Advances in Electrolysis for Affordable Green Hydrogen

Hydrogen is a vital chemical resource, not only as a promising alternative to fossil fuels, but also as a fundamental chemical feedstock for the production of valuable industrial chemicals, such as ammonia and methanol. The overwhelming majority of commercial hydrogen is currently produced by steam reforming of methane, an energy-intensive process that releases significant quantities of carbon dioxide as a byproduct.¹ However, in recent decades, substantial improvements in water electrolysis technologies have brought green hydrogen ever closer to a commercial reality.

Electrolyzers use electricity to break water molecules down into their component elements. At the cathode, H⁺ ions (or water, depending on the pH of the system) undergo a catalytic reaction to evolve hydrogen. Similarly, the OH^– ions (or water, at lower pHs) react at the anode to produce oxygen. The two sides of the cell are divided by a membrane to separate the resulting gases.² This overall process is usually powered by renewable electricity, making electrolysis an emissions-free route to hydrogen fuel.

Despite this clear advantage, the high production costs associated with electrolysis have so far limited its contribution to the hydrogen economy. The oxygen and hydrogen evolution reactions are chemically complex and generally require expensive precious metal catalysts. At the same time, competing side reactions and physical factors (such as the mobility of ions through the cell partition and the formation of bubbles on the electrode surfaces) can limit the cell’s overall efficiency, meaning additional electricity input is required to drive the reaction to completion.

With the cost of producing hydrogen by electrolysis being 1.5–6 times greater than steam reforming from fossil fuels, improving the economics of electrolysis will be key for the future widespread commercialization of this approach.³

Several different electrolyzer technologies are already available on the market, most notably alkaline water electrolysis (AWE) and proton exchange membrane (PEM) water electrolysis, with yet more variations (such as anion exchange membrane (AEM) water electrolysis) under development. Each technique has its own particular advantages over the others. But ultimately, improving the cost, efficiency and durability of these device components through advances in materials chemistry will be essential for the long-term future of green hydrogen production.⁴

Alkaline water electrolysis

The most mature of electrolyzer technologies, alkaline water electrolysis was used in the production of hydrogen before steam reforming and was later developed for the chlor-alkali process to produce chlorine and sodium hydroxide.^5,6 The alkaline conditions mean it’s possible to use cheaper transition metal catalysts, such as nickel or even stainless steel, on the electrodes rather than the precious metals required for acidic systems. ^5,7 However, lower current densities (from the slow diffusion of OH^– ions, leading to slower hydrogen evolution and lower efficiency), electrode corrosion under alkaline conditions and problematic gas permeation through the dividing membrane all remain significant drawbacks.⁵

High-performance electrodes

To address these challenges, Guo et al. developed cobalt-cerium MOFs as alternative catalytic electrodes.⁸ The combination of d and f block metals in the structure reduced the activation barrier for the sluggish oxygen evolution reaction by stabilizing the higher cobalt oxidation states required to catalyze the process.

Translated into a demonstration electrolysis cell, the team showed that this electrode substitution resulted in a notably lower energy consumption than the conventional Raney nickel catalyst and that the system remained stable, even under the fluctuating operational conditions associated with renewable energy. The cheap materials and easy fabrication, coupled with this enhanced operational efficiency, delivered a projected hydrogen cost on a par with the US Department of Energy’s target and substantially lower than those of existing alkali catalysts, including ruthenium dioxide and layered double hydroxide nickel compounds.⁸

Initial durability studies revealed that the alternative electrodes had a long-term stability of 5000 hours, though further work will be required to boost this lifetime to the 90,000 hours required of commercial devices.

Selectively permeable membranes

But it’s not just the electrodes themselves that influence the operational costs of electrolyzers. The permeable membrane between the anode and cathode is intended to prevent the mixing of any generated hydrogen and oxygen, while simultaneously permitting transport of relevant ions from one side to the other.

This is a delicate balance to manage – a porous membrane promotes ion conductivity, which reduces electrical resistance and increases the efficiency of the cell. However, porous membranes are also more permeable to gases, reducing the product purity and, at higher concentrations, creating a potentially explosive mixture. On the flipside, a dense membrane structure more effectively inhibits gas crossover, but likewise impairs the ability of the ions to move across the cell.

With a focus on this challenging selectivity issue, Abdel Haleem et al. prepared a membrane with two distinct faces.⁹ They coated a slurry containing inorganic particles and an organic polymer onto one side of a (polyphenylene sulfide) non-woven polymer support, forming a rough, impregnated face with large pores on the coated side and a dense, smooth surface with small pores on the untreated face.

After pre-soaking in an alkaline solution, the membrane demonstrated excellent ionic conductivity, which was unaffected by the orientation of the two faces. However, the two-faced partition did exhibit a notable selective gas barrier effect, with the treated side reducing permeation by a factor of three. While this system did not eliminate gas crossover entirely, the ability to selectively enhance gas purity in a particular cell compartment is nonetheless a valuable advance for these electrolyzer systems.

