The fuel cell stack, in theory, can weigh less than batteries.

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In the fuel-cell stack, hydrogen goes in and electricity comes out, which then powers the car.

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The gas is then fed into a stack of solid oxide fuel cells, where a chemical reaction between hydrogen and oxygen occurs, producing electricity, water vapor and a tiny amount of carbon dioxide.

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But a fuel-cell engine is more than a stack of cells.

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Now, if the industry could only figure out the touchy issue of cost and the still touchier issue of stack replacement (and several signs suggest that moment may arrive sooner rather than later), fuel cells could take off in mainstream markets.

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This stack inside a flow battery, where the electrolyte meets the electrodes, looks a lot like a fuel cell.

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