Perovskite ceramic membrane separator with improved biofouling resistance for yeast-based microbial fuel cells

Journal Pre-proof Perovskite ceramic membrane separator with improved biofouling resistance for yeast-based microbial fuel cells Domenico Frattini, Grazia Accardo, Yongchai Kwon PII:

S0376-7388(19)33379-4

DOI:

https://doi.org/10.1016/j.memsci.2020.117843

Reference:

MEMSCI 117843

To appear in:

Journal of Membrane Science

Received Date: 1 November 2019 Revised Date:

8 January 2020

Accepted Date: 12 January 2020

Please cite this article as: D. Frattini, G. Accardo, Y. Kwon, Perovskite ceramic membrane separator with improved biofouling resistance for yeast-based microbial fuel cells, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117843. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Author Statement

Dr. Domenico Frattini : Dr. Frattini mainly contributed to the conceptualization and data curation as well as formal analysis of this paper. He also played a main role in writing original draft. Dr. Grazia Accardo : Dr. Accardo mainly contributed to the conceptualization and she helped to write original draft. Prof. Yongchai Kwon : Prof. Kwon mainly contributed to supervision of this paper and conceptualization of this research, while he played a role in doing the final revision of original/revised draft. He also designed funding for this research.

Perovskite ceramic membrane separator with improved biofouling resistance for yeast-based microbial fuel cells

Domenico Frattinia, Grazia Accardob, and Yongchai Kwona*

1

Perovskite ceramic membrane separator with improved

2

biofouling resistance for yeast-based microbial fuel cells

3 4

Domenico Frattinia, Grazia Accardob, and Yongchai Kwona*

5 6 7 8 9

a

Graduate School of Energy and Environment, Seoul National University of Science and Technology 232

Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea. b

Center for Hydrogen Fuel Cell Research, KIST - Korea Institute of Science and Technology, Hwarang-ro

14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea.

10 11 12 13 14 15

*Corresponding authors. E-mails: [email protected] (Yongchai Kwon),

16 17 18

1

19

Abstract:

20

Ceramic-derived components in microbial fuel cells (MFCs) aim to substitute Nafion 117.

21

Ceramic membranes are a cheaper alternative and powders’ tailoring has beneficial flexibility for

22

proper surface morphology, porosity, controlled permeability, and water uptake. In this work,

23

differently from the fine fired ceramics or clay materials, barium-cerium-gadolinium oxides (BCGO)

24

powders, co-doped with lithium (Li) or cobalt (Co), are synthesized at a low temperature and then

25

sintered at 1400 and 1500°C to form the ceramic surface. Results show that the pore size is always

26

within a few microns, porosity can be varied greatly with firing temperature at fixed dwell time, and

27

the use of Li or Co can give smooth or corrugated porous particles, respectively. The permeability

28

tests show that the BCGO doped with Li cannot control the water flux from yeast extract-peptone-

29

D-glucose medium (YPD) with the yeast, and the absolute amount of biofouling is higher than that

30

of Nafion 117. BCGO doped with 5 mol% Co exhibits good permeability and lowers absolute

31

biofouling due to the unique surface morphology of parent powders. Thus, the ceramic separators

32

based on BCGO doped with Co can be an attractive alternative to expensive Nafion.

33 34

Keywords: ceramic membrane; barium-cerium-gadolinium oxide powder; permeability; perovskite;

35

yeast.

36

2

37

1. Introduction

38

Recently, microbial fuel cells (MFCs) have been recognized as profitable biodevices that

39

can generate energy from biomass [1]. The MFCs can be fed by pure organic compounds or waste

40

biomass, such as glucose or acetate, wastewater or food waste, while electricity can be directly

41

generated by them [2]. The key factor determining the performance of MFCs is the electron transfer

42

realization between bio-matter and conductive electrode [3]. For promoting the electron transfer,

43

there are two ways. First, the mediated electron transfer (MET), where mediators are required to

44

shuttle electrons effectively from the donor microorganism to the electrode acceptor, and second,

45

the direct electron transfer (DET) that electrons can be directly transferred to the anode. The

46

microorganism type and electrode materials determine the more effective electron transfer

47

mechanism [4,5]. Based on that, with the proper approach for anode modification [6–9] and an

48

efficient cathodic catalytic structure [10,11], MFCs can produce electricity from organic waste in a

49

direct way and at ambient temperature [12].

50

In terms of the economic feasibility breakdown [13] and the allometric scale-up approach

51

[14] indicated in the recent literature, developing anodes and membranes in a cost-effective way is

52

the first priority for the commercialization of large-scale MFCs in the near future. To accomplish

53

this goal, the exploration of new membrane material replacing the conventional Nafion must be

54

considered [15,16]. Nafion is a proton exchange polymeric material and is widely used in batteries

55

[17,18], and various fuel cells, including MFCs, for its superior properties [19]. However, this is

56

very expensive with the peak price of 2300 $·m-2 [9,16], and MFCs’ operating conditions are very

57

different from that of other fuel cells where Nafion performs well because this is best at relatively

58

high-temperature (>100ºC) and low humidity (20-30%) conditions in presence of concentrated

59

H2/H+ anodic gas streams. Thus, selecting or developing other proton exchange membranes for

60

MFCs is required [19]. As an alternative, ceramic-derived separators have been recently used

3

61

[21,22]. Fine Fired Clay (FFC) [23], earthenware [24], montmorillonite [25], and kaolinite [26]

62

belong to this category.

63

These ceramic-derived separators do not usually possess the desirable specific properties

64

that are related to proton transfer and ion selectivity, and so far, they are just used as

65

buffers/separators/filters [22,27] for catholyte production/anolyte cleaning in two-chamber MFCs.

66

In ceramic separators, the working principle is not based on the proton transport through sulfonated

67

side chain groups of Nafion and electro-osmotic drag across the membrane, but on water

68

permeation, diffusion, and evaporation through the microporous structure of the ceramic diaphragm.

69

In this prospect, the major concerns are associated with the resistance to biofouling, the selective

70

cation transport, and the high firing temperature of ceramics. To extend their availability to single-

71

chamber MFCs for power production, new ceramic-derived materials with advanced properties are

72

needed.

73

Therefore, differently from the simple FFC, kaolinite or earthenware used in literature, here

74

engineered and functional ceramics are designed. In this work, protons-conducting-only perovskite

75

systems derived from Barium Cerate (BaCeO3, BCO) co-doped with gadolinium (BCGO) and

76

lithium (Li) or cobalt (Co) are synthesized and characterized. To provide a common ceramic-like

77

microporosity, these electrolytes are sintered at two reduced temperatures, 1400 and 1500ºC, while

78

crystalline structure, surface morphology, and permeation phenomenology are deeply investigated.

79

The biofouling resistance, directly connected to the surface properties, in the worst possible

80

condition of direct and continuous contact between the separator/membrane and the microorganism,

81

the model yeast Saccharomyces Cerevisiae, is also examined both qualitatively and quantitatively.

