What happened

Easy come, easy go. Shares of hydrogen fuel stock Plug Power (PLUG -13.87%), which handily outperformed its fuel cell rivals Bloom Energy (BE -3.21%) and FuelCell Energy (FCEL -10.05%) in Thursday trading, are falling flat as of 2:30 p.m. EST Friday -- actually down 5.1%, while Bloom (up 3.3%) and FuelCell (up 9.8%) race higher.

Why is Plug stock underperforming its rivals?

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Image source: Getty Images.

So what

There's no news on the wires to explain why two of these hydrogen fuel cell stocks are up and the other is down. No upgrades (or downgrades). No price target hikes (or reductions) -- and nary an earnings report in sight.  

Profit-taking is probably one answer to why this is happening. All three of these stocks are pretty volatile, and investors have been trained over the past few months to anticipate that what was up today might be down tomorrow (and vice versa). Yesterday was Plug's turn to be up -- albeit only 2% at the closing bell. In contrast, Bloom stock flatlined on Thursday, and FuelCell actually closed down almost 5%.

It's only natural, then, that investors with more of a gambling mentality might be inclined to take their Plug profits today and invest them in the stocks that "haven't gone up yet" instead.

Now what

But while such a reaction would be understandable, it's probably not the best way to invest your money. Viewed objectively, none of these three alternative energy companies is profitable (nor ever has been). Bloom Energy, however, is the one that's gotten closest, with its losses shrinking this year and substantial positive free cash flow generated last year.

Of the three, it's the only one I'd be considering investing in today.