X E / N A S A / O 7 9 4 - 8 0 / 1 NASA CR-159738

(NASA-CC-159738) A D V A K E C SCI iEEblING G F N60-22777 E L E C l & O D E COUPLES ( N A S A ) 5 6 p HC A04 /BE A01

CSCL 1 0 2 Unclas

G3/44 16005

ADVANCED SCREEN I NG OF ELECTRODE COUPLES

J. G I N E R and

K. C A H I L L

GINER, INC, 14 S P R I N G STREET WALTHAM, MA 0 2 1 5 4

FEBRUARY 1980

PREPARED FOR :

PIATIONAL AERONAUT1 CS AND SPACE ADMIN I STKATION LEWI s FIESEARCH CENTER CONTRACT r J A S 3 - 2 0 7 9 4

U l S l DEPARTMENT OF ENERGY OFF ICE OF CONSERVATION AND SOLAR ENERGY PROGRAMS

https://ntrs.nasa.gov/search.jsp?R=19800014289 2020-04-01T12:45:21+00:00Z

ADVANCED SCREENING OF ELECTRODE COUPLES

3. Recipient's Catalog No

5. Report Date

1. Report No.

CR 159738 2. Government Accnrion No.

7. Author(sJ

J. Giner and K. Cah i l l

Q. Performing Organization Name md Address

I

15. Supplemmtary Nota Final repor t prepared urrder Interagency Agreement EC-77-A-31-1002. Project Manager, Y. A. Reid, Solar and Electrochemistry Div is ion, NASA-Lewis Research Center,

4. Title and Subtitle

8. Performing Orgmizat~on Report No

* 10. Work Unit No.

GINER, INC. 14 Spring Street Waltham, MA 02154

12. Spomoring Agency Name and Address

U. S. Department o f Energy Of f ice o f Conservation and Solar Programs Washington, DC 20545

16. Abstract The chromim (cr3+/cr2+ redox couple (e lec t ro l y te and electrode) was invest igated i n order to

determine i t s sui tab i 1 i t y as negativa electrode f o r the i r o n (~e3+/~e2+)-chromi um (cr3+/cr2+) redox f low battery. L i t e ra tu re search and laboratory invest iga t ion established tha t the solubi 1 i t y and s t a b i l i t y of aqueous ac id ic solut ions o f chromium(II1) ch lor ide and chromium(I1) chlor ide are suf f ic ien t f o r redox bat tery appl icat ion.

Four categories o f electrode mater ia ls were tested; namely, metals and meta l lo id mater ia ls (elements and compounds), a1 l o s, p la ted materials, and Teflon-bonded materials. I n a l l , the r e l a t i v e performance o f 26 -+ can idate electrode mater ia ls was evaluated on the basis of slow scan ra te 1 inear sweep vol t a m c t r y i n s t i r r e d solut ion. No sing1 e mater ia l tested gave both acceptable anodic and acceptable cathodic performance. However, the i den t i f i ca t i on o f lead as a good cathodic e lec t ro- ca ta lys t and gold as a good anodic e lec t rocata lys t l ed t o the invent ion o f the lead/gold conbination electrocatalyst . This type of ca ta l ys t can be fabr icated i n several ways and appears t o o f fe r the advantages of each metal without the disadvantages associated w i th t h e i r use as s ing le materials. This lead/gold e lec t rocata lys t has subsequently been tested by NASA-Lewis Research Center i n complete flowing, redox bat ter ies comprising a stack o f several ce l l s . A large improvement i n the bat tery 's coul ombi c and energy eff iciency has been observed.

I n addi t ion t o the above, f i ve dissolved organic compounds were tested as possible hydrogen evolut ion i nh ib i t o rs , including thiourea, palmi t i c acid, ce ty l alcohol, hexanol and n-propanol . F ina l l y , the cost of large l o t s o f t h i r t een common chemicals was estimated.

11. Contract or Grant No.

NAS3-20794 13. T y p of Rcport and Period Covered

Contractor Report

14. S~onrorino A w c v Report No. DCElNASAl0794-8011

7 . Key Words suggested by Author(sl J 18. Distribut~on Statement

energy storage, redox, chromi um, electrocatalysis, gold, lead

Unclassi f ied - Unlimited STAR Categories 25-04 and 44-02 DOE Category UC-94c

' For sale by the Natlonal Technical lnformat~on S e r v ~ c e , S p r ~ n e f ~ e l d . Vlrgma 22161

1

NASA-C-168 (Rev. 10-75)

19. Sacuritv Classif (of this report)

Unclassi f ied 20. security Classif. (of this page)

Unclassi f ied 21. No. of Pages

48

22. Rice*

NOTICE

This r e p o r t was prepared t o document work sponsored by t h e United S t a t e s Government. Nei ther t h e United S t a t e s nor i ts agen t , the United S t a t e s Department of Energy, nor any Federa l employees, nor any of t h e i r c o n t r a c t o r s , subcon t rac to r s o r t h e i r employees, makes any warranty , express o r implied, o r assumes any l e g a l l i a b i l i t y o r r e s p o n s i b i l i t y f o r t h e accuracy, completeness, o r use fu lness of any informat ion, appara tus , product o r process d i sc losed , o r r e p r e s e n t s t h a t i t s use would not i n f r i n g e p r i v a t e l y owned r i g h t s .

TABLE OF CONTENTS

ABSTRACT

I. INTRODUCTION

11. SOLUBILITY AND STABILITY OF AQUEOUS ACIDIC

CHROMIUM(I1) CHLORIDE SOLUTIONS

A. Introduction

B. Results

III. SOLUBILITY AND STABILITY OF AQUEOUS CHROMIUM(III)

CHLORIDE SOLUTIONS

A. Introduction

B. Results

IV . ELECTRODE MATERIAL EVALUATION

A. Introduction

B. Screening Approach

C. Results

D. Recommendations

V. EVALUATION OF DISSOLVED ORGANIC COMPOUNDS AS

HYDROGEN EVOLUTION INHIBITORS

A. Introduction

B. Results

C. Recommecdations

VI. PRELIMINARY COST ANALYSIS OF CHROMIUM CHEMICALS

A. Discussion

VII. REFERENCES

Page.

v

1

4

4

LIST OF TABLES AVD FIGURES

Figure 1-1 Thermodynamic Stability of ~ e ~ + / ~ e ~ + and cr3+/cr2+ Redox Couples

Table 11-1 Decomposition of Chromium(I1) Chloride

Figure 11-1 Chromous Chloride Solubility vs. Hydrochloric Acid Concentration

Figure 11-2 Gold Foil Electrode

Figure 11-3 Thermodynamic Stability of pb/pb2+ a d cd/cd2+ Redox Couples

Figure 111-1 Chromic Chloride Solubil.ity vs. Hydrochloric Acid Concentration

TaZ,le IV-1 -.

