Water electrolysis technologies for the production of green hydrogen

Green hydrogen is defined as hydrogen produced by water electrolysis using renewable energy. By applying an electrical current, water is split into its elemental constituents - oxygen and hydrogen - according to the equation displayed below. In the specific case of green hydrogen, the electrical energy comes from renewable energy sources such as wind and solar power.

To execute this process, we need an electrolyzer consisting of a series of electrochemical cells, each made of an anode and a cathode – the electrodes - separated by a diaphragm or a membrane. The electrodes are immersed in a solution of ions (the electrolyte). These two parts, called the half-cells, use electrical energy to initiate chemical reactions, specifically in our case, water splitting into oxygen and hydrogen. 

Depending on the electrolyte, the operating conditions, the materials, and the cell configuration, we have four different types of water electrolysis:
1. Alkaline water electrolysis
2. Polymer electrolyte membrane water electrolysis
3. Anionic exchange membrane water electrolysis and
4. Solid oxide water electrolysis cells.

Traditional AWE is a mature and cheap technology for producing green hydrogen.
The AWE electrolyzer works at low temperatures (60–80 ◦C) with a concentrated alkaline solution (5M KOH/NaOH).
The electrodes are made of a nickel-coated unit, and a diaphragm is used as a separator. The ionic charge carrier is hydroxyl ion (OH−), which passes through the porous structure of the diaphragm and is the energy carrier to sustain the electrochemical process.
AWE is suitable for large-scale applications; however, its major challenges are limited current densities (0.1-0.5 A/cm2) and corrosive (KOH) electrolyte usage.
Additionally, AWE produces low purity (99.9%) hydrogen because the diaphragm does not completely prevent the crossover of the gasses from one half-cell to the other.

AWE has been evolving since it was initially developed.
Different from the traditional technology, the advanced AWE boasts higher current densities, which reached values of 1.2 A/cm2, at lower electrical energy consumption. This is due to the use of electrocatalytic coatings applied on the electrodes, a zero-gap cell configuration, and an improved diaphragm that allows greater flexibility of the electrolyzer and better gas quality.

It uses a sulfonated polymer membrane as an electrolyte.
The ionic charge carrier is H+ which passes through the proton-conducting membrane to maintain the functionality of the electrochemical process.
Typically, PEM water electrolysis functions at low temperatures (50–80 ◦C) with high current densities (1.0-5.0 A/cm2) and produces high purity (99.999%) hydrogen. However, the major challenge associated with the PEM WE is the high cost of the components and electrocatalysts, both made of noble metals and iridium oxides, respectively.

AEM WE is an emerging technology for green hydrogen production. This technology is
similar to conventional AWE with the main difference being the replacement of the traditional diaphragms with an anion exchange membrane.
AEM WE offers several advantages such as cost-effective transition metal catalysts instead of using noble metal catalysts.
Additionally, distilled water/low-concentrated alkaline solutions (1M KOH) can be used as electrolytes instead of high-concentrated ones (5M KOH solution). Despite these significant advantages, AEM WE still requires further investigations into cell efficiency and longevity, which are essential features for large-scale or commercial applications.

The solid oxide water electrolysis cell (SOEC) is an emerging technology and was first conceptualized in the 1970s. Typically, the solid oxide water electrolyzer operates with water in the form of steam at high temperatures (500–850 ◦C).
SOEC offers two major advantages compared to the existing electrolysis technologies; the first one is the high operating temperature, which results in favorable and unrivaled conversion efficiencies.
Second, this technology can be thermally integrated with downstream chemical synthesis such as ammonia production.
Additionally, the solid oxide water electrolysis does not require the use of noble metal electrocatalysts to give high conversion efficiency. Despite these advantages, the cell's insufficient longevity has prevented the commercialization of solid oxide water electrolysis.

Is there a preferred technology to bring to market for large-scale production of green hydrogen? We at De Nora believe that each technology has challenges and opportunities.
However, AEM WE and SOEC are still in their infancy, and we still need research and investments to bring them to an acceptable level before commercialization. PEM and AWE are both mature technologies and have been on the market for a longer time. Although PEM works at higher current density and the electrolyzer is much easier to handle because of its smaller footprint, AWE is a much cheaper and robust technology, and here at De Nora, we believe that the advanced AWE is the optimal one to scale up and bring to market for commercialization and production of green hydrogen.