Perhaps some new catalyst or meta-material would enable a more efficient horse.

The current processes are not that inefficient, it’s currently commercially inefficient because the margins are small, but if there were issues in obtaining alternative fuels the margins would increase to make micro scale ammonia generation economically viable. There are supposedly already catalysts that exist to make small scale ammonia production competitive even at current margins and there is some interest in using this for solar power energy storage. For me this technology is still in the category of I’ll believe it when I see it. Most people who need ammonia today need a whole lot of it so small scale production wouldn’t make sense even if it was just as efficient. The other issues with ammonia are that it’s rather toxic and pretty easy to make into explosives. Eventually I’m planning on having an excess supply renewable energy so dumping that into ammonia production is a no brainer.