Figure IV-1

Figure IV-2

Figure IV-3

Figure IV-4

Figure IV-5

Figure IV-6

Figure IV-7

Figure IV-8

Figure IV-9

Figure IV-10

Figure IV-11

Figure IV-12

Figure IV-13

Figure IV-14

Summary of Electrode Yaterials

Hydrogen Evolution Characteristics of Several Pure Materials in IN HCl Solution

Chromic Reduction Characteristics on Several Pure Materials in 1N HC1 + 0.1M CrC13 Solution

Lead Foil Electrode

Copper Foil Electrode

Gold Foil Electrode

Antimony Electrode

Bismuth Disk Electrode

Bismuth Disk Electrode

Cadmium Electrode

Indium Electrode

Anodic Screening of Three Electrode Materials

Hydrogen Evolution Screening for Three Alloys in 1N HC1

Chromium Reduction on Three Alloys in 1N HC1 + 0.1M CrC13 Comparison of Hydrogen Evolution and Chromium Reduction Characteristics of Three Electrode Materials

Figure IV-15 CopperILead 90110 Electrode

Figure IV-16 Silicon Carbide Coated Graphite Electrode

Figure IV-17 Boron Carbide Electrode

t 6 , Figure IV-18 Tungsten Carbide on Titanium Screen Electrode

Figure IV-19 Tantalum Nitride Electrode (20% TFE)

Figure IV-20 Zirconium Carbide Electrode (20% TFE)

Figure IV-21 Zirconium Carbide Electrode (5% TFE)

Figure IV-22 Zirconium Nitride Electrode (20% TFE)

Figure IV-23 Titanium Carbide Electrode (20% TFE)

Figure IV-24 Titanium Nitride Electrode (20% TFE)

Fipre IV-25 Tantalum Carbide Electrode (20% TFE) - Figure IV-26 Chromiun Redox Reaction on AU/P~ Codeposited on a Graphite

Rod Electrode

Figure IV-27 Chromium Redox Reaction on AU/P~ Codeposited on a Graphite Rod Electrode

Figure IV-28 Chromium Reduction on Au/Pb Codeposited on s Graphite Rod Electrode

Figure IV-29 Comparison of H2-Evolution and Chromium Reduction on a Silver Foil Electrode and a Lead Plated Silver Foil Electrode

Figure IV-30 Comparison of H2-Evolution and Chromium Reduction m a Copper Foil and a Lead Plated Copper Foil Electrode

Figure IV-31 Chromium Redox Reaction on a Lead Plated Silver Wire Electrode

Figure IV-32 Chromium Reduction on a Lead Plated Silver Wire Electrode

Figure IV-33 Chromium Redox Reaction on a Lead Plated Gold Foil Electrode

Figure IV-34 Comparison of Chromium Reduction on a Graphite Rod and a Lead Plated Graphite Rod Electrode

Figure IV-35 Chromium Reduction on a Lead Plated Carbon Electrode

Figure IV-36 Chromium Reduction on a Gold and Lead Plated Carbon Electrode

Figure IV-37 Chromium Redox Reaction on a Gold and Lead Plated Carbon Electrode

Figure IV-38 Chromium Reduction on a Gold and Cadmium Plated Carbon Electrode

Figure IV-39 Chromium Reduction on a Gold and Cadmium Plated Carbon Electrode

Figure IV-40 Chromium Redox Reaction on a Copper and Lead Plated Carbon Electrode

Figure IV-41 Chromium Redox Reaction on a Silver and Lead Plated Carbon Electrode

Figure IV-42 Compari~~n of Chromic Reduction on a Lead Plated Graphite Electrode and an Indium Plated Graphite Electrode

iii

Figure IV-43 Chromium Redox Reaction on a Gold and Indium Plated Graphite Electrode

Figure IVT43 Chromium Redox Reaction on a Gold and Lead Plated Graphite Electrode

Figure V-1 Gold Foi l Electrode (Thiourea)

Figure V-2 Gold Foi l Electrode (Palmitic Acid)

Table VI-1 Costs of Common Chromium Compounds

ABSTRACT

The chromium (cr3+/cr2+) redox couple ( e l e c t r o l y t e and e l e c t rode) was

i n v e s t i g a t e d i n o rde r t o determine i t s s u i t a b i l i t y a s nega t ive e l e c t r o d e f o r t h e

i r o n ( ~ e ~ + / ~ e ~ + ) - c h r o m i u m (cr3+/cr2+) redox flow b a t t e r y . L i t e r a t u r e s e a r c h and

l a b o r a t o r y i n v e s t i g a t i o n e s t a b l i s h e d t h a t t h e s o l u b i l i t y and s t a b i l i t y of aqueous

a c i d i c s o l u t i o n s of chromium(I1I) c h l o r i d e and chromium(l1) c h l o r i d e a r e ' s u f f i c i e n t

f o r redox b a t t e r y app l i ca t ion .

Four c a t e g o r i e s of e l e c t r o d e m a t e r i a l s were t e s t e d ; namely, meta ls and meta l lo id

m a t e r i a l s (elements and compounds), a l l o y s , p l a t e d m a t e r i a l s , and Teflon-bonded

m a t e r i a l s . I n a l l , the r e l a t i v e performance of 26 candidate e l e c t r o d e m a t e r i a l s was

evaluated on t h e b a s i s of slow scan r a t e l i n e a r sweep voltammetry i n s t i r r e d s o l u t i o n .

