Transition metal phosphides for high energy efficiency electrocatalytic CO2 reduction: investigating mechanisms and structure-activity relationship
Creators: name: Doehl Calvinho, Karin Ute
Publisher: name: No Publisher Supplied
Year: 2020

description: Electrochemical reduction of carbon dioxide, powered by renewable electricity, enables the sustainable production of chemicals, polymers, and fuels, potentially displacing fossil carbon sources and mitigating the effects of global warming. However, the activation of CO2 is a kinetic bottleneck for this process. Low energy efficiencies and poor product selectivities prevent the commercial development of this technology. As such, we sought to develop viable catalysts for the CO2 reduction reaction (CO2RR) that 1) operate at high energy efficiency; 2) are capable of catalyzing C-C coupling for producing high-value chemicals; 3) are synthesized from earth-abundant materials, and 4) are robust and stable for extended lifetimes. Inspired by nature’s formate and carbon monoxide dehydrogenases, we investigated the CO2RR activity of seven different transition metal phosphides. Furthermore, we applied experimental and theoretical tools to unravel reaction mechanisms and extract design principles that can guide the development of next-generation catalytic materials.In Chapter 1, we report the application of five nickel phosphides for CO2RR, at ambient conditions in neutral electrolyte. The most selective nickel phosphides operate at exceedingly low overpotential (∼10 mV), yield no hydrogen by-product, and form non-volatile C3 and C4 products. Both products, methylglyoxal and furandiol, can be used as precursors for polymers. We propose a reaction mechanism that is initiated by hydride transfer to CO2, generating formate, which is further reduced to formaldehyde. Formaldehyde proceeds through a self-condensation mechanism, akin to the formose reaction, to yield methylglyoxal, and the aromatic compound 2,3-furandiol. The mechanism is supported by reduction of reaction intermediates that yield the same product ratios as the reduction of CO2. Nickel phosphide catalysts are affordable, abundant, highly active, and could represent a breakthrough in the sequestration of CO2 into fuels and chemical feedstocks for use in the polymer industry.In Chapter 2, copper phosphide (Cu3P) is investigated for CO2 reduction. Hydrogen is the major product detected, with less than 2% faradaic efficiency for formate. A detailed structural analysis of the Cu3P [001] facet identifies isolated Cu(I) sites as likely active sites for both H2 and formate production. This study shows that Cu(I) alone is insufficient to promote highly active CO2RR to C2+, and that stronger bidentate formate binding is necessary for CO2RR to outcompete H2 production.In Chapter 3, this thesis addresses the reactivity of Fe2P, iso-structural to Ni2P. Metallic iron has two fewer electrons than nickel in its d-orbitals, thereby binding the phosphorus ad-layer more strongly than Ni2P. Accordingly, binding of surface hydrides (P-H*) on Fe2P is weaker than on Ni2P, and therefore, they are predicted to be more reactive. Consequently, Fe2P catalyzes to CO2 reduction with a maximum of 53%. The major product is ethylene glycol (FE of 22% at -0.05 V), but formic acid (C1), methylglyoxal (C3), and 2,3-furandiol (C4) are also present. Phosphorus, hydroxide, hydride, CO2, and formate binding to Fe2P are investigated by Grand Canonical DFT (GC-DFT), accounting for the effects of the applied potential and solvent on electrocatalysis. Results reveal that weakly bound Fe3P-H surface hydrides on the P-reconstructed surface are the precursors to both CO2RR and HER. The surface hydrides become more hydridic as the bias increases, favoring high turnover of low barrier hydride transfer reactions, such as those that produce ethylene glycol, over C3 and C4 products, explaining the higher selectivity towards shorter chain products.Finally, in Chapter 4, the biased Ni2P surface is computationally modeled using GC-DFT and experimentally characterized using operando Raman spectroscopy. GC-DFT calculations confirm an earlier report of stable surface reconstruction that enriches phosphorus at the Ni3 hollow sites and predict the adsorption of two hydrides onto P* coupled to its displacement to a μ2-bridging site (Ni-P*-Ni) with tetrahedral coordination. Operando Raman spectroscopy provides support for these predictions, showing the dynamic behavior of the surface under applied bias at neutral, acidic, and basic pH. The assignment of experimental vibrational modes is validated with DFT phonon calculations. The deeper understanding of the surface which this study provides will inform mechanistic predictions and the rational design of catalysts, which are critical to improving the catalytic performance of the hydrogen evolution reaction and CO2 reduction.