There is at least one: Universal Hydrogen (hydrogen.aero). UH is looking at converting existing aircraft to hydrogen power. Their focus is as much on the logistics of hydrogen distribution as it is on aircraft design.
From their website:
“We are building a flexible, scalable, and capital-light approach to hydrogen logistics by transporting it in modular capsules over the existing freight network from green production sites to airports around the world. At the airport, the modules are loaded directly into the aircraft using existing cargo handling equipment. No new infrastructure is needed. Starting with regional airplanes, we are also developing conversion kits to retrofit the existing fleet with a hydrogen fuel cell powertrain.”