Development of a Controlled Humidity Differential Electrochemical Gas Monitoring System

Abstract

Lithium-air batteries (LABs) represent a promising future battery technology due to their high energy density, although several challenges hinder them from achieving their theoretical performance. These challenges include clogging of the porous air cathode by insoluble discharge products and high overpotentials during cycling [1, 2]. A recent proposal to address these issues involves the reversible formation and oxidation of LiOH in nonaqueous LABs. Of course, LiOH-based LABs require a proton source to form LiOH from the electrochemical reaction of oxygen and lithium ions [3]; humidity in air is an obvious choice for the proton source, making the relative humidity of LAB feed gas an important parameter to control and monitor during LAB electrochemical analysis.For LABs, galvanostatic charge/discharge coupled with quantitative gas evolution/consumption measurement is essential to understand the electrochemical processes during battery operation. Here, we report an advanced operando pressure measurement system that allows the control and monitoring of humidity in the cell headspace while also tracing overall gas consumption and evolution. Our gas handling system generates humid air using a water-containing chamber with a temperature control vessel to generate water vapor. A mass flow controller mixes dry air with humid air to generate specific relative humidity and send it to the cell. Using various low-volume in-line sensors, our system can track headspace humidity, temperature, and pressure within a custom-modified Swagelok-type cell. The total headspace volume must be small (~mL) since gas consumption occurs on a μmol scale in our lab-scale cells. To minimize headspace and dead volume within the Swagelok-type cell, a digital humidity and temperature sensor with exceptional accuracy was integrated into a customized printed circuit board (PCB). This PCB is attached to the small dead-volume Tee connection. To control the humidity inside the cell and achieve sensor data simultaneously, all sensors in the monitoring system are controlled by a LabVIEW program (National Instruments). To show the utility of this new system, we will present analyses of nonaqueous and solid-state LAB cells that operate at various humidities and current rates, while employing various known 2 e- and 4 e- oxygen reduction catalysts.