Hydrogen Production through Electrolysis
Hydrogen: An efficient alternative fuel
As companies looked toward a more sustainable future, hydrogen gained popularity as a viable alternative fuel. It is clear to see the appeal of hydrogen energy since it produces zero carbon emissions when it is combusted. The question then is, how is hydrogen gas produced? Most commonly, hydrogen is produced through reforming processes at a cost of around $1.40 USD/kg.[1] However, these processes produce large quantities of carbon emissions. An alternative way is through electrolysis at a cost of around $5.40 USD/kg.[2] Depending on the source of the electricity, this could be an effective way to reduce carbon emissions.
Electrolysis vs Reforming Processes
The major benefit of electrolysis over reforming processes are how quickly the production of hydrogen can be ramped up or down. Electrolysers can alter production rates almost instantaneously if flexibility is required.[3] Alkaline electrolysers are not a novel concept and have been used for several decades.[4,5] Its downsides include the operating temperature range of 70 to 150°C and the physical space required.[4,5] A typical industrial unit uses about 13.5m x 4m of floor space.[5]
Electrolysis Reaction
The electrolyser itself is a series of cells with an electrolysis reaction taking place:[6]
2 H20(l) ↔ H2(g) + O2(g) ΔHrxn = +286 kJ/mol H2
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Each electrolyser cell consists of an anode and a cathode separated by a membrane submerged in water. A power supply passes a current through the cell splitting water into hydrogen and oxygen.
Figure 1: A typical electrolysis cell.[7] |
Figure 2: An industrial-scale alkaline electrolysis cell.[8] |
Faraday's Law
Faraday’s law is used to properly size the electrolyser unit:[6]
NH2 = jAe / neF
Where,
NH2 = H2 production rate (mol/s)
j = Operating current density (A/m2)
Ae = Effective surface area of the alkaline electrolyser (m2)
ne = Number of electrons transferred in the chemical equation (2)
F = Faraday's constant (96,485 C/mol)
The design equation contains three variables: hydrogen production rate, surface area, and current density. The hydrogen production rate is determined by the process requirements. Therefore, either the current density or surface area must further be specified. A trade-off is apparent from this equation, higher current densities require less space. However, higher current densities are accompanied by greater resistance in the system, more inefficiencies.
How efficient is electrolysis?
The maximum efficiency that can theoretically be achieved by an alkaline electrolyser is ~66%.[9] The Figure below depicts the relationship between current density and efficiency. This should essentially provide the upper and lower bounds of current density. The lower bound would produce close to the maximum efficiency of 66% and the upper bound would be the point before a steep drop in efficiency with increasing current density. Where exactly the point should be made is dependant on logistics of installation of the electrolyser. The more space the electrolyser takes up, the greater efficiency it will have.
Figure 3: Efficiency as a function of current density for industrial alkaline electrolysers.[9]
References
[1] S. C. Bhatia, “Biohydrogen,” in Advanced Renewable Energy Systems, 1st ed. New Delhi: Woodhead Publishing Limited, 2014. pp. 636.
[2] F. D. Doty, "A Realistic Look at Hydrogen Price Projections,"⇗ Doty Scientific, Inc., Columbia, SC, 2004. [Accessed: Nov. 14, 2018].
[3] U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, "Current (2009) State-of-the-Art Hydrogen Production Cost Estimate Using Water Electrolysis - Independent Review - Published for the U.S. Department of Energy Hydrogen Program"⇗ National Renewable Energy Laboratory, Golden, CO, 80401-3393, 2009. [Accessed: Oct. 28, 2018].
[4] K. Chen, D. Dong, and S. P. Jiang, "Hydrogen Production from Water and Air Through Solid Oxide Electrolysis," in Production of Hydrogen from Renewable Resources, Vol. 5, Z. Fang, R. L. Smith, Jr., and X. Qi. Dordrecht, Netherlands: Springer Science+Business Media Dordrecht, 2015, pp. 223-242.
[5] S. A. Grigoriev and V. N. Fateev, "Hydrogen Production by Water Electrolysis," in Hydrogen Production Technologies Vol. 1, M. Sankir, and N. D. Sankir. Hoboken, New Jersey: John Wiley & Sons, Inc., 2017, pp. 231-298.
[6] Z. Houcheng, G. Lin, and J. Chen, “The performance analysis and multi-objective optimization of a typical alkaline fuel cell,”⇗ Energy, vol. 36, no. 7, pp. 4327-4332, 2011. [Accessed: Oct. 27, 2018].
[7] D. M. F. Santos, C. A. C. Sequeira, J. L. Figueiredo. Hydrogen production by alkaline water electrolysis.⇗ Química Nova, 36 (8), 2013. pp. 1176-1193. [Accessed 1 Oct. 2018].
[8] J. C. Koj et al., “Site-Dependent Environmental Impacts of Industrial Hydrogen Production by Alkaline Water Electrolysis,”⇗ Energies – Effects of Biofuels on Combustion and Pollutant Emissions, vol. 10, 2017. [Accessed: 30 Oct. 2018].
[9] Z. Houcheng, S. Su, G. Lin, and J. Chen, “Configuration Designs and Parametric Optimum Criteria of an Alkaline Water Electrolyzer System for Hydrogen Production,”⇗ International Journal of Electrochemical Science, vol. 6, no. 7, pp. 2566-2580, 2011. [Accessed: Oct. 28, 2018].
