There are quite a raft of regional airliners either being retrofitted with hydrogen fuel-cell-powered electric motors or complete clean-sheet designs under development now, especially in the 19-50+ seat market. However, take for example Universal Hydrogen's retrofitted ATR-72 and a good ten or so seats are replaced with containerised hydrogen tanks, so the revenue per seat mile will be somewhat diminished. Things like the Dornier 328eco are looking good (retains turbine engines but using alternative fuels), Heart Aerospace, Zero Avia and others, but can't imagine certification by 2025 and entry into service immediately thereafter. Also costs will be interesting - leasing an old Dash-8, ATR or Dornier, or perhaps say an old CRJ or EMB-145 is going to be a lot cheaper than anything new and/or expensively retrofitted. It's exciting to see, but perhaps people need to be a little more realistic on timescales?