catalysts Perspective Electrocatalysts for Using Renewably-Sourced, Organic Electrolytes for Redox Flow Batteries Robert S. Weber Pacific Northwest National Laboratory, Institute for Integrated Catalysis, Richland, DC 99352, USA; [email protected] Abstract: Biomass could be a source of the redox shuttles that have shown promise for operation as high potential, organic electrolytes for redox flow batteries. There is a sufficient quantity of biomass to satisfy the growing demand to buffer the episodic nature of renewably produced electricity. However, despite a century of effort, it is still not evident how to use existing information from organic electrochemistry to design the electrocatalysts or supporting electrolytes that will confer the required activity, selectivity and longevity. In this research, the use of a fiducial reaction to normalize reaction rates is shown to fail. Keywords: biomass; redox flow batteries; electrocatalysts 1. Introduction Catalysis is a key component of technologies that promise to satisfy the growing demand for clean energy. However, we do not yet have ready access to correlations for selecting or interpreting the performance of electrocatalysts for storing the quantities of Citation: Weber, R.S. Electrocatalysts electricity that will permit grid-scale use of renewable resources (e.g., wind power and solar for Using Renewably-Sourced, power). This article proposes the use of an infrequently employed approach to normalize Organic Electrolytes for Redox Flow electrocatalytic reaction rates, namely, a fiducial reaction. Here, fiducial means “faithful”, Batteries. Catalysts 2021, 11, 315. https://doi.org/10.3390/catal11030315 in the sense that the fiducial reaction faithfully tracks the number and rate of the active sites. While the discussion below was motivated by our study of electrocatalysts for redox Academic Editors: Justo Lobato and flow batteries, this article is not intended to be a review of that technology, nor to bear on Kelsey Stoerzinger the choice of the electrolyte. Rather, it introduces the idea of a fiducial reaction as a way to systematize the rates of electrocatalytic reactions. The examples shown and discussed Received: 29 December 2020 below illustrate an instance in which a fiducial reaction works and one in which it does Accepted: 22 February 2021 not, for the particular cases of electrolytes derived from biomass. Published: 28 February 2021 Projections from the International Energy Agency (IEA) [1] indicate that the installed capacity of renewable energy may soon surpass the installed capacity of coal- and natural Publisher’s Note: MDPI stays neutral gas-fired powerplants. However, because the availability of the renewable resources fluctu- with regard to jurisdictional claims in ates seasonally, diurnally and on even shorter times scales [2] (e.g., momentarily becalmed published maps and institutional affil- turbines, clouds passing overhead), the installed availability averages only about 40–70% of iations. the world’s installed capacity [3]. In the U.S., after nearly two decades of steady growth [4], there is sufficient battery capacity to cover about 0.1% of the power demand but only about 3 × 10−5% of the energy demand (Figure1). Therefore, supplying smooth, always-on power that the modern world demands will require technologies that can buffer multiple tranches Copyright: © 2021 by the author. of both power and energy. Consider that the characteristic power of a modern wind turbine Licensee MDPI, Basel, Switzerland. is on the order of 2 MW per generator [5]. The amount of energy to be buffered for a short This article is an open access article interruption in its operation is on the order of 10–100 MJ (= 2 MW × 5–50 s). distributed under the terms and Despite recent acceleration in the rate of growth of both capacities, projected devel- conditions of the Creative Commons opments in rechargeable batteries may satisfy power demands but not energy demands Attribution (CC BY) license (https:// across days or seasons. creativecommons.org/licenses/by/ 4.0/). Catalysts 2021, 11, 315. https://doi.org/10.3390/catal11030315 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 315 2 of 8 2. Flow Batteries In flow batteries, an electrochemically reversibly oxidizable and reducible species shuttles between the anode and cathode. Because the volume of the storage vessel can be independent of the size of the electrolysis cell, a flow battery, in principle, can satisfy the need for both large amounts of stored energy and large rates of energy delivery (power). Their features and limitations have already been reviewed elsewhere [2,6–9]. To store energy, the electrolyte in its oxidized form is reduced to a lower oxidation state. Then, when the energy is needed, the reduced electrolyte is directed to the anode of the electrolysis cell, where it is reoxidized. However, the species (or mixtures of them) that offer the right balance of rate and thermodynamics (potential difference) have proved to be costly and environmentally burdensome [6]. Here, we consider the implications for catalysis of substituting inorganic shuttles (e.g., V3+/5+, Br) with renewable energy carriers (e.g., liquids or solutes derived from biomass). Biomass-derived fuels are abundant enough to fulfill only about 6% of the global demand for fuel [10]. However, as redox looping agents, their annual production could nearly buffer the increase in the global demand for renewable power. About 160 Mt of lignin is produced each year for making pulp and paper [11]. Mostly, it is now burned for process heat. Instead, the lignin could be a feedstock for an organic electrolyte. Suppose, as an example, that a redox shuttle had a molecular weight in the range of 150 g/mol and that it could shuttle 2e− at 3 V cell potential. That high a voltage is in the range of recent developments in organic electrolytes [12–15]. Lignin comprises about 25% of the mass of the wood that is used for making pulp [16]. Therefore, its annual production could source an amount of electrolyte sufficient to store about half of the energy (Figure1) that is Catalysts 2021, 11, x FOR PEER REVIEWprojected to be produced from renewable, episodic sources like wind and solar [17]. That2 of is 8 enough energy to approximately buffer diurnal cycling. Figure 1. ConsumptionConsumption or or storage storage of of renewable renewable electricity. electricity. The The solid solid black black line line shows shows projected projected global consumption of renewable electricity [6]. The dashed line shows how much electricity global consumption of renewable electricity [18]. The dashed line shows how much electricity might might be stored in a redox flow battery whose electrolytes were derived from the 160 Mt of lignin be stored in a redox flow battery whose electrolytes were derived from the 160 Mt of lignin produced produced globally each year by the pulp and paper industry [7] if lignin were the source of a re- globallydox shuttle, each had year a molecular by the pulp weight and paper of 150 industry g/mol, and [11 ]could if lignin store were and the release source 2e− of and a redox an aspira- shuttle, − hadtional a molecular[8] cell voltage weight of 3 of V. 150 The g/mol, lowest and curve could is a storerecent and projection release 2efor theand production an aspirational of storage [12–15 ] cellbatteries voltage for ofall 3 applications, V. The lowest increasing curve is a at recent a compound projection annual for the growth production rate (CAGR) of storage of 21%. batteries for all applications, increasing at a compound annual growth rate (CAGR) of 21%. 2.1. Renewable Organic Redox Shuttles FlowMolecules Batteries that have been considered and tested [6,13,15,19] as organic electrolytes con- sist of heteronuclear or substituted aromatics. Some of them occur naturally as fragments In flow batteries, an electrochemically reversibly oxidizable and reducible species of lignin (Table1). shuttles between the anode and cathode. Because the volume of the storage vessel can be independent of the size of the electrolysis cell, a flow battery, in principle, can satisfy the need for both large amounts of stored energy and large rates of energy delivery (power). Their features and limitations have already been reviewed elsewhere [2,9–12]. To store energy, the electrolyte in its oxidized form is reduced to a lower oxidation state. Then, when the energy is needed, the reduced electrolyte is directed to the anode of the electrol- ysis cell, where it is reoxidized. However, the species (or mixtures of them) that offer the right balance of rate and thermodynamics (potential difference) have proved to be costly and environmentally burdensome [9]. Here, we consider the implications for catalysis of substituting inorganic shuttles (e.g., V3+/5+, Br) with renewable energy carriers (e.g., liquids or solutes derived from biomass). Biomass-derived fuels are abundant enough to fulfill only about 6% of the global demand for fuel [13]. However, as redox looping agents, their annual production could nearly buffer the increase in the global demand for renewable power. About 160 Mt of lignin is produced each year for making pulp and paper [7]. Mostly, it is now burned for process heat. Instead, the lignin could be a feedstock for an organic electrolyte. Suppose, as an example, that a redox shuttle had a molecular weight in the range of 150 g/mol and that it could shuttle 2e− at 3 V cell potential. That high a voltage is in the range of recent developments in organic electrolytes [8,14–16]. Lignin comprises about 25% of the mass of the wood that is used for making pulp [17]. Therefore, its annual production could source an amount of electrolyte sufficient to store about half of the energy (Figure 1) that is projected to be produced from renewable, episodic sources like wind and solar [18]. That is enough energy to approximately buffer diurnal cycling.