As discussed earlier, catalyst performance -including reaction rate-should be performed at low conversion in systems with known (facile) mass transport, at well-defined electrochemical potential and driving force. We offered suggestions for the composition of model electrolytes and controlled variables (𝑒 -passed NO a carbon matrix , the active site identity is reasonably well-known and controllable via metal precursor loading. For other systems such as metal nanoparticles, films, and alloys, assumptions and approximations must be made in choosing an appropriate strategy to normalize rate. The exposed surface area is typically used, in the absence of more detailed understanding of the system (e.g. that defect sites or a specific element is most active). Quantifying this area is challenging, and approaches can be system specific. For example, the surface area of Pt and Pd nanoparticles can be assessed via the underpotential deposited hydrogen (Hupd) charge . The stripping of deposited metals, such as Cu or Pb can also be used to estimate surface area in some cases (though care should be taken to avoid contamination resulting from this) . Measurement of the electrochemical surface area (ECSA), while often employed , is limited by a factor of ~7 uncertainty . However, its measurement is typically reproducible in comparison across research groups, and others recommend a specific capacitance of 0.035 mF cm -2 in 1 M H2SO4 and 0.040 mF cm -2 in 1 M NaOH for comparison across metallic systems .