82 83

2.1 Sol-gel synthesis and sintering of the co-doped BCGO ceramics

84

Perovskite powders with nominal composition of BaCe0.8Co0.05Gd0.15O3-δ (5CoBCGO) and

85

BaCe0.82Li0.03Gd0.15O3-δ (3LiBCGO) co-doped with Co (5 mol%) and Li (3 mol%) were prepared

86

from high-purity metal nitrate precursors of barium nitrate (Ba(NO3)2 >99.9%), cerium nitrate 4

87

hexahydrate (Ce(NO3)3·6H2O >99.9%), gadolinium nitrate hexahydrate (Gd(NO3)3·6H2O >99.9%),

88

cobalt nitrate (Co(NO3)2·6H2O >99.9%) and lithium nitrate (LiNO3, >98%). Nitrate precursors were

89

added to a small amount of distilled water (DIW) and mixed until they are fully dissolved at room

90

temperature [28]. The aqueous mixture was then heated up to 80°C and citric acid monohydrate

91

(C6H8O7·H2O >99.0%) was added to this mixture in a 1:1 molar ratio with the nitrates. A viscous

92

gel was formed after vigorous stirring for 30 min and the gel was decomposed by increasing the

93

temperature to 250°C for a few minutes to promote the combustion, in which citrate was the

94

complexing agent [29]. The sol-gel route is a convenient way to obtain a homogeneous and atomic

95

dispersion of the co-dopants to avoid the deleterious segregation and nucleation of metals without

96

using cryogenic temperatures during the synthesis [30,31]. All the reagents were purchased from

97

Sigma Aldrich (St. Louis, MO, United States), and used as received without further purification. A

98

reference perovskite powder without the second dopant (BCGO) was also prepared only for XRD

99

comparison.

100

After synthesis, powders were ground in an agate mortar and calcined at 1200°C for 2 h in

101

order to remove any impurity and to form the cubic perovskite structure. Then, pellets were

102

obtained by uniaxially pressing the powders at 150 MPa for 2 min and subsequently by sintering at

103

two different temperatures, 1400°C and 1500°C for 5 h, with heating and cooling rate of 3°C·min-1.

104 105

2.2 Structural and surface characterization of the co-doped BCGO ceramics

106

The lattice structures of the calcined ceramic powders and the Nafion 117 control sample

107

were investigated using XRD (Miniflex II, Rigaku Co., Tokyo, Japan) in the 2θ range of 5º – 90º

108

with a scan rate of 3º·min-1 and a scan width of 0.03º. Crystalline phase matching for the peaks’

109

identification was performed with PDXL software (Rigaku Co., Tokyo, Japan) and based on the

110

ICDD database. Rietveld refinement was used to estimate also the lattice parameters by using the

111

MAUD software [32,33].

5

112

The specific surface area according to the Brunauer–Emmett–Teller (BET) theory (SBET) of

113

the 5CoBCGO and 3LiBCGO calcined powders was determined from the complete nitrogen

114

adsorption isotherms at 77 K (ASAP 2010b absorptiometer, Micromeritics Instruments Co.,

115

Norcross, GA, United States).

116

Microstructure analysis and surface morphology of ceramic powders, pellets and of Nafion

117

were investigated by SEM in a high vacuum environment (Inspect F, FEI Co., Hillsboro, OR,

118

United States).

119

The outer surface of pellets was analyzed by X-ray photoelectron spectroscopy (XPS) to

120

determine single elements (Ce 3d and O 1s, 15 scans each, energy step size 0.10 eV, pass energy

121

50.0 eV) binding energy spectra in a high vacuum environment (K-Alpha+, Thermo Scientific Co.,

122

Waltham, MA, United States) on spots of 400 µm in diameter. The spectra were collected and

123

deconvoluted to interpret the chemical orbital bands of elements on the surface according to [34,35].

124 125

2.3 Permeation, water flux, and biofouling calculation.

126

The tested solutions for the liquid-vapor permeability experiments were distilled water

127

(DIW) or the yeast extract/Peptone/D-glucose (YPD) medium with active yeast. The YPD medium

128

composition was the same as previous work [36] and consisted of yeast extract (5 mg·mL−1),

129

peptone (2.5 mg·mL−1), D-glucose, and yeast (both at 13.18 mg·mL−1). Yeast extract, D-glucose,

130

and dried yeast from Saccharomyces Cerevisiae were purchased from Sigma Aldrich (St. Louis,

131

MO, United States), while Peptone was purchased from Duksan Pure Chemicals Co. (Seoul,

132

Gyeonggi-do, Republic of Korea). The liquid-vapor permeability testing procedure and calculations

133

for water flux, permeance, and dry/wet water uptake were referred from those described in the

134

literature [37,38]. A volume of 10 mL of the selected liquid, DIW or YPD+yeast, was poured in a

135

plastic vial and the ceramic separator or the Nafion 117 membrane sample was placed between the

136

vial and the cap; on the cap, there was a hole (diameter 7 mm) representing the pervaporation area

137

(38.47 mm2). The edge of the hole was sealed by an adhesive PTFE impermeable tape, while the 6

138

cap was sealed with parafilm and the bottom of the vial has an orifice to equilibrate the

139

internal/external pressures. For each sample and liquid, these tests were duplicated for

140

reproducibility's sake. The vials were placed upside down in a controlled environment, and water

141

losses were recorded every 30 min for 8 consecutive hours with a precision scale (ARG224

142

Adventurer 4 decimal digits, Ohaus Co., Parsippany, NJ, United States). The wet and dry water

143

uptakes were calculated as follow: −

=

−

= =

∙ 100

(1)

∙ 100

(2)

−

(3)

144

Where Utot and Udry are the total uptake and the dry water uptake of the ceramic separators

145

and Nafion 117 after 24 h, mi is the initial dry weight of the samples, mw is the final weight of the

146

samples after 24 h of wetting, and md is the dried weight of the samples after the 24 hours of testing

147

and a vacuum drying step to remove every trace of water, leaving only the solid residue

148

representing the biofilm. The net water uptake, Uwet is calculated simply As a difference of the total

149

and the dry uptake, as shown in equation 3. In fact, the Udry is referred to as the measure of the

150

biofouling when the YPD+yeast is used.

151

The permeability tests were carried out at the temperature of 26±2ºC (299±2 K) and 50±5%

152

of relative humidity (RH), and a small correction to the chemical potentials was necessary

153

according to the following equations for water flux, chemical potentials, and permeance [39]: =

( (

−

∙

) ∙ 60 1000 ∙ ∙ ∆!) 3.6

(4)

$%) &_( = $%) &_

*+,

+ .% & ∙ (/ − 298)

(5)

) ) $345_( = $345_

*+,

+ .345 ∙ (/ − 298)

(6)

) $345_6 = $345_( + 7/ ∙ ln :

A(_6 =

7; ∙

BBBBB

∆$

_6

<4 =345 4>?

