Recent progress of Ni-based catalysts for methanol electrooxidation reaction in alkaline media

The study is led by Ligang Feng (School of Chemistry and Chemical Engineering, Yangzhou University).

The rapid development of the economy driven by the large consummation of traditional fossil fuels is not sustainable, and global attention is shifted to the utilization of renewable energy sources, and biomass fuels. Methanol is considered a good biomass fuel to realize energy storage and conversion, which is convenient for storage and transportation; more importantly, it is much safer than other fuels such as gasoline, diesel, and natural gas. In addition, it can be prepared with wide sources in low-cost and environmentally friendly approaches. Especially, methanol is one of the target products of CO₂ reduction that plays an important role in the carbon cycle. Therefore, it is chosen as a competitive fuel in new energy, since it has a very high energy density, even much higher than that of lithium-ion batteries.

As an important hydrogen-rich fuel, the energy release and conversion of methanol can be realized in a methanol oxidation reaction, which is a key half-reaction for methanol-reforming of hydrogen production and direct methanol fuel cells (DMFCs). The theoretical oxidation potential of MOR is 0.016 V vs. RHE, which is much lower than that of the water oxidation; therefore, compared with the traditional electrochemical water electrolysis, the overpotential is greatly reduced in the hydrogen production by methanol electro-reforming, which is very helpful for the hydrogen release and circulation. In the DMFCs technique, which is not controlled by the Carnot cycle, the high power generation efficiency can be realized with the advantages of environmental friendliness and high specific energy. The MOR is also involved in some of the electro-oxidation processes for many organic materials. Therefore, the study of MOR is of great significance to the basic research of simulating the electrocatalytic process of various organics. In addition, the development of MOR has important implications for transforming methanol into value-added products such as formate. Thus, MOR is a significant reaction in the carbon cycle, energy conversion, and chemical industry. Note that the inherently slow kinetics of MOR is not favored in these methanol-based techniques, and highly efficient catalysts are required to drive MOR.

The noble metal-based materials of Pt and Pd are the most common catalysts for MOR, while the high cost of noble metal catalysts cannot support the commercial application on a large scale. More importantly, the poisoning effect during the process of MOR is adverse to the noble catalyst, as some intermediate products generated will strongly occupy the active sites, making it difficult for the catalytic reaction to continue. The MOR can be done in acid and alkaline electrolytes. Although MOR in acidic media is much easier to start than that in alkaline media, it has a strong dependence on noble metal-based catalysts, because non-noble metal catalysts are less stable in chemical properties resulting in their easy dissolution in acidic media. The MOR in an alkaline medium has the advantages of a less toxic effect, easy polarization at low anode overpotential, and much better oxidation kinetics than that in an acidic medium. Moreover, the deprotonation kinetics of methanol in an alkaline medium is favourable, which makes it possible for the application using non-noble metal-based catalysts as catalysts. Among the non-noble catalyst, nickel has attracted much attention due to its good catalytic activity and anti-poisoning ability. The easy conversion of Ni, Ni²⁺, and Ni³⁺ redox centres could facile promote the active species formation, thus, speeding up the MOR in the alkaline medium. As early as 1955, porous nickel and porous nickel silver were used as an anode and cathode to catalyze methanol oxidation in an alkaline medium. In the 1970s, the electrochemical oxidation of some organic compounds in alkaline solutions was systematically evaluated using nickel anodes by Fleischmann et al., and they found that the oxidation of Ni(OH)₂ to NiOOH was much faster than methanol oxidation catalyzed by NiOOH on Ni-based materials. Since then, Ni-based catalyst for MOR has been developed including Ni(OH)₂, Ni complexes, and various Ni alloys, as well as the carbon-supported Ni catalysts. For example, due to the inhibition of the α-Ni(OH)₂ phase by Cu, NiCu-based noble metal-free catalysts have been developed. The oxophilicity of Ni has a good adsorption and oxidation effect on CO, hybrid materials of Ni and noble metal-based catalysts have also been developed. In the past ten years, the newly developed Ni-based catalysts have become more and more complicated with varied structures but high catalytic performance such as core-shell structures, heterostructures, and composite hybrid Ni-based catalysts. In 2019, a review of ethanol oxidation catalyzed by Ni-based materials in alkaline media was done; the review progress of Pt-based and Pd-based, or the progress of MOR catalysts catalyzed by non-noble catalysts has been overviewed from time to time. The metal-based electrocatalysts of noble metals and non-noble metals for methanol oxidation in direct methanol fuel cells technique were also revised recently. While to the best of their knowledge, a comprehensive understanding of the progress of recent advances for MOR catalyzed by Ni in alkaline electrolytes is rare, especially considering the MOR-assisted water splitting for hydrogen generation.

Making a critical review of these achievements would be helpful in the new catalyst development, thus, an effort is highly required in this field. Considering the different properties of the Ni-based hybrid catalyst for the catalytic reactions, herein, the advances of the catalyst were displayed as Ni-based catalysts with noble metals and Ni-based catalysts without noble metals in the system. The reason is that the MOR catalyzed by Ni-based catalysts without noble metals required much higher potentials, higher than the potentials for oxygen reduction reaction, which makes it cannot be used in the DMFC technique but suitable for the methanol electrolysis for hydrogen generation and methanol oxidation in catalysis; while low potentials for MOR can be realized for the Ni-based catalysts with noble metals so that they can be employed in the fuel cell technique. In this review, the general mechanism of methanol oxidation was first introduced, and the catalysis effect of Ni-based material on MOR as well as their structure evolution involving the catalysis process in an alkaline medium was clarified. And then, the reasons why Ni-based materials can enhance MOR are introduced from four aspects: synergistic effect, electronic effect, defect structure, and surface reconstruction. Subsequently, the research progress of various Ni-based materials for MOR including Ni-based alloys, Ni-based compounds, and Ni-based composite hybrid catalysts in an alkaline medium was classified and discussed. Finally, the problems and challenges of nickel-based catalysts for methanol oxidation in alkaline media are summarized. They believe this summary helps readers understand the catalytic mechanism for MOR catalyzed by Ni-based catalysts, and it will be instructive to design and develop novel nickel-based catalysts for methanol oxidation in the alkaline electrolyte.

See the article:

Recent progress of Ni-based catalysts for methanol electrooxidation reaction in alkaline media

https://doi.org/10.1016/j.asems.2023.100055