It would be great if everyone could back up the intermittent power from wind and solar plants with energy stored as low-cost, zero-carbon hydrogen gas. But hydrogen can be hard to store. Last month, when the Royal Society advised the British government to start building underground caverns to store megatons of hydrogen gas, it noted that the United Kingdom would need to store 1,000 times as much energy in this way as its pumped hydropower reservoirs can hold, and far more than batteries can feas...