Heating, ventilation, and air conditioning (HVAC) systems account for more than 40% of the U.S. electricity usage, primarily utilizing vapor compression system (VCS) heat pumps, raising environmental concerns. The phase down of high GWP refrigerants, targeting an 85% reduction by 2035, prompts the exploration of novel heat pump technologies. Electrochemical looping heat pumps (ELHP) show promise by employing redox reactions for fluid compression to replace conventional compression technologies. However, energy-intensive redox interconversion necessitates catalysts to lower activation energy. Inspired by electrosynthesis principles, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) emerged as a successful homogeneous electrocatalyst for selective isopropanol oxidation to acetone for use in ELHP. HPLC and 13C NMR confirmed remarkable selectivity (~100%) at practical isopropanol concentrations. To enhance TEMPO's catalytic activity, various electron-withdrawing and donating groups were explored. TEMPO-OCH3, among seven derivatives, exhibited superior efficiency with a notable rate constant (6 M-1s-1) and turnover frequency (3.1 s-1). This study innovatively applies concepts from molecular catalysis for the development of efficient catalyst materials.

Automated, rapid electrocatalyst discovery techniques that comprehensively address the exploration of chemical spaces, characterization of catalyst robustness, reproducibility, and translation of results to (flow) electrolysis operation are needed. Responding to the growing interest in biomass valorization, we studied the glycerol electro-oxidation reaction (GEOR) on gold in alkaline media as a model reaction to demonstrate the efficacy of such methodology introduced here. Our platform combines individually addressable electrode arrays with HardPotato, a Python application programming interface for potentiostat control, to automate electrochemical experiments and data analysis operations. We systematically investigated the effects of reduction potential (E l) and pulse width (PW) on GEOR activity during the electrodeposition (Edep) of gold, evaluating 28 different conditions in triplicate measurements with great versatility. Our findings reveal a direct correlation between E l and GEOR activity. Upon CV cycling, we recorded a 52% increase in peak current density and a -0.25 V shift in peak potential as E l varied from -0.2 to -1.4 V. We also identified an optimal PW of similar to 1.0 s, yielding maximum catalytic performance. The swift analysis enabled by our methodology allowed us to correlate performance enhancements with increased electrochemical surface area and preferential deposition of Au(110) and Au(111) sites, even in disparate Edep conditions. We validate our methodology by scaling the Edep process to larger electrodes and correlating intrinsic activity with product speciation via flow electrolysis measurements. Our platform highlights opportunities in automation for electrocatalyst discovery to address pressing needs toward industrial decarbonization, such as biomass valorization.

Performing quantitative measurements of heterogeneous electron transfer reactions at electrodes (e.g., determining k0 and a) under conditions of high redox active molecule concentration in solution, such as those encountered in electrolytes for redox flow batteries (RFBs), presents significant technical challenges. For example, attempting to use transient voltammetry methodology quickly leads to limitations imposed by iR drop, as currents can be large and resistance, especially in non-aqueous systems, can be high. Additionally, using steady-state techniques based on convective principles, such as the rotating disk electrode (RDE), involve the use of relatively large equipment, with moving parts, and usually with requirements of tens to hundreds of mL of solution, which may be limiting for exploring new compounds at high concentration. To address these issues, here we introduce the use of scanning electrochemical microscopy (SECM) for the determining the rates of electron transfer of redox active species at high concentration in non-aqueous [1] and other unconventional systems, such as deep eutectic solvents. SECM enables the resolution of higher rates of electron transfer compared to transient voltammetry and RDE, and it also enables experiments with spatial resolution, enabling the exploration of reactivity on various sites of a heterogeneous electrode. In this presentation, we will describe theoretical and practical aspects of these measurements, and we will demonstrate their utility using two model systems. The first one consists on the observation of inherent rate asymmetries observed for ferrocene derivatives, ubiquitous redox probes, at the graphite/carbonate solvent interface. These asymmetries are not present on aqueous systems or metallic electrodes, demonstrating the ability of the SECM technique to capture unique behaviors. In the second case, we will discuss how SECM measurements can be used on deep eutectic solvents such as ethaline to address how interactions between redox actives and the electrode or the solvent systems modulate the rate of electron transfer. By systematically modifying the strength of interactions by means of molecule choice, solvent composition, and electrode type and structure, we can gain insight into the processes that determine reactivity at redox flow battery electrodes. This insight is critical for exploring the interfacial aspects such as adsorption, passivation, and double-layer effects on kinetics of electron transfer. [1] Gaddam, R.; Sarbapalli, D.; Howard, J.; Curtiss, L.A.; Assary, R.S.; Rodríguez-López, J. An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces Chem: Asian J., 2023, 18, 2, e202201120. Figure 1

Air conditioning, space heating, and refrigeration account for approximately 40% of electricity usage in the U.S. residential and commercial building sector. Therefore, the develop-ment of more energy efficient heat pump technologies remains an ongoing pursuit with widespread practical implications. Electro-chemical looping heat pump (ELHP) technology has emerged as an attractive alternative to conventional vapor compression systems (VCS), theoretically promising significant improvements in both performance and energy consumption. However, efficient ELHPs require selective facilitation of redox reactions that interconvert working fluids with minimal energy input. In this work, we report on the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation of isopropanol (IPA) to acetone as a promising system for ELHP. We characterized the performance of seven different TEMPO derivatives for mediated electrocatalytic IPA to acetone conversion. 4-Methoxy-TEMPO emerged as the most promising candidate which selectively oxidized IPA to acetone in basic pH as confirmed via electrochemical methods and 13C NMR. Coupling experimental results with DigiElch simulations helped in characterizing the rate constant for the reaction as 6 M-1 s-1 and a turnover frequency of 3.1 s-1 at pH 10. Bulk electrolysis measurements followed by 13C NMR highlighted the selectivity of the catalyst (similar to 100%), even at IPA concentrations as high as 0.5 M IPA, making them ideal for use in practical ELHP. This work introduces a methodology to identify suitable catalysts for efficient ELHP and other bulk conversion methods based on concepts of highly selective mediated electrocatalysis.

