Production of Chemicals from Air Through Electrocatalytic Nitrogen and Oxygen reduction

Abstract

The development of efficient energy conversion technologies, such as electrocatalysis process converting electricity derived from renewable energy to various forms of chemical energy, provides a highly desired pathway to produce transportable fuels and value-added chemicals from low-cost feedstocks like water and air, simultaneously to ease the fossil fuel reliance as well as the greenhouse gases emissions. Ammonia (NH3) and hydrogen peroxide (H2O2) are globally important chemicals as basic building blocks in industry and promising carbon-free hydrogen carriers. Production of NH3 from N2 (NRR) or H2O2 from O2 (2e– ORR) through the sustainable and energy-saving electrocatalysis process are therefore highly meaningful. A crucial step in conducting these processes is to develop efficient electrocatalysts for effective activation of the reactants and selective formation of the desired products. Therefore, this Thesis aims to design and synthesize novel nanostructured materials as efficient electrocatalysts for nitrogen and oxygen reduction reactions. Besides the electrocatalyst engineering, energy devices combining various reactions/techniques are also elaborately designed as demonstration for future practical applications. In this Thesis, a systematic review on the recent research progress for the application of main group elements on NRR is firstly provided by investigating their interaction with N2 and the strategies for suppression of the undesired hydrogen evolution reaction (HER) (Chapter 2). This chapter provides a concise but comprehensive understanding on various reaction pathways for NRR and strategies towards N2 activation and HER suppression. The first part of this Thesis (Chapter 3 and 4) focus on a comprehensive optimization and accurate evaluation of NRR performance by investigating aspects such as electrocatalyst and electrolyte. Firstly, semiconducting bismuth nanosheet was for the first time reported to be promising candidate for ambient NRR. The high NRR electrocatalytic activity of the Bi NS originates from the sufficient exposure of edge sites coupled with effective p-orbital electron delocalization. Secondly, trace amount of nitrate and nitrite were found to exist in some lithium salts such as Li2SO4 and LiClO4, which are usually used as electrolytes. Reduction of those nitrogen oxyanions (NOx –) causes false positive results for NRR. To avoid these false positive results and to make the best practice of NRR research, simple but versatile spectrophotometric methods were employed to quantitatively determine NOx – contaminations, followed by effective high-temperature annealing strategy to eliminate them. The second part of this Thesis (Chapter 5) focus on exploration of novel strategy for efficient nitrogen fixation other than the present one-step NRR process. It is proposed that fixation of N2-to-NH3 can be decoupled to a two-step process with one problem effectively solved in each step, in which facile activation of N2 to NOx – is realized by a non-thermal plasma technique and highly selective conversion of NOx – to NH3 by electrocatalytic reduction. In the third part of this Thesis (Chapter 6), the electrochemical reduction of O2 via a twoelectron reaction pathway for sustainable and decentralized H2O2 production was investigated on a nitrogen-rich few-layered graphene (N-FLG). A positive correlation between the content of pyrrolic-N and the H2O2 selectivity is experimentally observed. The critical role of pyrrolic- N is elucidated by the variable intermediate adsorption profiles as well as the dependent negative shifts of the pyrrolic-N peak on X-ray absorption near edge structure spectra. A practical device coupling electrochemical H2O2 production with furfural oxidation was then assembled to achieve high-value products on both anode and cathode with optimized energy efficiency.