No s i n g l e m a t e r i a l t e s t e d gave both acczp tah le anodic and accep tab le ca thod ic perfor-

mance. However, t h e i d e n t i f i c a t i o n of lead a s a good cathodic e l e c t r o c a t a l y s t and

gold as a good anodic e l e c t r o c a t a l y s t l ed t o t h e inven t ion of the l ead lgo ld combina-

t i o n e l e c t r ~ c a t a l ~ s t ( ~ ) . This tvpe of c a t a l y s t can be f a b r i c a t e d m s e v e r a l ways

and appears t o o f f e r the advantages of each meta l without t h e d isadvantages a s s o c i a t e d

wi th t h e i r use a s s i n g l e n a t e r i a l s . This lead/gold e l e c t r o c a t a l y s t h a s subsequently

been t e s t e d by NASA-Lewis Research Center i n conp le te , flowing, redox b a t t e r i e s

comprising a s t a c k of s e v e r a l c e l l s . A l a r g e improvement i n t h e b a t t e r y ' s coulombic

(9) and energy e f f i c i e n c y has been observed . Tn a d d i t i o n t o the above, f i v e d i s so lved o rgan ic compounds were t e s t e d a s

p o s s i b l e hydrogen evo lu t ion i n h i b i t o r s , inc lud ing th iourea , pa lmi t i c a c i d , c e t y l

a l coho l , hexanol and n-propanol. F i n a l l y , t h e c o s t of l a r g e l o t s of t h i r t e e n common

chemicals was es t imated.

I. INTRODUCTION

The iron (~e~+/Fe~')-chromium (cr3*/cr2+) rcdox flow battery is an attractive

system for bulk energy storage applications('). The primary objective of this

program was to contribute to thc development of the redox flow battery concept

by investigating the major currently recognized or suspected problems of the

chromium electrode. These are: the solubility of reactants, the stability of

chromium(I1) ion in solution, the efficient electrocatalysis of chromium(II1)

reduction and chromium(I1) oxidation, and the cost and availability of chromium

electrolyte.

Concentrated solutions of chromium(I1) chloride and chromium(I11) chloride

in hydrochloric acid are required in order to keep redox flow system costs at a

reasonably low level(2). Only a moderate amount .f effort in determining solubi-

lities was made under a previous c~ntract'~), so that further work was required.

The long term stability of the aqueous chromium(I1) ion is of concern because

thermodynamics predict its decomposition. This is illustrated by the Pourbaix

diagram of Figure 1-1 which shows that cr2+ is unstable with respect to oxidation

at low pH. The mode of decomposition is given by:

2cr2+ + 2~' + 2cr3+ + H2

where AGO = -18.8 kcal.

The driving force for oxidation of cr2+ by dissolved oxygen is, of course,

much larger than this value. It is significant that the process represented by

equation (1) is reported to be very slow when chemically pure solutions of cr2+ are

carefully stored in the absence of air (4,8)

The fundamental reason for the attractiveness of the cr3+/cr2+ electrode is

its negative rest potential with respect to the standard hydrogen electrode over the

acid pH range of interest (Figure 1-1). For the same reason, difficulties were

anticipated in finding stable electrocatalyst materials which exhibit good activity

for chromium(II1) reduction and chromium(I1) oxidation, as well as high hydrogen

-., t

! 1 overvoltage. By stable electrocatalysts is meant immunity from corrosion (most

electrode materials oxidize near the potential range where the cr3+/cr2+ redox

couple operates). Good activity means that the equilibrium exchange current

density (io) for the reaction considered is very high, so that activation polari-

zation will not be a significant source of energy inefficiency in the redox flow

battery. A high_ hydrogen overvoltage implies a low value of io for the hydrogen

evolution reaction so that the coulombic efficiency of the battery remains high.

For example, gold had been previously shown(3) to have good activity for the

cr3+/cr2+ reaction, but its hydrogen overvoltage is low.

The cost and availability of chromium ores as they relate to redox battery

operation had been reviewed by ~ahn('). The information on the cost of large

quantities of commonly available chromium chemicals was compiled.

Based on the above information and prior work, five technical tasks were

proposed and carried out as described in the following sections.

" t I

, i - b

' . t . i

11. SOLUBILITY AND STABILITY OF AQUEOUS ACIDIC CHROMIUM(I1) CHLORIDE SOLUTIONS

A. Introduce

In a prior contract0) we were unable to prepare a CrC12 solution in 1M HC1

as concentrated as one molar starting from samples of solid CrC12 obtained from

several commercial sources, We became convinced

were not genuine CrC12. Therefore, we conducted

CrC12 solubility early in the preseqt program to

work.

0. Results

that these commercial samples

a detailed literature search on

serve as a guide to the experimental

The literature search uncovered work done by Lux and 11lman('), which

indicated that E, solution 3M in CrC12 and .2M in 2C1 can be made by dissolving pure

chromium metal in 21% HC1 (approximately 6Y). These workers reported that

chromium(I1) ions remain stable for a long time provided air (02) is excluded.

It was deemed desirable to confirm the preparative method and findings of

Lux and Illman. To this end, chromium metal obtained from Atc'mergic Chemetals r

was dissolved in several concentrations of HC1. The amount of chromium(I1) 101

prodxed was determined by potentiometric titration with standard Ce(1V) solution.

The final HC1 concentration was calculated from the initial HC1 concentration and

the amount of chromiuin(I1) produced. The results presented in Figure 11-1 confirm

that solutions up to 3M in CrC12 could be prepared in up to 2.5M HCl and establish

the suitability for redox battery operation.

Our exploratory attempts to measure the solubility of CrC12 at higher HCl con-

centrations showed that the CrC12 solubility is only 0,6M in 5.3M HC1.

The stability of chromium(I1) chloride solutions was evaluated by comparison

of the amount of chromiua(1I) ion present in freshly prepared solutL%~s with that in

aged solutions. The results are given in Table 11-1. The small amount of decompo-

sition observed on long standing is probably due to small intrusion of air. The

thermodynamically feasible direct oxidation of H+ by cr2+ is kinetically inhibited.

In any event , the observed amount of decomposition is smal l enough t o be

manageable on the redox flow system l e v e l .

Table 11-1 a l s o g i v e s the r e s u l t s of t e s t s on t h e e f f e c t of cd2+ and Pb 2+

ions on t h e decomposition of cr2+ ions . These experiments i n d i c h t e no e f f e c t of

2-:. e i t h e r ions on decomposition of Cr ions. However, i n t h e c a s e of pb2+ we

no t i ced t h e presence of lead metal a t t h e t ~ t t o m of t h e r c a c t i o n chamber, c o n s i s t e n t

2+ with the thermodynamically favored reduct ion of Pb by cr2+ (Figure 11-3). Since

t h e t o t a l concen t ra t ion of pb2+ is very s n a l l r e l a t i v e t o cr2+, the r e s u l t s of

chemical a n a l y s i s a s repor ted i n Table 11-1 a r e unazfected by pb2+ wi th in exper i -

mental p rec i s ion .