Transitioning away from fossil-fuel-based chemical manufacturing is critical in advancing towards a carbon-neutral society. Chemical manufacturing processes that can be driven by renewable energy and use renewable feeds or waste products from other processes are of particular interest. This talk focuses on utilization of glycerol, a byproduct of biodiesel production and other processes, as a platform chemical for electro-organic synthesis of a number of value-added intermediates. Oxidation of glycerol can result in ten or more different products. Processes are needed to synthesize specific products of interest selectively. Based on market size and value, the four most promising oxidation products of glycerol are lactic acid, glycolic acid, oxalic acid, and dihydroxyacetone. Electrochemical reduction and oxidation processes can, in principle, be directed to produce different products by tuning reactor and reaction conditions such as catalyst identity, electrolyte/analyte concentration, flow/stir rates, electrolyte composition, etc. While a significant body of work has studied glycerol electrooxidation (GEOR) in electrochemical cells with the goals of unraveling mechanisms and identifying better catalysts (higher rates/selectivities), transitioning some of the most promising GEOR approaches to larger scales has received limited attention. In our work, we pursue glycerol electrooxidation in flow reactor configurations that could be scaled to industrial levels. Flow electrolysis approaches, in addition to providing a scalable chemical manufacturing platform, enhance mass transfer and kinetics while retaining the ability to tune key operational parameters that determine rates and selectivities. In this talk, we will report on two aspects: (i) Product speciation: what parameters can be used to optimize product selectivity towards key desired products (e.g., lactic acid, glycolic acid), and (ii) Feedstock composition: how to utilize raw glycerol (so-called ‘crude’ glycerol, containing varying amounts of water, NaOH, and methanol), available as a byproduct from a wide variety of plants, as the feedstock. For the former, we employ a systematic design approach that seeks to optimize the interplay between the different parameters that determine current density (conversion rate) and Faradaic efficiency (selectivity). For the latter, we explore which of the other ingredients (methanol, NaOH, water) of crude glycerol needs to be controlled or adjusted for reproducible, desired outcomes of flow glycerol electrolysis.

Batteries and fuel cells play a pivotal role in the advancement of portable electronics, the transportation sector, and supplying electricity across the grid. To produce high-performance and safe devices based on these technologies, it is critical to better understand the electrochemical processes, associated mass and charge transport, and material degradation chemistries during their operation. However, disentangling the complex surface and interfacial chemistry that characterizes them is quite challenging. Scanning electrochemical microscopy (SECM) has emerged as a powerful and versatile tool for understanding these complex processes with high-resolution imaging and local quantification of various electrochemical phenomena in situ and even operando. In this chapter, we discuss the applications of SECM toward ion batteries, metal-air batteries, redox flow batteries, and fuel cells, describing how the technique is used to elucidate the spatial and chemical heterogeneities and dynamic nature of the interfaces formed in the materials and components within these devices. We highlight the development of probes and measurement modes aimed at addressing specific processes in these technologies. While SECM can be considered a mature technique, its applications to batteries and fuel cells are incipient, and we hope this chapter is useful to both the novice and advanced users to identify new research frontiers.

The fundamental process in non-aqueous redox flow battery (NRFB) operation revolves around electron transfer (ET) between a current collector electrode and redox-active organic molecules (redoxmers) in solution. Here, we present an approach utilizing scanning electrochemical microscopy (SECM) to evaluate interfacial ET kinetics between redoxmers and various electrode materials of interest at desired locations. This spot-analysis method relies on the measurement of heterogeneous electron transfer rate constants (kf or kb ) as a function of applied potential (E-E0 '). As demonstrated by COMSOL simulations, this method enables the quantification of Butler-Volmer kinetic parameters, the standard heterogeneous rate constant, k0 , and the transfer coefficient, α. Our method enabled the identification of inherent asymmetries in the ET kinetics arising during the reduction of ferrocene-based redoxmers, compared to their oxidation which displayed faster rate constants. Similar behavior was observed on a wide variety of carbon electrodes such as multi-layer graphene, highly ordered pyrolytic graphite, glassy carbon, and chemical vapor deposition-grown graphite films. However, aqueous systems and Pt do not exhibit such kinetic effects. Our analysis suggests that differential adsorption of the redoxmers is insufficient to account for our observations. Displaying a greater versatility than conventional electroanalytical methods, we demonstrate the operation of our spot analysis at concentrations up to 100 mM of redoxmer over graphite films. Looking forward, our method can be used to assess non-idealities in a variety of redoxmer/electrode/solvent systems with quantitative evaluation of kinetics for applications in redox-flow battery research.