Carbon footprint comparison of liquid organic hydrogen carriers cycled thermochemically and electrochemically

Hydrogen (H2) has been proposed as a way to store energy for long durations with minimal carbon footprint. The challenges of using H2 to store energy are that it has very low volumetric density and diffuses rapidly through container walls. Liquid organic hydrogen carriers (LOHCs) have been proposed as a method of storing hydrogen in the molecular backbone of stable organic chemicals, addressing many of the concerns of molecular hydrogen. The majority of LOHC systems proposed have utilized thermochemical cycling for the hydrogenation (to store hydrogen) and dehydrogenation (to release hydrogen). Thermochemical cycling requires heating of the reactors, which results in an increased carbon footprint, and is not easily amenable to dynamic operating with the variable renewable electricity grid. Electrochemical LOHC cycling has been proposed as an alternative to thermochemical cycling because it can pair directly with the variable renewable grid and operate more dynamically. To understand the viability of the thermochemical and electrochemical processes, a comparative carbon footprint analysis is necessary. The analysis showed that the electrochemical LOHC cycling process achieves the lowest carbon footprint when using highly concentrated LOHCs as the feed or when a downstream separation process was not needed. The carbon footprint in electrochemical cycling of diluted LOHC was primarily contributed to by the LOHC distillation separation process. A sensitivity analysis showed the carbon footprint LOHC concentration dependence during the electrochemical cycling process. Moreover, the electrolyte composition significantly affects the carbon footprint during electrochemical LOHC cycling. Decisions regarding use of thermochemical versus electrochemical cycling need to include separation system boundaries, not just the reactors themselves.