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
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
2.1. 316L stainless steel electrode material and modification
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.
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
4%), and Methanobrevibacter spp, Bacteroidales, Clostridiales, Ruminococcaceae and
Sphaerochaeta each in relative abundance ≤2 % [25]. The SS/CC coupons were tested as
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
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 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
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 SSmaterial, 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 flameoxidation 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].
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
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
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
passive region Epit – Ecorr was 1054 ± 178 mV, whereas at day 65 the average range of EpitEcorr 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
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
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.
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
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.
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
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.
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
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 counterclockwise). 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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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
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