Furthermore, Frumkin effects depend on the pH, as it modulates the surface charge, i.e., the surface charge is more negative at higher pH. For the case of the OER at NiOOH, the effect of increasing the driving force is more pronounced. As a result, the activity exhibits an increase as the surface charge becomes more negative, and thus at higher pH. However, Frumkin effects have been revealed as being insignificant in the case of FAOR, as the competing effects more or less cancel each other out. Instead, the mass transport-induced local pH shift emerges as a crucial factor that influences observed pH effects. Although the FAOR has been studied for many decades as a model reaction, the relation between its activity and the pH remains controversial. Joo et al. firstly reported that the activity-pH relation of FAOR at Pt exhibits a bell shape with the peak at the pKa of formic acid (~ 4). Their study considered phosphate solutions with pH ranging from 0 to 12. It was explained that the activity increases with pH when pH < pKa since the concentration of HCOO -increases, which is the main reactant. The siteblocking effect of OH adsorption becomes significant when pH > pKa; therefore, the activity decreases with pH in this range. However, a trapezoidal-shaped activity-pH relation with a plateau between pH=5 and 10 was observed when perchlorate solution was used (Figure ). This observation challenged the previously proposed mechanism.