CHEM 521 Analytical Electrochemistry TOPIC 4 Nov 28, 2016 Electrochemical energy storage and conversion Batteries and Electrochemical Capacitors Daniel A. Scherson and Attila Palencsár The Electrochemical Society Interface Spring 2006
Zn/MnO 2 Battery Lithium Ion Battery Electrochemical capacitor Fuel cells and electrochemical catalysis 1800, the voltaic pile by Alessandro Volta, an assembly consisting of plates of two different metals, such as Zn and Cu, placed alternately in a stack-like fashion separated by paper soaked in an aqueous solution, such as brine or vinegar. 1859, lead-acid cell by Gaston Plante 1887, the first dry cell battery made of a zinc-carbon cell by Carl Gassner 1899, nickel-cadmium battery by Waldmar Jungner 1955, the common alkaline battery by Lewis Urry 1970s, nickel hydrogen battery 1980s, the nickel metal-hydride battery 1912, lithium batteries were first created 1996, the lithium ion polymer battery
Primary versus Secondary (or rechargeable) Batteries Primary - Irreversible, low self discharge rates, used in smoke detectors, flashlights, and most remote controls Secondary - Reversible, used as car batteries and in portable electronic devices Reduction: PbO 2 (s) + 3 H + (aq) + HSO 4- (aq) + 2e - PbSO 4 (s) + 2H 2 O(l) Oxidation: Pb(s) + HSO 4- (aq) PbSO 4 (s) + H + (aq) + 2e - Overall: PbO 2 (s) + Pb(s) + 2H + (aq) + 2HSO 4- (aq) 2PbSO 4 (s) + 2H 2 O (l)
Figures of merit Capacity (mah), e.g., 2500 mah (e.g., 250 ma discharge current for 10 h) E cell Time / h A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps. As the C rate is increased beyond a certain limit, however, the capacity of the battery can no longer be fully utilized. A fraction of valuable electrode material remains unused, an effect found for all battery systems for sufficiently high C rates.
Figures of merit the Ragone plot - a graph of specific energy (A V h/kg = W h/kg) vs. specific power (W/kg). other factors must also be considered, including reliability (critical for pacemakers), safety, self-discharge, temperature, and even humidity.
Zn/MnO 2 batteries Anode = Zn metal Cathode = MnO 2 Electrolyte = a strong alkaline aqueous solution in a fabric or separator Zn + 2NH 4 Cl + 2OH - Zn(NH 3 ) 2 Cl 2 + 2H 2 O + 2e - 2MnO 2 + 2H 2 O + 2e - 2OH - + 2MnOOH Zn + 2MnO 2 + 2NH 4 Cl Zn(NH 3 ) 2 Cl 2 + 2MnOOH Carbon is mixed with MnO 2 to increase conductivity Open circuit potential is ~1.5 V Battery potential drops with increasing current due to internal resistance and kinetics Battery potential drops with discharge
Li-ion batteries
Li-ion batteries Composite, semi-quantitative (arbitrary current scale) CVs for natural graphite and LiCoO 2 recorded independently at very slow scan rates. Data were obtained in a LiPF 6 solution in a mixture of alkyl carbonates. The arrows indicate the direction of the potential scans and the blue and magenta represent charge and discharge, respectively.
