Table 1.

Comparison of different characteristics of water electrolysis-based hydrogen production technologies.

	AECa	PEMa	SOEa
TRLa	9 [51]	8 [51]	6 [51]
Expected TRL 2050	9 [51]	9 [51]	9 [51]
Typical electrolyte	Aqueous potassium hydroxide (20–40 wt% KOH) [52]	Polymer membrane (e.g. Nafion) [38,39]	Yttria Stabilised Zirconia (YSZ) [53]
Anode	Ni or Ni–Co alloys	RuO2 or IrO2 [54]	LSM/YSZ [53]
Cathode	Ni or Ni–Mo alloys [38]	Pt or Pt-Pd [54]	Ni/YSZ [53]
Cell voltage (V)	1.8–2.4 [55]	1.8–2.2 [55]	0.7–1.5 [53]
Current density (A cm−2)	0.2–0.4 [54]	0.6–2.0 [54]	0.3–2.0 [53]
Cell area (m2)	<4 [52]	<0.3 [52]	<0.01 [52]
Voltage efficiency (%)	62-82 [55]	67-82 [55]	77-85 [42]
Operating temperature (◦C)	60-80 [39]	50-80 [55]	650-1000 [56]
Operating pressure (bar)	<30 [52]	30-80 [52]	<25 [52]
Production rate (m3H2h−1)	<760 [52]	<40 [52]	<40 [52]
Stack energy (kWhelm3H2−1)	4.2–5.9 [55]	4.2–5.5 [55]	>3.2 [52]
System energy (kWhelm3H2−1)	4.5–6.6 [57]	4.2–6.6 [57]	>3.7
Gas purity (%)	>99.5 [58]	99.99	99.9
Cold-start time (min.)	<60 [59]	<20 [59]	<60
System response	Seconds [52]	Milliseconds [52]	Seconds
Stack lifetime (h)	60,000–90,000 [57]	20,000–60,000 [57]	<10,000 [57]
Capital cost per stack 2020 (€2021/kW)	1000-1200d [59]	1860-2320d [59]	>2000d [59]
Capital cost per stack 2030 (€2021/kW, estimated)	611 [26]	978 [26]	1902 [26]
Stack efficiency (LHV) range 2020 (%)	58–70% [14]	58–65% [14]	81–83% [14]
Stack efficiency (LHV) range 2050 (%, estimated)	61–80% [60]	70–74% [60]	88–90% [60]
Advantages	Long life span	High current density	High system efficiency
	Minimal expense	Compact system layout	Less electricity utilization
	High technology readiness level	Fast response to current change	Expected cost reduction
	Large stack size		Integration with other technologies
Disadvantages	Low current density	Noble metal material requirement	Extraction and utilization of cathodic Lanthanide rare earth elements may cause environmental damage [43]
	Corrosive electrolyte	Short life span	Unstable electrodes
		High membrane expense	Sealing problems
Barriers for large-scale application	Accessibility to low cost and abundant electricity	Accessibility to low cost and abundant electricity	Accessibility to low cost and abundant electricity; immaturity of technology

b: The global share of renewable electricity in total electricity output was approximately 27% at the end of 2019, including 11% produced by wind turbines and solar photovoltaic, which potentially can be used to produce sustainable hydrogen [61].

c: Adequate renewable electricity for large-scale deployment of electrolysis is assumed to be available based on the existing net-zero commitments [62,63].

^aAEC: alkaline electrolysis cell; GHG: greenhouse gas; LHV: Low heating value; LSM: La_0.8Sr_0.2MnO₃; PEM: polymer electrolyte membrane; SOE: solid oxide electrolysis; TRL: technology readiness level; wt: weight.

^dUpdated capital cost according to Chemical Engineering Plant Cost Index (CEPCI). CEPCI₂₀₂₀ = 596.2; CEPCI₂₀₂₁ = 708.0. Calculation formula: cost at 2021 = cost at 2020 · $\frac{CEPCIindexat2021}{CEPCIindexat2020}$ [64]).