Table 1. H2 Production Characteristics of Steam Methane Reforming (SMR) versus Electrochemical (<100 °C) Water Electrolyzers (WEs).
| characteristic | SMR | WE |
|---|---|---|
| feed | fossil fuel | H2O |
| estimated CO2 emissions per kg H2 (kg/kg H2) | 9a | 0.4–0.9b |
| global CO2 emissions (MMT/year) (2019) | 720 | 32–72b |
| % of global CO2 emissions 2019 (est) | 1.7 | 0.08–0.17b |
| H2 production cost (2019) (€/kgH2) | 2 | >3.8c |
| driving force for reaction | heat | electrical energy |
| catalysts | sulfur- and coke-tolerant (nickel, nanosized nickel, platinum, rhodium) | acidic: Pt (cathode), IrO2 (anode) |
| alkaline: nickel-based or Pt (cathode), often nickel-based (anode) | ||
| H2 purityd (%) | 95–98 | PEMWE: 99.9–99.999e |
| AEMWE: 99.4 |
a
Taken from ref (15).
b
The data are for traditional alkaline water electrolyzers coupled with wind energy and estimated from refs (52−55). Fewer carbon-footprint studies are available for water electrolysis coupled with wind than from SMR processes. However, all of these studies consistently show water electrolyzers coupled with wind to be one of the lowest CO2 emitters.
c
An estimate of 3.8 €/kg H2 is for coupling with wind, minimal operating hours of 7000 of 8760 per year, i.e., 80% capacity, a CAPEX of 800 €/kW, WE efficiency of 80%, and renewable electricity cost of 70 € M/Wh.10
d
H2 purity without purification processes as pressure or temperature swing adsorption.
e
Typically at 30 bar outlet pressure.