(Invited) Alkaline Water Electrolysis Vs. PEM Water Electrolysis - Exploring Their Full Performance

The future of energy conversion and storage is expected to rely on the production and storage of hydrogen via water electrolysis¹. In this scenario, water electrolyzers will play a key role to the establishment of an energy matrix based on renewable but intermittent power sources (e.g. wind turbines and photovoltaics). Other technologies could also be used to store the surplus of energy, such as pumped hydro, geo thermal, batteries, air compression, and the like². However, none of those have the same value proposition as hydrogen. Hydrogen has the advantage of being able to drive multiple revenue streams like transportation, chemicals, green production of fertilizers, regeneration of electricity through fuel cells, and also initially supplement the energy gap through methanation, when coupling with CO₂ sequestration¹. Moreover, the production, storage, or distribution can be chosen to be centralized or decentralized and it is the only option with a multi-GWh storage capacity¹.

To date, only alkaline and polymer electrolyte membrane (PEM) water electrolyzers are commercially available^1,3. In order to meet the future demand for water electrolyzers, investment and operational costs still have to be reduced. Moreover, it is fundamental to develop electrolyzers that are able to operate at high current densities, variable partial load, overload, and on/off conditions. These requirements usually place PEM water electrolysis as the best alternative to couple with intermittent power sources. In any case the high costs of PEM water electrolysis components (based on Pt, Ir, and Ti materials) still hamper its large-scale commercial application³. Though consistent R&D one can pursue to considerably reduce the costs of PEM water electrolyzers, so that the loading of the expensive based materials can be reduced or even to the point of being completely substituted³.

On the other side, alkaline water electrolysis stays as a long-established, well matured, and comparatively low cost available technology and another approach could be pursued by improving the performance and operational characteristics of this KOH based system. The hydroxide transport across the diaphragm inside alkaline electrolyzers promoted by the KOH responds very slowly to the power input, limiting the efficiency of the electrochemical reaction, and consequently resulting in low current densities^1,3. Moreover, the porous structure of the diaphragm allows the diffusion and extensive mixture of the produced hydrogen and oxygen gases when operating at low current densities, limiting the safety range for its operation. Conventional electrodes used in alkaline electrolyzer also tend to possess low active surface area, poor catalyst utilization, and many associated voltage losses^1,3.

When conventional diaphragms or separators are replaced by thin polymer based separators or anion exchange membranes (AEM), the performance of the alkaline electrolyzers can be substantially enhanced. These are the so-called advanced alkaline electrolyzer units and are already expected to reach performance levels close to that of PEM water electrolyzers. The direct comparison between those three presented alternatives is absolutely not trivial and yet cannot be avoided. It is therefore essential to establish a fair performance range comparison when using standard and available materials for classic alkaline, alkaline PEM, and PEM water electrolysis. 

As an example for the performance behavior in PEM water electrolysis, by using thin PFSA based membranes (< 50 µm), performances reaching up to 10 Acm^-2 are obtained. Due to the low ohmic losses when using thin membranes, lower cell voltages are also found, mitigating the voltage inducing corrosion, allowing the use of less expensive material. Nonetheless, the thinner membrane shall increase the hydrogen permeation to the oxygen side limiting its partial load and differential pressure operation⁴. Another important point is the loading of noble metals used in the catalyst layer. To date, Ir loadings range between 2 and 3 mg_Ircm^-2, Pt loadings range between 0.8 and 1.5 mg_Ptcm^-2.^1,3By using advanced methods for the membrane electrode assembly (MEA) fabrication, loadings were dramatically reduced without statistically affecting the performance.

In conclusion, a new, robust and efficient benchmark study is presented showing the up-to-date performance behavior of classic alkaline, alkaline PEM, and PEM water electrolysis. This study shall be able to contribute to validate the R&D potential for each technology and its future incorporation into our energy matrix for energy storage and conversion.

[1]    J. Mergel, M. Carmo and D. Fritz. in Transition to Renewable Energy, D. Stolten, V. Scherer, Editors, p. 423-450, Wiley-VCH (2013)

[2]    W.F. Pickard et al Energy Reviews; 13(8), 1934 (2009).

[3]    M. Carmo, D. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 38, 4901 (2013)

[4]    M. Schalenbach, M. Carmo, D. Fritz, J. Mergel and D. Stolten, International Journal of Hydrogen Energy, 38, 14921 (2013)