Nitrogen-doped Carbon-CoOx Nanohybrids: A Precious Metal Free Cathode that Exceeds 1.0 W cm-2 Peak Power and 100 h Life in Anion-Exchange Membrane Fuel Cells

Abstract

Efficient and durable nonprecious metal electrocatalysts for the oxygen reduction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, a 2D planar electrocatalyst with CoO_x embedded in nitrogen-doped graphitic carbon (N-C-CoO_x ) was created through the direct pyrolysis of a metal-organic complex with a NaCl template. The N-C-CoO_x catalyst showed high ORR activity, indicated by excellent half-wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was also observed in operating AEMFCs where the kinetic current was 100 mA cm^-2 at 0.85 V. When paired with a radiation-grafted ETFE powder ionomer, the N-C-CoO_x AEMFC cathode was able to achieve extremely high peak power density (1.05 W cm^-2 ) and mass transport limited current (3 A cm^-2 ) for a precious metal free electrode. The N-C-CoO_x cathode also showed good stability over 100 hours of operation with a voltage decay of only 15 % at 600 mA cm^-2 under H₂ /air (CO₂ -free) reacting gas feeds. The N-C-CoO_x cathode catalyst was also paired with a very low loading PtRu/C anode catalyst, to create AEMFCs with a total PGM loading of only 0.10 mg_Pt-Ru cm^-2 capable of achieving 7.4 W mg^-1 _PGM as well as supporting a current of 0.7 A cm^-2 at 0.6 V with H₂ /air (CO₂ free)-creating a cell that was able to meet the 2019 U.S. Department of Energy initial performance target of 0.6 V at 0.6 A cm^-2 under H₂ /air with a PGM loading <0.125 mg cm^-2 with AEMFCs for the first time.

Keywords: anion-exchange membrane fuel cell; cobalt oxide; electrocatalysis; oxygen reduction reaction.