espace.library.uq.edu.au · Web viewPrior to testing, all parts were prepared as per the American Society for Testing Materials (ASTM) international standard G 102-89 [22]. Some SS

Oxidised stainless steel: a very effective electrode material for microbial fuel cell bioanodes but at high risk of corrosion

Pablo Ledezma*1, Bogdan C. Donose2, Stefano Freguia 1, 2 and Jurg Keller 1

1Advanced Water Management Centre, The University of Queensland, Brisbane QLD 4072,

Australia. 2Centre for Microbial Electrosynthesis, The University of Queensland, Brisbane

QLD 4072, Australia.

*Corresponding author: [email protected]

Phone: +61 (0)7 3346 3228

Fax: +61 7 3365 4726

Abstract

Bioelectrochemical systems (BES) are gaining momentum as biotechnological alternatives

for self-powered wastewater treatment and the recovery of valuable products from waste.

However, there is a strong need to reduce system costs and therefore some types of

stainless steel (SS) electrodes have been proposed as an alternative to carbonaceous

electrode materials, but their corrosion remains a strong concern. We hereby demonstrate

that a facile SS modification results in up to 45.367 mA cm-3 current production at room

temperature – the highest volumetric current density reported to date for BES bioanodes –

but that this benefit comes at a high risk of corrosion, which compromises the applicability

of SS-based Microbial Fuel Cells.

Keywords

Microbial fuel cell; bioanode; 316L stainless steel; biocorrosion

1. Introduction

Bioelectrochemical Systems (BES) are increasingly seen as a sustainable biotechnology

option for numerous niche applications, providing that a few barriers can be overcome [1].

BES comprise primarily Microbial Fuel Cells (MFCs) and Microbial Electrolysis Cells (MES), This is a post-print version of the following article: Ledezma, Pablo, Donose, Bogdan C., Freguia, Stefano and Keller, Jurg (2015) Oxidised stainless steel: a very effective electrode material for microbial fuel cell bioanodes but at high risk of corrosion. Electrochimica Acta, 158 356-360. doi:10.1016/j.electacta.2015.01.175

the former based on microbial oxidation of organic matter to produce of electricity with

bioanodes and the latter on the using biocathodes that drive reductive processes to recover

other valuable products [2]. In the last 15 years, massive improvements – both in

performance and fundamental understanding – have led to practical demonstrations of BES

as self-sustaining power sources for treatment/monitoring [3-5] and economically-viable

product recovery/synthesis from waste [6-8]. With such technical milestones attained, one

of the last hurdles keeping BES from real-world niche applications is high investment costs

and long payback times [1, 2]. In recent years, carbon-based materials have been utilised

extensively with excellent results [9], but unfortunately their high cost (which can be

≥US$150 m-2 projected exposed surface area, and significantly more if modified with the

latest nanoparticles) contributes significantly to the lack of economic viability.

More recently, the use of stainless steel (SS) has been proposed as an alternative material

for BES electrodes [10], due to lower costs and higher conductivity than e.g. carbon cloth

[11]. However, there is well-established evidence that exposing SS parts to bacteria – such

as iron-oxidising bacteria (IOB) and sulfate-reducing bacteria (SRB) – can favour corrosion

and part failure [12, 13]. Electroactive Bacteria (EAB), such as Geobacter spp can also

interact with metals but their role in corrosion remains unclear [14]. The use of SS anodes in

BES has resulted in the highest current density reported to date [15] but has also been

reported to suffer from enhanced corrosion [16-18], although in some circumstances a

degree of protection was observed [19].

Our investigations aim to clarify whether corrosion of 316L SS anodes does occur in the BES

anodic environment. We further explore a recent SS bioanode modification, and

subsequently show that the latter can enhance output levels to unprecedented values, but

that it also has severe corrosion implications.

2. Materials and Methods

2.1. 316L stainless steel electrode material and modificationThis is a post-print version of the following article: Ledezma, Pablo, Donose, Bogdan C., Freguia, Stefano and Keller, Jurg (2015) Oxidised stainless steel: a very effective electrode material for microbial fuel cell bioanodes but at high risk of corrosion. Electrochimica Acta, 158 356-360. doi:10.1016/j.electacta.2015.01.175

The austenitic stainless steel material utilised as anodic electrode was an AISI 316L sintered

fibre-felt – UNS31600 standard composition [20] with average weight percentage 16% Cr,

12% Ni, 3% Mo and 0.03% Carbon – which is manufactured as an air-filter for ventilation

ducts by LierFilter Ltd (China). This material can be obtained for ~US$50 m-2 in bulk and,

when compared to e.g. carbon cloth, it has comparable surface-area but superior

conductivity and mechanical strength [11, 21]. This felt had a 5 µm filter rating (achieved by

superposing layers of Ø10 µm SS fibres), a thickness of 0.3 ±10% mm, a total volumetric

density of 2 g cm-3 and an equivalent weight of 16.11 g eq-1. Prior to testing, all parts were

prepared as per the American Society for Testing Materials (ASTM) international standard G

102-89 [22].

