A switching diode, as Stan Meyer used it, is a component which blocks reverse current, up to a point, then 'switches' to its reverse mode, passing the backwards potential. This prevents excessive voltage build up on the fuel cell capacitor, even allowing it to discharge to some extent. I've found in the past, through working with one of my own circuits, that an effective application requires a diode with a PIV rating right at, or close to, the maximum desired potential. And the diode also has to be rated as "reversible". This means that it can pass an expected level of reverse current without burning out. The how and the why is detailed in Steve's link.
