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Jo ur nal o f E lectr o analy tical C hemistr y xxx (2018) xxx-xxx

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry

journal homepage: www.elsev ier.com

Electrosynthesis of hydrogen peroxide in a �lter-press �ow cell using graphite felt as

air-di�usion cathode

Tzayam Pérez , Gabriela Coria , Ignasi Sirés , José L. Nava , Agustín R. Uribe

Unive rsidad de Guanajuato, D e partamento de Ing e nie ría Química, D ivisión de Cie ncias Naturale s y Exactas, Noria Alta S/ N, C.P. 36050 Guanajuato, Guanajuato, Mexico

Departamento de Ing e nie ría Geomática e Hidráulica, Unive rsidad de Guanajuato, Av. Juáre z 77, Zona Centro, C.P. 36000 Guanajuato, Guanajuato, Mexico

Laboratori d'Ele ctroquímica de ls Mate rials i de l Medi Ambie nt, D e partament de Química Física, Facultat de Química, Unive rsitat de Barce lona, Martí i Franquès 1-11,

08028 Barce lona, Spain

A R T I C L E I N F O

Keywords:

Elec tr o sy n the sis

F il ter -p r ess cell

Gas-dif fu sio n elec tr o de

Gr aphite felt

Hy dr o gen per o x ide

Oxy gen r e duc tio n r e ac tio n

A B S T R A C T

The e lec trosyn the sis of H O via O re duc tion was fea si ble em ploy ing cheap, un mod i �ed graphitefe lt on top of car bon cloth as air -dif fu sion cath ode �t ted into an un di vided �l te r -pre ss ce ll. The ex ‐per i ments were per formed in a pre -pi lot plant with re cir cu la tion of 4 dm of 0.05 M Na SO so lu tions

at pH 3.0 upon con tin u ous air feed ing to the cath ode . The H O e lec tro gen e r a tion oc curred within

the range −0.30 } E }−0.01 V|SHE , be ing de pen dent on the mean lin ear �ow ve loc ity (u), which is

re lated to the mass trans por t of hy dro nium ions. Op ti mum con di tions achieved at E = −0.30 V|SHE

and u = 27.4 cm s yie lded 100.4 mg dm H O , with e f � ciency close to 100% and low en e rgy con ‐

sump tion.

1. Introduction

Hy dro gen per ox ide is listed as one of the 100 most im por tant

chem i cals in the world, be ing in volved in a large range of in dus trial

ap pli ca tions [1], in clud ing wa ter treat ment via ad vanced ox i da tion

processes like H O /O , H O /UVC and H O /Fe or elec tro chem i ‐

cal meth ods such as elec troox i da tion, elec tro-Fen ton and pho to ‐

elec tro-Fen ton at acidic pH ~3.0 [2]. Hy dro gena tion of alky l-9,10-

an thraquinone fol lowed by au toox i da tion in the pres ence of O is

the lead ing tech nol ogy for in dus trial syn the sis [3], al though co-gen ‐

er a tion of ex haust gas, toxic liq uid and solid waste is a ma jor con ‐

cern.

T he di rect elec tro chem i cal H O syn the sis ap pears as a much

greener method. It is less en ergy-in ten sive, es pe cially if cou pled

with re new able en ergy sources, than the chem i cal al ter na tives and

al lows min i miz ing risks and costs. Ac tu ally , it is a zero waste strat ‐

egy be cause it can be tai lored to pro duce only the re quired

amount of chem i cal just by con trol ling the elec trol y sis con di tions.

T wo main elec troly tic ap proaches have pre vailed for the two-

elec tron oxy gen re duc tion re ac tion (ORR) from air or pure O : (i)

feed ing of a gas-dif fu sion elec trode (GDE), and (ii) di rect sparg ing

into the elec troly te. T he ORR can oc cur in acidic and al ka line me ‐

dia [4]:

(1)

(2)

Car bona ceous ma te ri als act as op ti mum cath odes for H O elec ‐

tro gen er a tion ow ing to their high sta bil ity , con duc tiv ity , re sis tance,

non-tox i c ity and low cost [5]. In the �rst ap proach, GDEs are most

C o r r espo nding au tho r .

