I suggest a combination of Texas Instruments bq25504EVM-674 – Ultra Low Power Boost Converter and an LM385Z-2.5 voltage reference.
See:
Alexis X, Stinchcombe A, Greenman J, Ieropoulos I (2017) Urine transduction to usable energy : A modular MFC approach for smartphone and remote system charging. Appl Energy 192:575–581 . doi: 10.1016/j.apenergy.2016.06.006