Crabtree, Dresselhaus, and Buchanan reply: Peter Feibelman raises a good point in advocating NH3 as a hydrogen storage medium. He points out many advantages, including its high storage capacity, the significant ammonia infrastructure already in place, and our extensive chemical knowledge and industrial experience with ammonia.
The problem of effective hydrogen storage is one of the most challenging in the hydrogen economy, and we should pursue all promising options. The use of ammonia in a hydrogen economy has been discussed since at least the 1970s; Ali T-Raissi summarizes its history and its possibilities. 1 The subject remains vibrant today; new mechanisms for the release of hydrogen from ammonia over catalysts at acceptable temperatures are continuing topics of research. 2 A major challenge is toxicity, as Feibelman points out, but all hydrogen storage proposals come with safety issues.
Ammonia can be used effectively in other hydrogen storage media as well, 1,3 notably in combination with its borane analog, BH3. NH3BH3 releases more than 12% of its mass as H2 in decomposing to NHBH at low temperature and ambient pressure. Its release rate and decomposition chemistry can be significantly improved by nanoscale structuring in porous hosts. 3 This example shows how the richness of hydrogen chemistry and the influence of nano-patterning lead to new horizons in hydrogen storage.
Lewis Glenn correctly points out that the energy used to split water is only partially recovered on recombination of H2 and O2 to make water. No energy conversion process is 100% efficient; some energy will always be lost. The higher potential efficiency of fuel cells over internal combustion engines is an appealing advantage of hydrogen over gasoline. As a carrier of energy, hydrogen costs more to produce than gasoline, whose energy originates naturally in the crude oil from which it is refined. Although gasoline outperforms hydrogen in cost, hydrogen is the winner in the long-term sustainability of supply, security of access, and freedom from environmental pollution and climate change. These long-term quality-of-life issues are strong justification for strategic research now to enable the hydrogen economy in the future.
The switch from fossil fuel to hydrogen replaces emission of the greenhouse gas CO2 with emission of H2O, as Phil Stripling points out. Wouldn’t there be a potentially serious environmental impact from that additional water? The hydrogen required to supply the world’s energy for one year, 13 TW-yr, would make approximately 31 km 3 of water as “exhaust.” This is about twice the volume of Crater Lake in Oregon. The total water on Earth amounts to 1.4 × 109 km3, and that in the atmosphere to 12 000 km3. Thus even if all the exhaust water produced in one year from a hydrogen economy remained in the atmosphere, it would increase atmospheric water vapor by less than 1%. The actual increase would be much less, since the residence time of water vapor in the atmosphere is about nine days.