Electrochemical synthesis could provide an elegant and efficient means to exploit the largely available C1 building block CO2. The electrocarboxylation of dienes is, in particular, an attractive goal. However, the presently known electrocatalysts are inefficient and not very selective since they work around - 2.3 V vs. SHE. In order to identify more active catalysts, we need to better understand the reaction mechanism. In this contribution, we present prototypical experimental results for the electrocarboxylation of 2,3-dimethyl-butadiene on a Ni catalyst and quantify the side-products, namely carbonates, oxalic and formic acid. Together with the extensive, state of the art, first principles investigation of the mechanism at the atomic scale, we reveal a highly activated process around the onset potential of - ’1.3 V and a change in mechanism at the peak potential of - ’2.3 V, suggesting that a more active catalyst could be engineered by modifying the morphology in order to facilitate the œchemical€ CC coupling step.