Charge a supercapacitor safely using mobile charger
The simplest solution would be to use a linear regulator (LM1117) set to 2V. This pair with a current limiting resistor would prevent the regulator from blowing up. 

The next step would be to use a 555 timer to drive a PWM signal to a mosfet in between the LM1117 and the cap. This way you can limit the current without the need for a big current limiting resistor.