PEM vs Alkaline Electrolyzer Reference

Comparison of PEM, alkaline, and emerging electrolyzer technologies covering efficiency, load flexibility, capex, and balance-of-plant.

Electrolyzer Comparison

Electrolyzer Technology Comparison · Refs: IEA 2023, IRENA 2020, NREL 2022, Buttler & Spliethoff 2018

Parameter	Alkaline (AEL)	PEM	SOEC	Notes / Reference
Technology maturity (TRL)	9 (commercial)	8-9 (commercial)	6-7 (demo/early)	IEA 2023
Typical stack size (single)	0.5 - 6 MW	0.1 - 5 MW	up to 2.6 MW (Sunfire)	Vendor datasheets 2024
Cell stack lifetime	60,000 - 90,000 hr	50,000 - 80,000 hr	20,000 - 40,000 hr	IRENA 2020
Stack efficiency (LHV)	63 - 71%	60 - 68%	75 - 85%	NREL 2022
System efficiency (LHV)	51 - 60%	46 - 60%	76 - 81% (high-T)	With BoP losses
Specific energy	50-78 kWh/kg H2	50-83 kWh/kg H2	39-50 kWh/kg H2	At nominal load; theoretical LHV = 33.3 kWh/kg
Operating temperature	60-90 C	50-80 C	700-850 C	SOEC needs heat integration
Operating pressure	1-30 bar	15-80 bar	1-15 bar (prototype)	PEM highest pressure
Current density	0.2-0.6 A/cm2	1.0-2.5 A/cm2	0.3-1.0 A/cm2	Higher j -> smaller stack
Load range (% nominal)	15-100%	5-120%	30-125%	PEM best for variable renewable
Cold start time	50 min - 2 hr	5-15 min	hours	SOEC penalized by thermal cycling
Ramp rate	0.2-20%/s	up to 100%/s	slow	PEM best dynamic response
Stack CAPEX (2024)	$250-450/kW	$700-1400/kW	$2800-4700/kW	IEA 2023 price drop ongoing
System CAPEX (2024)	$500-1100/kW	$1100-2100/kW	$3500-6500/kW	Full package inc BoP
Key consumables	30% KOH	Platinum, Iridium, PFSA membrane	Ni cermet, YSZ ceramic	PEM PGM bottleneck
Water quality	Type II (5 microS/cm)	Type I / ASTM D1193 (>1 MOhm-cm)	Steam-quality	PEM strict
H2 purity (stack out)	99.5 - 99.9%	>99.99%	>99.9%	PEM less KOH carryover
H2 purity (after dryer)	>99.999%	>99.999%	>99.999%	PSA or TSA added
O2 impurity in H2	<2% (at turndown)	<0.1%	<0.5%	Alkaline limits low load
Footprint (rel. PEM)	1.5 - 2x	1x baseline	0.8 - 1.2x	Alkaline is largest
Response to curtailment	Limited	Excellent	Poor (thermal stress)	PEM best for solar/wind
Degradation rate	0.25-1.5 uV/hr	0.5-3 uV/hr	6-50 uV/hr	Bohlen CIRED 2019 meta
Safety concerns	Caustic KOH leaks; H2/O2 mixing at turndown	PFSA membrane fire risk; HF gas on decomposition	High T; thermal shock	Hazardous area Class 1 Div 2 minimum
SELECTION HEURISTICS
Grid-connected, steady baseload, >10 MW
Directly coupled to PV/wind with heavy curtailment
Industrial site with abundant 500-900 C waste heat
H2 fueling station (need >30 bar, compact)
Remote/marine site, minimal maintenance
Rapid deployment, modular skids
High purity for semiconductor / fuel cell grade

Source: PEM_vs_Alkaline_Electrolyzer_Reference_v1.xlsx

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