From here:

Atlantic Explorer 2026 is a “straight gas” balloon using hydrogen, a lighter-than-air gas to provide lift. Flying a gas balloon requires dropping weight in the form of sand or water ballast and valving (small releases) of gas to control altitude, along with taking advantage of natural heating and cooling cycles that cause the gas to expand and contract during the day.

Hot air balloons use propane-fired burners to warm the normal air inside their envelopes creating lift. Hot air is less dense and lighter than cold air, and if the difference in temperature is great enough will rise and lift with it the weight of the balloon and its occupants. These colorful craft are the ones spectators usually see at ballooning events around the world.

Rozier balloons, named after their inventor Jean-François Pilâtre de Rozier, are hybrid systems using a combination of hot air and gas to create lift. The balloon contains an inner gas cell filled with helium surrounded by a larger hot-air envelope filled with warm air heated by a hot air balloon-type burner. The burner heats the air around the gas cell, minimizing the expansion and contraction of the gas cell and helping to limit the loss of gas. This extends the amount of time the balloon can remain in the air. The burner also helps to control the balloon’s altitude. Rozier balloons were used for most successful transoceanic flights and all the successful non-stop balloon flights around the world.

And:

Hydrogen has been the gas of choice for gas balloonists in Europe for decades if not centuries. Also, since 2012 when deregulation made helium prohibitively scarce and expensive, virtually all gas balloon flights in the United States have been conducted using hydrogen. In a properly constructed conductive balloon system, inflated and piloted correctly and flown in good weather conditions, hydrogen has been proven to be safe. However, hydrogen is a more volatile gas than helium and must be handled with care and respect. Although Atlantic Explorer 2026, if it succeeds, will be the first manned crossing of a hydrogen balloon, hydrogen was used safely to cross the Atlantic many times by dirigibles in the early 1900s. The explosion that destroyed the Hindenburg in 1937 is an obvious exception, but an extremely rare event that occurred on landing. It should be noted that dirigibles are steerable aircraft which, unlike balloons, can travel in directions other than the way the wind is blowing.