Knowledg? gained about the p r o ~ e r t i e s of gold and lead e l e c t r o d e s (v ide i n f r a )

enabled us t o dev i se a s u i t a b l e v o l t m e t r y experiment t o determine i f PbC12 was

p resen t i n our C r C 1 2 s o l u t i o n i n g r e a t e r than t r a c e q u a n t i t i e s . A sample of C r C 1 2

s o l u t i o n was separa ted from t h e p r e c i p i t a t e d s o l i d m a t e r i a l and added t o an e l e c t r o -

chemical h a l f - c e l l a l r eady con ta in ing C r C 1 3 i n HC1. Cyclic voltammetry of t h i s

s o l u t i u n a t a gold f o i l e l e c t r o d e i s i l l u s t r a t e d i n Figure 11-2. Curve A e x h i b i t s

H2 evolut ion a s expected i f no PbC12 is presen t i n s o l u t i o n . A s a check, the

- 3 s o l u t i o n was made 10 Y i n PbC12 and f u r t h e r sweem were made. Curves R-1 and B-2

show t h e development of a l imit in: , c u r r e n t due t o cr3+ reduct ion i n p l a c e of H +

reduction. This i s a consequence of the ca thod ic d e ~ o s i t i o n of lead.

TARLE 11-1

Decomposition of Chromium(I1) Chloride

4 Days 5 Days 7 Days 9 Days 25 Days Original Solution Later Later Later Later Later - - -

OSolutrun r l

0 Sulut~un 8 2

O S u l u t w n # 3

A Solution r4

Data o f Lux 6 Illun

Figure 11-1

A W I C CURRENT -- / / ~ s t Sweep

Cold Foil Electrode

A -- IN HCl O.OW Cr

0.29M ~ r * ' (~b*'?)

I Sweep Hate: 0.05 Volts/ain c I U

Pot ,nr~al L d l r s r , 2CE ,Figure 11-2

7

Cd2+ + ?e - Cd. (cd2+) lo-%, c,, - -0.403 + 0.0295 lo# (Cd2+)

Figure 11-3 Thermodynamic Stability of pb/pb2+ and cd/cd2+ Couples.

111. SOLUBILITY AND STABILITY OF AQUEOUS ACIDIC CHROMIUM(II1) CHLORIDE SOLUTIONS

A. Introduction

There was general agreement that acidic CrC13 solutions are soluble and

stable enough for redox battery application. It was of interest, however, to

obtain information on the equilibrium solubility of CrC13 in various concentrations

of hydrochloric acid.

B. Experimental Results

Saturated solutions of CrC13 were prepared by dissolving reagent grade

chromic chloride hexahydrate in various concentrations of hydrochloric acid and

allowing the supernatant solution to remain in contact with excess solid material

at room temperature (25O~) for several days. The solution was then separated from

solid and analyzed for chromium using titration and spectrophotometric methods.

Results of chromium(II1) chloride solubility measurements are given in Figure 111-1.

In the HC1 concentration range of these experiments, the solubility of chromium(II1)

chloride decreases with increasing HC1 concentration.

I V . ELECTRODE MATERIAL EVALUATION

A. In t roduc t ion

Our o b j e c t i v e i n t h i s por t ion of t h e program was t o a c q u i r e enough

knowledge about t h e e l e c t r o c a t a l y s i s of t h e cr3+/cr2+ redox couple t o enable us

t o recommend s u i t a b l e e l e c t r o c a t a l y s t s f o r f u l l c e l l t e s t i n g a t NASA-Lewis Research

Center. This t a s k was accomplished by screening a number of cand ida te e l e c t r o -

c a t a l y s t m a t e r i a l s . The candidates i n v e s t i g a t e d included meta l s and meta l lo id

m a t e r i a l s , a l l o y s , Teflon-bonded m a t e r i a l s and m a t e r i a l s p l ~ t e d on var ious s u b s t r a t e s .

S u i t a y l e e l e c t r o c a t a l y s t s should meet a l l of t h e fol lowing c r i t e r i a :

(1) good a c t i v i t y f o r cr3+ reduct ion;

(2) good a c t i v i t y f o r cr2+ oxida t ion ;

(3) h igh hydrogen overvol tage; and

(4) good r e s i s t a n c e t o anodic corros ion.

To screen p o t e n t i a l l y s u i t a b l e e l e c t r o c a t a l y s t s , we devised a s imple t e s t

procedure t h a t cons i s ted of s e l e c t i v e slow scan r a t e l i n e a r sweep voltammetry.

This procedure provided d i sc r imina to ry information f o r t h e above mentioned four

c r i t e r i a . Most of t h e m a t e r i a l s t e s t e d were found u n s a t i s f a c t o r y on one o r more

counts. This required ob ta in ing o r prepar ing and t e s t i n g of a l a r g e number of

metals , compounds and a l l o y s i n bulk form, supported and composite s t r u c t u r e s .

B. Screening Approach

The screening was conducted i n t h e following s t e p s :

Step 1. The hydrogen evo lu t ion r e a c t i o n of each e l e c t r o c a t a l y s t was

measured by performing sweeps i n 1N HC1. A t y p i c a l sweep begins a t some r e l a t i v e l y

p o s i t i v e p o t e n t i a l where only a small r e s i d u a l cur ren t is observed and progresses

toward more nega t ive p o t e n t i a l s where cathodic cur ren t due t o hydrogen evo lu t ion is

eventual ly observed. The c u r r e n t w i l l r i s e a s t h e sweep i s continued, al though no

l i m i t i n g c u r r e n t w i l l be achieved. The hydrogen overvol tage of t h e

d i f f e r e n t e l e c t r o d e s t e s t e d was compared by no t ing t h e p o t e n t i a l a t which t h e

cathodic c u r r e n t f i r s t exceeds t h e a r b i t r a r i l y chosen value of 1 mA. This

cu to f f p o t e n t i a l was then compared wi th t h e open c i r c u i t p o t e n t i a l of t h e

cr3+/cr2+ redox couple.

