More than 50 million metric tonnes of hydrogen is required to be used in petroleum refining, ammonia production and metal reforming yearly [1]. Moreover, hydrogen as an energy carrier can be converted into electrical energy by using fuel cells, for both transportation and electricity distribution purposes. Various resources including steam reforming of methane, partial oxidation of hydrocarbons, coal gasification, biomass and water electrolysis can be employed to obtain hydrogen. Steam Methane reforming (SMR) is currently the main hydrogen production technology, which exploits non-renewable fossil fuels and contributes to green-house gas emissions. Considering the growing demand for hydrogen and assuming that carbon capture and storage is not a large-scale option, the water-electrolysis-dominant scenario in Hydrogen Roadmap Europe [2] predicts that water electrolysis will be the main low-cost technology for sustainable hydrogen production after 2030.

Water electrolysis is splitting of water into hydrogen and oxygen in an electrochemical cell (Figure 2). This process is thermodynamically non-spontaneous and therefore, an external electrical work is required to split water in the electrochemical cell. If renewable energy sources such as sun, wind and hydropower are employed to provide the power for water electrolysis, the hydrogen produced is so-called “green hydrogen”.

Based on the electrolyte and separator in the electrochemical cell, electrolyzer technology is classified into alkaline electrolyzer, polymer electrolyte membrane (PEM) electrolyzer and solid oxide electrolyzer. Alkaline water electrolyzer (AWE) uses aqueous KOH or NaOH as electrolyte and a porous diaphragm to separate the anode and cathode. AWE has the advantageous of relatively lower cost, being a mature technology at industrial scale and having capability to use natural water without further purification [3].

The actual voltage required in an electrolyzer is higher than the theoretical voltage due to existence of loss terms in an operating water electrolyzer unit. Overvoltage is the term used for the difference between the operating cell voltage (E) and theoretical voltage (E_rev) and originates from ohmic overpotential, concentration overpotentials and activation overpotentials. As a result, the energy efficiency of water electrolyzers are typically in the region 60-80 %.

Ohmic overpotential originates from resistance towards transport of ions in the electrolyte and electrons within the electrodes and electrical circuit. Concentration overpotential is the depletion of reactants at the electrode due to insufficient mass transfer or accumulation of products at the electrode. Activation overpotential or charge transfer overpotential is the potential necessary to overcome the activation energy of the electrochemical reactions at the electrode surfaces to reach a desired current. Activation overpotential depends on the electrode properties such as electronic structure and geometry of the electrode material. In addition, topography, lattice defects, crystal orientation and the interaction between neighboring species affect the rate of reactions [4]. Therefore, it is important to employ electrodes with low activation overpotential to increase the efficiency of an electrolyzer. Moreover, it is of great importance to use cheap and stable electrodes to minimize the capital and operational costs.

Nickel is used as a base electrode material in modern industrial alkaline electrolyzers and contributes significantly to the overall cost in these systems. As a PhD research fellow in the electrochemistry group at NTNU, I am working to develop cheaper alternatives to the current alkaline water electrolyzers’ electrodes. The electrodes need also to be more active and durable. My research includes surface engineering, corrosion investigation and electro-catalytic activity evaluation for both oxygen evolution reaction and hydrogen evolution reaction in alkaline media.

References:

1. Zeng, K. and D. Zhang, Recent progress in alkaline water electrolysis for hydrogen production and applications. Progress in energy and combustion science, 2010. 36(3): p. 307-326.
2. Fuel Cell, H.J.U., Hydrogen Roadmap Europe: a sustainable pathway for the European energy transition. 2019.
3. Millet, P. and S. Grigoriev, Water electrolysis technologies. Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety, 2013: p. 19-41.
4. Kjartansdóttir, C.K., Development of hydrogen electrodes for alkaline water electrolysis. 2013: DTU Mechanical Engineering.