Fuel cells are highly efficient and hardly polluting. They have a long history of use in space, and as a power source for submarines. They are beginning to appear powering city buses and intercity trains, at least in Europe, but not so much in the US or Canada. The business case for fuel cells is that they provide clean electric power to the train or bus, without the need for overhead wires. Avoiding wires helps make up for the high cost of hydrogen as a fuel. The reluctance to switch to fuel cells is the US is due to the longer distances that must be covered. The very low volumetric energy density of hydrogen means you need many filling stations with hydrogen fuel cells, and many fill ups per trip.

On a mass-basis, hydrogen is energy dense, with 1 kg providing the same energy as 2-3 kg of gasoline. The problem with hydrogen (aside from the cost) is that its mass density is very low, less than 50g/liter, even at high pressure. This is terribly un-dense on a volume basis. It would take 20 liters of high pressure hydrogen (about 5 gallons) to take a car or bus as far as with one gallon of gasoline. Even with a huge tank of high pressure hydrogen, 150 gallons or so, a cross country trip would require some 12 fill ups, one every 250 miles, and this is an annoyance, besides being an infrastructure problem.

Then there is cost. In California, hydrogen costs far more than gasoline, between $12 and $15 per kg. That’s ten times as expensive as gasoline on a weight basis and 4 times as expensive on an energy basis. What’s needed is a cheaper, more energy-dense version of hydrogen, ideally one that can be used in both fuel cells and IC engines, and the version I’d like to suggest is hythane, a mix of methane (natural gas) and 20-30% hydrogen.

Hythane has about 3 times the volumetric energy density of hydrogen, and about 1/3 the price. It makes less CO and CO2 pollution because there is far less carbon. On an energy basis, hythane costs just slightly more than gasoline, and requires less infrastructure. Natural gas is cheap and available, delivered by pipeline, without the need for hydrogen delivery trucks. Because hythane has about three times the volumetric energy density of hydrogen, the tank described above, that would give a 250 mile ride with hydrogen, would give 750 miles with hythane. This means a lot fewer fueling stations are needed, and a lot fewer forced stops. As a bonus, hythane can be used in (some) IC engines as well as in fuel cells.

Hydrogen for hythane-automotive use can be made on site, by electrolysis of water. Because there is relatively little hydrogen in the mix, only 25% by volume, or 8% on an energy basis, there is relatively little burden on the electric grid, and fueling will be a lot faster than with battery chargers. Hythane is already in use in buses in China and Canada. These are normal combustion buses but hythane works even better — more efficiently — with fuel cells (solid oxide fuel cells) and thus hythane provides a path to efficiency and greater fuel cell use.

Natural gas does not work as well in fuel cells; it requires a pre-reformer to make some H2, and even then tends to coke. To be used in most fuel cells, the methane has to be converted, at lest partially into hydrogen and this takes heat energy and water.

CH4 + H2O + energy –> 3H2 + CO

Reforming is a lot easier with hythane; it can be done within the fuel cell. Within a SOFC, the hydrogen combustion, H2 + 1/2 O2 –> H2O, provides heat and water that helps feed the reforming reaction and helps prevent coking. Long term, fuel cells will likely dominate the energy future, but for now it’s nice to have a fuel that will work well in normal IC engines too.

Robert Buxbaum, April 28, 2021