Email addre sse s: t. p er ezsegur a@ ugto . mx (T. Pér ez); g. co r iar o dr iguez@ ugto . mx (G. C o r ia); i. sir es@ ub . edu (I. Sir és); jlnm@ ugto . mx (J.L . Nav a); u r ib e@

ugto . mx (A.R. Ur ibe)

ISE Activ e Member .

h ttp s://do i.o r g/10 . 1016/ j. jelechem. 2018 . 01. 054

Receiv ed 12 December 2017; Receiv ed in r ev ised fo r m 29 Januar y 2018 ; Accep ted 30 Januar y 2018

Av ailab le o n line xxx

1572 -6657/ © 2017.

Short communication

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T. Pé re z e t al. Journal of Ele ctroanalytical Chemistry xxx (2018) xxx-xxx

usu ally pre pared from raw car bon black mixed with PT FE to im ‐

part hy dropho bic ity [6–10]. T hey y ield the high est amount of

H O thanks to both, the ex is tence of a triple phase bound ary

(T PB) and pres sur iza tion, which fa vor the two-elec tron ORR over

the HER. T his, in turn, shifts the cath ode po ten tial to more pos i tive

val ues [3]. Sig nif i cant ef forts are spent to si mul ta ne ously in crease

the ac tiv ity , se lec tiv ity and sta bil ity of elec tro cat a lysts by mod i fy ‐

ing the car bon par ti cles with quinones, Pd-Au or Co-based com ‐

pounds [11–13]. Nonethe less, GDEs pre sent some draw backs: (i)

com plex se tups needed to pre vent �ood ing; (ii) costly com mer cial

cath ode ma te ri als; and (iii) mod est me chan i cal re sis tance.

Fol low ing the sec ond ap proach, a lower H O con cen tra tion is

at tained due to lim ited sol u bil ity of gaseous O and mass trans port

lim i ta tions, which could be a dis ad van tage for some in dus trial ap pli ‐

ca tions but not for wa ter treat ment. How ever, sim pler se tups and

less strict con trol are re quired, and ma te ri als are usu ally cheaper.

T he great est H O con tents are ob tained with three-di men sional

car bons like graphite felt [14–16], retic u lated v it re ous car bon

[15], �bers [17], nan otubes [18], hi er ar chi cally porous car bon

[19] and graphene [20]. Mod i � ca tion of the setup or re ac tor has

also been ad dressed: (i) greater mass trans port of O to ward the

cath ode sur face can be pro moted with a ro tat ing cy lin der elec ‐

trode thanks to the tur bu lent regime [21], whereas (ii) su per-sat u ‐

ra tion of O can be fos tered at am bi ent pres sure with a jet aer a tor

[22] or at high pres sure re ac tors [23]. T he main draw backs of

these lat ter sys tems are the sup ply of dirty air that poi sons the

cath ode, the high me chan i cal stress un der gone by the 3D car bon or

the high cost of pres sur iza tion.

In this work, a novel ap proach has been fol lowed for the H O

elec tro gen er a tion from re duc tion of O at con stant po ten tial in

4 dm of 0.05 M Na SO so lu tions at pH 3.0 us ing a pre-pi lot �ow

plant. Com mer cial car bon cloth has been hy dropho bized and put

in con tact with raw graphite felt to con vert a 3D cath ode into an

ef � cient GDE by in creas ing the elec tro chem i cal con tact area and

fa vor ing the mass trans port in side the porous ma te r ial. Air was sup ‐

plied through a cham ber added to an un di v ided FM01-LC �ow cell.

T he cath ode po ten tial and liq uid �ow rate were eval u ated as main

op er a tion pa ra me ters.

2. Mater ials and methods

An a ly t i cal grade reagents from Sigma-Aldrich and Fer mont and

deion ized wa ter were em ployed. T he char ac ter is tics of the FM01-

LC �l ter-press re ac tor can be found else where [24]. Here, the

con ven tional un di v ided FM01-LC re ac tor, which was mod i �ed to

in clude an air cham ber (Fig. 1a), was equipped with a T i|Pt plate

an ode man u fac tured fol low ing the Pe chini method and a stain less

steel frame (4 cm height, 16 cm length, 0.30 cm thick ness) for the

elec tri cal sup ply to the cath ode. A novel GDE as cath ode was as ‐

sem bled by us ing a com mer cial car bon cloth of 64 cm area, which

was hy dropho bized with PT FE [7,25], on top of which a graphite

felt par al lelepiped (4 cm height × 16 cm length × 0.15 cm,

10–100 × 10 Ω cm elec tri cal con duc tiv ity , 651 cm cm vol ‐

u met ric area, 0.97 poros ity ) was placed with out any glue. T he

thick ness of the strands was 19 μm. It is im por tant to re mark that

the pres sure ex erted by the �l ter-press and the tur bu lence pro ‐

mot ers was high enough so as to en sure a rea son able elec tri cal con ‐

tact. T he cloth and graphite felt were from ROOE Group. T he GDE

was in con tact with the air cham ber fed with at mos pheric air (De ‐

walt D55168 air com pres sor) un der over pres sure of 0.7 bar reg u ‐

lated with a gauge back-pres sure to elec tro gen er ate H O . T wo

plas tic routed meshes (pro mot ers type D) were used [24], one be ‐

tween the an ode and cath ode and the other into the air cham ber.