Electrochemical capacitors
Electrochemical capacitors
Electrochemical capacitors Schematic construction of a wound supercapacitor 1.Terminals, 2.Safety vent, 3.Sealing disc, 4.Aluminum can, 5.Positive pole, 6.Separator, 7.Carbon electrode, 8.Collector, 9.Carbon electrode, 10.Negative pole
Carbon 2000 m 2 /g Assuming a C dl ~10 μf/cm 2 200 F/g Radial style of a lithium-ion capacitor for PCB mounting used for industrial applications Typical button capacitor for PCB mounting used for memory backup
Electrodes for EDLCs carbon (activated carbon (AC), carbon fibre-cloth (AFC), carbide-derived carbon (CDC), carbon aerogel, graphite (graphene), graphane and carbon nanotubes (CNTs). Electrolytes Water w/ sodium perchlorate (NaClO 4 ), lithium perchlorate (LiClO 4 ) Acetonitrile w/ tetraethylammonium tetrafluoroborate Separators ~1500 m 2 /g
Fuel cells and Electrocatalysis Electrocatalyst Issues: Cost of Pt or Pt/Ru; CO poisoning Membrane Issues: Low operating temperature (< 100 o C); cost of Nafion Transport Issues: Water management; three-phase region Fuel Issues: H 2 transportation and storage
Fuel cells Polymer Electrolyte Membrane (PEM) Fuel Cells Direct Methanol Fuel Cells Alkaline Fuel Cells Phosphoric Acid Fuel Cells Molten Carbonate Fuel Cells Solid Oxide Fuel Cells
PEM Fuel cells Also called Proton Exchange Membrane Fuel Cells High-power density, ~600 ma/cm 2 Low weight and volume Voltage per cell = 0.7 V Power density = 0.25 KW/kg Life time = 5000 h Cost per KW = $500 1000 Porous carbon electrodes Operates at lower temperatures, <80 C Platinum electrocatalyst CO poisoning Hydrogen storage solid polymer H 2 = 2H + + 2e - O 2 + 4e - + 2H 2 O = 4OH -
Alkaline Fuel cells Power density, ~400 ma/cm 2 Voltage per cell = 0.8 V Power density = 0.17 KW/kg Life time = 4000 h Cost per KW = $150 Catalysts (Nickel, silver) High performance >60% efficiency High temperature (90-120 o C) CO 2 poisoning a KOH solution
Solid Oxide Fuel Cells (SOFCs) A hard, non-porous ceramic compound as the electrolyte Very high operating temperature, >1000 o C High efficiency, >80% No precious metals are needed as electrocatalysts Electrode stability greatly enhanced a slow startup and thermal shielding Yttria-stabilized zirconia (YSZ) a ceramic of zirconium dioxide + yttrium oxide.
Electrocatalysts Activity Stability Selectivity Primary figures of merit for electrocatalyst activity: Exchange current density, i o (ma/cm 2 ) Tafel slope, b (mv/decade) Current density at a given over potential: i E (V vs. RHE) (ma/cm 2 ) Overpotential needed to reach a given current density: η (when i=10 ma/cm 2 (mv))
Tafel plots for various HER catalysts Chen Z, Jaramillo T.F., et al. NanoLetters 11, 10 (2011)
M.T.M. Koper, H.A. Heering, in Fuel Cell Science, Eds. A. Wieckowski, J.K. Nørskov, Wiley (2010), p. 71 110
Fig. 2. (a) Fuel cell volcano plot showing theoretically predicted site-specific ORR activities on Pt/Rh(111) in comparison with measured activities of several other bimetallic systems 1 4. (b) Bilayer islands with maximized occurrence of near-b-edge sites (yellow). For a series of such islands with different lateral widths, the predicted ORR activities are plotted in the graph. - See more at: http://www- ssrl.slac.stanford.edu/content/science/highlight/2012-06- 22/catalyst-design-x-rays-cross-examine-fuel-cell-volcanoplot#sthash.oUGEg7q0.dpuf http://www-ssrl.slac.stanford.edu/content/science/highlight/2012-06- 22/catalyst-design-x-rays-cross-examine-fuel-cell-volcano-plot
Langmuir 2007, 23, 11901-11906 Effect of Particle Size on the Kinetics of the Electrocatalytic Oxygen Reduction Reaction Catalyzed by Pt Dendrimer-Encapsulated Nanoparticles Five different G6-OH(Ptn) (n=) 55, 100, 147, 200, 240) DENs Sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers
Electrocatalytic activity for ORR Onset potential = 1.0 V for Pt bulk Steady-state, mass-transfer-limited region = 0.15 to 0.7 V Larger particles have better activity! Figure 2. Rotating disk voltammograms (RDVs) for G6-OH(Pt n )-modified GCEs (n= 240, 200, 147, 100, and 55) and for a bulk Pt electrode. The rotation rate was 900 rpm, the scan rate was 5 mv/s, and the geometric areas of GCE and bulk Pt electrodes were 0.196 cm 2. The electrolyte solution consisted of aqueous 0.1 M HClO 4 saturated with O 2.
Nanoparticle surface area determination Figure 3. Cyclic voltammograms for G6-OH(Pt n )-modified GCEs (n= 240, 200, and 147) and a bulk Pt electrode showing (a) CO oxidation and (b) H-adsorption/desorption. For each CV, the electrode potential was scanned from 0.05 to 1.1 V, back to 0.05 V, and then to 1.1 V again at a rate of 100 mv/s. For clarity, each CV was divided into the two parts shown in parts a and b. The electrolyte solution consisted of aqueous 0.1 M HClO4 saturated with N2. The electrode preparation is described in the corresponding section of the text. The geometric areas of GCE and bulk Pt electrodes was 0.196 cm 2.
Specific activity