Step 2. The a c t i v i t y of e l e c t r o c a t a l y s t s f o r r educ t ion of cr3+ was

determined i n an experiment i d e n t i c a l w i t h t h a t performed i n S tep ( I ) , except

t h a t t h e e l e c t r o l y t e was made .1M i n CrC13. The t o t a l Faradaic c u r r e n t observed

i n t h e s e experiments is usua l ly t h e sum of t h e c u r r e n t s due t o reduct ion of C r 3+

and t h e evo lu t ion of H2. I n a few cases t h e r e is a con t r ibu t ion from an anodic

corros ion process. I n one c a s e (bismuth) t h e r e i s a con t r ibu t ion from a

cathodic cor ros ion process. A w e l l def ined l i m i t i n g cur ren t due t o cr3+ reduct ion

was observed when t h e c u r r e n t s due t o competing processes were r e l a t i v e l y low.

Such cases were judged t o have f a i r a c t i v i t y . An example i s Ag-Hg ( ~ i g u r e IV-13).

I f , i n a d d i t i o n t o t h e above, t h e shape of t h e cur ren t -po ten t i a l curve i n d i c a t e d

low p o l a r i z a t i o n , t h e e l e c t r o d e m a t e r i a l was judged t o have 500d a c t i v i t y f o r

cr3+ reduct ion. An exapple i s Pb (Figure IV-3). I n some cases no l i m i t i n g c u r r e n t

due t o cr3+ reduc t ion was observed. The e l e c t r o d e m a t e r i a l s i n quest ion were then

categor ized a s no t a c t i v e . An example is B 4 C a s shown by Figure IV-17.

Step 3. For completeness, anodic cor ros ion s t u d i e s i n 1 M H C 1 were done on

a l l t h e m a t e r i a l s t e s t e d i n S teps (1) and (2). The s u i t a b i l i t y of t h e candidates

screened was evaluated on t h e b a s i s of t h e p o t e n t i a l a t which anodic c u r r e n t f i r s t

exceeds t h e a r b i t r a r i l y chosen va lue of la. This cu to f f p o t e n t i a l was then

compared wi th t h e open c i r c u i t p o t e n t i a l of t h e cr3+/cr2+ redox couple.

Step 4. Those m a t e r i a l s which s t i l l appeared promising a f t e r s t e p s ( I ) ,

(2) and ( 3 ) , were inves t iga ted f o r t h e i r a c t i v i t y towards the ox ida t ion of cr2+ 2+ using a s o l u t i o n 1 N i n HC1 and .09M i n C r . A l l t he m a t e r i a l s t e s t e d i n t h i s

s t e p showed a w e l l def ined l i m i t i n g c u r r e n t due t o cr2+ ox ida t ion and were,

t h e r e f o r e , judged t o have good a c t i v i t y .

Step 5. More d e t a i l e d i n v e s t i g a t i o n s were undertaken on s e l e c t e d promising

e l e c t r o d e m a t e r i a l s emerging from t h e f i r s t four s c r e e n i n g s t e p s .

C. Resu l t s

The i d e n t i t y of each candidate e l e c t r o d e , a cross-reference t o

Figures I V ( see below), i t s phys ica l c h a r a c t e r i s t i c s , and a b r i e f sumnary of our

conclusions a s t o i ts performance ( e . ~ . good a c t i v i t y bu t low hydrogen overvol tage

and poor cor ros ion behavior) i n the four screening s t e p s considered a r e presented

i n Table IV-1.

Resul ts of t h e voltammetry experiments on which sc reen ing was based a r e

presented i n Figures IV-1 t o IV-44 which a l s o inc lude s i m p l i f i e d Pourbaix diagrams.

I n s e r t s and/or cap t ions i n d i c a t e t h e cond i t ions under which an experiment was run.

Each f i g u r e is a l s o marked wi th one o r more of t h e sc reen ing co?e numbers (1, 2, 3,

4 and 5) i n o rder t o i n d i c a t e t h e type of information shown. These sc reen ing code

numbers correspond t o t h e screening s t e p s descr ibed i n d e t a i l i n Sec t ion 1 V . B . of

t h i s r epor t .

D . Recommendat ions

Based on t h e f i r s t four screening s t e p s , we s e l e c t e d a s t h e most promising

e l e c t r o c a t a l y s t s the combination e l e c t r o d e s Ag/Pb, Cu/Pb and Au/Pb. Fur ther

i n v e s t i g a t i o n (screening Step 5) i n d i c a t e s t h a t a p o s s i b l e problem with Ag is the

i r r e v e r s i b l e formation of an oxide so t h a t t h e e l e c t r o d e may l o s e i ts good performance

on repeated cycling. Both t h e Au/Pb and Cu/Pb e l e c t r o d e were found t o have good

r e s i s t a n c e t o anodic corros ion, but Au/Pb is even more r e s i s t a n c e than CuIPb. Of

course, Au is t h e most expensive m a t e r i a l , but i t is f e l t t h a t loadings a t microgram

l e v e l s w i l l be needed t o f a b r i c a t e p r a c t i c a l e lec t rodes . Therefore, i n s p i t e of a

small c o s t inc rease , a s s w e d r e l i a b i l i t y warrants t h e s e l e c t i o n of t h e Au/Pb

e i e c t r o d e a s most s u i t a b l e fo r f u l l c e l l t e s t i ~ l g a t NASA-Lewis Research Center.

The i n v e s t i g a t i o n of d i f f e r e n t types of Au/Pb c a t a l y s t s , including layered

( sequen t ia l depos i t ion of the two m e t a l s ) , codeposited and underpo ten t ia l p l a t e d

was begun i n t h i s progrcn. Here codeposi t ion is used i n i t s most general sense -- a

TABLE IV-1

Po

i 1

Wi

re

(In

go

ld)

Low

Low

Fa

ir

Fa

ir

Fo

il

Ver

a, Good

Ver

y fbod

Lo

u

rood

Fa

ir

Ro

d,

wra

pp

ed

wit

h T

efl

on

C

d

tap

e to

ex

po

se e

nd

(V

en

tro

n)

lliq

h

c (v

itre

ou

s c

ar-)

R

ou

qh

enrd

. ro

tati

ng

dis

k

Fa

ir

ele

ctr

od

e t

ip

(f

leck

man

) Good

C

(gra

ph

ite

)

C

(qra

oh

ite

)

Wen

tro

n

rod

, w

rap

ped

P

oo

r w

ith

Te

flo

n t

ap

e

to

ex

po

se

en

d

(Ult

ra C

arb

on

) ro

d,

Fa

ir to

Cb

d

Po

or

wra

pp

ed

wit

h T

efl

on

ta

pe

to

ex

po

se e

nd

II ig

h

Low

Ver

y

Po

or

Fa

ir

Fo

i 1

(Alf

a)

Fo

il

Hig

h

Ver

y

Po

or

Hydrogen

Phy8ical

cathodic

Anodic

evolution

Corrosion

Figure

Ele

ctr

c de

Charactaristics

Performance

Performance

Overvoltage

Characteristics

No.

