Electrochemical CO2 reduction on nickel phosphides
Creators: name: Li, Yifei
Publisher: name: No Publisher Supplied
Year: 2024

description: Electrochemically converting CO2 into valuable chemicals and fuels presents a promising avenue for closing the carbon cycle and efficiently storing intermittent renewable energy. Bioinspired nickel phosphide electrocatalysts have demonstrated the ability to produce more complex multi-carbon products compared to natural photosynthetic enzymes. However, several key challenges in this field remain including controlling the selectivity of carbon product formation and, revealing the reaction mechanism as well as the commercial application of CO2RR on nickel phosphides.When traditional anodes like the oxygen evolution reaction (OER) are used, it leads to both significant energy consumption and the production of oxygen, which has limited commercial value. On the other hand, the chlor-alkali process is a well-established and profitable electrochemical method that generates chlorine gas (Cl2) and alkali products such as potassium hydroxide (KOH). In Chapter 2, we demonstrate the feasibility of integrating the chlorine evolution reaction (CER) with CO2 reduction reactions (CO2RR) to simultaneously produce C4 (2,3-furandiol) and C3 (methylglyoxal) products. At a cell current of 1 mA, our system achieves a Faradaic efficiency (FE) of approximately 80% to produce Cl2 and 100% for CO2RR, with no losses due to crossover or cathode degradation. We further enhance the Faradaic efficiency of CO2RR products at higher current densities by utilizing a nickel phosphide Ni2P catalyst, which we synthesized through a low-temperature soft-template method. Additionally, we present a technoeconomic assessment that quantifies the economic advantages of integrating these two electrochemical processes, showcasing the economic benefits of co-producing methylglyoxal and Cl2.In Chapter 3, we present a notable alteration in the distribution of products generated through the electrochemical reduction of carbon dioxide (CO2) when using Ni2P in the presence of boric acid or borate, which act as soluble Lewis acid/base co-catalysts. In the absence of a co-catalyst, the reduction of CO2 on Ni2P results in a mixture of methyl glyoxal (C3), 2,3-furandiol (C4), and formic acid (C1) with a 100% Faradaic efficiency for carbon products. However, the introduction of boric acid or borate as a co-catalyst leads to a shift in product selectivity towards the formation of ethylene glycol (EG), with an 85% C-Faradaic efficiency (at 10 mM, 0 V vs. RHE). Additionally, a portion of the products includes the aforementioned C1, C3, and C4 compounds. This change in selectivity is attributed to the co-catalyst's role in activating a reaction between surface *hydride and *glycolaldehyde on Ni2P while inhibiting the aldol C-C coupling reaction that typically generates C3 and C4 products. An intermediate borate-EG-diester, [(OCH2CHO)2B]-, is detected via 11B-NMR spectroscopy, and this compound subsequently undergoes hydrolysis to release EG as the final product. Extended electrolysis of boric acid results in the modification of the Ni2P surface, leading to the formation of *BO3-Ni2P, as confirmed by X-ray photoelectron spectroscopy (XPS). CO2 electro-reduction on *BO3-Ni2P, in the absence of free boric acid, exclusively yields ethylene oxide (EO), which gradually hydrolyzes into EG in the presence of a bicarbonate electrolyte. When using free boric acid as a co-catalyst and *BO3-Ni2P as the cathode, the combined Faradaic efficiencies for CO2RR products EO and EG reach 88% (at 0V vs RHE), marking a record for carbon selectivity. This research demonstrates the feasibility of utilizing Lewis acid/base co-catalysts to alter the established chemical reaction mechanism of an electrocatalyst, resulting in the formation of a new, predictably chemical product with high yield.Atomically flat single-crystal surfaces with well-defined and uniform surface structures can provide detailed mechanistic information on the surface interaction of adsorption and intermediate species, which makes it ideal model electrode for in-situ studies. In Chapter 4, we investigate the CO2RR performance on single crystal NiP2 and employ in situ Raman spectroscopy to elucidate the reaction pathways. By detecting the catalyst under reaction condition in real time, this work provides essential information for the rational design of efficient catalysts for CO2RR.