@

(7) (8)

7

154

Where JLVP is the molar liquid-vapor permeation flux (mol·m-2·s-1), mtn and mtn+1 are the

155

relative weight losses (g) at the time point tn and tn+1, PMH2O is the molecular weight of water

156

(18.015 g·mol-1), ALVP is the exposed pervaporation area (mm2), ∆t is the time interval (30 min)

157

between tn and tn+1. For the chemical potentials, µ0liq_T and µ0vap_T, are the chemical potentials of

158

liquid water and water vapor at the temperature T (299 K); µ0liq_298K and µ0vap_298K are the standard

159

chemical potentials of liquid water and water vapor at 298 K and 1 atm (–237.18 and –228.59

160

kJ·mol-1, respectively); the coefficients γliq and γvap are the linear temperature coefficients for water

161

(liquid water –0.06985 kJ·mol-1·K-1, water vapor –0.18874 kJ·mol-1·K-1); µvap_RH is the chemical

162

potential of water vapor corrected for the actual value of RH (0.5) and the Psat-vap and Pamb are the

163

saturated vapor pressure at the experimental temperature (25.2 mmHg) and the ambient pressure

164

(760 mmHg), respectively.

165

The differential chemical potential across the separators and the membrane, ∆µLVP_RH, used

166

to calculate the permeance ϑT_RH in equation 8 (mol2·m-2·s-1·kJ-2) is the difference between the

167

corrected chemical potentials of liquid (eq. 5) and vapor (eq. 7), while BBBBB is the average molar

168

flux after 8 h. At the completion of the permeability test, the attached biofilms on the wet sample

169

were simply photographed, while the residual biofilm on vacuum-dried samples was optically

170

inspected by SEM to evaluate the yeast cells’ extend and attachment on the surface.

171

172

173

3. Results and Discussion 3.1 Preliminary characterization of the co-doped BCGO ceramic powders

174

It is important to characterize the calcined parent powders to inspect the results of the sol-gel

175

synthesis. The XRD patterns of the two co-doped BCGO powders are reported and compared with

176

those of a reference BCGO fabricated without the second dopant, and Nafion 117 (Fig. 1).

177

As expected, the XRD pattern of Nafion 117 shows a scattered background and broad peaks,

178

ascribed to its polymeric backbone (polytetrafluoroethylene, ICDD card N. 00-054-1595) that is 8

179

typical of this kind of material (Fig. 1a). In Fig. 1b, the XRD patterns of the BCGO systems,

180

together with the structural representation of the lattice cell, are reported. These systems have all

181

complex orthorhombic cell structures derived from the reference Barium Cerate BCO crystal

182

because all the peaks of the BCGO powders match with those of the orthorhombic perovskite

183

BaCeO3 (ICDD card N. 01-089-8268).

184 185

Fig. 1. XRD patterns of a) Nafion 117 membrane; b) BCGO and co-doped BCGO calcined powders.

186 187

The two co-dopants, i.e. gadolinium and lithium or cobalt, have different positions and

188

distribution in the lattice cell. In fact, it is well-known that Gd+3 cations substitute Ce+4 cations in

189

vertex positions, creating oxygen vacancies due to the different charges, but also stabilize protonic

190

defects thanks to the Ba2+ cation acceptors located at the center of the cell. By this phenomenon,

191

BCGO oxides can show the highest protonic conductivity at high temperatures (above 400-500°C),

192

but low chemical stability, and a low conductivity at low temperatures in presence of humidity and 9

193

water [40]. This is the reason why a second dopant cation is necessary and why Li and Co, which

194

have a tendency to become hydrated even at low temperature and to reduce the sintering

195

temperature, are selected.

196

In Nafion 117, the proton conduction mechanism is based on the H+ in water that transfers

197

through the membrane by the cation exchange sites on the sulfonated side chain. This is the proton

198

hopping mechanism or the Grotthus mechanism, while another mechanism is also possible, such as

199

the vehicular mechanism, in which protons are dragged through water nanochannels established

200

inside the polymeric matrix of Nafion [41] due to the chemical structure of Nafion examined by

201

XRD. The porous ceramic proton-conducting materials with (i) a perovskite crystal structure, (ii)

202

insertion of doping and (iii) co-doping acceptor cations, and (iv) high concentration of oxygen

203

vacancies can have similar behaviors to Nafion even at low-temperature ranges. The modifications

204

induced to lattice by the co-doping with Li and Co can be evaluated by the calculation of lattice

205

constant parameters with respect to the reference BaCeO3 system as reported in Table 1.

206

Table 1. Lattice constants of BCGO, 3LiBCGO, 5CoBCGO, and the ICDD reference Barium Cerate (IV).

a (Å)

b (Å)

c (Å)

BaCeO3 (database)

6.2517

8.7906

6.2771

BCGO

6.2363±1.1·10-3

8.7719±1.2·10-3

6.2638±1.1·10-3

3LiBCGO

6.2245±0.8·10-3

8.7697±1.0·10-3

6.2606±0.8·10-3

5CoBCGO

6.2383±0.5·10-3

8.7811±0.5·10-3

6.2216±0.5·10-3

207 208

Considering the effective ionic radii from Shannon [42] of Li+ (0.92 Å, eight-fold

209

coordination), Co2+ (0.90 Å, eight-fold coordination) and Co3+ (0.55 Å, six-fold coordination), the

210

most distorted cell is 5CoBCGO and it is because the radii of Co cations in 5CoBCGO are distant

211

from those of Ce4+ (0.97 Å, eight-fold coordination) and Gd3+ (1.05 Å, eight-fold coordination)

212

cations. In 3LiBCGO, the most distorted cell parameters are a, along the x-direction, and b, along

10

213

the y-direction, indicating that the small Li+ cations probably occupy a vacancy on the basal face of

214

the cell. Similarly, in 5CoBCGO, considering the higher amount of co-dopant, two parameters are

215

largely distorted (bc on the yz plane), and this indicates that the small Co2+/Co3+ cations can

216

substitute Ba2+ cations (1.42 Å, eight-fold coordination) at the center of the cell and/or occupy a

217

vacancy on the lateral face of the cell. The exact determination of the position of Li and Co in the

218

lattice cell is beyond the scope of this work and the main aim is to demonstrate successful co-

219

doping by considering the crystal structure and the unit cell parameters.

220

The morphology of 3LiBCGO and 5CoBCGO powders is observed by SEM (Fig. 2). In both

221

powders, hard aggregates (5-8 µm) of smaller particles (0.5-1 µm) are observed, and this means that

222

channels and pores between the particles are created and this is a beneficial feature for the vehicular

223

proton transport mechanism. Each aggregate is formed approx. by 10-12 particles or more.

11

224 225

Fig. 2. SEM of calcined co-doped BCGO powders a,c) 3LiBCGO 20000x, b) 5CoBCGO 20000x, and d)

226

5CoBCGO 30000x.

227 228

Regarding morphology, that of each sample is different. 3LiBCGO aggregates have a

229

smooth and rounded surface while sometimes the internal surfaces can show some roughness. The

230

5CoBCGO aggregates are a little bit smaller and show a less curved shape with more edges,

231

microchannels and the presence of small protuberances on the surface. This is a unique feature of

232

BCGO doped with Co, not observed in 3LiBCGO or BCGO. The specific surface area and average

233

pore size of these powders observed by BET measurements are given inTable 2.

