No abstract

In the past decades, the surplus of atmospheric CO2 concentration has drawn tremendous political and scientific attention for its negative impacts, such as the greenhouse effect and ocean carbonation. To mitigate such CO2 issues, a combination of various strategies is required. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative to convert CO2 into carbon-based chemicals and fuels, and electricity generated from the renewable sources (solar and wind) could be employed to sustain this transformation. At the current moment, the technological viability of this process is still contingent on finding affordable and efficient catalysts. In this thesis, a family of catalyst materials composed of abundant elements, in particular, non-precious metals, nitrogen, and carbon, typically referred to as precious group metal (PGM)-free "M-N-C" catalysts, were synthesized and mechanistically investigated-both experimentally and computationally-as catalyst candidates for the CO2RR. MNC catalysts feature hemoglobin-like single-site metallated porphyrin moieties with great impact on the catalytic reactivity and selectivity of the CO2RR. Among our studied M-N-C catalysts, the Ni-functionalized one exhibits great efficiency for CO yielding at high potentials and current densities. In particular, employment of Ni-N-C-based gas diffusion electrodes (GDE) combined with micro flow cells, allowed high CO evolution that could exceed 80% faradaic efficiency at 250 mA cm-2 current density, outperforming the industry commonly used Ag benchmark. By coupling our experimental observation and density functional theory (DFT) simulation, the reaction path from CO2 to CO over this sort single site catalyst could be deduced. Unlike the Ni-N-C catalyst, the Fe-N-C shows selective CO production only at low potentials. Further, due to relatively strong interaction with CO*, it opens the chance for hydrocarbons formation, yet showing little selectivity. To understand the mechanism behind this kind of selectivity, we carried out a series of studies, discussing catalytic tests, in-operando spectroscopic analysis, and computational modeling. Towards material research, operando-XAFS measurements identified an unusual Fe-N3, possibly a Fe I-N3 state, which appears to enable CH4 evolution. Further mechanistic studies included the electrocatalytic reduction of CO and CH2O as possible reactive intermediates for CH4 production. By combining the experimental and computational results, we suggest a reaction network for CO2 reduction into a variety of carbon-based products over the Fe-N-C catalyst. This contributes to the overall mechanistic understanding of CO2RR over the M-N-C catalysts and delivers perspectives to evolve and design novel catalysts to produce hydrocarbons of high value.

Significant reduction of the precious metal catalyst loading is one of the key challenges for the commercialization of proton-exchange membrane water electrolyzers. In this work we combine IrOx nanofibers with a conventional nanoparticle-based IrOx anode catalyst layer. With this hybrid design we can reduce the iridium loading by more than 80% while maintaining performance. In spite of an ultralow overall catalyst loading of 0.2 mgIr/cm2, a cell with a hybrid layer shows similar performance compared to a state-of-the-art cell with a catalyst loading of 1.2 mgIr/cm2 and clearly outperforms identically loaded reference cells with pure IrOx nanoparticle and pure nanofiber anodes. The improved performance is attributed to a combination of good electric contact and high porosity of the IrOx nanofibers with high surface area of the IrOx nanoparticles. Besides the improved performance, the hybrid layer also shows better stability in a potential cycling and a 150 h constant current test compared to an identically loaded nanoparticle reference.

Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co3O4), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir>4+ during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co3O4 exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO2 in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.

No abstract