T o pro v ide a con stant liq uid �ow through the plant un der re cir cu ‐

la tion batch mode, a mag netic pump and a �owme ter were in ‐

stalled (Fig. 1b). All tri als were made at con stant cath ode po ten tial

pro v ided by a BK Pre ci sion 1621A power source, which di rectly

dis played the po ten tial dif fer ence be tween the an ode and cath ode

(E ). T he elec trode po ten tials were mea sured against a sat u rated

sul fate ref er ence elec trode (SSE), in serted into a Lug gin cap il lary ,

us ing an Ag i lent 34,410 high im ped ance mul ti me ter. All elec trode

po ten tials are re ferred to stan dard hy dro gen elec trode (SHE).

In our pre v i ous study [15], the re duc tion of dis solved O for

mass-trans port con trolled H O elec tro gen er a tion at graphite felt in

sul fate oc curred within the do main −0.40 < E < −0.10 V|SHE,

high light ing that at more neg a tive po ten tial the HER oc curs to

much larger ex tent. In the pre sent work, dif fer ent cath ode po ten ‐

tials be tween −0.30 } E } −0.01 V|SHE and vol u met ric �ow rates

(q, in cm s ) have been tested. T he mean lin ear �ow ve loc ity (u,

in cm s ) is de ter mined as q/A ε, be ing A the trans verse area

wherein the elec troly te �ows (A = BS, where B and S are the

thick ness and width of the chan nel, in cm), and ε is the over all

voidage (di men sion less). T he H O con tent was de ter mined on a

Perkin-Elmer spec tropho tome ter from light ab sorp tion at

λ = 408 nm [26].

3. Results and discussion

Fig. 2a shows the H O ac cu mu la tion as a func tion of the elec ‐

trol y sis time un der po ten tio sta tic con di tions at dif fer ent ap plied ca ‐

thodic po ten tials, namely −0.01, −0.15, −0.20 and −0.30 V|SHE

at vol u met ric �ow rate of 3.0 dm min (27.4 cm s ) for 180 min.

A greater ac cu mu la tion was achieved at a given time as the ap ‐

plied po ten tial was in creased up to −0.30 V|SHE, which is re lated

to its quicker pro duc tion from Eq. (1) re sult ing in cur rent val ues

be tween 0.13 and 0.20 A along the elec trol y sis. T he ap pli ca tion of

more neg a tive po ten tial val ues does not en hance the H O pro duc ‐

tion, which can be ex plained by the oc cur rence of the par a sitic

HER [15].

Car bona ceous ma te ri als are suit able be cause the ex tent of HER

at their sur face is min i mized, as con �rmed up to the op ti mal value

of −0.30 V|SHE. T his par a sitic re ac tion is detri men tal, since it

causes a de crease in the cur rent ef � ciency of the ORR to y ield

H O . Fig. 2b and c shows the cor re spond ing cur rent ef � ciency and

en ergy con sump tion, which were eval u ated as de scribed else where

[4]. T he curves of Fig. 2b are in good agree ment with the trends

shown in Fig. 2a, with cur rent ef � ciency in creas ing at more neg a ‐

tive po ten tials up to −0.30 V|SHE, be cause from that po ten tial the

re ac tion (1) is fa vored [15]. At that po ten tial, the ef � ciency

reached a max i mum of 88% dur ing the �rst hour of elec trol y sis;

where upon it de cayed down to 67% at 180 min. At more neg a tive

po ten tials than −0.30 V|SHE, the HER be comes more rel e vant.

