(47b) Carbon-Negative Hydrogen Via Bio-Derived Feedstock Electrosplitting

Hydrogen production from fossil resources is CO₂-intensive, yet these feedstocks account for 96% of global hydrogen production; while electrified H₂ synthesis from water is potentially less carbon-intensive but remains electricity-intensive. A (cradle-to-gate) carbon-negative alternative is to split partially-reduced terrestrial biomass—which incorporates CO₂ from the atmosphere—into separated streams of H₂ for use and CO₂ for sequestration. Unfortunately, biomass thermochemical gasification, which uses O₂, generally requires > 280 MJ/kg H₂ because of a approximately < 50% biomass-to-H₂ yield. The balance of biomass becomes biochar and H₂O. Here we report biomass electrosplitting, wherein a membrane reactor produces H₂ on the cathodic gas stream and CO₂ (for sequestration) on the anode. Unfortunately, in initial studies employing known catalysts, we found that the full cell voltage drifted upward over the course of a few hours’ operation. Only by developing catalysts that enhance metal:hydroxyl group binding to stabilize C₃ adsorbates (Raman + DFT) were we able to lower the reaction barrier to the rate-limiting (and hence full-cell-voltage-determining) step—glycerol oxidation to glyceric acid. We report that when separated streams of H₂ and CO₂ (each 98 mol%) enter the system, electrical energy consumption drops to 205 MJ/kg H_2. When we analyze life cycle GHG emissions and cost as a function of candidate feedstocks and sources of electricity we identify two cases of interest. Firstly, solar electricity plus either soy glycerol or corn stover glucose feedstocks lead to cradle-to-gate GHG emissions of −8 and −10 kgCO₂/kgH₂, respectively, with a plant-gate levelized cost <= $2/kg H₂. Secondly, because of the low production GHG emissions, soy glycerol or corn stover glucose achieve negative net emissions (−0.1 and −2 kgCO₂/kgH₂, respectively) even when powered using electricity having the carbon intensity of present-day US mix.