Rediscovering Cr2O72−, an Oxidant with Unrivaled Power and Energy Density, for Affordable, Next-Generation Energy Storage and Conversion David A. Finkelstein* and Héctor D. Abruña* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States S Supporting Information *
ABSTRACT: In the search for electrochemical energy storage systems with higher energy and power at reduced cost, alternative oxidants are rarely explored. Here we revisit the oxidant dichromate (Cr2O72−), once used in primary batteries for telegraphs, and carry out the first assessment of its suitability for fuel cells and flow batteries. We show that Cr2O72− delivers 100 times the maximum current density from O2 at just 1/70th of Cr2O72−’s 7.1 M solubility and yields the same voltage at carbon as O2 does at Pt. Furthermore, we show that Cr2O72− produces current densities that are higher than those of virtually all liquid-phase oxidants considered for energy storage. While the use of some highpower oxidants (e.g., H2O2, MnO4−) is impractical due to deleterious side reactions, Cr2O72− reduction proceeds unhindered, enabling its use at high current densities. With these promising properties and an exceptionally low materials cost, Cr2O72− likely provides a path to affordable energy storage.
T
scaling for these parameters (eq 2). Dioxovanadium (VO2+) and cerium (Ce4+), used in vanadium and Zn/Ce flow batteries, respectively, had surprisingly low values for D, which when combined with their low n of 1 e− resulted in low current densities (Figure 1A and Table 1). While the traditional highpower oxidants hydrogen peroxide (H2O2) and permanganate (MnO4−) had high n, Cmax, and D, their use at high concentration (C) proved infeasible due to bubble-producing decomposition and precipitation, respectively. The reduction of hypochlorite (ClO−) generated Cl−, which poisoned Pt, Au, and glassy carbon (GC) electrodes. Equation 1, where W, L, and H are channel width, length, and height, respectively (cm); F is Faraday’s constant (96 487 C/mol); Q is flow rate (cm3/s); and the units for iL, Cmax, and D are A/cm2, mol/cm3, and cm2/s, is
he transition from fossil fuels to renewable-yetintermittent wind and solar energy requires electrochemical energy storage systems to stabilize their output to the electric grid in order to avoid using natural gas or coal for backup power. Technologies ranging from Li ion batteries to H2/O2 fuel cells and vanadium flow batteries are too expensive for widespread deployment,1 especially for smallscale renewables operators. Increasing affordability requires either (1) decreases in the cost of energy storage materials and/ or catalysts or (2) improvements in system energy and power density, which in turn reduce the amount of storage material, cell stack size, or catalyst required. Both Li ion batteries and H2/O2 fuel cells are limited by their oxidants, whether in terms of materials cost and energy density (Li ion) or catalyst cost and power density (H2/O2). Vanadium flow batteries have low energy and power densities for both fuel and oxidant. We previously investigated a number of alternative oxidants in the search for higher-power chemistries for fuel cells and flow batteries.2 However, none of the oxidants we studied exhibited exemplary characteristics for higher power via increased current density. To achieve high mass transport limited current (iL) under laminar flow in an electrochemical cell, an oxidant must have high values for the following parameters: n, the number of e− transferred per formula unit; Cmax, the maximum concentration (solubility); and D, the diffusion coefficient (eq 1). Either n or D is easily discerned via rotating disk electrode (RDE) voltammetry, which has identical © XXXX American Chemical Society
iL ≈
⎛ DWL ⎞2/3 1/3 3 ⎟ nCmax ⎜ FQ ⎝ H ⎠ 2
(1)
Equation 2, where ν is kinematic solution viscosity (cm2/s), A is electrode area (cm2), and ω is rotation rate (rad/s), is iL = 0.62nCD2/3ν−1/6FAω1/2
(2)
Received: March 29, 2017 Accepted: May 19, 2017
1439
DOI: 10.1021/acsenergylett.7b00274 ACS Energy Lett. 2017, 2, 1439−1443
