Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes.

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Bayesian-Optimization-Assisted Laser Reduction of Poly(acrylonitrile) for Electrochemical Applications.

Laser reduction of polymers has recently been explored to rapidly and inexpensively synthesize high-quality graphitic and carbonaceous materials. However, in past work, laser-induced graphene has been restricted to semiaromatic polymers and graphene oxide; in particular, poly(acrylonitrile) (PAN) is claimed to be a polymer that cannot be laser-reduced successfully to form electrochemically active material. In this work, three strategies to surmount this barrier are employed: (1) thermal stabilization of PAN to increase its sp2 content for improved laser processability, (2) prelaser treatment microstructuring to reduce the effects of thermal stresses, and (3) Bayesian optimization to search the parameter space of laser processing to improve performance and discover morphologies. Based on these approaches, we successfully synthesize laser-reduced PAN with a low sheet resistance (6.5 Ω sq-1) in a single lasing step. The resulting materials are tested electrochemically, and their applicability as membrane electrodes for vanadium redox flow batteries is demonstrated. This work demonstrates electrodes that are processed in air, below 300 °C, which are cycled stably over 2 weeks at 40 mA cm-2, motivating further development of laser reduction of porous polymers for membrane electrode applications such as RFBs.

Toward a Mechanically Rechargeable Solid Fuel Flow Battery Based on Earth-Abundant Materials.

Metal-air batteries are a promising energy storage solution, but material limitations (e.g., metal passivation and low active material utilization) have stymied their adoption. We investigate a solid fuel flow battery (SFFB) architecture that combines the energy density of metal-air batteries with the modularity of redox flow batteries. Specifically, a metallic solid electrochemical fuel (SEF) is spatially separated from the anodic current collector, a dissolved redox mediator (RM) shuttles charges between the two, and an oxygen reduction cathode completes the circuit. This modification decouples power and energy system components while enabling mechanical recharging and mitigating the effects of nonuniform metal oxidation. We conduct an exploratory study showing that metallic SEFs can chemically reduce organic RMs repeatedly. We subsequently operate a proof-of-concept SFFB cell for ca. 25 days as an initial demonstration of technical feasibility. Overall, this work illustrates the potential of this storage concept and highlights scientific and engineering pathways to improvement.

Non-Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes.

Porous carbonaceous electrodes are performance-defining components in redox flow batteries (RFBs), where their properties impact the efficiency, cost, and durability of the system. The overarching challenge is to simultaneously fulfill multiple seemingly contradictory requirements-i.e., high surface area, low pressure drop, and facile mass transport-without sacrificing scalability or manufacturability. Here, non-solvent induced phase separation (NIPS) is proposed as a versatile method to synthesize tunable porous structures suitable for use as RFB electrodes. The variation of the relative concentration of scaffold-forming polyacrylonitrile to pore-forming poly(vinylpyrrolidone) is demonstrated to result in electrodes with distinct microstructure and porosity. Tomographic microscopy, porosimetry, and spectroscopy are used to characterize the 3D structure and surface chemistry. Flow cell studies with two common redox species (i.e., all-vanadium and Fe2+/3+ ) reveal that the novel electrodes can outperform traditional carbon fiber electrodes. It is posited that the bimodal porous structure, with interconnected large (>50 µm) macrovoids in the through-plane direction and smaller (<5 µm) pores throughout, provides a favorable balance between offsetting traits. Although nascent, the NIPS synthesis approach has the potential to serve as a technology platform for the development of porous electrodes specifically designed to enable electrochemical flow technologies.

Designer interphases for the lithium-oxygen electrochemical cell.

An electrochemical cell based on the reversible oxygen reduction reaction: 2Li+ + 2e - + O2↔ Li2O2, provides among the most energy dense platforms for portable electrical energy storage. Such Lithium-Oxygen (Li-O2) cells offer specific energies competitive with fossil fuels and are considered promising for electrified transportation. Multiple, fundamental challenges with the cathode, anode, and electrolyte have limited practical interest in Li-O2 cells because these problems lead to as many practical shortcomings, including poor rechargeability, high overpotentials, and specific energies well below theoretical expectations. We create and study in-situ formation of solid-electrolyte interphases (SEIs) based on bromide ionomers tethered to a Li anode that take advantage of three powerful processes for overcoming the most stubborn of these challenges. The ionomer SEIs are shown to protect the Li anode against parasitic reactions and also stabilize Li electrodeposition during cell recharge. Bromine species liberated during the anchoring reaction also function as redox mediators at the cathode, reducing the charge overpotential. Finally, the ionomer SEI forms a stable interphase with Li, which protects the metal in high Gutmann donor number liquid electrolytes. Such electrolytes have been reported to exhibit rare stability against nucleophilic attack by Li2O2 and other cathode reaction intermediates, but also react spontaneously with Li metal anodes. We conclude that rationally designed SEIs able to regulate transport of matter and ions at the electrolyte/anode interface provide a promising platform for addressing three major technical barriers to practical Li-O2 cells.