Some SS coupons were flame-oxidised with a handheld butane torch – similarly to Guo and

colleagues’ method [21]. Both treated (SSt ; see Fig. 2e) and untreated (SSu ; see Fig. 2f)

materials, were additionally compared to plain carbon cloth (CC, from FuelCellStore, USA) as

a reference material.

2.2. BES and Electrochemical cell tests

2.2.1. BES

Six coupons – two of each SSu, SSt and CC – each with a projected surface area of 2 cm2 (0.5 x

4 cm x 0.03 cm, projected volume 0.24 cm3), were tested for bioanode performance in three

conventional 200mL borosilicate BES reactors [23]. Using degassed M1 medium (9 mM

(NH4)2SO4, 5.7 mM K2HPO4, 3.3 mM KH2PO4, 2 mM NaHCO3, 1 mM MgSO4, 0.5 mM CaCl2 and

1 mL L-1 trace elements stock solution [24]; pH 7.4) with 20 mM sodium acetate as sole

carbon source, the reactors were inoculated with20 mL of OD540nm = 0.250 effluent from a

long-term MFC enrichment (>5 years continuous reactor operation). This enrichment had

recently been analysed via 16s RNA pyrosequencing and was found to be composed

predominantly of Geobacter spp (avg. 81 % relative abundance), Chlrobaculum spp (avg.

4%), and Methanobrevibacter spp, Bacteroidales, Clostridiales, Ruminococcaceae and

Sphaerochaeta each in relative abundance ≤2 % [25]. The SS/CC coupons were tested as

This is a post-print version of the following article: Ledezma, Pablo, Donose, Bogdan C., Freguia, Stefano and Keller, Jurg (2015) Oxidised stainless steel: a very effective electrode material for microbial fuel cell bioanodes but at high risk of corrosion. Electrochimica Acta, 158 356-360. doi:10.1016/j.electacta.2015.01.175

working electrodes in chronoamperometric mode under constant stirring and poised at -

0.205 V vs Ag/AgCl 3.5M (equivalent to 0.0 V vs SHE) with Pt-wires as counter electrodes

and medium replaced approx. every 10 days.

2.2.2. Electrochemical tests

A further five SS coupons of each type (SSu/SSt) were tested in two 3-pin electrochemical

cells (20 mL vol. from BASi, USA) – one left in open circuit and the other poised at 0.0 V vs

SHE – with M1 medium, inoculated with EAB and with media replaced as above. These parts

were subject to Cyclic Potentiodynamic Polarisation (CPP) at 0.1667 mV s-1, starting from -20

mV vs Eoc and pursued towards +2.0 V vs Ag/AgCl until a limit of 2 mA cm-2 was attained, at

which point the cycles were reversed towards Eoc until repassivation was attained [21]. The

information obtained was used to understand the corrosion behaviour of these coupons

over 70 days’ exposure to EAB and to determine their respective free corrosion (Ecorr), pitting

or onset (Epit) and repassivation (Erep) potentials along with the observable passive regions

(between Ecorr and Epit, as exemplified on Fig. 2a). Furthermore, data from the CPPs for

treated coupons was used to estimate the corrosion current icorr using the Stern-Geary

method [12] with EC-Lab v10.32 (Bio-Logic, France).

Before and between all electrochemical tests, the anodic potentials were stabilised at Eoc for

30 min. Each coupon was used only once. All experiments were carried at room

temperature (22 ± 3 °C). SEM micrographs (secondary electron) were obtained using a JEOL

NeoScope or a Phillips XL30 at 10 kV accelerating voltage. Size determination of pore-like

features on biofilm micrographs were obtained with ImageJ v1.48 [26].

3. Results and Discussion

3.1. BES performance

The electrical output results (Fig. 1a) confirm the excellent performance of SSt as a bioanode

material. In less than 20 days, a maximum current density of 2.722 ± 0.187 mA cm-2 was This is a post-print version of the following article: Ledezma, Pablo, Donose, Bogdan C., Freguia, Stefano and Keller, Jurg (2015) Oxidised stainless steel: a very effective electrode material for microbial fuel cell bioanodes but at high risk of corrosion. Electrochimica Acta, 158 356-360. doi:10.1016/j.electacta.2015.01.175

observed for SSt (see Section 3.3.2. for evidence that this current levels are not due to SSt

corrosion), whereas CC peaked at 1.501 ± 0.129 mA cm-2 at day 15. The maximal output of

our SSt was slightly higher than previously reported [21] (probably due to differences in SS-

material, preparation and inoculum), but again amongst the highest surface-projected

current densities attained so far. In volumetric terms however, this amounts to an average

of 45.367 ± 3.111 mA cm-3, to best knowledge the highest volumetric current density

reported to date. Although the SSt is not strictly a three-dimensional (3D) material, Chen et

al. [27] have already shown that cumulative addition of thin layers can achieve current

densities superior to true 3D materials. Accordingly, we believe our projection is valid, but

aim to demonstrate this with future experiments.