----

----

----

----

.---

----

----

----

----

----

-..-

----

-.--

----

----

----

----

----

----

----

----

----

----

----

----

----

----

----

----

----

---+

----

----

----

----

----

----

--

Sh

sic

(Fisher) Poil

(alfa) Supported Poil

(Ultra Cubon) Rod,

wrapped with T8flon tap

to expose end only

(Ventron) Poil

Wire

very

Coo

d

Pair

Poor t

o Pair

Fair

Not

active

mt active

Not

active

Etched A

g Poi1 dipped

Fair

in Hg

(high polarization)

cu Foil left in Iig

Fair

(high polarization)

~t

ch

ed

Cu w

t

dipped

Fair

in 2\ H~(ND~)~

' H20

(high polarization)

wire solder

Coo

d

High

nedira

nigh

nediu

Lcw

very Lcw

Very Lcw

poor

Fair

Very High

Very High

Fair

nigh

High

IV-1,

-2.

-3.

-14

1V-6

IV-16

IV-1,

-2

IV-1

IV- 1

IV-1

IV-12. -13

1V-12. -13

Cd

Cd/h

Cu/Pb

Pb

8.

Graphite Substrate

r(u/Pb

Au/In

In

m

C.

Other Substrates

Aq

fPb plated)

hq IPb plated)

Au (Pb plated)

d2

added to electrolyte

~d*~,

added

to

electrolyte

a+,

m+2 .d

dd to

electrolyte (10 rps)

pb+2 a

dded

to electrolyte

Rad

, w

rapped

with Teflon

tape to expose end

AU

+~

, 1n

+2 addui to

electrolyte

pb+2 a to electrolyte

Ag foil electrode, Pb*2

8dded

to electrolyte

Ag wire electrode. w*~

added

to electrolyte

Au

mil, pb42

added

to

electrolyte

c" Foil, pb+2 a

ddad

to

electrolyte

bod

Goo

d

Coo

d

Cood

Goo

d

Goo

d

Coad

Good

Gaad

Good

Cood

Good

Good

Goo

d

Cmd

High

High

nigh

High

High

nigh

High

High

nigh

High

Mi

u

High

simultaneous electrodeposition of Au and Pb from a common solution. In such event,

an intimately dispersed mixture of Au and Pb will be deposited; and under some

favorable, yet undefined, conditions, either a solid solution of Au and Pb or

goldllead intermetallics may be formed. Further work on this aspect of electro-

catalysis is anticipated under NASA Contract No. DEN3-97.

Screening Code 1

.

Several Pure K :.als

Figure IV-1

Screeninn Code 2

Potentbrl. Yoltr v s SCE

19

Screening Code 1,2,3 '&AD POfL ELECI'RODE

0

. - . . . IN ICI after ~ r * f

Potoatial. Volts vs. Se6 Fl'gure IV-3

Figure IV-4

Screeninn Code 1.2

-0.4 -0 .6

Potential, V o l t s vs. SCE

Figure IV-5 Potontid, Volts vr. sa

mrnov~ ~~a Screening Code 1,2,3

- - - - - - - - - - - - -.

-- In acl

-LM IICI + 0.111 ~ r + ~

. . . ..IN BCI attar ~ r + 3 nr..p

- 0.2 - 0.1 - 0.6 - 0.8 - 1.0 - 1.2

U P ~ ~ P P P I P I ~ Screening Code 1,2,3 I I I I I I I I I I

J

' 9 "

Figure IV-7 ~0tmti.l. volt* VS. sa

b09 After activation at +l.Z volts

- - . . . .Aftor actiwation at +.4 volts

Figure IV-8

Screening Code 1

I I I I

Figure IV-12 ~ o t m t l a l . Vol tsvs SCE

Screening Code 2 I I I I I I *-I

. . . . . . . CuHl ---a PbSn

Solution - UO BCL + O . l W CrCl,

/--- -., #' '8

I '. --.

i i

Figure IV-13 POTENTIAL. VGLTS vs. SCE

m I I I I I I I - - - I)

II

I

- - -

. . . . . . CulPb (90110)

-0.4 -0.6 -0.8 -1.0 -1.2

Figure IV-14 r~te~tirt. volts VS. SCE

C o P P W w W/lO EUcTnoOe Screening Code 1,2,3

- .* - .. *'/

' 0 0 '

10 - 3 -

: / - / -

- -

1.0 = 1 - - - - - - 0.S - - - - -

-

-0.2 -0.4 -0.6 -0.8 -1 .0

Figure I V - 1 5 Poccnt i a l , V vr. SCE

26

S c e n Code 1.2 I I I I I I -

..I -

.I

-0.8 -1.0 -1.2

Potential, Volts vr SCE

Screening Code 1.2

Figure IV-18 potential. Volts VS.