234

Table 2. BET specific surface area and average pore size of calcined powders.

12

SBET (m2·g-1)

Average pore size (nm)

3LiBCGO

0.631

5.15

5CoBCGO

0.539

5.96

235 236

The pore size of both powders is close to 5 nm, and this is a suitable size for the vehicular

237

transport mechanism that is related to the transport of protons in presence of gas/liquids, resembling

238

Nafion [41]. However, due to the formation of hard aggregates, the specific surface area is greatly

239

reduced to 0.6 m2·g-1. Usually, the specific surface area of these materials is 6-8 m2·g-1 [43],

240

approximately ten times higher than those of Table 1. Namely, the actual surface area of co-doped

241

BCGO powders is reduced by one-tenth of the normal value because each aggregate is constituted

242

by at least 10-12 particles as observed in Fig. 2.

243

In terms of the sintering behavior of the powders at different sintering temperatures, Fig. 3

244

represents the micrographs of the sintered surfaces of 3LiBCGO and 5CoBCGO at 1400 and

245

1500°C for 5h.

13

246 247 248

Fig. 3. SEM of sintered pellets at 1500°C for 5 h a) 5CoBCGO; b) 3LiBCGO; and at 1400°C for 5 h c) 5CoBCGO; d) 3LiBCGO.

249 250

The selection of the correct sintering temperature and of the sintering aid are critical for the

251

fabrication of co-doped BCGO ceramic separators. In fact, in Fig. 3a, the 5CoBCGO sintered at

252

1500°C shows a fully densified structure with very large grains and no visible porosity. This

253

structure is proper for high-temperature proton-conducting electrolytes based on pure hopping

254

Grotthus conduction mechanism, but not for prospective ceramic separators for MFCs applications

255

where some vehicular transport is needed [26]. On the contrary, in the structure of 3LiBCGO

256

sintered at 1500°C, the surface is not fully densified (Fig. 3b), the grains are still forming, and many

257

micrometric pores are clearly visible, meaning that this structure is more suitable as a separator for

258

MFCs. By using the same sintering aids and amounts for various Gd/Sm doped ceria electrolytes,

259

Spiridigliozzi et al. [44] have found that both Li and Co can increase the relative density of 14

260

electrolytes with cubic fluorite structure, but this seems not true for the actual non-cubic and

261

orthorhombic BCGO, and the choice of the sintering aid, along with the sintering temperature, is

262

not a trivial matter. When the sintering temperature is reduced to 1400°C, the structure of

263

5CoBCGO is still constituted by large grains but there are visibly some spaces between the grains

264

not fully attached each other (Fig. 3c), not forming a diffused porous structure, but a network of

265

interspaces with size < 1 µm, indicating that the 5CoBCGO sintered at 1400°C is proper for using

266

in MFCs and is fabricated with large well-sintered zones and few voids. For the 3LiBCGO sintered

267

at 1400°C (Fig. 3d), the structure reveals that the surface is still partially sintered with much more

268

pores than those at 1500°C, although the pore size is similar. Therefore, the co-doped ceramic

269

separators sintered at 1400°C for 5h are selected for the XPS analysis and the permeability tests.

270 271

The deconvolution and area quantification of the Ce 3d spectra of the two co-doped perovskites are shown in Fig. 4.

15

272 273

Fig. 4. Deconvoluted XPS Ce 3d spectra of BCGO, 3LiBCGO and 5CoBCGO pellets sintered at 1400°C for 5 h.

274 275

The Ce 3d spectrum is rather complex and according to Aliotta et al. [34], due to the

276

presence of both Ce3+ and Ce4+, ten components of 5 doublets pairs (V, V0, V’, V’’ and V’’’ for the

277

Ce 3d5/2 orbital, and U, U0, U’, U’’ and U’’’ for the Ce 3d3/2 orbital) can be found but usually the

278

pair V0-U0 for the Ce3+ is zeroed in doped electrolytes, leaving only the pair V’-U’ for the Ce3+,

279

suggesting a limited reduction of cerium. In fact, one benefit of doping and co-doping of ceria is

280

that, depending on the oxidation state of the dopants, the reduction of Ce4+ to Ce3+ can be limited

281

thus keeping more oxygen vacancies [45,46]. The deconvolution of spectra shows that this is the

282

case of 3LiBCGO and 5CoBCGO because, by comparing the areas ascribed to the components of

283

Ce4+ and Ce3+, there is more Ce3+ in BCGO rather than 3LiBCGO and 5CoBCGO.

16

284 285

Important information about the role of surface oxygen vacancies for the protons’ hopping Grotthus mechanism is visualized in the deconvoluted O 1s spectra in Fig. 5.

286 287

Fig. 5. Deconvoluted XPS O 1s spectra of BCGO, 3LiBCGO and 5CoBCGO pellets sintered at 1400°C for 5 h.

288 289

These spectra are simpler and characterized just by two well-separated Gaussian peaks,

290

representing two specific bonds of oxygen, i.e. the metal oxide bond M-O (approx. at 529 eV) and

291

the (metal) hydroxide bond (M)-OH (approx at 532 eV). The first bond is the structural one

292

involved in the lattice of the ceramic oxide (Olattice), representing the backbone of the material; the

293

second bond is the one ascribed to the surface defects due to the co-doping (Osurface) [35] and the

294

hydration of the oxygen vacancies as the preliminary step for proton conduction [40]. Differently

295

from non-perovskite ceria systems [34,35], and the reference BCGO prepared here, there many 17

296

more defects on the surface of the 5CoBCGO, and the peak at 532 eV is higher. Moreover, the

297

5CoBCGO perovskite has a different distribution of the defects because the relative area of the

298

Osurface peaks is larger in 3LiBCGO and BCGO, and the peak is visibly shifted by 0.5-1 eV toward

299

higher energy values. This means that the (M)-OH surface bonds of 5CoBCGO are more and

300

stronger than those of 3LiBCGO, probably improving the selective uptake of protons and water

301

thanks to the specific type and dosage of the co-dopant, providing a good morphology to the

302

sintered pellet even at the reduced sintering temperature of 1400ºC.

303 304

3.2 Permeability and biofouling behavior of Nafion 117 membrane and co-doped BCGO

305

separators

306

A Nafion 117 membrane and the 3LiBCGO and 5CoBCGO ceramic separators sintered at

307

1400°C for 5h are prepared for the permeability and biofouling tests in DIW and YPD+yeast. For

308

each replicated test, values of weight losses and molar flux are recorded.