The exceptional results obtained with SSt are, firstly the result of long-term MFC enrichment

[28]. Secondly, the flame-oxidation treatment was clearly effective with regards to

enhancing bacterial attachment. As hinted by the SEM in Fig. 1c, EABs were unable to attach

to SSu in large numbers even after 120 days; this may explain why the output of these

bioanodes never surpassed 0.2 mA cm-2 (see Fig. 1a). Conversely, Fig. 1b points to a highly

developed biofilm being present on SSt. It has previously been demonstrated that the flame-

oxidation treatment results in a uniform layer of iron nanoparticles [21]; such nanoparticle

coatings have shown excellent electrocatalytic properties in other fuel cells [29], so these

may have a role – whether in providing numerous anchoring points or in facilitating direct

electron transfer – resulting in the levels of attachment observed.

A further favourable element observable in Fig. 1b, is the porous nature of the SS material

which helps maintain pore sizes of Ø5-30 µm (measured with ImageJ), allowing for better

nutrient distribution to the biofilm and thus more uniform and faster metabolic rates, and

consequently higher electrical output [24]. Only very recent studies recognise the

importance of using porous materials for biofilm development [15, 24, 27, 30, 31], despite

the fact that the benefits doing so were demonstrated over 25 years ago [32].

3.2. Electrochemical tests


In confirmation of all aforecited studies where sterile medium for EAB was used, the blank

SSu/SSt coupons displayed an initial Ecorr of -815 ± 29 mV vs Ag/AgCl (see Fig. 2a and 2b), the

treatment causing no major electrochemically-measurable differences at this point, except

for a passivation peak at -223 ± 29 mV vs Ag/AgCl observed for most SSu coupons (see Fig.

2a and 2c), but absent for all SSt coupons (see Fig. 2b and 2d), thus very likely corresponding

to Fe oxidation.

Furthermore, although their CPP hysteresis loops differed slightly, both blanks showed

typical oxygen-evolution peaks reaching the voltage limit of +2.0 vs Ag/AgCl rather than the

current density limit, which is indicative of strong protection against corrosion [12, 33, 34],

as previously observed for uniformly-oxidised SS by Okado et al. [35], although their

treatment was by inductively-coupled oxygen plasma.

Following inoculation, significant ennoblement ensued for all coupons – again in

confirmation of numerous previous studies with exposures to EAB, SRB and IOB (e.g. [17, 36,

37] respectively) – although the CPP behaviour became markedly different depending on

testing conditions.

3.2.1. Results for SS coupons in open circuit

For treated and untreated non-poised coupons (SStnp and SSunp , respectively), Ecorr

ennoblement was Δ+485 ± 32 mV from five days after inoculation and remained so after 2

months. For SSunp (Fig. 2a), the corrosion onset Epit was at +475mV vs Ag/AgCl after 5 days

and increased to +756 mV at day 65. For SStnp (Fig. 2b) however, Epit reached a very high

+1022 mV from day 5 and remained significantly unchanged (at +1011 mV) afterwards.

Despite these differences, it can be generally concluded that the risk of corrosion for SSnp

coupons was lowered by the presence of EAB, given that (i) the coupons’ Epit was ennobled

pointedly for SSunp (Δ+371 mV) and more significantly so for SStnp (Δ+919 mV) over time and

(ii) their passive regions were maintained and even slightly expanded: at day 0, the average

passive region Epit – Ecorr was 1054 ± 178 mV, whereas at day 65 the average range of Epit-Ecorr

reached 1193 ± 181 mV.


No significant bacterial attachment was observed for all SSnp coupons at the end of the

testing period (SEMs not pictured, but very similar to Fig. 2c), which is understandable since

EAB are not able to utilise open-circuit electrodes for direct electron transfer [38-40].

3.2.2. Results for poised SS coupons

Conversely, poising the coupons at -205mV vs Ag/AgCl allowed the EAB to respire (whether

directly or indirectly) via the working electrodes, leading to different potentiodynamic

behaviour. At day 5, SSu (Fig. 2c) displayed very similar levels of Ecorr/Epit ennoblement to its

non-poised counterpart (Δ+475 and Δ+65 mV respectively), although the hysteresis loop

was much narrower than that of SSunp (see day 5 in Fig. 2a), indicating a lower incidence of

localised corrosion [34]. In a similar fashion, SSt displayed little or no loop hysteresis at day 5

(see Fig. 2d), with its Epit - Erep = 20 mV, suggesting minimal or no localised corrosion [22, 34].

