Conductive-Matrix-Mediated Alkaline Fe(III/VI) Charge Transfer:  Three-Electron Storage, Reversible Super-Iron Thin Film Cathodes

Abstract

An extended conductive matrix facilitates a 100-fold enhancement in charge storage for reversible Fe(III/VI) super-iron thin films. These films were deposited, by electrochemical reduction of Na2FeO4, with an intrinsic high capacity 3 e- cathodic storage of 485 mAh g(-1). Whereas 3 nm Fe(III/VI) films exhibited a high degree of reversibility (throughout 100 charge/discharge cycles), thicker films had been increasingly passive toward the Fe(VI) charge transfer. Films were alternatively deposited on either smooth or on extended conductive matrixes composed of high-surface-area Pt, Ti, and Au and probed galvanostatically and via cyclic voltammetry. A 100 nm Fe(VI) cathode, on the extended conductive matrixes, sustained 100-200 reversible three-electrode charge/discharge cycles, and a 19 nm thin film cathode sustained 500 such cycles. With a metal hydride anode, full cell storage was probed, and a 250 nm super-iron film cathode film sustained 40 charge/discharge cycles, and a 25 nm film was reversible throughout 300 cycles. Fe(VI) salts exhibit higher cathodic capacity and environmental advantages, and the films are of relevance toward the next generation charge storage chemistry for reversible cathodes.

Advances in electrochemical Fe(VI) synthesis and analysis

“…Whereas ultra-thin super-iron films can sustain an extended reversibility, thicker films were not rechargeable due to the irreversible buildup of passivating (resistive) Effect of apparent current density on purity, yield and current efficiency [55] Fe(III) oxide, formed during film reduction. In 2006, Licht's group probed that preparation of Fe(III/VI) films on an extended conductive matrix can facilitate the thick film's reversibility [41]. Substrates for the film preparation are Pt or Ti foil.…”

A High Capacity Li-Ion Cathode: The Fe(III/VI) Super-Iron Cathode

“…Development Reference 1999 introduction of super-iron charge storage & super-iron alkaline battery [5] 2000** introduction of super-iron lithium primary (single discharge) battery [7] 2001 demonstration of the solid state stability of the hexavalent iron [8] 1999-5 chemical syntheses of an array of super-iron salts [5,7,[9][10][11][12][13][14][15][16] 2000-4 inexpensive, electrochemical syntheses of super-iron salts [17][18][19][20][21][22][23][24][25][26] 2003-5** electrolyte optimization for super-iron lithium batteries [27,28] 2003 reversibility of alkaline, nanothick (3 nm) Fe(VI) cathodes [29] 2006 rechargeable alkaline super-iron battery [30] 2006** reversibility of non-aqueous, nanothick (3 nm) Fe(VI) cathodes [31] 2007-8 zirconia encapsulation-stabilization of alkali super-irons [32][33][34][35] 2009** rechargeable super-iron lithium battery, 4 V cathode [6] **=lithium super-iron battery development.…”

“…FTIR provides not only a specific "fingerprint" distinguishing the various Fe(VI) oxides, as shown in Figure 5, but importantly we have also developed it as a quantitative technique to determine the Fe(VI) salt purity through the addition of a standardized BaSO 4 salt [8]. Discharge of cathode replaced, commercial alkaline button cells provides rapid screening of the redox activity of alternative salts [6,10,11,14,15,19,[27][28][29][30][31][32][33][34][35].…”

Transformation of Solid Potassium Ferrate(VI) (K2FeO4): Mechanism and Kinetic Effect of Air Humidity

“…[50] The assignment of the Fe III phase to Fe(OH) 3 explains its amorphous and speromagnetic nature originally determined by XRD, Mössbauer, and magnetic measure- ments in the decomposition of K 2 FeO 4 in humid air. Moreover, considering the starting atomic ratio of potassium/iron (2:1), KHCO 3 and Fe(OH) 3 should be formed in a molar ratio of 2:1 according to Equation (2 Both these decomposition steps are probably partially overlapped, and the decomposition of Fe(OH) 3 clearly dominates at higher temperatures [see the step assignments in Figure 6, right]. In summary, a room-temperature transformation of potassium ferrate(VI) in humid air containing water and carbon dioxide resulted in the formation of KHCO 3 and Fe(OH) 3 in a molar ratio of 2:1 with the simultaneous evolution of 3/4 mol of oxygen [Equation (2)].…”

“…Iron(VI) compounds [ferrates(VI), FeO 4 2-] represent an advanced class of compounds, which can be used in many promising electrochemical, environmental, and chemical applications such as high-energy-density rechargeable batteries [1][2][3][4][5] and cleaner ("greener") technologies of organic syntheses, [6][7][8] "environmentally friendly" oxidant useful in innovative technologies for water treatment. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Among multitudes of other advantageous properties it is worth to emphasize the coagulation and disinfection effects.…”