On-site hydrogen peroxide electrosynthesis via partial oxygen reduction reaction enabled by advanced carbon-based electrocatalysts and electrochemical flow reactors

Abstract

Pursuing sustainable alternatives to fossil fuels for our energy and chemical demands is crucial for mitigating climate change and environmental pressures. Electrochemical processes have become an attractive route for chemical synthesis, owing to their inherent sustainability, as they enable the direct use of renewable energy sources such as solar or wind. Along with operating at mild temperature and pressure, electrochemical processes are also highly efficient, resulting in less by-product wastes than conventional energy-intensive thermochemical processes. Since electrochemical transformations and processes are driven by the electrode potential, optimizing the cell potential can significantly influence reaction kinetics and pathways. Thus, rational design of electrocatalysts and optimization of electrochemical processes are expected to play an essential role in our future energy and chemical production. Hydrogen peroxide (H2O2) synthesis via electrocatalytic partial oxygen reduction has attracted growing interest in recent years. This electrochemical process offers an efficient and sustainable route for on-site generation of H2O2 under ambient conditions. Achieving remarkable performance of H2O2 synthesis is often associated with the activity of the electrocatalyst. A promising electrocatalyst needs to be affordable, highly selective and, active, while also exhibiting high stability and durability in long-term operation. The goal of my Ph.D. research is primarily focused on designing carbon-based electrocatalysts for the cathodic hydrogen peroxide electrosynthesis via partial oxygen reduction with the aim of making electrochemical approaches feasible alternatives. Carbon compounds are attractive catalysts for two-electron ORR (2e- ORR) because of their abundance, moderate selectivity, and desirable stability under reaction conditions. More importantly, the carbon's surface and morphology are highly tunable, which can enhance the electrochemical properties and hence the 2e- ORR selectivity. It is worth noting that the electrolyte pH has a substantial impact on the activity and selectivity toward H2O2 generation, even for the same electrocatalyst. Thus, diverse 2e- ORR electrocatalyst mechanisms in different pH media motivated me to rationally design and finely tune the suitable active sites for H2O2 synthesis in different pH electrolytes. In my Ph.D. study, I explored carbon-based electrocatalysts' ORR activity in acidic media as they often exhibit high ORR overpotential. To address this issue, we hypothesized that by developing cobalt-nitrogen (Co-Nx) activity sites on the carbon catalyst framework, the ORR overpotential might be reduced. However, incorporating (Co-Nx) activity sites requires complicated syntheses and high-cost precursors. In my work, using a facile and cost-effective method, I successfully synthesized nitrogen-doped carbon featuring catalytically active cobaltnitrogen (Co-Nx) sites. Electrochemical measurements in an acidic media demonstrated that the present material significantly enhanced the ORR current density, accompanied by a positive shift in the onset potential and durable performance. Additionally, the present catalyst has shown approximately 90% selectivity toward H2O2 over a broad potential range. In the second project, my goal was to synthesize H2O2 in an alkaline media using a practical flow cell. It is noteworthy that ORR is kinetically facile in alkaline media; therefore, the inclusion of metals is unnecessary. Creating a metal-free carbon catalyst that is both highly active and durable became a major challenge for this project. In order to achieve optimal catalytic performance, I proposed a carbon catalyst featuring an improved structure with tuned nitrogen dopants. I used a simple, solvent-free technique to synthesize metal-free nitrogen-doped ordered mesoporous carbon (N-OMC) by in situ transforming glycine (carbon and nitrogen precursors) in a mesoporous SiO2 template (KIT-6), followed by thermal treatment at various temperatures. The improved structural properties with the optimal N-pyrrolic/N-graphitic, P/G carbon ratio provided remarkable electrocatalytic activity boosting H2O2 with high selectivity and generation rate in alkaline media. Furthermore, its practical capability was demonstrated in our self-designed flow cell, where it produced 9.43 mol gcat-1 h-1 H2O2 at 0.35 VRHE and nearly 100% FE at a cathode potential of 0.6 VRHE for 12 hours without degradation. The final phase of my research involved advancing H2O2 electrosynthesis technology toward more practical applications and validating its use at industrially relevant production rates. I focused primarily on synthesizing H2O2 in a neutral pH, particularly in its pure aqueous form collected in deionized water (DI). Pure aqueous H2O2 electrosynthesis is the most desirable approach, as it is ready-to-use and pH-adjustable. Recently an innovative standard solid-electrolyte flow cell with dual membranes (SE-FCAEM/CEM) was reported, in which the anode and cathode "sandwiched" the cation-exchange-membrane (CEM) and anion-exchange-membrane (AEM) layers, separated by a solid-electrolyte, thus allowing H+ and HO2– ions to recombine to form pure H2O2 in DI water stream. One key research needs to effectively deploy this flow cell is to address the stability and engineering difficulties of using an AEM, which creates significant drawbacks in cell performance and lifespan. In this study, I report a modified SE-FC without involving AEM (SE-FCAEM-FREE) to achieve better performance of H2O2 electrosynthesis. To validate SE-FCAEMFREE for industrial-relevant production rates. First, I further enhanced the performance of our nitrogen-doped carbon catalyst in the new settings by combining glycine, the nitrogen precursor, with affordable and highly conductive commercial carbon black in various nitrogen-to-carbon ratios. Among all samples, the catalyst N-C(2:3) contains high carbon and a proper nitrogen precursor that boosted its activity, resulting in excellent half-cell performance with faradaic efficiency (FE) above 90% at different pH-electrolytes. Secondly, we optimized the catalyst microenvironment by applying a PTFE layer. The Layered-PTFE (5wt.%) arrangement suppressed hydrogen evolution reaction (HER) and exhibited high 2e-ORR activity with a high current density of 380 mAcm-2 (about 6.53 mmol cm-2 h-1) at 90% FEH2O2 with no degradation for a 50-hour durability test.