Hot plasma mixed with hydrogen will most likely turn hydrogen into plasma - but it depends on the plasma properties (how hot it is) and on the relative amounts of both. This is simple heat balance.
(Well, if it's hot enough to convert the hydrogen into plasma, wouldn't the hydrogen then leave the cell, since plasma has the capabilities to travel through solids, and the hydrogen is now plasma too?)
As long as the fuel cell didn't have oxygen, there is no real reason for the cell to explode. But it depends on a lot of other factors.
(But wouldn't the plasma entering the fuel alone cause oxygen to enter when the plasma enters or exits it?)