In This Issue
Winter Bridge on Frontiers of Engineering
December 15, 2022 Volume 52 Issue 4
From novel applications of microbes to DEI in engineering to the potential for hydrogen energy, Frontiers of Engineering participants tackle today’s challenging world issues. The winter issue of The Bridge showcases research by early-career engineers as shared at the 2022 US FOE symposium.

Green Hydrogen: The Cutting Edge in Clean Electrolysis

Thursday, December 15, 2022

Author: Xiong Peng

Advances in electrolysis techniques, materials, design, and diagnostic tools can make green hydrogen more economically competitive and accessible.

Hydrogen is a chemically stable energy carrier that can be used to decarbonize various sectors. However, hydrogen produced by reformation or gasification of fossil fuels—and thus associated with a large quantity of CO2 emissions—accounts for more than 90 percent of the global hydrogen supply.

As the energy landscape shifts toward renewable sources, green hydrogen, which is generated by renewable energy or from low-carbon power, can play a significant role in decarbonization and the achievement of carbon neutrality (Pivovar et al. 2021). Compared to electrons, hydrogen can help decarbonize sectors traditionally considered hard to decarbonize, including heavy-duty transportation, heavy oil upgrading, iron and steel, ammonia, synthetic fuel, and other chemical feedstock productions.

Water electrolysis using renewable electricity is one way to produce green hydrogen. The central piece to link renewable electricity to various decarbonization applications of hydrogen is a water electrolyzer, which uses electricity to split water into hydrogen and oxygen. With the growing availability of low-cost carbon-free electricity as a feedstock for electrolyzers, a positive feedback loop should be created: green hydrogen needs cheap renewables, and more renewables can be managed on the electrical grid by making green hydrogen. However, only about 2 percent of the world’s hydrogen is produced by water electrolysis. Industrial-scale green hydrogen production requires large-scale electrolyzer deployment, which is associated with cost, efficiency, and durability challenges.

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Types of Water Electrolyzers

There are two types of commercially available and technically mature water electrolyzers: alkaline water electrolyzers (AWEs) and proton exchange membrane water electrolyzers (PEMWEs).

AWEs are essentially massive chemical plants with a lifetime of 20–30 years. They have changed little since their first commercialization more than a century ago: they are still fed with a high concentration of potassium hydroxide (KOH) and typically operated at current densities of 0.2–0.6 A/cm2 (ampere per square centimeter) and single cell voltage of 1.8–2.4 V. Because of their low energy efficiency, low operating current, and poor compatibility with renewable electricity, large-scale deployment of AWEs is not imminent. Their biggest advantage is low capital cost due to the use of inexpensive materials such as nickel, stainless steel, and KOH as electrolyte. In short, AWEs are old technologies that need innovations to improve their efficiency, dynamic operation, operating current densities, and partial differential operating pressure.

With the increasing need for pressurized hydrogen production at high purity, PEMWEs are gaining more attention, thanks to their zero-gap design and the use of a highly conductive polymer electrolyte, enabling dry cathode operation at high current density and efficiency. PEMWEs can also offer great synergy with highly intermittent renewable energies to utilize low-cost electrons to produce low-cost hydrogen, due to the capability of dynamic operation, as the electricity price dominates the electrolyzer operational cost.

But PEMWEs are operated in highly corrosive and oxidative environments, which require the use of expensive cell components such as platinum group metal (PGM), including iridium as anode catalyst and platinum as cathode catalyst, and platinized titanium (Ti) porous transport layers (PTL) and flow fields. PEMWEs are therefore typically used in niche markets such as submarine systems or hydrogen refueling stations. Their large-scale deployment likely faces challenges in a materials supply bottleneck, as the global annual supply of iridium is only a few tons. A critical research need for PEMWEs is to reduce PGM catalyst loadings while maintaining electrolyzer performance and durability.

Efforts to Improve Electrolyzer Performance

In my group we conduct fundamental research to understand how to achieve low PGM loading while maintaining PEMWE performance (figure 1a).

We have found that both the PTL/catalyst layer and catalyst/polymer-electrolyte interfaces play important roles in determining PEMWE performance (Peng et al. 2021). For example, there is an optimal interfacial contact area between the PTL and catalyst layer to balance the latter’s in-plane electric conductivity and the water supply to the reacting zone. At the catalyst/polymer-electrolyte interface, advanced tools such as in situ grazing incidence small-angle X-ray scattering are used to characterize the polymer electrolyte adsorption behaviors at various catalyst surfaces.

