Water electrolysis: Divide and conquer.

news & views the eventual transition to homogenous 3ʹ,5ʹ-linked genetic material. T echnologies for generating electricity from renewable resources, such as sunlight and wind, are on a steady trajectory of falling cost. Within the next decade, solar electric power is expected to reach grid parity, the point at which its cost will be equivalent (on a per-watt basis) to that of power generated from coal, nuclear fission or natural gas. This sounds like good news for a sustainable energy future — with its economic, geopolitical and environmental consequences — but there is a catch. Intermittent and fluctuating power from solar and wind are not well matched to demand, so massive energy storage (on a scale of gigawatt-hours in North America, Europe and Asia) will be needed 1. Further, electricity represents only about 20% of our energy use now, with fuels making up the balance. Although the gradual electrification of the transportation energy sector can help 2 , fuels are essential now and will remain so well in to the future for heating, aviation and other uses. We do not yet have an economic way to make fuel from electricity; in fact, the high cost of electricity relative to fossil fuel energy reflects the thermodynamic penalty — a hefty 65% — of generating (high quality) electricial energy from (low quality) cheap heat. To go the other way and compete with the low cost of fossil fuels, one would need to start with very cheap electricity and not pay much to convert it. One of the front-running technologies for storing electrical energy and converting it to fuel is the electrolysis of water to hydrogen and oxygen. Hydrogen can either be used directly as a fuel, converted catalytically to liquid fuels such as methanol 3,4 , or used to make electricity in fuel cells. The latter is the reverse process of electrolytic energy storage. Electrolysis is an old and well-developed technology, the scale of which is not limited by the availability of raw materials or chemical feedstocks. However, the use of electrolysis in energy conversion is hampered by the high cost of the catalysts, membranes and hardware that comprise the electrolyser, and also by the low efficiency of the anodic half-reaction, in which water is oxidized to oxygen. For energy storage, the best way to beat down the cost of the electrolyser is to improve its duty cycle (the fraction of time that …