However, though the Ecorr ennoblement of Δ+666 mV for SSt was the highest for all coupons

at day 5, its passive region was also the smallest (Epit – Ecorr = 377 mV).

This conflicting effect – ennoblement of Ecorr on the one hand and reduction of the passive

region on the other – would become exacerbated over time for poised coupons. At day 65

and as shown in Figs. 2c and 2d, the hysteresis-loop dynamics changed drastically

(formation in a ‘counter-clockwise direction’) since pitting and repassivation potentials

became inversed (Epit < Erep) , whereas in all previous tests Epit > Erep (resulting in ‘clockwise’

loops).

For other materials and conditions, Epit < Erep need not be deleterious, and can in fact be an

indication that no localised corrosion (particularly pitting) occurs during the

potentiodynamic tests [34]. However, for the latter to be valid, a broad passive region – i.e.

Epit >> Ecorr – is also required in oxidising environments. Yet as can be observed in Figs. 2c and

2d, the passive regions for SSu and SSt were greatly diminished to 210 and 128 mV

respectively after 65 days of poising/EAB-exposure. Given that these two negative effects

occurred simultaneously, during those two CPPs we actually observed the transpassive

dissolution [41] of our treated/untreated coupons (see Fig. 2g and 2h respectively).


The CPPs is Fig. 2h show that if the SSt working potential could be kept below its fully

ennobled Ecorr of +35 mV vs Ag/AgCl, then the electrodes would remain in the cathodic

region and thus protected from corrosion [41]. Even at Ecorr, the corrosion current icorr was

only 3.488 µA cm-2 after 5 days (Ecorr: -170 mV) and decreased to 0.358 µA cm-2 after 65

(estimations via Stern-Geary method). Accordingly, the exceptional current output of SSt

discussed in Section 3.1. could not have been the consequence of SS corrosion because of

the difference in magnitude vs observed icorr and moreover because the coupons were

poised at -205 mV vs Ag/AgCl (i.e. the cathodic region for SSt after 5+ days’ ennoblement).

In practical terms however, anodic potentials are not potentiostatically-controlled in MFCs,

and given that there are a vast number of passive (e.g. permeation of dissolved oxygen [42])

and operational conditions (transient nutrient depletion [43]) where the anodic potential

could easily shift ≥+200 mV, the risk of corrosion is very high (see extensive damage in Fig.

2h). This poses a serious problem for the long-term viability of SS-based BES.

Interestingly, the SEMs showed no observable bacterial attachment for SSu but appreciable

biomass accumulation over SSt (not pictured since analogous to Fig. 1b/1c), yet their

inversed CPP dynamics were very similar. Therefore, we hypothesise that the change in

corrosion behaviour in the mid-term was not caused by attachment, but by the conditions

brought about by the bacteria [44]. From previous investigations, we know that EAB are

capable of secreting mediators [45] which may become adsorbed to the electrode surface

[46]. If such mediators were to remain adsorbed to the electrode (rather than returning to

the bulk liquid to be re-reduced), then they could possibly alter in a significant way the

corrosion dynamics of an SS electrode [47]. The desiccation steps for SEM observation

inherently affect such molecules, so these experiments need to be repeated to determine

whether this is the case.

Accordingly, although using oxidised SS can lead to unprecedented anodic current levels at a

lower cost than carbon-based materials, the CPP results highlight that greater emphasis

must be placed on SS corrosion behaviour over time and under variable conditions, before


SS-based BES can be implemented outside the laboratory.

Acknowledgments

This work was funded by the Australian Research Council project DP 120104415. S.F. is

supported by the ARC fellowship DE130101168.


Figures

Figure 1.a. Chronoamperometric performance of the first 33 days post-inoculation for the three tested materials. Arrows indicate anolyte replacement (*arrow: SSt was not fed). b. SEM for SSt and c. SEM for SSu 120 days after inoculation.


Figure 2. CPP tests on SS coupons (blanks: coupons in sterile M1 medium for 24h; days 5 and 65 are timed after EAB inoculation). Circular arrow marks next to each trace’s legend indicate the direction in which the hysteresis loop was formed (clockwise or counter-clockwise). Black arrows matching each trace type indicate the direction of the polarisation. Red arrows exemplify the positions of Epit, Erep, Ecorr and the passive region for SStnp blank. a. SSu and b. SSt coupons left in open circuit over 65 days. c. SSu and d. SSt coupons poised at -0.205 V vs Ag/AgCl for 65 days. e. SSu coupon and f. SSt coupon after CPP on day 5. g. SSu and h. SSt coupon after CPP on day 65; note the abundance of corrosion by-products.

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espace.library.uq.edu.au · Web viewPrior to testing, all parts were prepared as per the American Society for Testing Materials (ASTM) international standard G 102-89 [22]. Some SS