T he fact that the ef � ciency be comes lower than 100%, even be ‐

tween −0.01 and −0.30 V|SHE where the HER is min i mized, can

be due partly to the po ten tial dis tri b u tion in the cath ode ma trix be ‐

cause part of the cur rent is lost by ca pac i tive phe nom ena as a re ‐

sult of its high vol u met ric area [27]. In ad di tion, the use of an un di ‐

v ided cell fa vors the grad ual ox i da tion of H O to O at the an ode

sur face, which is ac com pa nied by the for ma tion of HO as an in ‐

ter me di ate [4]. On the other hand, the ef � ciency loss as the elec ‐

trol y ses are pro longed can be due to the pro mo tion of ad di tional

par a sitic re ac tions upon so lu tion re cir cu la tion. First, the elec tro ‐

chem i cal re duc tion at the cath ode sur face from Eq. (3) can oc cur

to a cer tain ex tent, de spite the fact that car bona ceous ma te ri als

min i mize this phe nom e non. Sec ond, dis pro por tion a tion of H O in

the bulk via Eq. (4) also con tributes to an even tual lower ac cu mu la

2

2 2

2 2

2

2 2

2

2

2 2

2 3

2 4

2

−3 −1 −1 2 −3

2 2

cell

2

2 2

3 −1

−1 T T

T

2 2

2 2

3 −1 −1

2 2

2 2

2 2 2

2

2 2

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F ig . 1. (a) Ex p lo ded v iew dr aw ing o f the no v el F M01-L C �l ter -p r ess r e ac to r . (b ) Sketch o f the r e c ir cu la tio n p r e-p i lo t p lan t.

tion than ex pected [4].

(3)

(4)

As can be ob served in Fig. 2c, higher cur rent ef � cien cies are

cor re lated with lower en ergy con sump tions, be cause of the grad ual

min i miza tion of par a sitic HER. T here fore, for the best con di tion

(i .e ., E = −0.30 V vs . SHE), the en ergy con sump tion at the end of

elec trol y sis was 5.4 kWh (kg H O ) .

A sec ond set of tri als was per formed to as sess the ef fect of the

liq uid �ow rate. It is well known that any elec tro chem i cal process

that is lim ited by mass trans port could be po ten tially im proved by

en hanc ing the hy dro dy nam ics in side the cell. In this work, the oxy ‐

gen from air pumped through the air cham ber to reach the GDE is

con sid ered in ex cess (0.7 bar). T he new cath ode de sign of fers a key

ad van tage, in ad di tion to feed ing of the gaseous O at the ex act

place where it is con sumed (typ i cal in GDEs, in con trast to the ap ‐

proach based on air sparg ing in the so lu tion bulk): the 3D porous

graphite felt struc ture acts as gas dis perser, which in turn may en ‐

hance the tur bu lence within the pores and plau si bly im proves the

3

2 2 −1

2

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F ig . 2. (a) H O co n cen tr a tio n , (b ) cur r en t ef � c iency and (c) en er gy co n ‐

sump tio n per kg H O vs . time at dif fer en t ca tho dic po ten tials: (◊) −0 .01,

(}) −0 .15 , (�) −0 .20 , and (○) −0 .30 V vs . SHE. C o n di tio ns: 4 dm o f 0 .05 MNa SO so lu tio ns at pH 3.0 and u = 27.4 cm s , w ith air fed at 0 .7 bar .

mass trans port [28]. Con versely , the con cen tra tion of pro tons in

the bulk com ing from a so lu tion ad justed to pH 3.0 (1 mM H ) is cer ‐

tainly di luted and, con se quently , the mass trans port of these cations

to ward the cath ode sur face may limit the pro duc tion of H O from

Eq. (1). T here fore, pro tons, which can be also trans ported once

gen er ated at the an ode, could con sti tute the lim it ing reagent in the

pre sent sys tem. Fig. 3a de picts the H O ac cu mu la tion as a func tion

of the elec trol y sis time at dif fer ent in �ow ve loc i ties, namely 14.6,

21.0, and 27.4 cm s (q of 1.6, 2.3 and 3.0 dm min ), ap ply ing the

op ti mal cath ode po ten tial (−0.30 V vs . SHE) for 240 min. T he

curves sug gest that an in crease in the in �ow ve loc ity up to

27.4 cm s leads to a greater H O pro duc tion from Eq. (1), at tain ‐

ing a max i mum con cen tra tion of 100.4 mg dm at 240 min. T his

agrees with the pro gres sive en hance ment of the re sult ing cur rent

val ues as the ve loc ity was in creased, within the range of

0.20–0.26 A (for u = 14.6 cm s ), 0.25–0.28 A (for

u = 21.0 cm s ) and 0.27–0.30 A (for u = 27.4 cm s ). When the

tri als were pro longed for more than 240 min (not shown), the H O

con cen tra tion tended to reach a plateau, which can be mainly ac ‐

counted for by the an odic de struc tion that leads to an equi lib rium

be tween the gen er a tion and de struc tion rates [4]. Fur ther more, it

is im por tant to men tion that ve loc i ties above 27.4 cm s were not

ben e � cial to at tain a greater amount of H O . T his means that, un ‐

der these con di tions, the hy dro dy nam ics could not im prove the

mass trans port in side the cell in an ef fec tive man ner, prob a bly be

F ig . 3. (a) H O co n cen tr a tio n , (b ) cur r en t ef � c iency and (c) en er gy co n ‐

sump tio n per kg H O vs . time at dif fer en t v e lo c i ties: (◊) 14.6 , (}) 21.0 , and

