Thermally regenerative ammonia batteries (TRABs) are electrochemical energy conversion devices that convert low-grade waste heat into electrical power. To date, reported TRABs have suffered from poor performance due to their reliance on dissolution and deposition redox reactions with transition metals. Here we present a new TRAB chemistry that uses ligands to stabilize aqueous Cu(I) and Cu(II) ions, thereby creating the first reported all-aqueous TRAB. Rotating disc electrode studies were conducted to evaluate thermodynamic and kinetic parameters of prospective anolyte and catholyte chemistries. The use of NH 3 (aq) and Br − (aq) ligands resulted in a cell potential difference of 695 ± 2 mV with rate constants of 101 ± 5 μm s −1 and 819 ± 236 μm s −1 , respectively. Single-cell tests achieved power densities up to 350 W m −2 which are the highest reported for single metal TRABs at 25 °C. Coulombic efficiencies exceeded 90% and their energy storage densities were two to four times of those reported for alternative TRAB chemistries.
Efforts to reduce the cost of long-duration energy storage systems have drawn increased interest in all-iron redox flow battery systems due to their inexpensive electrolytes. Previous studies have shown that the kinetics of the positive electrode for the Fe(II, III) reaction in these systems are sluggish on carbon surfaces and require significant overpotentials. Here, we identify how changes to the electrode surface and electrolyte composition can influence the electrochemical rate constants to reduce required overpotentials using a rotating disc electrode (RDE) assembly. The impacts of solution composition in terms of pH, chloride content, and cation species were investigated using a range of electrolyte compositions reported for all-iron flow battery systems. The electrode materials studied were glassy carbon, pyrolytic graphite basal plane, and pyrolytic graphite edge plane. Untreated and electrochemically oxidized electrode materials were quantified using X-ray photoelectron spectroscopy (XPS), and the reaction rate constants were quantified using electron impedance spectroscopy (EIS). Rate constants of the Fe(II, III) reaction on untreated electrode materials were largest on platinum and lowest on glassy carbon with pyrolytic materials providing comparable results. For the materials examined, increases electrolyte chloride content generally decreased the obtained rate constant.
Redox flow batteries are emerging as a promising method to provide grid-scale power and long-duration energy storage safely and economically. The thermally regenerative ammonia battery (TRAB) is a new redox flow battery category that can be recharged using low-grade waste heat rather than electric energy, adding further flexibility to the applicability of flow battery systems. Recently, a new TRAB with all-aqueous electroactive species (referred to as the Cuaq-TRAB), as opposed to deposition-dissolution reactions, was found to have superior energy and power densities relative to competing TRABs. The use of bromide and ammonia stabilizes for both Cu(I) and Cu(II) oxidation states, while also creating a cell potential of up to 1.0 V. The potential can be recovered by thermally separating ammonia from the electrolyte and adding it back to alternate electrolyte chambers in successive cycles. Potential improvements in overall cell performance are possible by reducing ohmic losses associated with membrane selection. However, the ideal membrane for the Cuaq-TRAB is not obvious and raises interesting transport questions relative to the dominant redox-active copper species being negatively charged in the catholyte but positively charged in the anolyte. Furthermore, membrane crossover of ammonia, a small, uncharged molecule, was previously shown to be a primary source of parasitic losses for TRABs. Therefore, we investigated how different membrane types (cation, anion, and non-selective) affected ion transport and TRAB performance. A batch symmetry cell was used to determine membrane conductivity in the Cuaq-TRAB environment and membrane diffusion coefficients of each species in the electrolytes. Flow cell experiments were also conducted to find peak power, energy density, average power during discharge, and capacity fade over successive electric charge/discharge cycles. Tradeoffs between membrane conductivity and permeability observed in the symmetry cell are manifested in flow cell results as either high peak and average power or high energy density and low capacity fade. These results expand on potential methods for controlling transport in redox flow batteries and demonstrate the potential for non-selective membranes for electrochemical energy technologies.
Terrawatt-hours of low-grade (t < 150 °C) waste heat are lost to the environment each year through inefficiencies in manufacturing, power generation, and waste management processes. Though several methods have been proposed to capture and convert this unused resource into electricity, most have exhibited low power densities (0.5-12 W m-2, normalized to membrane area) which has been a major roadblock to harnessing this energy stream. Recently, thermally regenerative ammonia batteries (TRABs) have overcome this limitation by producing power densities greater than 100 W m-2. However, the most promising TRABs still exhibit poor coulombic efficiencies, are susceptible to dendrite formation, and have low energy storage densities. To overcome these limitations, we show via Cu(I, II)-based half-reactions that it is possible to design an all-aqueous TRAB chemistry with distinct advantages over the conventional TRABs. To prove the utility of this approach, we used a rotating disc electrode (RDE) to quantify fundamental performance parameters of the new Cu(I, II)-based TRAB, including equilibrium potentials, limiting current densities and rate constants of the half-reactions at each electrode. Cell coulombic efficiency, power density, and energy storage densities were quantified using a conventional zero-gap flow battery system. Single cell tests achieved power densities up to 350 W m-2 (normalized to membrane area) with cell coulombic efficiencies greater than 90%, and energy densities four times larger than conventional TRABs.
For meeting ambitious carbon-free electricity goals, low-cost electrical energy storage solutions are needed. Although redox flow batteries are a promising option to satisfy this need, the cost of these devices must decrease before widespread adoption can take place. As the electrolytes are the most significant expense for MW-scale flow batteries, new chemistries derived from inexpensive metals is one approach that may alleviate this bottleneck towards a carbon-free power grid. Here we use an electrochemical approach by applying a rotating disc electrode (RDE) to quantify how ligand chemistries can favorably manipulate the electrochemical parameters of the Cu(I/II) redox reaction, progressing these chemistries forward as potential low-cost electrolytes for redox flow batteries. In this research, multiple ligands were used to analyze such impacts on aqueous copper metal complexes by using both a platinum and carbon tipped electrode installed into the RDE. Electrochemical Impedance Spectroscopy (EIS), Linear Sweep Voltammetry (LSV) and Open Circuit Potential (OCP) were the three techniques utilized to analyze the effects of these metal complexes. The experimental data was analyzed using the Butler-Volmer and Nernst equations. As step reactions are mostly unknown for these reactions, LSV and EIS data gave insights into what may be occurring at the electrode interfaces. OCP data quantified how different ligands modified the standard electrode potentials of the Cu(I/II) redox reaction.
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