The paper you cite refers to metal hydride. That is where I would expect the source of heat is from. Basically an expensive battery.
Hydrides are a favorite tool of scam artists. They have high energy density, look like rocks, and release energy when exposed to water. They get passed off as an energy source, but are no such thing. Rather they are an energy storage medium, and so long as the ‘charging’ process is done off stage, short demonstrations can be very convincing and misleading.
Now, some references to the scam artists and their tricks ~ bring them on. Always interested in what they're up to.
If they can drop HUGE amounts of energy into these small needle shaped devices they've certainly got some winning batteries.
I don't recall encountering any reports that had large sheets of hydrides involved.