Amorphous cobalt alloy electrodes for aqueous electrochemical devices

A material for use as an electrode, and particularly the cathode (30) of an electrochemical device, such as a pseudocapacitive device (10). The material is a substantially amorphous material having a host matrix material selected from the group of cobalt, nickel, iron, and combinations, and a modifier element or elements selected from the group of boron, phosphorous, carbon, silicon, aluminum, manganese, copper, chromium, vanadium, titanium, molybdenum, zirconium, tungsten, and combinations thereof.

TECHNICAL FIELD 
This invention relates in general to the field of electrochemical devices, 
and more particularly to materials which may be employed as electrodes 
therein. 
BACKGROUND 
Electrochemical capacitors are a class of high-rate energy 
storage/discharge devices which use electrolytes and electrodes of various 
kinds in a system similar to that of conventional batteries. 
Electrochemical capacitors, like batteries, are essentially energy storage 
devices. However, unlike batteries, they rely on charge accumulation at 
the electrode/electrolyte interface to store energy. Charge storage in 
electrochemical capacitors therefore is a surface phenomenon. Conversely, 
charge storage in batteries is a bulk phenomenon occurring within the bulk 
of the electrode material. 
Electrochemical capacitors can generally be divided into two subcategories: 
Double layer capacitors in which the interfacial capacitance at the 
electrode/electrolyte interface can be modeled as two parallel sheets of 
charge; and pseudocapacitor devices in which charge transfer between the 
electrolyte and the electrode occurs over a wide potential range. These 
charge transfers are the result of primary, secondary, and tertiary 
oxidation/reduction reactions between the electrode and the electrolyte. 
These types of electrochemical capacitors are being developed for 
high-pulse power applications. 
Most of the known pseudocapacitor active materials are based on noble metal 
elements such as ruthenium and iridium. These materials are generally 
quite expensive. Material expense thus poses a significant hurdle to the 
wide-spread commercialization of this technology. Other less expensive 
materials have been tried, but have been less than successful. For 
example, workers in the field have attempted to fabricated devices using 
pressed powder cobalt and cobalt oxide electrodes. However, these types of 
electrodes have failed for numerous reasons including, for example, poor 
life cycle performance, and inability to achieve desired electrochemical 
performance characteristics. 
Accordingly, there exists a need for pseudocapacitive electrode materials 
which deliver good performance in terms of energy storage, power density, 
and cycle life. Moreover, such materials should be abundant in nature, 
inexpensive in cost, readily processable into devices, and relatively 
benign environmentally.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While the specification concludes with claims defining the features of the 
invention that are regarded as novel, it is believed that the invention 
will be better understood from a consideration of the following 
description in conjunction with the drawing figures, in which like 
reference numerals are carried forward. 
Referring now to FIG. 1, there is illustrated therein a representation of a 
high energy density electrochemical capacitor device (10) in accordance 
with the instant invention. The device (10) includes an anode (20), a 
cathode (30) and a separator (40) operatively disposed between the anode 
and the cathode. Also disposed between the anode (20) and the cathode (30) 
is an electrolyte (50), which as illustrated in FIG. I is an aqueous 
(liquid) electrolyte disposed entirely about both the anode (20) and the 
cathode (30). 
In one preferred embodiment of the invention, the cathode (30) is 
fabricated of an amorphous cobalt alloy material such as that described 
hereinbelow. The anode material may be fabricated from any of a number of 
different materials known in the art. Examples of such materials include 
vanadium oxides, chromium oxides, manganese oxides, iron oxides, cobalt 
oxides, nickel oxides or their corresponding sulfides, selenides, 
tellurides, and combinations thereof. 
The electrolyte used in connection with the electrochemical capacitor 
device in accordance with the invention may be any aqueous electrolyte, 
such as an alkaline electrolyte, neutral electrolyte, acid electrolyte, 
and combinations thereof. In one preferred embodiment, the electrolyte is 
31% KOH. Similarly the separator (40) may be fabricated of a number of 
known separator materials as are practiced in the art. Specific examples 
of such separators includes but are not limited to porous cellulose, 
porous silica, glass wool, glass fiber, polypropylene, and combinations 
thereof. 
The schematic representation of the capacitor device as shown in FIG. 1 is 
used to explain the redox processes occurring at the anode and the 
cathode. During charging, electrons, for example, (22, 24, 26, 28), flow 
to the anode (20) as shown, and the active material from which the anode 
is formed undergoes a reduction process. The resulting charge imbalance, 
here, an excess of negative charge, is balanced by the migration of 
positively charged ions (32, 34, 36, 38) from the electrolyte to cathode 
(30) as shown. 
