Progress toward Metal-Air Batteries: Mechanistic Investigation of the Effect of Water on the Oxygen Reduction Reaction at Carbon-Conductive Polymer-Silver Composite Air Electrodes

Abstract

The oxygen reduction reaction (ORR) at the carbon-conductive polymer-silver (C-cp-Ag) composite electrode in non-aqueous electrolyte with small amounts of added water is the subject of this study. The contributions of the various components of the composite electrode were assessed by employing four electrodes: (1) glassy carbon (C), (2) polypyrrole coated glassy carbon (C-cp), (3) silver disk (Ag), and (4) carbon-polypyrrole-silver composite (C-cp-Ag). Notably, with 5000 ppm of water in non-aqueous solution, the ORR reaction at Ag and C-cp-Ag shows an n = 4 reduction, while ORR at C and C-cp display an n = 1 reduction. The results show that the use of a multilayer C-cp-Ag composite electrode provides the opportunity to achieve the four electron reduction of one O 2 molecule, with a low precious metal (Ag) loading. In metal air batteries, the cathode consists of the electroactive cathode material, oxygen (O 2 ) and the remainder of the inert electrode consisting of the current collector and oxygen reduction reaction (ORR) catalyst. Metal air batteries fall into a special category as the electroactive cathode material, oxygen (O 2 ), is available in excess from outside the battery. While the cathode current collector, the ORR catalyst and the oxygen reduction products all add mass, 1,2 metal air batteries still provide the opportunity for high energy densities relative to sealed battery technologies. 3,4 Notably, the structures and chemistries of the inert electrode (air electrode) can be varied to address ORR kinetic issues. Typically, a composite air electrode consists of an electrical conductor mixed with an ORR catalyst, often strengthened with a binder 5,6 and a support such as a metal mesh. 7,8 A disadvantage of the conventional air electrode fabrication strategy is that catalyst particles positioned within the electrode interior often have limited access to oxygen.With an earlier article, we introduced a new composite electrode paradigm for metal air batteries, and reported the preparation, characterization, and electrochemical activity of a carbon current collector-conductive polymer-silver (cc-cp-Ag) composite electrode. Enhanced oxygen reduction activity for our composite electrode was observed relative to coated glassy carbon or silver disk electrodes, at a low silver loading of < 0.3 mg cm −2 . Specifically, the role of the current collector toward the electrochemical reduction of oxygen, the role of the conductive polymer in improving the structural integrity of the composite electrode, and a quantitative study of the silver loading effect on ORR activity were all investigated. A notable advantage of the electrodeposition based strategy we developed is the ability to easily generate silver coated three-dimensionally structured composites via use of three dimensional electrically conductive substrates. Three dimensional electrodes can increase the active surface area, enabling reduction of more oxygen per unit planar area. This approach was utilized to prepare three-dimensionally structured carboncondu...