beauchemin, Byron D. Gates, and Gregory Jerkiewicz, Electrochemically Active Nickel Foams as Support Materials for Nanoscopic Platinum Electrocatalysts, ACS Applied Materials & Interfaces 2014 6 (15), 12046-12061 [3] P. J. Ferreira, G. J. la O,, Y. Shao-Horna, D. Morgan, R. Makharia, S. Kocha, and H. A. Gasteiger, Instability of Pt ∕ C Electrocatalysts in Proton Exchange Membrane Fuel Cells A Mechanistic Investigation J. Electrochem. Soc. 2005 volume 152, issue 11, A2256-A2271 [4] Serhiy Cherevko, Aleksandar R. Zeradjanin, Gareth P. Keeley and Karl J. J. Mayrhofer, A Comparative Study on Gold and Platinum Dissolution in Acidic and Alkaline Media J. Electrochem. Soc. 2014 volume 161, issue 12, H822-H830 Figure 1