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters Table 1. Oxidant Reduction Reactions oxidant Ce4+ ClO− CrO42− Cr2O72− H2O2 IO3− MnO4− O2 S2O82− VO2+
reduction reaction Ce4+ + e− ⇆ Ce3+ HClO + H+ + 2e− ⇆ Cl− + H2O HCrO4− + 7H+ + 3e− ⇆ Cr3+ + 4H2O Cr2O72− + 14H+ + 6e− ⇆ 2Cr3+ + 7H2O H2O2 + 2H+ + 2e− ⇆ 2H2O IO3− + 6H+ + 6e− ⇆ I− + 3H2O MnO4− + 8H+ + 5e− ⇆ Mn2+ + 4H2O O2 + 4H+ + 4e− ⇆ H2 O S2O82− + 2H+ + 2e− ⇆ 2HSO4− VO2+ + 2H+ + e− ⇆ VO2+ + H2O
E0 (V vs Ag/AgCl)
D (×10−5 cm2/s)
1.52 1.29
0.36 1.10
2.6 10.7
1.15
0.92
5.4
1.03
0.96
7.1
1.58
1.50
42.4
0.89
1.10
0.5
1.31
1.20
7.3
1.03
2.40
0.001
1.93
0.60
2.3
0.79
0.25
3.0
Cmax (M)
Figure 1A, and our analytical findings of their values for n, Cmax, and D are listed in Table 1. Table S1 additionally includes the combined metric VnCmaxD2/3. The values of iL at C = 0.1 M for these oxidants were much larger than those for our previous oxidants with n = 1 e− (Ce4+, ClO−, VO2+). Indeed, the iL values were so large that significant voltage drops (Vd) were evident in our RDE analytical cell from solution resistance (Rs; Vd = iRs), leading to the linear regions in the RDE voltammograms in Figure 1A. These regions of voltage drop cause the misleading appearance of high reaction overpotentials, but they are actually artifacts from overwhelming analytical cells with high currents. The behavior of some oxidants proved problematic. S2O82− adsorbed onto Pt and was reduced as a surface species in H2SO4 and HClO4. Hence, it is shown at GC in NaOH. IO3− formed the poison I−, resulting in electrode passivation. CrO4−, with half the e− of Cr2O72−, also generated half of Cr2O72−’s current. Our attention rapidly converged on Cr2O72−, which produced the highest limiting current density (655 mA/cm2) of all the oxidants studied. Its n value of 6 e− allowed it to outpace even MnO4− with an n value of 5 e−, despite MnO4−’s 25% larger D (Table 1). Cr2O72− has the same E0 as O2 (1.03 V vs Ag/AgCl). Figure 1B shows that at Au, Cr2O72− has a halfwave potential (E1/2) similar to O2 at Pt, while at GC, it has the same onset potential (Eonset) as O2 at Pt. Thus Cr2O72− electrochemical cells should exhibit the same voltage as O2based fuel cells and flow batteries (H2/O2, Li air, Zn air, etc.) while providing >100 times more current at just 0.1 M, or 1/ 70th of Cr2O72−’s Cmax of 7.1 M (0.1 M Cr2O72− in Figure 1A versus saturated O2 in Figure 1B). Figure 2 shows that Cr2O72−’s reduction is catalyzed by a number of electrode materials, with the highest E1/2 (and thus highest efficiency) at Au and Pd and highest Eonset at Au and GC. Cr2O72−’s ability to be reduced at carbon-based electrodes at low overpotentials allows a cost structure that is entirely different than that available for efficient O2-based systems. In Figure 2, Cr2O72−’s kinetic rate of reduction at GC appears qualitatively slow, yet it is far superior to that of VO2+ at GC (Figure S1). Modified carbon materials show greatly enhanced
Figure 1. (A) RDE voltammetric metacomparison of alternative oxidants for fuel cells and flow batteries [0.1 M oxidant in 1 N electrolyte, 20 mV/s, and 3000 rpm. Conditions: CrO42−, Cr2O72−, IO3−, and VO2+ at Au in H2SO4; ClO−, H2O2, and MnO4− at Pt in H2SO4; Ce4+ (as Ce(NH4)2(NO3)6) at Au in HNO3; S2O82− at GC in NaOH with a 0.059 V/pH unit corrective potential shift applied for comparison]. (B) RDE voltammograms of 5 mM Cr2O72− at Au and GC versus saturated (1 mM) O2 at Pt in 0.5 M H2SO4, 20 mV/ s, and 3000 rpm, showing that Cr2O72− at Au has rapid kinetics and E1/2 similar to O2, while at GC it has an Eonset similar to O2. Note: Axes are reversed from IUPAC norm throughout.
While none of the above oxidants proved ideal, the results provided a clear guideline: Achieving high power via high current densities requires oxidants with large Cmax and n ≥ 2 e−, since variations in D are typically small and cannot offset the multiplying factor of n. A significant improvement in power cannot realistically arise from higher voltage (V), either, since cell voltages for these oxidants and typical fuels varies by