309

Firstly, the permeability of the three samples is tested with DIW (Fig. 6). In DIW, only

310

water is transported through the sample due to the liquid-vapor permeation phenomenon. In this

311

case, the internal side of the sample is in direct contact with the liquid, whereas the external side is

312

exposed to air, hence liquid water slowly diffuses from side to side of the sample and then

313

evaporates in air due to the concentration gradient between the sides of sample and the humidity of

314

the environment. Fig. 6a shows an interesting weight loss behavior of the two ceramic separators

315

compared to Nafion 117. The weight loss of 3LiBCGO is higher than that of 5CoBCGO and Nafion

316

117 due to the higher porosity and the wettability of the surface and acts like a ceramic filter under

317

constant pressure. In Nafion 117, a remarkable weight loss is observed, while the weight loss of

318

5CoBCGO, after an initial stage, is low. Meanwhile, the water molar fluxes of the three samples

319

(Fig. 6b) confirm that the samples have different behavior. Here, 3LiBCGO becomes instantly

320

hydrated and reaches an almost stable water flux after 30 min regardless of the thickness (approx. 1

321

mm). Nafion 117 (thickness approx. 200 µm) shows also fast hydration behavior, but this is not 18

322

finalized completely, and the molar flux slowly increases over time and reaches an almost stable

323

flux after 5-6 h. The behavior of 5CoBCGO is dissimilar from previous samples. After fast

324

hydration like Nafion, its flux is reduced to the minimum and the water transport takes more time

325

due to probably small porosity, wettability, and the thickness of the ceramic separator that is far

326

thicker (approx. 1 mm) than Nafion 117 (200 µm).

327 328 329

Fig. 6. Permeability results with DIW for: a) relative weight losses; b) water flux of Nafion 117 and ceramic separators.

330 331

Taken together, this is attributed to (i) different porosity of 3LiBCGO and 5CoBCGO and (ii)

332

different wettability of the internal surface of the samples exposed to the liquid. In proton-

333

conducting ceramics, the hydration of vacancies is an important step of the transport process, and

334

dopants are added to stimulate hydration, especially at low temperatures. Combined with the

335

excessive porosity and pore size of the BCGO doped with Li, it is explained that the water transport

336

in 3LiBCGO is almost free and uncontrolled, whereas the water transport in 5CoBCGO is restricted

337

and can be controlled after an initial stage. 19

338 339

340 341 342

This hypothesis is confirmed when DIW is changed into the YPD+yeast medium. The permeability tests performed with YPD+yeast are represented in Fig. 7.

Fig. 7. Permeability results with YPD+yeast for: a) relative weight losses; b) water flux of Nafion 117 and ceramic separators.

343 344

The first important findings by observing Fig. 7 and Fig. 6 is that the water flux of

345

3LiBCGO has a tenfold increase, while both Nafion and 5CoBCGO increase just twice. In the

346

presence of the YPD+yeast liquid, the water flux data and weight loss are widely ranged. Here, the

347

YPD+yeast is a reacting medium, in which the active yeast converts the glucose substrate,

348

producing H+, electrons, extra H2O and gaseous CO2 [47]. To simulate the worst situation for

349

biofouling, the vials are placed upside down and the liquid is in direct contact with the sample. The

350

extra water and gaseous CO2 byproducts are locally produced at the internal surface of the sample

351

and an overpressure may raise, pushing water and/or gases through the membrane/separator. If the

352

porosity, pore size, and wettability are suitable, these products can easily diffuse through the

353

thickness of the sample. Based on that, it is confirmed that 3LiBCGO is not eligible as a separator 20

354

for the MFCs, especially for the single-chamber design, due to its surface structure. Its porosity,

355

wettability, and filter-like behavior induce free percolation in the presence of the YPD+yeast, and

356

this does not promote a controlled pervaporation of water when the real anodic broth of yeast-MFCs

357

is used. On the contrary, Nafion and 5CoBCGO can well resist to the overpressure attributed to the

358

YPD+yeast placed on the internal side of the membrane/separator and, even if the flux increases,

359

the percolation of liquid to the external side is not observed. However, extra sealing for gas was

360

always adopted during the tests for the vials when the YPD+yeast medium was used in order to

361

minimize the eventual gas leakage from the screw cap. According to the equations 4-8, from weight loss and water flux data, it is possible to

362 363

calculate the apparent permeance at the experimental conditions (26°C/299 K, RH 50%), A

364

The calculations for the two liquids are listed in Table 3.

365

**_C) .

Table 3. Calculated permeances at 26°C and RH 50% for Nafion, 3LiBCGO, and 5CoBCGO.

Permeance (mol2·m-2·s-1·kJ-1) DIW

YPD+yeast

Nafion 117

1.16·10-3

2.33·10-3

3LiBCGO

1.65·10-3

9.05·10-3

5CoBCGO

3.35·10-4

1.86·10-3

366 367

The values for YPD+yeast also consider the eventual gas evolved from yeast reactions and

368

diffused through the samples, and are used only for this comparison. The 3LiBCGO has always the

369

highest apparent permeance, but the transport of liquid is percolative and not diffusive, especially

370

when the liquid is changed from DIW to YPD+yeast. The 5CoBCGO and Nafion 117 have almost

371

the same permeance when tested with YPD+yeast, and they have similar transport behavior, being

372

the 5CoBCGO more apt to transport water and gas under YPD+yeast condition (1.86·10-3 mol2·m-

373

2

374

the 5CoBCGO is an appropriate ceramic separator material for the MFCs.

·s-1·kJ-1) rather than under DIW (3.35·10-4 mol2·m-2·s-1·kJ-1). With that, it can be determined that

21

375

In terms of the resistance to biofouling, after the permeability test, the vials were left upside

376

down for 24 h and then opened to observe the surface conditions of the internal and external sides

377

and to quantify the uptake of the membrane/separator. The qualitative observations of the surfaces

378

after 24 h of contact with the YPD+yeast medium are shown in Fig. 8.

379 380

Fig. 8. Photographs and SEM of biofilm on samples after 24 h of pervaporation with YPD+yeast.

381 382

Briefly, the biofouling is observed in the internal side of the samples, whereas only Nafion

383

has the biofilm eventually on the external side. This is due to (i) the micrometric thickness of the

384

membrane and (ii) the strong adhesion and penetration of yeast cells that can be transported with

385

ions and water through the nanochannels of the polymeric matrix. This is not observed for the

386

ceramic separators due to the denser thickness and this is a benefit of the ceramic-derived materials.

387

However, the two ceramic materials do not have the same biofouling resistance because, in

388

3LiBCGO, a thick fouling biofilm on the internal side is located exactly like that of Nafion, while in

389

5CoBCGO, the fouling biofilm is thin, not compact and not restricted to a precise area. The SEM

390

images of the biofilm formed on the internal sides of samples after drying show that Nafion 117 and

391

3LiBCGO have a dense and compact biofilm, while the biofilm of 5CoBCGO has many breaches, 22

392

breaks, and a not-continuos structure. The original and clean ceramic surface of the separator is

393

visible in some points. Such a strong resistance to biofouling is another benefit of the ceramic-

394

derived materials.

395

396 397

These optical observations are supported by the wet and dry uptake calculations (Fig. 9).

Fig. 9. Relative wet and dry water uptakes of Nafion 117 and co-doped BCGO with a) DIW; b) YPD+yeast.