While the reduction process occurs at the anode, a complimentary oxidation 
process takes place at the cathode, resulting in the ejection of an 
electron. Both the electrode processes occur at or near the 
electrode/electrolyte interface. During this process, ions pass through 
the porous separator (40). The entire scenario is then reversed during the 
discharge process of the pseudocapadtor electrodes. 
In accordance with the present invention, there is provided an electrode 
material for use as, for example, the cathode in an aqueous 
electrochemical capacitor. The electrode material comprises a disordered, 
single-phase, multicomponent material having a host matrix material and at 
least one modifier element. The host matrix material may be selected from 
the group of materials consisting of cobalt (Co), nickel (Ni), iron (Fe), 
and combinations thereof. Similarly, the modifier element may be selected 
from the group of silicon (Si), boron (B), phosphorus (P) carbon (C), 
aluminum (Al), manganese (Mn), copper (Cu), chromium (Cr), vanadium (V), 
titanium (Ti), molybdenum (Mo), zirconium (Zr), Tungsten (W), and 
combinations thereof. Indeed, it is contemplated that more than one 
modifying element will routinely be added to the host matrix material. 
It is contemplated that the multicomponent material, will be amorphous; 
alternatively, the material may be either microcrystalline or 
polycrystalline, so long as it is lacking in long range compositional 
order. The material may also include crystalline inclusions. The 
substantially amorphous materials of the instant invention lack long range 
structural order, but possess short-range structural and chemical order. 
As a result, these materials have a higher concentration of coordinatively 
unsaturated sites (CUS). These CUS are very active sites for surface 
reaction and surface catalytic/electrocatalytic reactions. It is a 
fundamental characteristic of electrochemical capacitors such as 
pseudocapacitors that the electrochemical reactions be substantially 
surface phenomenon. Accordingly, higher concentrations of active surface 
sites, such as the CUS, will yield better electrochemical activity. 
Exemplary materials according to the instant invention include but are not 
limited to: 
Co.sub.66 Fe.sub.4 Ni.sub.1 Si.sub.15 B.sub.14, Co.sub.69 Fe.sub.4 Ni.sub.1 
Mo.sub.2 Si.sub.12 B.sub.12, Co.sub.63 Cr.sub.27 Ni.sub.13 Fe.sub.3 
W.sub.4 and Co.sub.72 Ni.sub.18 P.sub.10. The materials are expressed in 
atomic percent. 
EXAMPLES 
The invention is further discussed by offering specific examples of the 
electrode material. 
PREATION OF THE AMORPHOUS COBALT MATERIAL 
Materials having the compositions described herein were mixed and 
pre-melted to form an ingot. Thereafter, the materials were subjected to a 
re-melted and melt-spinning process in which a stream of liquid metal is 
ejected from a nozzle in the crucible in which the ingot was re-melted. 
The nozzle has a thin slit which directs the molten metal onto a cold 
rotating metal wheel. As the metal flows from the slit, it is rapidly 
quenched at a rate of approximately 10.sup.6 .degree. C./second. As a 
result of rapid quenching, the material forms an amorphous alloy ribbon. 
Melt spinning techniques such as described herein are well-known in the 
art. 
Amorphous cobalt alloy materials as described herein may then be pretreated 
by for example: a) immersing in alkaline solution for a long period of 
time (e.g. 24 hr) to form a mixed oxide/hydroxide film; b) etching in 
acids to increase surface area; and c) anodizing in alkaline solutions. In 
the present case, the material were immersed in 31% KOH for approximately 
24 hours, and anodized at a range of between -0.8 and -0.5 V versus Hg/HgO 
for between 10 and 30 seconds prior to electrochemical testing. The 
anodization process leads to the formation of cobalt and iron (in iron 
containing alloys) oxides and hydroxides. 
Testing of alloys prepared as described above was carded out in a standard 
3-compartment electrochemical cell containing 31% KOH solution, a large 
area nickel gauze counter electrode, and a Hg/HgO reference electrode. 
Electrochemical experiments were carried out on an EG&G M273 potentiostat. 
EXAMPLE I 
A first amorphous cobalt alloy having the composition Co.sub.69 Fe.sub.4 
Ni.sub.1 Mo.sub.2 Si.sub.15 B.sub.12 was tested to determine its 
electrochemical properties. The material is commercially available as a 
soft magnetic material, and is known as Metglas 2705M. A first sample of 
the material had the following dimensions: thickness of 50 .mu.m, weight 
of 0.0107 g, and area of approximately 0.92 cm.sup.2. 
