Q: Why is it that potential difference decreases in thermistor when temperature of circuit is increased? Why is it that potential difference decreases in thermistor when temperature of circuit is increased? From my understanding when temperature increases, there are more free electrons ($n$) so according to $I=navq$, current increases as $n$ is greater. But why is it that potential difference decreases; if $V=IR$ shouldn't pd also increase? 

A: You are confusing cause and effect.  The current is a function of the applied voltage, or the other way around.  It is incorrect to say that a thermistor changes in voltage or current as a function of temperature.  That's dependent on the circuit it is connected to.
The external property that changes in thermistors as a function of temperature is the resistance.  R goes up or down with increased temperature, depending on the kind of thermistor you have.  Most things just called a "thermistor" exhibit decreased resistance with rising temperature.  There are also such things as PTC (positive temperature coefficient) thermistors that exhibit the opposite effect.
As you say, V = IR.  If R goes down, the V will go down at the same I.  Conversely, if V is held constant, the I goes up.  Both can be legitimate ways to run a thermistor.  Probably the most common way is to put the thermistor in series with a fixed voltage and a fixed resistance.  In that case, both I and V go down as R goes down.

A: if the temperature decreases the resistance of the thermistor will increase so the potential different will increase because V=IR so if R is increased then the total answer for V will increase
