Alkaline water electrolysis is the most mature water electrolysis technology available to date, having been in use for over a century. Pioneering green hydrogen projects in the 1920s utilised AWE technology. Numerous companies around the world are currently manufacturing AWEs. This technology is also beneficial for electrolysing brine or saltwater to produce caustic soda, a crucial intermediate in the chlor-alkali industry.

Some of the benefits of AWE are:

1. Mature, commercialised, and proven technology which has been in existence for over 100 years.

2. Cheaper system cost ($800-1000/kW) owing to minimal usage of precious Platinum Group Metals (PGMs).

3. Suitable for continuous or baseload operation like hydropower, geothermal etc. (though some new-generation AWEs can have a high ramp up rate to match the renewables).

Some of the disadvantages of AWE are:

1. High startup time (~60 minutes) during a cold start.

2. Hydrogen available at atmospheric pressure (though new generation of Pressurised alkaline water electrolysers (PAWE) can offer a ~40 bar pressure).

3. Purification system, especially lye separator needed to purify hydrogen.

4. Bulky equipment, unsuitable for compact applications

PEM is a relatively new technology, having been commercialised about 20-30 years ago. PEM electrolysers offer to reduce some of the disadvantages associated with alkaline electrolysers in terms of size, flexibility towards variable renewables or VRE. PEM is the next most dominant electrolyser technology after alkaline in terms of manufacturing facilities dedicated to this. 

Some of the benefits of PEM are:

1. Commercialised, and newer technology which has been in existence for about 20-30 years.

2. Accommodates variability in RE due to its faster response time.

3. Can be started from cold condition to 100% load in a matter of a few minutes

4. Compact and suitable for applications having size constraints like automotive hydrogen refueling stations.

5. Provides hydrogen output at higher pressure (~50 bar).

6. Provides a very high degree of purity of hydrogen (~99.99% and more purity).

Some of the disadvantages of PEM are:

1. Expensive (~$1400/kW) due to the usage of PGMs

2. Highly sensitive to catalyst poisoning due to the slightest entry of impurities in water

SOEC is a new technology in pre-commercialisation phase. This technology offers the highest efficiency of electrolysis, reaching as high as 90% efficiency levels. This technology uses zirconia based ceramics for high temperature steam electrolysis to produce green hydrogen.

Some of the benefits of SOECs are:

1. Very high efficiency (~90%)

2. Suitable for operations involving the availability of waste heat, like nuclear, geothermal, etc.

3. Can reduce even CO2 to CO, which may further be used as a raw material for methanol, ethanol and SAF production.

4. Suitable for combusting methane or natural gas efficiently and producing a higher electrical output as compared to traditional use involving IC engine or turbine.

5. Can work reversibly as a fuel cell as well.

Some of the disadvantages of SOECs are:

1. Expensive (~$2000/kW) due to the nascency of the technology (though this may reduce during widespread manufacturing of SOECs)

2. Suitable only for processes involving waste heat availability.

3. Takes several hours during a cold start. Hence, it’s preferable to keep SOECs continuously running.

4. Extended baseload high temperature operations at 500-600^OC range may cause thermal stresses on the sub-components and may negatively affect durability.

AEM is a new technology in pre-commercialisation phase, which combines the good features of both AWE and PEM. This technology neither uses expensive PGMs, nor does suffer from lack of flexibility in case of VRE. 

Some of the benefits of AEMs are:

1. Cheaper than PEM electrolysers owing to lack of PGMs

2. Accommodates variability in RE due to its faster response time.

3. Can be started from cold condition to 100% load in a matter of a few minutes

4. Compact and suitable for applications having size constraints like automotive hydrogen refueling stations.

5. Provides hydrogen output at high pressure (~35-40 bar).

Some of the disadvantages of AEM are:

1. Lack of visibility on price points as the electrolyser technology is quite nascent.

2. At present, the electrolyser suffers from durability issues when compared with established technologies like AWE or PEM