An optimal control approach to a simplified reaction diffusion system describing cardiac defibrillation is proposed that allows for joint optimization of shape and duration of defibrillation pulses. Within the framework, optimized multiphasic pulses with low energy, short duration and/or low amplitude can be designed according to specific needs. The approach is based on a novel time optimal control formulation for the monodomain model, which takes into consideration the dynamical system properties of the uncontrolled equation. The highly complex dynamics requires a consistent discretization of first and second order information to guarantee effective optimization schemes leading to successful defibrillation. Numerical examples underline the efficiency of the proposed method.