The most interesting region of the activity volcano is the transition region of the ratelimiting step from CO 2 adsorption to CO desorption. The activity of maximal TOF lies in this sweet spot and is characterized by a non-vanishing coverage of *COOH (cf. Sup-plementary Fig. ). Cu lies according to our model closest to this region with a binding of CO being neither too week nor too strong. This leads to a partially CO-covered surface (Supplementary Fig. and ref. 64) and also the highest predicted TOF of all transition metals. This is different from the conventional model suggesting Au to have the highest TOF (cf. Fig. ). Since Cu has a PZC not much different from the applied potential in the figure, surface charging cannot explain the deviation between the two models. Instead, the presence of the CO 2 intermediate in our model which needs to be stabilized by a stronger adsorption force of the metal leads to the observed shift. Experimentally, further reduced products like C 1 (methane) and C 2 (ethylene) products are usually observed on Cu, depending on the potential region and pH. Our calculations suggest, however, that Cu is actually a very excellent catalyst for producing CO, indicating that a rather significant amount of higher reduced products might be formed by re-adsorption of CO. Also the combination with a higher PZC metal like Pd might be promising as it moves the system into the higher activity region, and corresponding alloys showing high CO selectivity have been recently reported. Our volcano suggests, that even more active catalysts might exist beyond the bare metallic ones. The most straightforward way to design such materials would be the combination of features from two different metals in the form of bimetallic catalysts. Such catalysts could be formed by doping a substrate with a different metal e.g. in the form of single atoms, forming an overlayer, forming high entropy alloys in homogeneously mixed alloy phases, forming pure alloy crystal phases, nano-particles, etc. In Fig. we tested these possibilities at the example of Au-Cu bimetallic alloys to discuss how such structural motifs could help to navigate in our newly discovered descriptor space. CO will always adsorb on the most strongly binding site and thus the presence of Cu sites in the surface will always set the CO adsorption energy descriptor to a value close to Cu. Variations of this adsorption energy across the bimetallics is rather small, only about ±0.1 eV. This suggests that the tuning of material properties based on adsorption energies may not be particularly successful to actually design better catalysts. We stress that this simplified treatment does not take into account coverage distributions across active sites, saturation of the Cu sites, etc. However, in contrast to the comparably difficult modulation of the CO adsorption energy, the PZC can be much more easily tuned over a wide range. Looking at this descriptor in more detail, one can first note by comparing analogous surface configurations that the less noble metal dominates the PZC. In order to increase the PZC of Cu to make it more active for CO formation, the most efficient way is to dope Cu into a Au substrate. Also having small Cu clusters on a Au substrate can be beneficial up to a full monolayer of Cu. In contrast, high entropy alloy or single crystalline crystals promise much less tunability of the PZC, fixing the value to the less noble metal. The importance of the substrate in tuning the PZC of a catalytically active site on the surface has been so far relatively rarely discussed but could indeed play a much important role than previously expected.