Sounding out green H2

C&I Issue 1, 2023

Read time: 2 mins

Author: Maria Burke

Hydrogen is likely to be pivotal in the transition to a clean-energy economy.

The most climate-friendly option – green hydrogen – is made by electrolysing water using energy from renewable sources, but currently only accounts for a small percentage of hydrogen production because it requires expensive electrocatalysts. Now, Australian researchers have come up with an approach that could make green hydrogen more attractive commercially.

Hydrogen production via electrolysis is presently predominantly carried out in strongly acidic/alkaline electrolytes using state-of-the-art platinum group metal electrocatalysts to attain industrially relevant current densities. However, acidic electrolytes can generate highly corrosive environments and contaminate the product. Alkaline electrolysis, on the other hand, is often plagued by unstable electrocatalysts and the need for expensive pH-tolerant membranes.

Instead, researchers are investigating electrolysis using neutral or near-neutral electrolytes (pH 5–9) with cheaper electrocatalysts. The challenge is that, under these conditions, the hydrogen production rate is significantly lower.

A team of Australian engineers from RMIT University says they have solved this problem by using sound waves to boost production. They used a neutral electrolyte (sodium phosphate) on polycrystalline gold electrodes (doi: 10.1002/aenm.202203164). The use of high-frequency 10MHz hybrid sound waves during electrolysis resulted in 14 times more hydrogen being released – in terms of current density – compared with conventional electrolysis without sound waves; and produced a net-positive energy saving of 27%.

‘With sound waves making it much easier to extract hydrogen from water, it eliminates the need to use corrosive electrolytes and expensive electrodes such as platinum or iridium,’ says team leader Amgad Rezk. ‘As water is not a corrosive electrolyte, we can use much cheaper electrode materials such as silver.’

Producing hydrogen is notoriously difficult in neutral electrolytes, he explains. But in this case, the acoustic waves ‘frustrate’ the tetrahedrally-coordinated hydrogen bond network of water molecules at the electrode-electrolyte interface. The result is a high concentration of ‘free’ water molecules that are more readily able to access catalytic sites on the unmodified polycrystalline electrode.

The method also prevented the build-up of hydrogen and oxygen bubbles on the electrodes, the team reports. This gas layer minimises the electrodes’ activity and significantly reduces their performance. The result was greatly improved conductivity and stability.

‘In terms of efficiency (lowest energy loss), the acidic system is the most efficient because its reaction kinetics (catalytic activity) is faster,’ explains Jinho Park of Georgia Tech Research Institute, US. ‘But the system is highly corrosive and needs noble metal catalysts on the electrodes and anticorrosive coatings for stack parts.’ Most commercialised electrolysers use alkaline systems based on cheaper transition metal-based catalysts. But a major concern is a lack of high-performance and safe membrane separators, so it requires an external gas pressure control system to reduce the risk of explosion, he adds.

‘A neutral electrolyte system has lower efficiency [than acidic/alkaline] due to the intrinsic difficulty of reactions to generate hydrogen and oxygen from neutral molecules,’ Park says. ‘Low ion transfer rate of the system also limits its performance. However, the system durability of neutral electrolysers is superior to the others because of neutral pH.’ In this study, the researchers showed they could increase reaction kinetics by using sound waves, he notes. ‘The results are striking, but the research is still at a fundamental level. Many more issues have to be addressed [before commercialisation]. So, it is perhaps hasty to call this technology a “game changer”.’

‘The search for novel improvement in electrolyser stacks is critical to lower the overall cost of green hydrogen, which is in part driven by the cost of the capital cost of the stack itself,’ comments Laurie Heyworth, RenewableUK’s Senior Policy Analyst on Emerging Technologies. ‘Unlike fossil-fuel derived hydrogen, the cost of green hydrogen production has significantly fallen in the past few years largely due to cheap renewables and reductions in the costs of electrolyser facilities; but further cost reductions are needed. Scientific breakthroughs which avoid the use of scarce materials while retaining efficiency and flexibility in operation are therefore paramount if we are to establish a competitive global industry.’

Meanwhile, a team at Loughborough University, UK, has concluded that Africa has solar power, wind power, and biohydrogen resource potential for significant hydrogen production. Their new study predicts Africa could produce a total of 16,000m t/year of hydrogen (Renewable & Sust. Energy Rev., 2022, 167, 112705).

The long-distance export of green hydrogen to markets is critical in propelling the hydrogen economy in Africa and can be done so using a method known as ‘high voltage direct current system’, which is three times cheaper than exporting liquid hydrogen and 20 times cheaper than exporting compressed hydrogen, the study says. While several studies have evaluated the potential of hydrogen in Africa, this is the first to look at hydrogen potential at country level and assess water and energy access needs, transportation systems and costs.

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