The new method allows larger-than-ever quantities of hydrogen to be produced at atmospheric pressure using lower power loads, typical of those generated by renewable power sources. It also solves intrinsic safety issues which have so far limited the use of intermittent renewable energy for hydrogen production.

Of the alternative methods for H₂ production that are not linked to fossil resources, the electrolysis of water stands out as a mature, scalable technology for which the only required inputs are water and energy (in the form of electricity). Hence, if the energy source is renewable, H₂ can be produced sustainably from water using electrolysis.

Renewable energy inputs tend to be sporadic and fluctuating, and thus the systems that are developed to harness this energy and convert it to H₂ [such as proton exchange membrane electrolyzers (PEMEs), solar-to-fuels systems, and artificial leaves] must be able to deal with varying energy inputs effectively and have rapid startup times. At the low power loads that are characteristic of renewable power sources, the rate at which H₂ and O₂ are produced may in fact be slower than the rate at which these gases permeate the membrane. At the very least, this will severely affect the amount of hydrogen that can be harvested from such devices, and in extreme cases could give rise to hazardous O₂/H₂ mixtures. The PEME is the most mature technology cited for renewables-to-hydrogen conversion, but prevention of such gas crossover at low current densities remains a challenge.

… There is thus a need to develop new electrolyzer systems that can prevent product gases from mixing over a range of current densities and that make more effective use of the precious metal catalysts they employ, in order to make renewables-to-hydrogen conversion both practically and economically more attractive.

—Rausch et al.

The research team was led by Professor Lee Cronin of the University of Glasgow’s School of Chemistry.

The process uses a liquid that allows the hydrogen to be locked up in a liquid-based inorganic fuel. By using a liquid sponge known as a redox mediator that can soak up electrons and acid we’ve been able to create a system where hydrogen can be produced in a separate chamber without any additional energy input after the electrolysis of water takes place. The link between the rate of water oxidation and hydrogen production has been overcome, allowing hydrogen to be released from the water 30 times faster than the leading PEME process on a per-milligram-of-catalyst basis.

—Professor Cronin

A schematic of silicotungstic acid–mediated H2 evolution from water. At the anode, H2O is split into O2, protons, and electrons, while the mediator is reversibly reduced and protonated at the cathode in preference to direct production of H2. The reduced H6[SiW12O40] (dark blue) is then transferred to a separate chamber for H2 evolution over a suitable catalyst and without additional energy input after two-electron reduction of the mediator to H6[SiW12O40]. Click to enlarge.

The use of a redox mediator that can be reversibly reduced in an electrolytic cell (as water is oxidized at the anode) and then transferred to a separate chamber for spontaneous catalytic H₂ evolution leads to a device architecture for electrolyzers that has several important advantages, the team noted.

1. It allows the electrochemical step to be performed at atmospheric pressure, while potentially permitting H₂ to be evolved at elevated pressure in a distinct compartment.

2. Virtually no H2 is produced in the electrolytic cell itself, which (taken with the feature above) obviates the need to purge H₂ from the anode side of the cell and could significantly reduce ROS-mediated membrane degradation and the possibility of explosive gas mixtures forming at low current densities or upon membrane failure.

3. H₂ evolution from such a system is no longer directly coupled to the rate of water oxidation, and thus the decoupled H₂ production step can be performed a rate per milligram of catalyst that is more than 30 times faster than that for state-of-the-art PEMEs.

4. The hydrogen produced has the potential to have an inherently low O₂ content, both on account of its production in a separate chamber from water oxidation and by virtue of the fact that the reduced mediator reacts rapidly with O₂ in solution. This final point could render the H₂ produced suitable for applications requiring high-purity H₂ such as fuel cells or the Haber-Bosch process, without the need for post-electrolysis purification or built-in recombination catalysts.

Comparison of the rate of H2 production possible with silicotungstic acid–mediated electrolysis and a selection of leading PEMEs from the current literature. Literature values are based on the highest rate of H2 production reported in those works. A table with more examples and a full description of how these metrics were calculated can be found in the paper’s supplementary materials. Rausch et al. Click to enlarge.

The research was produced as part of the University of Glasgow Solar Fuels Group, which is working to create artificial photosynthetic systems which produce significant amounts of fuel from solar power.

Around 95% of the world’s hydrogen supply is currently obtained from fossil fuels, a finite resource which we know harms the environment and speeds climate change. Some of this hydrogen is used to make ammonia fertilizer and as such, fossil hydrogen helps feed more than half of the world’s population. The potential for reliable hydrogen production from renewable sources is huge. The sun, for example, provides more energy in a single hour of sunlight than the entire world’s population uses in a year. If we can tap and store even a fraction of that in the coming years and decrease our reliance on fossil fuels it will be a tremendously important step to slowing climate change.

—Professor Cronin

The University of Glasgow’s Dr. Greig Chisholm, Dr. Mark Symes and Benjamin Rausch also contributed to the paper.

Resources

• Benjamin Rausch, Mark D. Symes, Greig Chisholm, and Leroy Cronin (2014) “Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting” Science 345 (6202), 1326-1330. doi: 10.1126/science.1257443