Overview Basic Electrolyzer Types and System Components Anode liquid feed Cathode liquid feed Cathode vapor feed Applications Challenges XXX

Bennett, Bill

NASA Technical Reports Server

PEM Electrolyzer Types
Mars Workshop and ISRU tag-up
July 31 - August 2, 2017
Bill Bennett NASA GRC 
Photovoltaic and Electrochemical Systems Branch
https://ntrs.nasa.gov/search.jsp?R=20170009573 2019-08-31T01:50:52+00:00Z
Overview
Basic Electrolyzer Types and System Components
• Anode liquid feed
• Cathode liquid feed 
• Cathode vapor feed
Applications
Challenges
Anode Liquid Feed
Not Restricted
Cell characteristics:
Liquid water fed to oxygen side (anode)
Product O2 and H2 contain liquid water
Proton drag carries substantial water to H2 side
• ~2 moles H2O per mole of H2 (or more)
Protonic water is saturated with H2
System characteristics:
Greatest rate capability
Most complicated system
Anode Liquid Feed
Most complicated system
• 2 Pumps 
• 2 phase separators
• H2 degas / H2O recovery
System schematic borrowed from John Graf, EC - June 3, 2011
Cathode Liquid Feed
Cell characteristics:
Liquid water fed to hydrogen side (cathode)
Water diffuses to O2 side through PEM
• H+ drag limits rate capability
• Keeps  O2 free of liquid water
H2 contains liquid water
System characteristics:
Moderate rate capability
Less complicated system
Cathode Liquid Feed
Less complicated system
• 1 Pump 
• 1 phase separator
Diffusion
barrier
Cathode Vapor Feed
Cell characteristics:
Liquid water fed to water chamber (cathode side)
Water diffuses to O2 side through H2 cavity and PEM
• H+ drag limits rate capability
• Keeps  O2 free of liquid water
H2 contains no liquid water
System characteristics:
Lowest rate capability
Simplest system
H2 
pump
Cathode Vapor Feed
Simplest system
• 1 Pump (for cooling) 
• no phase separators
Electrolysis Applications
Hydrogen Generation for Electric Vehicles
Oxygen Generation for Life Support
• Navy Submarines
• International Space Station
Regenerative Fuel Cells
• High-Altitude Long-Endurance (HALE) 
Unmanned Aerial Vehicles
• Surface Power Systems
ISRU
System-Level Challenges
Accumulation of impurities feed water
MEA degradation
Corrosion/shunt currents
Drying of product gases 
Degassing (in anode feed systems)
Gas/liquid separation
Gas crossover/purity
Complexity


PEM Electrolyzer Types

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