Table 1. Comparative Summary of Various Hydrogen Production Technologies.

hydrogen production technologies	reactions involved	advantages	challenges
source: water
electrolysis	2H2O → O2 + 4H+ + 4e–; ΔH = 285 kJ/mol, ΔG = 237 kJ/mol	• carbon-free H2 generation	• low efficiency of electrolyzer over a range of operational conditions
		• possibility of using renewable/nuclear feedstock	• high capital and production costs
			• low operational life of electrodes
			• in early stages of development
photochemical	2H2O → O2 + 4H+ + 4e– ; ΔH = 285 kJ/mol, ΔG = 237 kJ/mol	• ability to generate negligible or no greenhouse gas emissions	• only developed at lab-scale
		• no intake of fossil fuels	• low efficiency and stability of photoelectrodes/photocatalysts due to high reaction impedance
		• low operating temperatures	• high risk of photocorrosion of active materials
			• spontaneous and rapid back reactions
			• overwhelming production costs
thermochemical	2H2O → O2 + 4H+ + 4e– ; ΔH = 285 kJ/mol, ΔG = 237 kJ/mol	• potentially negligible or no greenhouse gas emissions	• highly energy intensive with requirement of temperature up to 2000 °C
		• follows a closed loop cycle consuming only water and producing only H2 and O2	• low efficiency of thermochemical reactors and materials
		• chemicals can be reused within a cycle	• high cost of solar concentrating mirrors
			• developed at lab scale only
source: organic matter
biomass gasification	C6H12O6 + O2 + H2O → CO + CO2 + H2 + other byproducts (taking glucose molecules only)	• high efficiency and rapid process	• emission of COx and other components such as H2S, NH3 and tar
		• availability of abundant and cheap raw materials	• requirement of gas separation and COx removal processes
		• low cost syngas production	• high reactor and feedstock costs
			• quality of H2 produced is poor
			• demonstrated at lab scale only
photobiological	2H2O → O2 + 2H2 ; ΔH = 285 kJ/mol, ΔG = 237 kJ/mol	• can be used in a wide range of water conditions	• in early stages of development
		• the process is self-sustaining	• low efficiency and sustainability of microorganisms
			• designing robust reactor configurations that can use sunlight effectively and produce H2
source: fossil fuels (natural gas/methane)
steam methane reforming (SMR)	CH4+ H2O ↔ CO + 3H2; ΔH298K = 206 kJ/mol	• low-cost H2 production technology	• emission of greenhouse gases in to atmosphere
		• high H/C ratio	• requirement of downstream separation and purification processes
		• high efficiency	• a complex system of reactions including water-gas shift reaction and pressure-swing adsorption reactions
dry reforming of methane (DRM)	CH4 + CO2 ↔ 2CO + 2H2; ΔH = 247 kJ/mol	• utilizes CO2 to produce H2	• more energy intensive than SMR
		• can be used for the Fischer–Tropsch process	• high equipment cost
			• complex downstream processes for H2 purification
			• emission of greenhouse gases
partial oxidation of methane (POM)	CH4 + 1/2O2 ⇆ CO + 2H2; ΔH = −36 kJ/mol	• high efficiency and selectivity	• high emissions of COx and possibility of emissions of NOx
		• very short residence time	• formation of soot byproduct
			• requirement of pure O2
catalytic decomposition of methane (CDM)	CH4 → C(s) + 2H2; ΔH298K = 75.4 kJmol	• simple and one-step process	• in early stages of development
		• no COx or NOx emissions	• low stability of catalysts, requiring effective regeneration techniques
		• produces high quality H2	• unreacted methane in out stream
		• produces solid carbon as a byproduct, which is easier to separate
		• solid carbon in the form of value-added nanocarbons is generated