(Sensor Division Outstanding Achievement Award) Mixed Potential Sensors for Hydrogen Safety and Automotive Applications

Abstract

Mixed potential sensors are a class of non-equilibrium electrochemical sensors capable of detecting the presence of reducing or oxidizing gases. These devices usually consist of two electrodes supported on an electrolyte, and exposed to a sensing gas without the need for a separate reference electrode. The potential at each of the electrode/electrolyte interfaces is determined by the rates of electrochemical oxidation and reduction reactions. Since electrode composition, morphology and sample temperature control the rates of these reactions, it is theoretically possible to select a combination of electrodes and an operating temperature to provide sensitivity and selectivity to a desired gas species. Over the past 15 years Los Alamos National Laboratory (LANL) has developed yttria stabilized zirconia (YSZ) electrolyte based mixed potential sensors for various applications. Novel 4-electrode experiments were used to unravel the working principle of these sensors and it was determined that the sensor sensitivity was primarily controlled by the rate of oxygen reduction reaction (ORR) and amount of heterogeneous catalysis of the reducing/oxidizing gas. Electrodes with poor ORR kinetics and low heterogeneous catalysis rates yielded the highest mixed potentials. This observation led to the development of dense electrode and porous electrolyte morphologies for optimal sensitivity. Furthermore, the use of dense electrodes also improved the morphological stability of these devices leading to exceptional long-term durability. The LANL sensor design is unique and does not use the dense electrolyte and porous electrodes configuration that most sensors use. In this talk the development of new techniques including thin film deposition, tape casting and high temperature ceramic co-fired (HTCC) processes for the manufacturing of these novel sensors will be discussed. While tape casting and thin film deposition yield sensors with high sensitivity and long-term durability, they are not easily scalable for low cost manufacturing. LANL, in collaboration with ESL ElectroScience, has developed a HTCC process to manufacture low cost sensors that retain the unique advantages of the LANL design. This talk will focus on the development of nitrogen oxide (NOx), ammonia (NH3) and hydrogen sensors. The sensing electrodes for the nitrogen oxide, ammonia and hydrogen sensors are La1-xSrxCrO3- d, Au and Sn1-xInxO2- d respectively. All sensors use a Pt counter electrode and the LANL patented design, where dense electrodes are partially covered with a thin YSZ electrolyte layer. While the hydrogen and ammonia sensors are operated in the open circuit mode, the nitrogen oxide sensor is operated under a current bias to minimize selectivity to hydrocarbons and improve selectivity to both NO and NO2. This talk will also discuss the evaluation of these sensors in end-user applications. Data from dynamometer testing of NOx and NH3 sensors at Oak Ridge National Laboratory’s National Transportation Resource Center will be presented. Hydrogen safety sensor data from field trials at a hydrogen fueling station in California will also be presented. Finally the barriers and hurdles for the successful commercialization of these sensors will be discussed.