Referring now to FIG. 2, there is illustrated therein a cyclic voltammagram 
(CV) of the sample of material described above. The CV illustrated by line 
60 was taken after 4000 cycles of the material, and demonstrates a 
symmetrical profile at a sweep rate of 200 mV/s. The symmetrical peaks 
seen in the CV suggests a multi-electron transfer oxidation/reduction 
reaction over a certain potential range. These electron transfer 
electrochemical reactions are kinetically fast and reversible, which is a 
characteristic of pseudocapacitive behavior. CV testing was repeated after 
12,000 cycles, and is illustrated by line 62. The results indicated 
stable, reproducible, pseudocapacitive behavior. 
Referring now to FIG. 3, there is illustrated therein the constant current 
charge/discharge curves for the instant material. In contrast to battery 
materials which show a relatively flat potential vs. time relationship, 
the curves in FIG. 3 show a linear potential decline which is a typical 
capacitive transient. The symmetrical charging/discharging transient is 
indicative of a material that may be readily charged and discharged at a 
rapid rate. FIG. 3 also demonstrates that the electrochemical behavior of 
the material is stable as the transient after 200 cycles was faster and 
more symmetrical than that after 60 cycles. 
In order to determine whether or not the amorphous structure of the cobalt 
alloys material contributed to the pseudocapacitive performance, the 
material of this example was partially re-crystallized by annealing. After 
annealing at 550.degree. C. for 1 hour, the sample became very brittle 
indicating the formation of a multiphase crystalline material. Referring 
now to FIG.4, there are two CV curves, from before (64) and after (66) 
annealing, respectively. The electrode was tested via the same procedure 
as the amorphous alloy. The results showed that the CV was asymmetrical, 
with poor chargeability. The results in FIG. 4 strongly suggest that the 
superior electrochemical activity illustrated in FIGS. 2 and 3 are 
attributable to the amorphous structure of the alloys. 
EXAMPLE II 
A second amorphous cobalt alloy having the composition Co.sub.66 Fe.sub.4 
Ni.sub.1 Si.sub.15 B.sub.14 was tested to determine its electrochemical 
properties. The material is also commercially available as a soft magnetic 
material and is known as Metglas 2714A. A first sample of the material had 
the following dimensions: thickness of 40 .mu.m, weight of 0.0097 g, and 
area of approximately 0.90 cm.sup.2. 
Referring now to FIG. 5, there is illustrated therein a chart comparing 
charge density versus cycle number for an amorphous cobalt alloy electrode 
material as described above. Specifically, the material was scanned over 
20,000 CV cycles, and measured for charge density. As may be appreciated 
from FIG. 5, the charge density increased consistently with cycle number, 
until the test terminated at 20,000 cycles. This increase in charge 
density was believed to be attributable to the formation and development 
of a hydrated mixed metal oxide film which is electronically/ionically 
conductive and can facilitate multi-electron transfer oxidation/reduction 
faradaic reactions. The CV's which were run to compile information for 
FIG. 5 also indicated that the material demonstrates a symmetric profile 
indicative of pseudocapacitive behavior. This is illustrated in FIG. 6, 
which is the CV of the material after 20,000 cycles, at a sweep rate of 
100 mV/s. 
To test the importance of the crystallographic structure on electrochemical 
performance, a sample of material was again recrystallized by annealing. 
After annealing at 550.degree. C. for 1 hour, the sample became very 
brittle indicating the formation of a multiphase crystalline material. 
Referring now to FIG.7, there are two CV curves, from before (68) and 
after (70) annealing, respectively. The electrode was tested in the same 
procedure as that on its amorphous counterpart. The results showed that 
the current density of crystalline alloy was much less than that of its 
amorphous counterpart, indicating a relatively poor activity in the 
crystalline alloy. 
COMATIVE RESULTS 
Amorphous cobalt alloy materials as described herein afford significant 
performance and price advantages over the materials commonly used in the 
prior art. These advantages are illustrated in the following table, in 
which measurements are based on flat, planar electrodes. 
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MATERIAL CHARGE DENSITY CAITANCE 
mC/g mC/cm.sup.2 
F/cm.sup.2 
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MG2714A 13,000 100 0.2 
MG2705MN 6,700 70 0.06 
Iridium 200 12 0.04 
Ruthenium 2,400 60 0.06 
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In terms of cost, amorphous cobalt alloys are commercially available at 
typical costs of approximately $250/kg, versus approximately $9,500/kg for 
ruthenium, and $64,500/kg for iridium. It may thus be appreciated that the 
instant amorphous cobalt alloys enjoy a substantial cost and performance 
advantage over the materials of the prior art. 
While the preferred embodiments of the invention have been illustrated and 
described, it will be clear that the invention is not so limited. Numerous 
modifications, changes, variations, substitutions and equivalents will 
occur to those skilled in the art without departing from the spirit and 
scope of the present invention as defined by the appended claims.