3-Electrode sensor potentiostatic circuitry
All op-amps require a power supply.  They do not work without external power.
Where the power supply is connected to the op-amps depends on the specific op-amp you are using.
The required operating voltage depends on the specific op-amp you are using.  Typically, you would use +-3.3V or +-5V.  The exact voltage depends on what your op-amp allows, how well your op-amp functions when its input or output voltages are close to the power supply voltages, and probably many other concerns.
Which op-amp you choose depends on many things.  Supply voltage is only the first of many things.
The 1.5V &quot;Maximum  voltage  in  normal  operation&quot; mentioned on page 3 of MEM1 does not refer to supply voltage.  That 1.5V is the maximum voltage that a sensor will produce. You don't supply power to it.  It generates a (tiny) amount of power when exposed to the gas it is supposed to detect. The information on page three is in a section titled &quot;Intrinsic  Safety  Considerations.&quot;  It is telling you that the sensors meet the requirements to be considered intrinsically safe.  This is important given that some of the sensors are used to detect explosive gases.
Judging by the very small currents mentioned, building a functional and accurate circuit will require op-amps with low leakage and low offset.  Given the slow reaction time of the sensors, drift will be a concern - you'll want an op-amp with low drift.
The small currents will also require careful layout to keep external interence out of your measurements.