Storing energy in chemical bonds - Strategies for electrochemical conversion of organic waste to petrochemicals and fuels

Sustainable transportation is one of the grand issues of our day. Transitioning from an economy based on depletable carbon to a circular and connected economy that reuses carbon will provide new opportunities but will require science and technology that is economical in small distributed processing facilities and that match availability of renewable power. Furthermore, the strategies for storing energy in chemical bonds need to consider the larger context of energy systems. Sustainable transportation is a vexing issue in the United States, a country that uses 30 EJ of primary energy for transportation and 100 EJ of total primary energy each year. There is a gap between the amount of electrical energy generated and the energy in liquid fuels used. In 2018 the U.S. generated 15 EJ of electricity (from 40 EJ of primary energy) while consuming 27 EJ of gasoline, diesel, and jet fuel. Increasing the fuel economy of vehicles is a critical step for reducing energy and greenhouse gasses in the transportation sector. Various levels of electrification provide one means for improving fuel economy. Yet many modes of transportation have little opportunity for electrification (heavy trucking, shipping, and aviation) and combined these modes use 10 EJ per year in the U.S., about four times the amount of electricity generated from renewables last year. Today there is excitement at the prospects of re-using CO₂ as a carbon source for fuels. However, if it were practiced it would result in elevated energy consumption for transportation. To produce 1 EJ of fuel would require 1.8 EJ of electricity to satisfy thermodynamics for CO₂ and H₂O conversion into just synthesis gas. Its conversion into liquid fuels would incur additional inefficiencies. At large scale, the amount of energy needed has implications on land use. Solar power requires 3.2 hectare/ MW; under ideal conditions 56 GW of power would be needed over a year to meet the thermodynamic demands of synthesis gas required to make 1 EJ of fuel. The cost of the electricity as a contributor to fuel cost is also high, often higher than the value of the fuel. There are other carbon sources in the form of organic waste, such as polymers (“man-made” carbon resources) and biogenic waste (“organic” carbon resources) that still contain energy and which are a better match for the amount of renewable electrons available. The electrochemistry for converting organic carbon is challenging, requiring reducing mixtures of carbonyls, phenolics, and carboxylic acids into alcohols and hydrocarbons. In this talk we will consider a number of strategies in context of the energy system for storing energy in chemical bonds and electrochemical production of fuels from organic waste in circular and connected economies of the future.