Rapid inkjet printing of high catalytic activity Co3O4 /N-rGO layers for oxygen reduction reaction

The accurate and reproducible large-scale production of catalyst layers containing low-cost, abundant electrocatalysts gains in importance. Herein, pivotal factors are discussed that need to be considered when a combined inkjet printing and photonic curing platform is used as a promising fabrication method for catalyst layers based on a model low-cost catalyst, i.e., nitrogen-doped reduced graphene oxide supported cobalt oxide nano sheets (Co3O4/N-rGO), specifically prepared to formulate an inkjet ink. The ink is stable for weeks and can reproducibly be printed with piezoelectric printheads. Ink composition and printing parameters are optimized to achieve high-resolution printing and good adhesion on glassy carbon substrates. Polyvinylpyrrolidone and ethyl cellulose are used as catalyst stabilizers in the ink and must be removed through thermal post-processing to avoid a decrease of the electrical conductivity of the catalyst layer and a degradation of the catalytic activity of the Co3O4 nanocrystals. Conventional slow oven curing (i.e., hours) and photonic curing with a Xe flash lamp (seconds) are compared to generate temperatures above 400 °C under ambient conditions. Both techniques can increase the size of the Co3O4 nanocrystals from ~7 nm up to ~15 nm. Photonic curing with pulses above 2 J·cm–2 shot energy density initiates the reduction of the oxidation states of cobalt from (II,III) to (II). Residues and side products of polymeric stabilizers can be found using photonic curing pulses below 10 J·cm–2. This work highlights the advances made in digital printing and post-processing for catalyst layer production and demonstrates the importance of proper design of the ink, the printing and the post-processing for the large-scale production of catalyst layers for the ORR based on low-cost materials. The findings can be transferred to other metal and mixed metal oxide nanocatalysts.