David G. Goodwin Memorial Lecture

Protonics for Sustainable Energy Technologies

Sossina M. Haile, Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University

Over the past decade, the costs of solar and wind electricity have plummeted, declining by about 90%. The challenge in achieving sustainable energy goals thus no longer lies in creating electricity technologies with negligible carbon footprint, but instead in creating methods for storing the electricity for use when the sun isn't shining or the wind isn't blowing. Electrolysis of water, or using electricity to split the H₂O molecule into hydrogen and oxygen, has garnered renewed interest due to the suitability of hydrogen for long term energy storage. Subsequent use of that hydrogen in fuel cells generates electricity without carbon emissions. Two challenges have prevented the realization of hydrogen as an energy storage medium. First, the electrochemical cells suitable for electrolysis have historically been unsuited to electric power generation (fuel cell operation), limiting system flexibility for combining local hydrogen storage with its utilization. Second, a hydrogen delivery infrastructure, as required for transporting locally produced hydrogen to point of use, remains to be developed. Here we describe recent advances in electrochemical cells based on solid state proton conducting electrolytes that tackle both of these challenges. Our protonic ceramic electrolyte systems enable reversible operation to both generate hydrogen from electricity and generate electricity from hydrogen, effectively functioning like rechargeable batteries. Our superprotonic solid acid electrolyte systems enable electrochemical conversion of ammonia into nitrogen and ultra-high purity hydrogen. Ammonia has emerged as an attractive carrier of hydrogen that is entirely carbon-free and can be easily liquified. Its local conversion to high purity hydrogen provides a fuel source for automotive polymer electrolyte membrane fuel cells which operate at high power densities but have little tolerance to impurities.