Dreams of a “hydrogen economy” where renewable energy is stored as gas have come a step closer with the announcement of a cheap and efficient catalyst for water electrolysis. The new technology could also offer a cleaner source of hydrogen for existing uses.
Renewable energy sources such as the sun and wind are now often cheaper than fossil fuels, even without pricing in the cost of pollution. However, they still face the hurdle of intermittency, the question of what to do when the sun doesn't shine or the wind doesn't blow.
While battery technology is making great strides, there is a strong argument for hydrogen as a way to store energy. In this model, we would use surplus energy on sunny days to split hydrogen through electrolysis and store it to be burnt or used in fuel cells when required.
One of the major obstacles has been finding appropriate electrodes to do the splitting, particularly the anode. Efficient electrodes exist, but they involve oxides of platinum group metals such as iridium or ruthenium to act as catalysts. Iridium costs more than half as much as gold, and even ruthenium is far too expensive for large-scale use.
Associate Professor Chuan Zhao of the University of New South Wales says cheaper cathodes work well enough to use. However, in Nature he writes, “The production of H2 at the cathode is severely constrained by the sluggish kinetics of the oxygen evolution reaction on the anode.” Currently anodes can be cheap or fast, but not both.
Zhao thinks that is about to change. His Nature paper reports on the electroplating of a nickle-iron catalyst onto nickel foam. The foam is filled with holes twice as wide as a human hair, while the catalyst forms pores a quarter of that size. “The three-dimensional architecture of the electrode means it has an enormous surface area on which the oxygen evolution reaction can occur," says Zhao.
Credit: UNSW Australia. The porous structure of nickel foam provides a huge area to catalyze reactions. Scale bar is 200 µm.
Moreover, Zhao adds, “The larger bubbles of oxygen can escape easily through the big holes in the foam.” On the other hand, the surface is very hydrophilic, attracting water and dispersing oxygen in the process. It is capable of carrying the high currents needed to drive oxygen production.
Zhao says more work is required to understand “why the metals are so active,” and this may lead to still better versions. Nevertheless, he thinks the efficiency is high enough to investigate expanding the systems to industrial scale. Nickel foam is cheaply available commercially and, as Zhao says, “Nickel and iron are two of the most common metals on Earth.”
The system is unlikely to be suitable for splitting seawater, but Zhao sees a bright future for splitting water in closed systems or to power hydrogen cars. Current hydrogen production mostly uses fossil fuels since they are still cheaper than electrolysis, but this may change if Zhao's system can be applied commercially. Electrode manufacturing and energy inputs are the two main costs keeping electrolysis expensive.
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