image: In alkaline electrolyte, glycerol is converted to formate by electrocatalytic oxidation in alkaline electrolyte over Ni-MnO2 catalyst. The incorporation of Ni element in MnO2 accelerates the hydrogen transfer rate in glycerol oxidation.
Credit: ©Science China Press
Developing high-performance, non-precious metal electrocatalysts is a critical step in glycerol oxidation research. This requires an in-depth understanding of the catalytic reaction mechanisms. The strength of surface adsorption and the rate of hydrogen transfer are key factors that determine catalytic efficiency. And the C-H bond activation and dehydrogenation processes in biomass molecules are usually shown to occur via lattice oxygen-mediated hydrogen transfer.
The excessively strong d-p coupling at metal centers (such as Mn sites) can lead to an overoccupation of the lattice oxygen orbitals, which indirectly reduces the reactivity of lattice oxygen in the subsequent hydrogen transfer processes of organic molecules. However, by employing transition metal elements with weak d-p coupling properties to achieve precise regulation of the active centers of the double octahedra, it is possible to regulate the orbital properties of the lattice oxygen to accelerate hydrogen transfer while maintaining the strong d-p coupling adsorption properties of the single site (such as Mn sites).
Regulatory strategy
Researchers selected MnO2 with strong d-p coupling as the catalyst model, in which a series of transition metal elements (Fe, Co, Ni, and Cu) with relatively weak d-p coupling were added to precisely modulate the hybridization of the double octahedral d-p orbitals. This method significantly accelerated the hydrogen transfer rate in glycerol oxidation. The effectiveness and multifunctionality of this method were confirmed through a combination of in-situ spectroscopic studies and DFT calculations.
Electrocatalytic GOR performance
Researchers have confirmed that replacing OER with GOR can significantly reduce the anode potential and the energy consumption of the electrolytic cell. The multi-atom doped MnO2 catalyst requires a much lower onset potential for GOR (1.0 V vs. RHE) compared to OER (1.5 V vs. RHE). Moreover, as the local coordination environment evolves from Mn-Mn to Mn-Cu, the anode potential for glycerol oxidation exhibits a trend of initially decreasing and then increasing. Notably, Ni-doped MnO2 shows the optimal GOR catalytic activity. Ni-MnO2 only requires 1.16 V vs. RHE to achieve a current density of 10 mA cm-2, which is significantly lower than the 1.38 V vs. RHE required by the original MnO2. Additionally, Ni-MnO2 exhibits excellent stability, with negligible activity decay after continuous testing for 80 hours. In addition, the main product of glycerol electrooxidation is formate, and its Faradaic efficiency reaches 99.7%.
Ni-MnO2 shows universal applicability in the electro-oxidation of biomass to produce formate. Ni-MnO2 has good oxidation performance for both methanol and ethylene glycol, and the Faradaic efficiency of the oxidation product formate reaches over 95%.
Mechanistic insights on glycerol oxidation
Researchers proposed a glycerol oxidation pathway based on product analysis. Glycerol is first oxidized to glyceraldehyde and glycerate. Subsequently, glycerate undergoes cleavage, producing formate and glycolate. Finally, glycolate is further oxidized to generate two molecules of formate.
Researchers found that during the glycerol oxidation process, Ni-MnO2 underwent continuous phase transitions (α-MnO2→Mn3O4→δ-MnO2/MnOOH), with MnOOH being the main active phase. Compared to the original MnO2, the surface of Ni-MnO2 can spontaneously activate glycerol molecules, and continuous hydrogen transfer attacks at the lattice oxygen sites lead to the spontaneous phase transition of α-MnO2 to Mn3O4. Additionally, at lower potentials, Mn3O4 continues to phase transition to MnOOH. The introduction of Ni elements can significantly improve the hydrogen transfer efficiency at the lattice oxygen sites.
Researchers have further elucidated the reaction mechanism of glycerol oxidation through density functional theory calculations. The incorporation of Ni significantly reduces the hydrogen transfer energy barrier and accelerates the deprotonation process during the glycerol oxidation process. Furthermore, Ni doping leads to d-orbital broadening, which modulates the p-orbital distribution near the oxygen Fermi energy level in the lattice. This produces relatively higher empty orbital states, which in turn facilitates the hydrogen transfer process.
This work provides new insights into balancing the adsorption and activation of biomass molecules, while offering a universal modulation strategy for designing efficient biomass oxidation electrocatalysts.