Proton exchange membrane water electrolysis

Another mature technology, proton exchange membrane water electrolysis, was developed to overcome the low current density and gas leakage problems of alkali systems.² The anode and cathode are separated by a proton exchange membrane – a solid polymer film which permits the passage of protons but not electrons or gases. However, the resulting acidic conditions mean these cells typically require precious metal catalysts, making the initial investment prohibitively high for many commercial applications.

Surface design for lower catalyst loading

The oxygen evolution reaction, which is the rate-limiting step of the overall process, is mechanistically complex and requires either iridium or ruthenium oxides on the anode surface, both of which are substantially more expensive than gold. What’s more, the sluggish surface reaction kinetics mean relatively high catalytic loadings are also required to achieve an acceptable reaction rate, further contributing to the cost. Reducing catalytic loadings and the dependence of this chemistry on expensive metals is key to making these acidic systems more appealing to investors.

Dong et al. made important progress in this area, developing a gradient-loaded anode with around ten times less iridium than the standard electrode.¹⁰ Through careful control of the catalyst morphology, they exposed the metal active sites to increase the overall electrochemically active area by 4.2 times, enabling the reagents to more efficiently utilise the catalyst surface.

The team prepared a catalyst ink containing iridium, a charged polymer support and solvent, and loaded this onto an anode aluminium oxide (AAO) template. The ink was allowed to settle, then the system dried under vacuum to create a highly-ordered porous catalytic structure, which the team evaluated with scanning electron microscopy and X-ray photon spectroscopy.

The resulting regularity of the anode surface facilitates easy movement of reagent ions, reducing efficiency losses through resistance by up to 48%. Preliminary stability testing also showed that the device was stable at standard current densities for 300 hours. The team is now focused on extending this lifetime for practical deployment.

Swapping out precious metals

Likewise, under acidic conditions, the hydrogen evolution reaction at the cathode also requires a precious metal catalyst, in this case, platinum.¹¹ Decades of research have already uncovered several alternative catalyst classes, most notably molybdenum sulfides; In 2005, Hinnemann et al. realised that the Mo-edge of these clusters is similar to the active site of nitrogenase enzymes, meaning it has the potential to act as a modest hydrogen evolution reaction catalyst.¹² However, it is only this edge region that is catalytically active, so there is thriving active research into the design of different morphologies of molybdenum sulfides to improve this baseline activity.¹¹

Gonell et al. designed a series of amorphous surfaces whose disordered structure created active sites within the plane of the cluster, as well as the naturally reactive edge (thereby producing a material that exhibits greater activity across a larger area of the catalyst than previous cathode systems).¹³ The team combined different molybdenum clusters, including Mo₃S₄ and Mo₃S₇, exploiting the influence of varying compositions on the catalytic activity, and found that a higher proportion of Mo₃S₇ (which contains an S–S bridge) was most effective at enhancing the hydrogen evolution reactivity. The resulting amorphous catalyst surface was characterised by X-ray and microscopy methods, revealing well-defined active sites in previously inactive regions of the catalyst.

Anion exchange membrane water electrolysis

Anion exchange membrane systems are still under development, but are a promising alternative to alkali water electrolysis and proton exchange membrane water electrolysis. The method combines non-noble metal catalysis with high current density, using membranes formed of a polymer support with cationic head groups to promote the exchange of hydroxide ions from one cell compartment to the other.¹⁴ 

A major challenge is striking a balance between facilitating easy transport of hydroxide ions and the stability of the dividing membrane under alkaline conditions. Like alkali water electrolysis, the separating barrier must permit the passage of slow-moving hydroxide ions, but the charged surface is vulnerable to chemical degradation by nucleophilic reaction with these very particles.

Cross-linked nanochannel membranes

Crosslinking – the formation of chemical connections between adjacent polymer chains – is a straightforward method to boost the strength and durability of polymeric materials and preliminary research on anion exchange membrane systems has shown it can also reduce the susceptibility of these partitions to degradation.

Wu et al. developed one such diaphragm in 2022, forming a crosslinked nanochannel membrane from chitin biowaste (e.g., seafood shells).¹⁵ The chitosan polysaccharide derived from the waste contains free amino groups, which can coordinate hydroxide ions, but the crystalline structure makes it unsuitable for use in membranes. By combining the polysaccharide chains with Cu²⁺ ions, the group formed cross-links between the amino and hydroxyl groups of adjacent chains, thereby altering the wider crystal structure of chitosan to create a helical chain arrangement with 1nm nanochannels. The resulting channels exhibit high hydroxide conductivity and greater mechanical strength than the unlinked material, with the crosslinking also suppressing swelling of the membrane to boost the stability of the test cell.

The future of hydrogen production

With mature technologies already available and exciting alternatives on horizon, it’s estimated that global electrolyzer capacity will reach 520 gigawatts by 2030, more than 700 times that recorded in 2023.⁴ However, reducing the upfront and operational costs remains one of the biggest challenges to address to facilitate more widespread uptake of electrolysis technologies. Materials innovations will play a key role in this story.