398 399

In Fig. 9, the relative uptakes of the three samples are compared and the wet/dry

400

contributions are quantified and split. As expected, in DIW the uptake is simply given by the wet

401

contribution because only water is adsorbed and then almost completely removed upon drying. The

402

situation is a bit different when YPD+yeast is the liquid (Fig. 9b). This medium contains dissolved

403

solids and the floating yeast cells that deposit on the membrane/separators and are adsorbed inside

404

pores and not removed upon drying, thus representing the dry uptake, a permanent contribution

405

leading to biofouling, in case of the yeast cells. From the relative point of view, the percentages

406

show that Nafion 117 has always the highest uptake and the incidence of biofouling is approx. 6%,

407

one-tenth of the total uptake, in presence of YPD+yeast medium. For the two ceramic separators, 23

408

the biofouling contribution is still almost one-tenth of the total uptakes, and this represents less than

409

1%. This would mean that the two ceramic separators have a far higher resistance to biofouling, but

410

this is partially true because the relative uptakes do not consider the absolute weight and thickness

411

of the materials that are completely different from Nafion 117, as demonstrated in Table 4 and

412

Table 5.

413

Table 4. Absolute wet and dry uptakes of Nafion, 3LiBCGO, and 5CoBCGO with DIW.

Nafion 117

3LiBCGO

Weight ∆wt Weight (mg) (mg) (mg)

∆wt (mg)

5CoBCGO Weight ∆wt (mg) (mg)

Initial

80.6

-

1047.0

-

1041.3

-

Wet

133.5

52.9

1172.1

125.1

1063.1

21.8

Dry

80.7

0.1

1047.1

0.1

1041.8

0.5

414 415

The data of Table 4 for the DIW case shows that the final uptake is almost zero for all the

416

samples, but the uptake of Nafion is not higher than the ceramics. Considering the total weights, the

417

3LiBCGO can adsorb more water than Nafion in the wet condition, while 5CoBCGO adsorb less

418

water than Nafion, suggesting the possibility of its partial wetting behavior. The total wet and dry

419

uptake weights measured after the YPD+yeast permeability tests are listed in Table 5.

420

Table 5. Absolute wet and dry uptakes of Nafion, 3LiBCGO, and 5CoBCGO with YPD+yeast.

Nafion 117

3LiBCGO

Weight ∆wt Weight (mg) (mg) (mg)

∆wt (mg)

5CoBCGO Weight ∆wt (mg) (mg)

Initial

81.3

-

1025.1

-

1042.4

-

Wet

125.2

43.9

1173.5

148.4

1072.6

30.2

Dry

86.4

4.1

1033.1

8.0

1045.6

3.2

421 422

Important findings are brought by the biofouling behavior of the two ceramic materials.

423

According to Fig. 9, 3LiBCGO has a lower uptake than Nafion, but from data in Table 4, the

424

opposite result is true. In fact, 3LiBCGO adsorbs more water than Nafion (148.4 mg vs 43.9 mg)

24

425

and the final dry uptake, the biofouling, is double (8 mg vs 4.1 mg). In other words, after 24 h, on

426

3LiBCGO more fouling biofilm can be deposited than on Nafion 117, meaning that Nafion may

427

have a better biofouling resistance than 3LiBCGO. On the contrary, the amount of dry biofilm in

428

5CoBCGO (3.2 mg) is slightly smaller than that in Nafion (4.1 mg), indicating that 5CoBCGO has

429

better resistance to biofouling than Nafion 117. This result is very important because a ceramic

430

material, which has similar permeability and better biofouling resistance than Nafion, can be

431

prepared, and the 5CoBCGO material can be used as a separator in yeast-MFCs and other types of

432

MFCs with single-chamber architecture. The BCGO doped with Co exhibited the best behavior and

433

resistance to biofouling, representing a very interesting and absolutely novel advanced ceramic

434

material for MFCs.

435 436

4. Conclusions

437

In this work, two advanced ceramic materials derived from proton-conducting perovskite

438

electrolytes co-doped with Li or Co were suggested to enhance proton and water transport when

439

used as membrane/separator for single-chamber yeast-MFCs. The parent perovskite system was the

440

Barium Cerate, BaCeO3, basically doped with gadolinium (BCGO), and this was directly

441

synthesized in a one-pot sol-gel synthesis in this work. Similarly, the insertions of Li (3LiBCGO)

442

and Co (5CoBCGO) atoms were done in the same one-pot synthesis to obtain the co-doped powders.

443

These co-dopants deeply modified the lattice and the unit cell, inducing measurable distortions and

444

enhancing the proton conductivity.

445

For the application as ceramic separators in MFCs and the optimization of fabrication and

446

sintering cycle, preliminary characterizations were conducted. Sintering temperature, porosity and

447

pore size were controlled and reduced. As a result, the sintering temperature of 1400°C and the

448

dwell time of 5 h were selected as optimal conditions for both 3LiBCGO and 5CoBCGO.

25

449

The 5CoBCGO has shown excellent permeability behavior and surface morphology,

450

compared to 3LiBCGO. Very high water flux and losses in the presence of the YPD+yeast medium

451

were observed from 3LiBCGO separator. On the other hand, 5CoBCGO has shown better resistance

452

to biofouling due to the minimal uptake even in the worst condition. Based on the experimental

453

evidence so far, it can be determined that the 5CoBCGO is an eligible alternative material to Nafion

454

117 for using as a separator in yeast-MFCs and other types of MFCs with single-chamber

455

architecture.

456 457

Declaration of interest

458

The authors declare no conflicts of interest.

459 460

Acknowledgment

461

Dr. Domenico Frattini was supported by the Korea Research Fellowship through the National

462

Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Republic of

463

Korea (No. 2017H1D3A1A01013887) and this work was also supported by the NRF and the

464

Ministry of Science, ICT and Future Planning (MSIP) (No. 2016M1A2A2937143).

465

26

466

References

467

[1]

applications. A review, J. Power Sources. 356 (2017) 225–244. doi:10.1016/j.jpowsour.2017.03.109.

468 469

C. Santoro, C. Arbizzani, B. Erable, I. Ieropoulos, Microbial fuel cells: From fundamentals to

[2]

P. Pandey, V.N. Shinde, R.L. Deopurkar, S.P. Kale, S.A. Patil, D. Pant, Recent advances in the use of

470

different substrates in microbial fuel cells toward wastewater treatment and simultaneous energy

471

recovery, Appl. Energy. 168 (2016) 706–723. doi:10.1016/j.apenergy.2016.01.056.

472

[3]

K.D.Z. Duarte, D. Frattini, Y. Kwon, High performance yeast-based microbial fuel cells by

473

surfactant-mediated gold nanoparticles grown atop a carbon felt anode, Appl. Energy. 256 (2019)

474

113912. doi:10.1016/j.apenergy.2019.113912.

475

[4]

Microbiol. 14 (2006) 512–518. doi:10.1016/j.tim.2006.10.003.

476 477

[5]

B.E. Logan, Exoelectrogenic bacteria that power microbial fuel cells, Nat. Rev. Microbiol. 7 (2009) 375–381. doi:10.1038/nrmicro2113.

478 479

B.E. Logan, J.M. Regan, Electricity-producing bacterial communities in microbial fuel cells, Trends

[6]

M. Christwardana, D. Frattini, G. Accardo, S.P. Yoon, Y. Kwon, Early-stage performance evaluation

480

of flowing microbial fuel cells using chemically treated carbon felt and yeast biocatalyst, Appl.

