Electrocatalytic Refinery toward Green Production of Chemicals through N2 and CO2 Electroreduction

Abstract

The depletion of fossil fuels and rapid environmental deterioration call for the development of clean and sustainable energy supplies for the future. Recently, the electrocatalytic refinery (e-refinery) is emerging as a more sustainable and environmentally benign strategy to harness renewable energy for the production of value-added chemicals. As the second most produced chemical in the world, ammonia has played a significant role in agriculture and chemical engineering, and now severs as one promising medium for hydrogen storage, due to its higher energy density. Thus, exploration of strategies to produce ammonia (NH3) powered by renewable energy with decreased carbon emissions has raised widespread interests among researchers. Up till now, two main routes with lower carbon emission levels have been proposed: (1) steam methane reforming coupled Haber-Bosch process (SMR-HB) with carbon capture and storage technologies (SMR-HB + CCS) and (2) direct electrocatalytic nitrogen (N2) reduction reaction (NRR) in aqueous conditions. For the CCS, electrocatalytic reduction of carbon dioxide (CO2) to other carbonaceous products is regarded as the highly effective pathway. However, the difficulty to achieve these two routes is to develop electrocatalysts for effective activation and conversion of the reactants (like N2 and CO2). Therefore, this Thesis aims to design and synthesize novel nanostructured materials as efficient electrocatalysts for N2 and CO2 reduction reactions. In conjunction with the electrocatalyst engineering, detailed investigations on catalyst “structure-to-performance” correlations are also elaborated to guide the design of catalysts for future applications in other fields. In this Thesis, a systematic review on the roadmap towards electrocatalytic green NH3 production is provided (Chapter 2). This chapter critically evaluates the challenges of NRR and discusses several emerging strategies for green ammonia synthesis beyond conventional catalyst design and engineering, including electrocatalytic nitrogen oxidation, electrocatalytic nitrate reduction, bioelectrocatalysis and redox-mediated electrocatalysis. The first study of this Thesis (Chapter 3) focuses on a comprehensive and accurate evaluation of NRR performance by employing in situ fragmented bismuth nanoparticles as a promising candidate for ambient NRR. NRR performance is rigorously evaluated in both neutral and acid electrolyte through ionic chromatograph and isotope labelling testing. Online differential electrochemical mass spectrometry (DEMS) detects the production of NH3 and N2H2 during NRR, suggesting a possible pathway through two-step reduction and decomposition. The second section of this Thesis (Chapter 4 and 5) explores novel strategies for efficient CO2 fixation to produce value-added chemicals via electrocatalytic CO2 reduction (CRR). In/ex situ characterizations unravel the in-depth understanding on the complicated surface reconstruction of Bi-MOFs under CRR conditions, which can be controlled using electrolyte and potential mediation. The intentionally reconstructed Bi catalyst exhibits excellent activity and selectivity for formate production. It is also revealed that unsaturated surface Bi atoms are formed during reconstruction and become the active sites. The optimized CO2-to-CO performance is achieved on the asymmetric dual-atom NiCu catalysts which are distributed within threshold distance on the N-doped graphene. The simulation results and electrocatalytic experiments unravel the inter-metal interaction with a threshold effect. The random distribution algorithm and mathematical modelling establish the relationship between diatomic distance and metal loading amount, benefiting the design of dual-atom catalysts for other electrocatalytic reactions and applications. In the third part of this Thesis (Chapter 6), a simple and straightforward strategy based on the “adsorption-sonication” route is developed for the synthesis of noblemetal single atom catalysts (SACs) on C3N4, including Pd, Pt, Ag, and Au. It is found the Pt SACs on the C3N4 (Pt-CN) exhibit promising electrocatalytic nitrate reduction for ammonia production. The Pt-CN achieves the maximized FE for ammonia of 80.42% at 0.45 V versus reversible hydrogen electrode (vs RHE) and the highest ammonia yield of 10.65 μmol cm2 h1 at 0.8 V in 0.1 M KNO3/0.5 M K2SO4 electrolyte. This work deepens the understanding of the growth mechanism of SACs, explores some new insights into employing traditional wet chemistry in SACs preparation, and also paves the way for their future industrial production and application.