(�) 27.4 cm s . C o n di tio ns: as in F ig. 2 , at −0 .30 V vs . SHE.

cause of: (i) ex ces sive trans port of pro tons to ward the cath ode sur ‐

face, thus pro mot ing the side HER over the ORR; (ii) faster trans ‐

port of elec tro gen er ated H O to ward the an ode sur face, thus in ‐

creas ing its de struc tion rate; (iii) a too short res i dence time of the

elec troly te so lu tion within the re ac tor, lim it ing the par tic i pa tion of

pro tons re quired for Eq. (1); and/ or (iv ) an in crease in the hy ‐

draulic pres sure that forces a greater pen e tra tion of the liq uid

phase into the porous elec trode, which could change the area of

the gas-solid-liq uid in ter face, even tu ally en hanc ing the ca thodic

H O de struc tion. In a pre v i ous pa per em ploy ing the same pre-pi ‐

lot plant but us ing just a graphite felt as cath ode to pro duce H O

from dis solved O (8 mg dm ) in 0.050 mol dm Na SO so lu tion at

pH 3, a con cen tra tion of 50.3 mg dm H O was at tained at con ‐

stant cur rent den sity of −0.16 mA cm and 17.6 cm s . T his low

con ver sion was at trib uted to the low con cen tra tion of dis solved

oxy gen [29].

Fig. 3b and c shows the cor re spond ing cur rent ef � ciency and

en ergy con sump tion per kg of H O pro duced vs . elec trol y sis time.

T he pro �les shown in Fig. 3b agree with those of Fig. 3a and are

very sim i lar to those re ported in Fig. 2b, sug gest ing the oc cur rence

of the par a sitic re ac tions, al though the cur rent ef � ciency was close

to 100% af ter 30 min as a re sult of the en hanced hy dro dy nam ics.

T he best elec trol y sis in terms of H O pro duc tion (100.4 mg dm )

was achieved at con stant cath ode po ten tial E = −0.30 V|SHE and

4

2 2

2 2 3

2 4 −1

+

2 2

2 2

−1 3 −1

−1 2 2

−3

−1

−1 −1

2 2

−1

2 2

2 2

2 2 −1

2 2

2 2

2 2

2 −3 −3

2 4 −3

2 2 −2 −1

2 2

2 2 −3

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T. Pé re z e t al. Journal of Ele ctroanalytical Chemistry xxx (2018) xxx-xxx

liq uid �ow rate of 27.4 cm s , giv ing rise to an en ergy con sump tion

of 6.4 kWh (kg H O ) af ter 240 min of elec trol y sis.

4. Conclusions

T his work demon strates that H O can be elec trosyn the sized in

a pre-pi lot plant us ing a mod i �ed, un di v ided �l ter-press FM01-LC

re ac tor upon feed ing of a cheap, pur pose-made car bon cloth/

graphite felt cath ode with com pressed air, ob tain ing high cur rent

ef � cien cies at low en ergy con sump tions. T he ex per i ments were

per formed at pH 3.0 for fu ture ap pli ca tion in wa ter treat ment by

Fen ton-based elec tro chem i cal processes. T he H O pro duc tion was

op ti mal at −0.30 V, be ing en hanced as the mean lin ear �ow ve loc ‐

ity was in creased from 14.6 to 27.4 cm s , ow ing to the en hanced

mass trans port of hy dro nium ions. T his con sti tutes a �rst ap proach

to ef � ciently pro duce H O in a greener man ner, al though CFD

sim u la tions con sid er ing bipha sic �ow within the graphite felt will

be needed.

Acknowledgements

Fi nan cial sup port from pro jects CT Q2016-78616-R (MINECO,

Feder, EU) and 869/ 2016 (Uni ver si dad de Gua na ju ato, Mex ico)

and PhD schol ar ship No. 366128 (CONA CyT ) to G. Co ria are ac ‐

knowl edged.

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