The hydrogen economy was always sold as a technology story. Increasingly, it is an interest-rate story — and Plug Power is where that tension is most visible.The fuel-cell developer's shares were changing hands around €1.86 to €1.89 on Tuesday, a decline of roughly 2.4 to 4 percent depending on the pricing reference, as the yield on ten-year US Treasuries crept back toward its own 52-week...