My group also uses high-throughput manufacturing methods to engineer electrode structures with low transport resistance (Peng et al. 2020) and conduct catalyst ink characterization (Berlinger et al. 2021) to understand how ink rheology can affect catalyst layer structure and electrolyzer performance. For materials, we develop new materials such as advanced PTL (figure 1b) with porosity gradients and highly active and durable supported catalysts (figure 1c) for PEMWE application (Chatterjee et al. 2021).

On the alkaline side, a device that may combine benefits from both AWEs and PEMWEs is an anion exchange membrane water electrolyzer (AEMWE; figure 2). AEMWEs can use inexpensive cell materials, such as non-PGM catalysts and stainless steel PTLs, and thus avoid both high capital costs and the raw material supply bottleneck. AEMWEs also have a zero-gap design, which enables high operating currents and efficiency. AEMWE-related research has focused on developing highly conductive and durable anion exchange membranes and anion exchange ionomers. With the progress in developing anion exchange electrolytes, some startups are taking steps toward mass producing these materials.

Peng fig 2.gif

In my group we use diagnostic tools such as microelectrodes, a 3-electrode membrane electrode assembly, and multiphysics modeling to understand complex transport behaviors (figure 2a), sources of over-potentials from different components (figure 2b), and the role of supporting electrolytes in AEMWE performance and durability. We have achieved excellent initial cell performance (e.g., 4 A/cm2 at < 2.1 V) (figure 2c) and high current and durable operation (2.0 A/cm2 for > 500h) using very dilute supporting electrolytes (0.1 M KOH or 1 wt percent KHCO3) with complete PGM-free electrolyzer components. 

Other emerging water electrolysis technologies (e.g., solid oxide electrolysis cells, photoelectrochemical electrolysis cells, and solar thermochemical electrolysis cells) have also been gaining a lot of research attention, because of their potential to reduce green hydrogen production costs by integrating with solar energy or waste thermal energy. Many of these emerging technologies are at lower technical readiness levels and require significant R&D efforts before deployment.

Looking Ahead

Future water electrolysis technologies will require highly efficient and durable systems that can be deployed at large scale (gigawatt to terawatt) and exhibit fast response to renewable electricity, so that future systems are not necessarily operated at a constant load but more likely in dynamic modes. This implies challenges for electrolyzer durability, as dynamic operation is likely to induce more degradation to electrolyzer components (Alia et al. 2019). Enhanced understanding of degradation mechanisms and mitigation strategies are needed to prevent electrolyzer failure.

To make green hydrogen economically more competitive and accessible, efforts are needed to further reduce electrolyzer capital costs and improve operational efficiency. This requires not only the use of cheap and durable materials but also the establishment of automated manufacturing facilities for electrolyzer stacks and balance of plant. Cheaper renewable electricity prices are also essential to enable a green hydrogen production price of less than $1/kg in the near future.


Alia SM, Stariha S, Borup RL. 2019. Electrolyzer durability at low catalyst loading and with dynamic operation. Journal of the Electrochemical Society 166(15):F1164.

Berlinger SA, McCloskey BD, Weber AZ. 2021. Probing ionomer interactions with electrocatalyst particles in solution. ACS Energy Letters 6(6):2275–82.

Chatterjee S, Peng X, Intikhab S, Zeng G, Kariuki N, Myers D, Danilovic N, Snyder J. 2021. Nanoporous iridium nanosheets for polymer electrolyte membrane electrolysis. Advanced Energy Materials 11:2101438.

Peng X, Taie Z, Liu J, Zhang Y, Peng X, Regmi YN, Fornaciari JC, Capuano C, Binny D, Kariuki N, and 4 others. 2020. Hierarchical electrode design of highly efficient and stable unitized regenerative fuel cells (URFCs) for long-term energy storage. Energy & Environmental Science 13:4872–81.

Peng X, Satjaritanun P, Taie Z, Wiles L, Keane A, Capuano C, Zenyuk IV, Danilovic N. 2021. Insights into interfacial and bulk transport phenomena affecting proton exchange membrane water electrolyzer performance at ultra-low iridium loadings. Advanced Science 8(21).

Pivovar BS, Ruth MF, Myers DJ, Dinh HN. 2021. Hydrogen: Targeting $1/kg in 1 decade. Electrochemical Society Interface 30(4):61–66.

About the Author:Xiong Peng is a research scientist, Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory.