In early December, I focused on Plug Power (PLUG -3.44%) after one of the most optimistic management conference calls I'd tuned into in quite some time. Since then, shares of Plug Power jumped over 200%. I've previously mentioned my excitement over Plug (the company) and over the potential for hydrogen's use in transportation and refrigeration. However, my concerns still remain related to the leftover CO2 once hydrogen is actually split from natural gas. 

Hydrogen infrastructure is still a young, albeit promising, industry -- that's for sure. I'd like to see wider applications for using that leftover CO2 for enhanced oil recovery and more generation of hydrogen through renewable opportunities to produce electricity through solar and wind. For that reason the recent strength in shares of Plug Power may be a good opportunity to take some money off the table in this name and wait for a deeper pullback to add more shares. Also, while Plug recently announced a deal with FedEx (FDX -0.77%) that led to an additional 70% gain in its shares, it's worth noting that the company was expecting that deal to happen in 2014, so the news was not out of left field. I'm still a big fan of Plug and the stock could go higher, but longs may be hard pressed to pass up cashing in some chips in a name that was exchanging hands at $0.80 just last month.