(273c) The Hunt for New Catalysts: Continuous Electrochemistry

Efficient energy storage is a major enabling technology for the widespread adoption of solar energy. A proposed solution is to store the energy chemically and split water into hydrogen and oxygen gas. For this method to be realized, catalysts must be found that make electrolysis more favorable. As the search for these materials continues, a better tool for synthesizing potential catalyst and subsequently analyzing these materials is needed.

The properties of microreactors make them an attractive solution for this problem. Potential catalysts can by synthesized in a microfluidic system with online detection tools capable of measuring electrochemical properties. A system of this nature would integrate synthesis and analysis in parallel. Moreover, the high surface area to volume ratio in microreactors enables faster heat and mass transfer than found in batch reactors. This allows for greater control over reaction conditions. In addition, if a material is made and analyzed continuously, the process can be optimized in situ. This scenario of synthesizing and analyzing material continuously also makes kinetic studies possible. By changing the flow rate in the microreactor one is able to look at conditions up and down the reaction coordinate, thereby obtaining kinetic information.

A key issue to running electrochemistry continuously is the reliability and reproducibility of the reference electrode. To obtain reliable data the reference electrode must remain stable, undergo no reactions and maintain a constant potential. A typical in-channel reference electrode, when operated continuously, will drift and react with the flowing analyte. This makes the continuous data inconsistent and nearly impossible to use. A unique microfluidic, electrochemical cell has been designed to address this problem. A separate reference channel has been incorporated fully on-chip, within a PDMS device that allows for consistent data while maintaining a simple fabrication process. This device can be operated continuously, enabling many experiments to be performed in one day. An advantageous example system, with an electrochemically active reactant and catalytically active product, the synthesis of IrO₂ was examined using this device to study kinetics by altering temperature and flow rate. Additionally, water-splitting catalysts developed by the Nocera group¹ were investigated to further understand their water-splitting capabilities. Novel physical studies that arise from the combination of microfluidic tools and traditional electrochemical methods will also be discussed.

1. Kanan, M.W.; Nocera, D.G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co²⁺. Science 2008, 321, 1072-1075