Electrochemical processes for energy technology

The energy challenge

Finding the energy to power human civilization without ruining the environment is the greatest challenge facing humanity this century. World energy demand is projected to double by mid-century and triple by the end of the century. The overwhelming majority of this power is provided by the combustion of fossil fuels. Their combustion emits CO2 into the atmosphere. This must be ramped down and replaced by carbon-free sources of energy, such as wind, solar, biomass, and maybe nuclear. 

Electrochemical acceleration of chemical weathering for CO2 capture

We have studied an approach to accelerating the earth's natural chemical weathering process for capturing CO2 from the atmosphere and transferring it to the ocean where it is permanently stored in the innocuous form of dissolved bicarbonate. Our approach would engineer the same process and bring it to industrial rates through a sequence of chemical and electrochemical processes (Fig. 1). [Ref. mja184]. This process looks like it will be too expensive to attract investors in today's climate, but it may prove valuable in the future.

Fig. 1. Electrochemical weathering. HCl is electrochemically removed from the ocean and neutralized through reaction with silicate rocks. The alkaline solution resulting from the removal of HCl is neutralized by capturing atmospheric carbon dioxide and is returned to the ocean where the carbon is stored primarily as HCO3- without further acidifying the ocean. The overall process is equivalent to the earth's natural chemical weathering process of silicate rocks.

Electrochemical energy storage for the grid

The ability to inexpensively store large amounts of electrical energy is of increasing importance with the growing fraction of electric generation from intermittent renewable sources such as wind and solar. As this fraction increases, problems associated with the mismatch between power supply from wind and solar and grid demand (Fig. 2) become more severe. We analyzed the requirements that storing photovoltaic or wind power place on a storage device in order to provide power when it is needed and the wind isn't blowing and the sun isn't shining [Ref. mja212]. We did this for a simplified, prototypical storage device in which the instantaneous losses are proportional to the instantaneous current (e.g. Ohmic losses). In this case, any device can be characterized by a single scalar figure of merit -- its peak discharge power capability. It then shows how much storage power and storage energy is needed per MW of wind or solar power, for any specified round-trip system efficiency.

Fig 2. A: Electrical power calculated using the performance curve of a commercial wind turbine from real wind speed data, showing a windy week, an intermediate week, and a calm week. B: Solar PV power output, showing sunny days, cloudy days, and -- of course -- dark nights. C: Demand on the electrical grid. The problem: A and B don't look like C. What would it take to solve this problem with storage? [mja212]

Our analysis shows that traditional (solid-electrode) batteries are grossly inadequate for this role. They need to be able to discharge at full power for tens of hours whereas they are typically drained after tens of minutes. Much more promising are flow batteries, in which the power capability and the energy storage capability can be sized independently, thus enabling tens of hours, or even days, of discharge at peak power. In a flow battery the energy is stored in fluids held externally in arbitrarily large, inexpensive storage tanks. These fluids pass by the electrodes where they undergo an electrochemical charge/discharge reaction, and back out to the storage tanks. The electrodes, whose size is built to match the required power delivery capacity, don't participate in the reaction, although they may catalyze it.

Flow batteries for massive electrical energy storage

Our current research is aimed at developing breakthrough chemistries and materials for flow batteries, which could be used for massive electrical energy storage -- up to grid-scale. In a flow battery the power capability and the energy storage capability can be sized independently, thus enabling tens of hours, or even days, of discharge at peak power. In a flow battery the energy is stored in fluids held externally in arbitrarily large, inexpensive storage tanks. These fluids pass by the electrodes where they undergo an electrochemical charge/discharge reaction, and back out to the storage tanks. The electrodes, whose size is built to match the required power delivery capacity, don't participate in the reaction, although they may catalyze it.

 

Fig. 3. A flow battery.  (a) Schematic. During charging, electrical energy is input, H+ is taken from (say) HBr electrolyte (orange) and driven across a Proton Exchange Membrane (PEM) to be taken up by molecule AQDS to make AQDSH2 (green electrolyte). During discharging, these reactions are reversed. The arrows here show the processes during discharging.  (b) Our first "baby" organic molecule flow battery. Energy-storing chemicals (hydroquinone and bromine) are stored in containers in the background. To discharge the battery they are pumped through the energy conversion hardware in the foreground, converting chemical energy to electrical energy and becoming low-energy molecules (quinone and hydrobromic acid). To recharge the battery electrical energy is pushed into the energy conversion hardware, converting low-energy chemicals back into high-energy chemicals which are stored in these containers until electrical energy is needed again. The negative (quinone+hydroquinone) electrode is the left side of the battery and the positive electrode (bromine+hydrobromic acid) is the right side. In order to store more energy, the energy conversion hardware does not need to change: only the container size and the amount of chemicals needs to increase.

The least developed component of the engineered weathering {link to the section on "Electrochemical Acceleration of Chemical Weathering for CO2 capture"} process is the hydrogen-chlorine fuel cell, which converts hydrogen and chlorine gas to hydrochloric acid and generates electricity in the process. (In Fig. 3, the left side would be just H2, and the right side would be HCl + Cl2). As we were working on this cell, we discovered its potential of becoming a reversible cell that could be run backwards to store electricity, i.e. a flow battery. We developed high-performance catalyst materials that were successfully implemented in a cell, creating a class-leading device capable of very high power densities (> 1 W/cm2) and operating at very high efficiencies (running at 0.4 W/cm2 at 90% voltage efficiency) [Ref. mja226]. We have also developed models for both hydrogen-chlorine [Ref. mja217] and hydrogen-bromine [Ref. mja234] flow batteries to see how well we might expect them to perform with further R&D.