481

Energy. 222 (2018) 369–382. doi:10.1016/j.apenergy.2018.03.193.

482

[7]

M. Christwardana, D. Frattini, K.D.Z. Duarte, G. Accardo, Y. Kwon, Carbon felt molecular

483

modification and biofilm augmentation via quorum sensing approach in yeast-based microbial fuel

484

cells, Appl. Energy. 238 (2019) 239–248. doi:10.1016/j.apenergy.2019.01.078.

485

[8]

Y. Ahn, K.S. Yoo, L.-H. Kim, Y. Kwon, Development of biofuel cell adopting multiple

486

poly(diallyldimethylammonium chloride) layers immobilized on carbon nanotube as powerful

487

catalyst, Int. J. Hydrogen Energy. 41 (2016) 17548–17556. doi:10.1016/j.ijhydene.2016.07.124.

488

[9]

M. Christwardana, Y. Kwon, Yeast and carbon nanotube based biocatalyst developed by synergetic

489

effects of covalent bonding and hydrophobic interaction for performance enhancement of

490

membraneless microbial fuel cell, Bioresour. Technol. 225 (2017) 175–182.

491

doi:10.1016/j.biortech.2016.11.051.

27

492

[10]

S. Kang, K.S. Yoo, Y. Chung, Y. Kwon, Cathodic biocatalyst consisting of laccase and gold

493

nanoparticle for improving oxygen reduction reaction rate and enzymatic biofuel cell performance, J.

494

Ind. Eng. Chem. 62 (2018) 329–332. doi:10.1016/j.jiec.2018.01.011.

495

[11]

Y. Chung, J. Ji, Y. Kwon, Performance evaluation of enzymatic biofuel cells using a new cathodic

496

catalyst containing hemin and poly acrylic acid promoting the oxygen reduction reaction, J. Mater.

497

Chem. C. 7 (2019) 11597–11605. doi:10.1039/C9TC03071A.

498

[12]

Clean. Prod. 194 (2018) 359–371. doi:10.1016/J.JCLEPRO.2018.05.155.

499 500

[13]

[14]

J. Greenman, I.A. Ieropoulos, Allometric scaling of microbial fuel cells and stacks: The lifeform case for scale-up, J. Power Sources. 356 (2017) 365–370. doi:10.1016/j.jpowsour.2017.04.033.

503 504

V.G. Gude, Wastewater treatment in microbial fuel cells - An overview, J. Clean. Prod. 122 (2016) 287–307. doi:10.1016/j.jclepro.2016.02.022.

501 502

M.N.I. Siddique, Z.A. Wahid, Achievements and perspectives of anaerobic co-digestion: A review, J.

[15]

M. Jung, W. Lee, C. Noh, A. Konovalova, G.S. Yi, S. Kim, Y. Kwon, D. Henkensmeier, Blending

505

polybenzimidazole with an anion exchange polymer increases the efficiency of vanadium redox flow

506

batteries, J. Memb. Sci. 580 (2019) 110–116. doi:10.1016/j.memsci.2019.03.014.

507

[16]

C. Noh, M. Jung, D. Henkensmeier, S.W. Nam, Y. Kwon, Vanadium Redox Flow Batteries Using

508

meta -Polybenzimidazole-Based Membranes of Different Thicknesses, ACS Appl. Mater. Interfaces.

509

9 (2017) 36799–36809. doi:10.1021/acsami.7b10598.

510

[17]

M. Jung, W. Lee, N. Nambi Krishnan, S. Kim, G. Gupta, L. Komsiyska, C. Harms, Y. Kwon, D.

511

Henkensmeier, Porous-Nafion/PBI composite membranes and Nafion/PBI blend membranes for

512

vanadium redox flow batteries, Appl. Surf. Sci. 450 (2018) 301–311.

513

doi:10.1016/j.apsusc.2018.04.198.

514

[18]

S. Jeong, L.-H. Kim, Y. Kwon, S. Kim, Effect of nafion membrane thickness on performance of

515

vanadium redox flow battery, Korean J. Chem. Eng. 31 (2014) 2081–2087. doi:10.1007/s11814-014-

516

0157-5.

517 518

[19]

D. Huang, B.Y. Song, Y.L. He, Q. Ren, S. Yao, Cations Diffusion in Nafion117 Membrane of Microbial fuel cells, Electrochim. Acta. 245 (2017) 654–663. doi:10.1016/j.electacta.2017.06.004.

28

519

[20]

H.B. Khalili, D. Mohebbi-Kalhori, M.S. Afarani, Microbial fuel cell (MFC) using commercially

520

available unglazed ceramic wares: Low-cost ceramic separators suitable for scale-up, Int. J. Hydrogen

521

Energy. 42 (2017) 8233–8241. doi:10.1016/j.ijhydene.2017.02.095.

522

[21]

cells, Bioresour. Technol. 215 (2016) 296–303. doi:10.1016/j.biortech.2016.03.135.

523 524

[22]

V. Yousefi, D. Mohebbi-Kalhori, A. Samimi, Ceramic-based microbial fuel cells (MFCs): A review, Int. J. Hydrogen Energy. 42 (2017) 1672–1690. doi:10.1016/j.ijhydene.2016.06.054.

525 526

J. Winfield, I. Gajda, J. Greenman, I. Ieropoulos, A review into the use of ceramics in microbial fuel

[23]

I. Gajda, A. Stinchcombe, I. Merino-Jimenez, G. Pasternak, D. Sanchez-Herranz, J. Greenman, I.A.

527

Ieropoulos, Miniaturized ceramic-based microbial fuel cell for efficient power generation from urine

528

and stack development, Front. Energy Res. 6 (2018) 1–9. doi:10.3389/fenrg.2018.00084.

529

[24]

P. Theodosiou, J. Greenman, I. Ieropoulos, Towards monolithically printed Mfcs: Development of a

530

3d-printable membrane electrode assembly (mea), Int. J. Hydrogen Energy. 44 (2019) 4450–4462.

531

doi:10.1016/j.ijhydene.2018.12.163.

532

[25]

Cost Microbial Fuel Cells, ChemSusChem. 9 (2016) 88–96. doi:10.1002/cssc.201501320.

533 534

G. Pasternak, J. Greenman, I. Ieropoulos, Comprehensive Study on Ceramic Membranes for Low-

[26]

A.N. Ghadge, M.M. Ghangrekar, Development of low cost ceramic separator using mineral cation

535

exchanger to enhance performance of microbial fuel cells, Electrochim. Acta. 166 (2015) 320–328.

536

doi:10.1016/j.electacta.2015.03.105.

537

[27]

E. Yang, K.J. Chae, I.S. Kim, Assessment of different ceramic filtration membranes as a separator in

538

microbial fuel cells, Desalin. Water Treat. 57 (2016) 28077–28085.

539

doi:10.1080/19443994.2016.1183523.