Screening Code 1,2

100 - I I I I I I I I I ,' m

*I - - - - - -

"a -10 = I - - -

d/' : I - - -

--- 1N HC1 1.0 -

-0.6 -1.0 -1 .2 -1.0

Potent ia l . Vol t r vm. SCE

Figure IV-19

2 8

Screening Code 2 I

.I) - m

I

n

10-

1.0 - -

I 1, I I I I I I I I I I -0. -0.6 -0.6 -1.0 -1.2 -1.4

? o t a t h l . V o l u w. BCB Figure IV-22

Screening Code 2

Figure I V - 2 3

Figure IV-24 lot.ati.l* Volt. VB. 0ce

Screening Code 1.2 . I I

m

m

m

I)

- 10 -

"8 = 3 =

1. D

.I - - - - L .I

I, -

- I I I I I I I- I I I

4 . 4 -0.6 -0.0 -1.0 s b . 2 -1.4

- -

ElrctrnJe - .\u/?b LodwwsItd E lmrodw (on i:-rocb)

Elcctrolgtu- IN IKI O.W cr*' U.IJW ~ r * '

k e p lrtr - 0.0) Volts/r ln -I.-- F l ~ t #WWPI

ma-.-- S a d sweeps - fhlrd wreps

Figure IV-26 Potwnthl, i d t s vs SCE

r o t o a t i l l , vol ts v s SCL Figure IV-27

-0.4

Figure IV-26

Au-P

b Coieposited Electrode (on graphite rod)

Both sweeps obtained in first

solution

---

Second Solution

1N HCl

.* 0.1M CrC13

--*-*

- TLird

Solution

1N HC1

+ 0.1M CrC13

---n

Fourth S

olutim

13 HC1

+ 0.1M CrC13

-0.8

-1.0

Potential, Volts vs SCE

Screening Code 1,2

- - - - - - - - -

- 0 . 4 - 0 . 6 - 0 . s - 1 . 0 - 1 . 2 - 1 . 4

Fotu l l t l : l l , t'a3lt.i v s \CI

Figure IV-30 34

Screening Code 2 . 4 104 I i I I I I I I

-

"& - - -

3 - - - - h

- 'J

- - C

H -

P - -

E b

- - 8 Silver Wire Electrode

1.0 - Solution: lN HCl

- 0. IW cr" - - - - -

0.1 I I I I f I I I -0.4 -0.6 -0.8 -i.O

FIGURE IV-31 Potential, volts vs. SCE

Screening Code 1 , 2 I I I I I I I 1 I

Figure IV-32 Potential, koltr vr SCE

35

Scr

een1

ng C

odes

2,4

I I

I I

I I

I I - I

ANOD

IC CU

RREN

T

I Swe

ep Rate:

0.0

5 Volts/min

-- .

Fig

ure

IV-33

Pote

ntia

l Vo

lts Vs SCE

Screening Code 1 ,2 I I I I I I I I I I I

mtati.r. volt. v.. SCE Figure IV-34

Screening Code 2

100

- I I I I I I I I I-

- - - - - - - - - - - - - - - / - 40-

/-

- Vitreous Carbon Electrode -- solution IN HCI + 10-GM ~b*'

0 .1 w ~ r * 3

- sweep from 0.45V t o -1.SSV

sweep from - 1.33V t o -0.4SV

Sweep Rate - O.OSV/min.

- - . I

- a

- - - - - - a

a

I I I I I I I I I I , -

-0 .6 -0 .8 -1.0 - 1 . 2 1.4 Potmti?l , Volts vr SCE

Figure IV-35

37

Screening Code 2

Vitrrws Cubon Electrode -- Solution: IN HC1 + 1.W W 1 3

+ lo-% d3 + 10-311

w+* Svnp Rate: 0.0s volts/.in

Rotation Rate: -0 RPS

- 4 RPS

-L -6 -0: 8 -1.0 -1.2 1.4

Potential, Volts vs SCE

Figure IV-36

Screening Code 2 ,4 - 1

Figure IV-37 Po..rtitial, V o l t ? vs SCE

Screening Code 1,2 I I 1 I I I I I I I I

-.- Potential, Volts vs SCE

Figure IV-38

Screening Code 1,2

Vitreous Carbon Electrode ~ 0 ~ ~ ~ 0 R P S S n r p

rate 0.05 Voltr/min swoeping .ore positive

-1N HCl

--IN HCI + ~ o - ~ M d2 *3

--IN HCl Cd 0.1~ ~ r * 3

---1N HCl + 10.~~1 ~d*'

0.1M cre3 + 10-'M A I J * ~

I I I 1 1 I I I 1 I -0.b -0.8 -1.0 -1.2 -1.4

Potential, Volts vr SCE

Figure IV-39

Screening Code 2 ,3 ,4

L I I I I I I I I I I I

Figure IV-40 ~ot-tid, volt* tn. scc

Screening Code 2 -4

I I I I I I I ' I I I I

VITREOUS CAMOW ELECTRODE

1I HC1 + .WU ~ r + ~ -09 !I ~ r + l + 10% A(+' + 10% Pb"

(sump rate = O.1VImin)

-- Sweap from -1.2SV to -0.1V v*. SCC - Sweep from -0.1V to -1.2SV

I I I 1 I I I I I I I I -0,4 -0.6 -0.8 -1.0 -1.2

Potential. Volt* vr. SCE

Figure IV-41

I I t I I I I I -0.6 -0.8 -1.0 -1.2. -1.4

Potential, Volts vS SCE

Figure IV-42

Screening Code 2,4

I ' I I I I I I I I - I

1000 , .I - 0 - - - 0 - ANODIC CURREWT CATHOOIC CURReM - -- - .I - I - .I

- .I - - -

~rsphite Rod Electrode - Sweep Rate: 0.05 voltr/min - Solution: 1N HC1 0.0W

~r*' 0.09~ ~r+' I

- I O - ~ W h+3 + s x I

lo-% --First Sweep

10 = ? I S e c o n d Sweep I -

I - X I .I - I

'2 9 U - \

N - b - 1 I i I 1 I I I I I I ,

-0.6 4.8 -1.0 -1.: -0.4 Potential, Voltr vr SCE

~ i ~ u r e IV-43 4 1

i I Screening Codes 2, 4 I I I 1 I-*

j h - I I I I I I I

I

! -

VITREOUS CARBON ELECTRODE

1N HCl + 10% + 10'~ A U + ~

+ 0.09H ~ r + 3 + 0.09K ~ r + ~

- Sweep Rate - 0.05 V/ain - Sweep rate = 0.025 Vldn

Potential. Volcr vs. SCE

Figure IV-44 -

V. EVALUATION OF DISSOLVED ORGANIC COMPOUNDS AS HYDROGEN EVOLUTION INHIBITORS

A. Introduct ion

Five organic compounds mentioned by ~ e t t e r ( ( ) a s hydrogen evolution

inh ib i to re were t e s t ed wi th the objec t ive of thereby increasing the number of

s u i t e b l e e l ec t roca t a lys t s . The organic i n h i b i t o r s considered were thiourea,

palmit ic ac id , c e t y l alcohol, hexanol and n-propanol.

B. Experimental Resul ts

The f i r s t mater ia l t o be t e s t ed was thiourea. Solutions of ~o-'M thiourea

(x 1, 2, 3, 4, 5) i n I N HC1 were prepared. .Is shown i n Figure V-1, concentrations

above lo1% enhanced hydrogen evolution. Solutions containing and 10- '~

thiourea i n lN HC1 and 0.1M C r C 1 3 were tes ted . The addi t ion of thiourea a t

these l eve l s had v i r t u a l l y no e f f e c t on the gold electrode.

The work reported i n t he l i t e r a t u r e on palmit ic acid was performed i n 6N HC1.

In attempting t o prepare a 10-%l so lu t ion , we found palmit ic acid (as i n the case

with most a l i p h a t i c carboxi l ic ac ids) t o be v i r t u a l l y insoluble. Upon t e s t i n g the

solut ion, which was l e s s than lo-?# palmit ic ac id , we found t h a t hydrogen evolution

was only s l i g h t l y enhanced a s shown i n Figure V-2.

We attempted t o study the e f f e c t of c e t y l a lcohol addi t ion t o a so lu t ion of

0 .m CrC13 i n 1 N HC1 containing PbCIZ and AuC13. The purpose was t o s e e i f the

onset of H2 evolution on the gold-lead plated e lec t rode would be a l t e r ed by the

presence of t he c e t y l alcohol. We found t h a t c e t y l alcohol did not measurably

diesolve. Consequently, the current-potent ial curves showed no d iscern ib le change.

The c i t e d use of t h i s i n h i b i t o r was i n 6N HC1, and we suspect t h a t a s t ronger acid

than we employed is required t o so lub i l i ze c e t y l alcohol.

A s imi la r experiment was conducted with hexanol a s was performed with c e t y l

alcohol. The e o l u b i l i t y of hexanol a l s o appears t o be q u i t e low. When % 10%

hexanol vaa added, no e f f e c t on H2 evolution was observed. 10-%¶ hexannl a l s o

d id not show any e f f e c t . Upon addi t ion t o the c e l l of an amount of hexanol which

would exceed i ts s o l u b i l i t y , i t was noted t h a t the e lec t rode appeared t o be

poironed, and the chromic ion reaction was ouppreseed.

Finally, m experiment was performed to determine whether the addition of

n-propanol would have any effect on the hydrogen evolution characteristics of

a carbon electrode plated with gold and lead and acting as a catalyst for chromic

ion reduction. Linear potential sweeps were applied at a rate of 100 mV/dn

and limiting currents were observed. The rolutions that were tested were

1N HCl + 10% pb2+ + 10'~ A? + 0.1M cr3+ and the same eolution with additions of 10-%l n-propanol and 10-51 n-propanol. We did not observe any change in the

hydrogen evolution characterie tics of the electrode with the addition of n-.propa*iol.

With this experiment the eolution used was freshly prepared, and there did

seem to be a problem with plating occurring. We have noticed this before with

plating solutions and feel that more investigation into plating could be quite

benef icial for future study.

C. Recommendations

Based on the work done eo far, a useful hydrogen evolution inhibitor has

yet to be identified.

-&I. b

Figure V 2

V I . PRELIMINARY COST ANALYSIS OF CHROMIUM CHEMICALS

A. Discuerion

Some preliminary cost information war obtained under t h i s contract.

The resu l t8 a r e presented i n Table I. The (dollar per pound) pr ice of la rge

lo to of the variour chromium chemicale l i e t e d wre obtained from Chemical Marketin&

Reporter. The pr ice of chromium metal was quoted by Union Carbide Corp.,

Metalr Div., Marietta, Ohio. The prices were converted t o the un i t r do l l a r per

pound of chromium i n order t o f a c i l i t a t e compariaonr.

TABLE VI-1

COST OF COEPI)N CHROMIUM COMPOUNDS (large quntitieb)

Potassium Chropute K2Cr04 (194.2)

Potm~sium Chromium Sulfate KCr(S04)2 ' 12H20 (499.41) Potash Chromium Alum

Sodium Chromate NapCrO4 (252.05) Tetrahydrate

Sodium Dichromrte Na2Cr207 2H20 (298;

Lead Compounds:

Chrome Ysllw PbCr04 (323.18)

Chrome Orange Pb2(0H)2 Cr04 (564.39)

Chrome Green Fe4(Fe(CN)6) ' PbCr04 (758)

Chromium Fluoride CrF3 (108.99)

Chromim Oxide Green Cr203 (151.99)

Zinc Chromate ZnCr04 (233.36)

Chromium Cr (52.00)

V I I . REPFRENCES

1. Redox P r o j e c t Of f ice , NASA-Lewis Research Center, Cleveland, Ohio, "Redox

Electrochemical Storage Systems f o r Storage Appl icat ions ," January 1979.

2. M. Warshay and L. 0 . Wright, "Cost and S i z e Est imates f o r an Electrochemical

Bulk Energy Storage Concept," Ed. J. B. Berkowitz and H. P. Silverman,

Electrochemical Socie ty , 1976.

3. J. Giner, L. Swette and K. C a h i l l , "Screening of Redox Couples and E lec t rode

Materials ," Contract No. NAS3-19760, f o r NASA-Lewis Research Center,

September 1976.

4. A. Adin and A. G. Sykes, "The Kine t i cs of t h e Oxidation of Europium(I1) wi th

Vanadium(II1) and Chromium(I11) i n Aqueous P e r c h l o r i c Acid Solut ions ,"

J. Chem. Soc., - A, 1230 (1966).

5. R. F. Gahn, NASA TM-78995, "Supply of Reactants f o r Redox Bulk Energy

Storage Systems," September 1978.

6. K. J. Ve t te r , Electrochemi_cal Kine t i cs , Academic Press , 1967.

7. J. Giner and K. Cahi l l . "Cata lys t Surfaces f o r t h e Chromous-Chromic Redox

Couple" (pa ten t pending) . 8. H. Lux and J. I l lman, Zur Ke-ntis d e r Chrom(I1) - Salze und des Chrom(I1)-

oxyds, I," Chem. Ber., 91, 2143 (1958).

9. L. H. T h a l l e r , Redox Flow C e l l Energy Storage Systems," Presented a t T e r r e s t i a l

Energy Systems Conference (AIM), Orlando, Fla . , June 4-6, 1979.