Building on this work, the same group proposed another way to enforce a concerted mechanism for the weakly conjugated amide-linked cobalt catalyst. Chemically modified electrodes derived from the covalent anchoring strategy through an amide linkage are supposed to adopt a stepwise and redox-mediated reaction pathway during electrocatalysis. The metal centers reach their active states by gaining or losing electrons from the electron flow through the electrostatic potential gradient within the electrical double layer. The electron transfer process is then followed by chemical reactions of the substrates at these active metal sites in the electrolyte. However, this commonly adopted redox mediation mechanism does not hold in the case of solid surface coupling in previous studies on graphite conjugated catalysts which on the contrary, adopted the concerted pathway. Interestingly, although the amide linkage was considered to provide weak coupling compared to the pyrazine linkage, it could undergo a concerted pathway based on the choice of solvent. To understand the requirements for a surface-attached metal complex to behave like a metal electrode versus a molecular catalyst (serve as a redox mediator), a glassy carbon electrode surface-attached cobalt complex in different solvent systems was investigated (Figure ). In this work, the authors utilized an aliphatic tether to anchor a cobalt porphyrin on the surface of a pre-oxidized glassy carbon electrode. In acetonitrile, an outer-sphere electron transfer corresponding to the Co II/I redox event was observed, consistent with the analogous homogenous system. The hybrid system was found to be catalytically active in catalyzing hydrogen evolution reactions via a stepwise pathway, which is commonly adapted by molecular catalysts. In contrast, no cobalt-based redox event was observed by cyclic voltammetry in aqueous media with various pH values. The key difference between the molecularly modified electrode in the two systems is that in acetonitrile, the homogenous analogue and the anchored catalyst remained soluble and thus are considered freely solvated and stay outside the double layer. In contrast, the molecular cobalt complex and the anchored catalyst remained insoluble in the aqueous solution and were forced to stay close to the electrode within the double layer (Figure ) due to extra surface interactions (axial coordination and π-π interactions, Figure ). Therefore, the redox potential of cobalt porphyrin will depend on the change of the Fermi level of the carbon electrode modulated by the external potentiostat. No electron transfer to the cobalt center can happen due to the lack of potential drop between the carbon electrode and covalently anchored cobalt porphyrin. However, hydrogen evolution reactions can still proceed in aqueous media with their proposed concerted proton-electron transfer mechanism. Under the circumstance, a proton could travel across the electrical double layer and bind to the cobalt center because of the electrostatic potential difference between the active cobalt sites and the bulk electrolyte. At the same time, the abovementioned strong surface interactions can enable electron flow within the same time scale of proton transfer and binding. This hydrogen evolution reaction enabled by the concerted pathway involving proton binding and electron transfer made the chemically modified glassy carbon electrode act like a metallic electrode and distinct from the redox-mediated reaction pathway in organic media or its soluble analogue.