A metal-free organic-inorganic aqueous flow battery

The opportunities are immense in organic-based energy storage systems. In particular, a class of molecules known as quinones provides a number of benefits when used in a battery system due to their special properties. Quinones undergo rapid and reversible two-electron, two-proton redox reactions in aqueous solution (Fig. 4). A big advantage of organic molecules is the tuning of solubilities and redox potentials with constituent substitutions such as the –R1 and –R2 groups in Fig. 4(b); the –R1 groups enhance the solubility and the –R2 groups shift the redox potential to a more negative voltage.

Fig. 4. Cyclic voltammetry of quinone oxidation and reduction. (a) Schematic of cyclic voltammetry measurement. The working electrode starts at high potential (with respect to the reference electrode) and the potential slews downward. As the potential crosses the reduction potential of the quinone, two electrons squirt out of the working electrode into the molecule, and two protons from the acid join it, to make the reduced form (called hydroquinone), in which the =O's have become -OH's. When the potential is subsequently raised, the hydroquinone is re-oxidized to quinone as the working electrode sucks up the electrons and the protons go back into solution. (b) A plot of the cycle, starting at the right, swinging to low potential and then back again to high. The presence of the R­2 groups (dashed voltammogram) shifts the reduction potential to more negative voltage.

 

Fig. 5. (left) cell voltage vs. current density and (right) power density vs. current density for our first "baby" quinone-bromine flow battery.

As described in a publication appearing in Nature in January 2014, we have successfully implemented a quinone in a flow battery using the quinone "QR" on one side and bromine on the other side [Ref. mja240] (Fig. 3). The cell is capable of reaching galvanic power densities of 0.6 W/cm2: this is a world-class power density and we haven't even tried to optimize it yet!  The cell has been cycled about 800 times (so far) without any sign of degradation, indicating the robustness of the molecules. Current efforts are geared towards evaluating cycle life, increasing the cell voltage, and synthesizing other quinones with desirable properties for use in an all-quinone flow battery.

Fun facts about quinones!

  1. Hydroquinone cream is used to bleach dark spots, moles, etc off of skin

  2. Rhein (1,8-dihydroxy-3-carboxy-anthraquinone) is obtained from Rhubarb and used as a laxative and an antibacterial agent

  3. Plastoquinone shuttles electrons as part of photosystem II, and is found in all green plants

  4. Blatellaquinone is a sex pheromone female cockroaches use to attract males

Relevant publications

mja240. B. Huskinson, M. Marshak, C. Suh, S. Er, M. Gerhardt, C. Galvin, X. Chen, A. Aspuru-Guzik, R. Gordon, and M.J. Aziz. “A Metal-Free Organic-Inorganic Aqueous Flow Battery.” Nature, in press (January 2013). In this paper we describe a class of energy storage materials that exploits the favorable chemical and electrochemical properties of a family of molecules known as quinones. We demonstrate a flow battery free of redox-active metals and free of catalytic precious metals, based on the redox chemistry of a substituted anthraquinone.  Click here to see the paper.

mja212. J. Rugolo, and M.J. Aziz. “Electricity storage for intermittent renewable sources.” Energy & Environmental Science, 5, 7151–7160 (2012). In this paper we present a framework to determine the required storage power as
a function of time for any power production profile, supply profile, and targeted system efficiency, given the loss characteristics of the storage system. We apply the framework to the electrochemical storage of intermittent renewable power.  Click here to see the paper.

mja226. B. Huskinson, J. Rugolo, S.K. Mondal, & M.J. Aziz. “A high power density, high efficiency hydrogen–chlorine regenerative fuel cell with a low precious metal content catalyst.” Energy & Environmental Science, 5(9), 8690–8698 (2012). In this paper we report the performance of a hydrogen–chlorine regenerative fuel cell (essentially a flow battery) with a chlorine electrode employing our low precious metal content alloy oxide electrocatalyst.  Click here to see the paper.

mja217. J. Rugolo, B. Huskinson, & M.J. Aziz. “Model of Performance of a Regenerative Hydrogen Chlorine Fuel Cell for Grid-Scale Electrical Energy Storage.” Journal of The Electrochemical Society, 159(2), B133–B144 (2012). In this paper we develop a model for a regenerative hydrogen-chlorine fuel cell (rHCFC) including four voltage loss mechanisms: hydrogen electrode activation, chlorine electrode activation, chlorine electrode mass transport, and ohmic loss through the membrane. We identify chlorine electrode activation as the dominant contribution to the loss for low current density, high-efficiency operation and membrane resistance as the dominant contribution to the loss at maximum galvanic power density.  Click here to see the paper.

mja184. K.Z. House, C.H. House, D.P. Schrag, & M.J. Aziz. “Electrochemical Acceleration of Chemical Weathering as an Energetically Feasible Approach to Mitigating Anthropogenic Climate Change.” Environmental Science & Technology, 41(24), 8464–8470 (2007). In this paper we describe an approach to CO2 capture and storage from the atmosphere that involves enhancing the solubility of CO2 in the ocean by a process equivalent to the natural silicate weathering reaction.  Click here to see the paper.

mja234. B. Huskinson, & M.J. Aziz. “Performance Model of a Regenerative Hydrogen Bromine Fuel Cell for Grid-Scale Energy Storage.” Energy Science and Technology, 5(1), 1–16 (2013). In this paper we develop a performance model for a polymer electrolyte membrane based regenerative hydrogen-bromine fuel cell (rHBFC). The model is also compared to published experimental results on the performance of a hydrogen-bromine cell.  Click here to see the paper.