540

[28]

G. Accardo, C. Ferone, R. Cioffi, Influence of Lithium on the Sintering Behavior and Electrical

541

Properties of Ce 0.8 Gd 0.2 O 1.9 for Intermediate-Temperature Solid Oxide Fuel Cells, Energy

542

Technol. 4 (2016) 409–416. doi:10.1002/ente.201500275.

543

[29]

G. Accardo, D. Frattini, H.C. Ham, S.P. Yoon, Direct addition of lithium and cobalt precursors to

544

Ce0.8Gd0.2O1.95 electrolytes to improve microstructural and electrochemical properties in IT-SOFC

545

at lower sintering temperature, Ceram. Int. 45 (2019) 9348–9358. doi:10.1016/j.ceramint.2018.07.209.

29

546

[30]

K. Huang, L. Zhang, T. Xu, H. Wei, R. Zhang, X. Zhang, B. Ge, M. Lei, J.-Y. Ma, L.-M. Liu, H. Wu,

547

−60 °C solution synthesis of atomically dispersed cobalt electrocatalyst with superior performance,

548

Nat. Commun. 10 (2019) 606. doi:10.1038/s41467-019-08484-8.

549

[31]

K. Huang, R. Wang, H. Wu, H. Wang, X. He, H. Wei, S. Wang, R. Zhang, M. Lei, W. Guo, B. Ge, H.

550

Wu, Direct immobilization of an atomically dispersed Pt catalyst by suppressing heterogeneous

551

nucleation at −40 °C, J. Mater. Chem. A. 7 (2019) 25779–25784. doi:10.1039/C9TA07469D.

552

[32]

G. Accardo, G.S. Kim, H.C. Ham, S.P. Yoon, Optimized lithium-doped ceramic electrolytes and their

553

use in fabrication of an electrolyte-supported solid oxide fuel cell, Int. J. Hydrogen Energy. 44 (2019)

554

12138–12150. doi:10.1016/j.ijhydene.2019.03.052.

555

[33]

L. Lutterotti, M. Bortolotti, G. Ischia, I. Lonardelli, H.-R. Wenk, Rietveld texture analysis from

556

diffraction images, Zeitschrift Für Krist. Suppl. 2007 (2007) 125–130.

557

doi:10.1524/zksu.2007.2007.suppl_26.125.

558

[34]

C. Aliotta, L.F. Liotta, V. La Parola, A. Martorana, E.N.. Muccillo, R. Muccillo, F. Deganello, Ceria-

559

based electrolytes prepared by solution combustion synthesis: The role of fuel on the materials

560

properties, Appl. Catal. B Environ. 197 (2016) 14–22. doi:10.1016/j.apcatb.2016.02.044.

561

[35]

Formaldehyde Oxidation, Environ. Sci. Technol. 49 (2015) 8675–8682. doi:10.1021/acs.est.5b01264.

562 563

H. Tan, J. Wang, S. Yu, K. Zhou, Support Morphology-Dependent Catalytic Activity of Pd/CeO2 for

[36]

M. Christwardana, D. Frattini, G. Accardo, S.P. Yoon, Y. Kwon, Optimization of glucose

564

concentration and glucose/yeast ratio in yeast microbial fuel cell using response surface methodology

565

approach, J. Power Sources. 402 (2018) 402–412. doi:10.1016/j.jpowsour.2018.09.068.

566

[37]

membranes, J. Memb. Sci. 520 (2016) 155–165. doi:10.1016/j.memsci.2016.07.021.

567 568

X. Luo, S. Holdcroft, Water transport through short side chain perfluorosulfonic acid ionomer

[38]

M.A. Izquierdo-Gil, V.M. Barragán, J.P.G. Villaluenga, M.P. Godino, Water uptake and salt transport

569

through Nafion cation-exchange membranes with different thicknesses, Chem. Eng. Sci. 72 (2012) 1–

570

9. doi:10.1016/j.ces.2011.12.040.

571

[39]

M. Adachi, T. Navessin, Z. Xie, F.H. Li, S. Tanaka, S. Holdcroft, Thickness dependence of water

572

permeation through proton exchange membranes, J. Memb. Sci. 364 (2010) 183–193.

573

doi:10.1016/j.memsci.2010.08.011. 30

574

[40]

J. Melnik, J. Luo, K.T. Chuang, A.R. Sanger, Stability and Electric Conductivity of Barium Cerate

575

Perovskites Co- Doped with Praseodymium, Open Fuels Energy Sci. J. 1 (2008) 7–10.

576

doi:10.2174/1876973x00801010007.

577

[41]

review, J. Polym. Sci. Part B Polym. Phys. 44 (2006) 2201–2225. doi:10.1002/polb.20861.

578 579

[42]

R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A. 32 (1976) 751–767. doi:10.1107/S0567739476001551.

580 581

N.W. DeLuca, Y.A. Elabd, Polymer electrolyte membranes for the direct methanol fuel cell: A

[43]

A. Bassano, V. Buscaglia, M. Viviani, M. Bassoli, M.T. Buscaglia, M. Sennour, A. Thorel, P. Nanni,

582

Synthesis of Y-doped BaCeO3 nanopowders by a modified solid-state process and conductivity of

583

dense fine-grained ceramics, Solid State Ionics. 180 (2009) 168–174. doi:10.1016/j.ssi.2008.12.026.

584

[44]

L. Spiridigliozzi, M. Biesuz, G. Dell’Agli, E. Di Bartolomeo, F. Zurlo, V.M. Sglavo, Microstructural

585

and electrical investigation of flash-sintered Gd/Sm-doped ceria, J. Mater. Sci. 52 (2017) 7479–7488.

586

doi:10.1007/s10853-017-0980-2.

587

[45]

G. Accardo, D. Frattini, H.C. Ham, J.H. Han, S.P. Yoon, Improved microstructure and sintering

588

temperature of bismuth nano-doped GDC powders synthesized by direct sol-gel combustion, Ceram.

589

Int. 44 (2018) 3800–3809. doi:10.1016/j.ceramint.2017.11.165.

590

[46]

L. Spiridigliozzi, G. Dell’Agli, G. Accardo, S.P. Yoon, D. Frattini, Electro-morphological, structural,

591

thermal and ionic conduction properties of Gd/Pr co-doped ceria electrolytes exhibiting mixed

592

Pr3+/Pr4+ cations, Ceram. Int. 45 (2019) 4570–4580. doi:10.1016/j.ceramint.2018.11.144.

593

[47]

M. Christwardana, D. Frattini, G. Accardo, S.P. Yoon, Y. Kwon, Effects of methylene blue and

594

methyl red mediators on performance of yeast based microbial fuel cells adopting polyethylenimine

595

coated carbon felt as anode, J. Power Sources. 396 (2018) 1–11. doi:10.1016/j.jpowsour.2018.06.005.

596

31

Highlights

• • • •

BCGO perovskites doped with Li and Co are compared with Nafion for MFCs Synthesis, powders surface and sintering are optimized to control porosity/pore size Permeability tests simulate single-chamber worst conditions for biofouling 5CoBCGO shows better biofouling resistance due to minimal uptake

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: