In this work, we use ab initio simulations to explore neutral and charged Frenkel pair (FP) formation inside HfO2. FP plays a crucial role in the conductive filament (CF) formation. We explore two possible mechanisms for the FP formation, namely electron injection and electron detrapping. The existence of one of the two mechanisms or both depends on the nature of the metal electrodes and potential applied. The results show that at high voltage operation electron detrapping may occur which strongly degrades the HfO2 dielectric layer. Contrary to electrodes with high work function (Wf) and non-reactive with oxygen, we show that oxygen active electrodes with low Wf avoid electron detrapping, thus improving device variability and CF thermal stability.