- Email: [email protected]
Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures Xuejie Li a, Peng Zhou a, Kevin Ogle a,**, Sebastian Proch b,*, €rgen Westlinder b Manas Paliwal b, Anna Jansson b, Jo a b
Chimie ParisTech, PSL University, CNRS, Institut de Recherche Chimie Paris (IRCP), F-75005, Paris, France Sandvik Materials Technology, Sandviken, Sweden
highlights Stainless-steels
graphical abstract
exhibit
highly
transient dissolution properties. Dissolution
via
passive-
transpassive-passive
transitions
(z100 s). Decoupled bipolar plate potentials will lead to ultra-low metal ion dissolution. Potentiostatic ex-situ screening of stainless-steel
bipolar
plates
seems unsuitable.
article info
abstract
Article history:
Stainless steels, due to their scalability, formability, and cost effectiveness, are, today, used
Received 16 August 2019
as bipolar plates in proton-exchange membrane fuel cells (PEMFCs). The release of cationic
Received in revised form
metal species during fuel cell operation may have an impact on fuel cell durability if
18 October 2019
accumulated in the ionomer phase (membrane electrode assembly). However, the mech-
Accepted 24 October 2019
anisms and time scales of this release from stainless-steel surfaces are not fully under-
Available online xxx
stood, hence, a systematic study was carried out. Time-resolved dissolution measurements of bright-annealed stainless steels in a simulated fuel cell show measurable ion dissolution
Keywords:
exclusively under changing potentials (passive-transpassive-passive transition, transient
Proton exchange membrane fuel
transpassivity, 100s). Together with shielded (decoupled) bipolar plates these findings
cells (PEMFCs)
suggest the absence of continued metal ion release from stainless-steel bipolar plates and,
Metallic bipolar plates (BPPs)
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Ogle), [email protected] (S. Proch). https://doi.org/10.1016/j.ijhydene.2019.10.191 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
2
international journal of hydrogen energy xxx (xxxx) xxx
Stainless steel
in turn, ultra-low Fe dissolution. Moreover, prolonged potentiostatic tests seem unsuitable
Atomic emission
since they will probe the passive state with low or absent dissolution for the most part.
spectroelectrochemistry (AESEC)
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Transient transpassivity
Introduction Proton-exchange membrane fuel cells (PEMFCs) are imagined as a vital part of locally carbon emission-free automotive propulsion [1,2]. The reversible voltage of a single-cell PEMFC is only 1.23 V [3,4] which is, moreover, substantially reduced down to 0.5e0.9 V by the sluggishness of the oxygen reduction reaction (ORR) taking place at the PEMFC cathode [1,5e8]. In contrast, practical automotive applications require several hundreds of volts, e.g., the 2017 Toyota Mirai has an open circuit potential (OCP) of 315 V, which can be boosted up to 650 V. This is achieved by connecting 370 single cells in series resulting in a fuel cell stack [9]. A PEMFC stack is commonly constructed in a bipolar architecture [7] and the bipolar plates (BPPs), connecting the single cells, are desired to have the following properties. (a) gas tight (separation of H2 and O2) (b) high conductivity (low interfacial contact resistance [ICR] to minimize ohmic losses) (c) high stability against dissolution (no trace-metal contamination of the cell/stack) (d) high heat conductivity (to facilitate easy cooling) (e) light-weight and thin (high power density) (f) good mechanical and forming properties (stable stack, mass production). Metallic, especially stainless-steel, BPPs fulfill many of the above properties [10] and are, furthermore, scalable and massproduction compatible for a market encompassing millions of vehicles per year [11]. Bare, i.e., uncoated, stainless steels typically lack in properties (b) and/or (c) and, thus, they are usually coated to ensure low interfacial contact resistance and low metal (cat)ion dissolution [12e14]. Stainless steels are multicomponent iron alloys and their corrosion stability is brought about by a thin chromium oxide film (passive film) [15]. Common stainless-steel compositions are presented in Table 1. For uncoated stainless steels ICR and
Table 1 e Elemental compositions of common stainlesssteel grades. Given are the maximum allowed values while Fe is obtained from the balance. Elements in low concentration, e.g., Si, C, N, etc. have been neglected. elemental content
Cr (wt%) Mn (wt%) Fe (wt%) Ni (wt%) Cu (wt%) Mo (wt%)
dissolution are strongly potential dependent as follows. High positive potentials lead to anodic (oxide) film formation on stainless steels via migration and, hence, high contact resistance [15e18]. Moreover, the various alloy components in stainless steels exhibit individual, potential- and pHdependent dissolution patterns which cause time-dependent (transient) phenomena that are absent for pure metals [15,18,19]. Metal cations can replace membrane protons in the proton exchange membrane (PEM) and the ionomer which leads to higher proton resistance and, therefore, lower voltages [8,20]. More severely, some metal ions (Ti, Fe, Cu) are “fenton-active” [21] and create radicals that degrade the PEM and the ionomer chemically (membrane thinning) [22e25], Fe2þ is particularly dangerous in this respect. Membrane thinning can be mitigated by the addition of radical scavengers like Ce and Mn [26]. The effect of metal cations on PEMFCs is summarized in Table 2. A comparison between Tables 2 and 1 presents a significant overlap in relevant metal cations, especially fenton-active Fe and Cu, rendering stainless steels a potential threat to PEMFC durability [22]. However, a systematic, time-resolved study of stainless-steel dissolution taking “multicomponent effects” [15,18,19] into account is lacking so far. What would be the relevant potential range (transients) for such a study? One assumption is that the BPP experiences the same potentials (transients) as the fuel cell electrodes (catalyst layers). The anode potential is usually at ±0 V vs. reversible hydrogen electrode (RHE) while the cathode potential is load dependent and ranges between þ0.5 (high load) and þ0.9 V vs. RHE (low load) [1,8]. However, both anode and cathode can be at potentials above of þ1.5 V vs. RHE during transient events due to conditions called global [8] and local [27,28] hydrogen starvation. In contrast, research by Hinds et al. using an in-situ reference electrode during fuel cell operation has demonstrated that potentials experienced by the BPP are very mild due to a strong shielding of the BPP from the fuel cell electrodes [29,30]. The shielding effect is enhanced in the presence of microporous layers (MPLs) [31]. MPLs as part of the gas diffusion layer (GDL) are standard in
Table 2 e Overview of the effects of metal cations on PEMFCs. replacement of membrane protons
steel grade 304L
316L
904L
20 2 65 12 e e
18 2 61 15 e 3
23 2 39 28 2 5
Cr Ni Co Cu Fe Mn …
membrane degradation
mitigation
Ti Fe Cu
Mn Ce
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
3
international journal of hydrogen energy xxx (xxxx) xxx
Table 3 e Wavelengths used for ICP-AES and detection limits of the investigated elements.
Wavelength (nm) Detection limit (ppb)
Fe
Cr
Ni
Mn
Mo
259.940 3.3 ± 0.5
267.716 5.8 ± 1.0
231.604 12.4 ± 0.9(Poly)/3.7 ± 0.9(Mono)
257.610 0.7 ± 0.1
202.032 8.1 ± 0.4(Poly)/3.3 ± 1.0(Mono)
PEMFC technology [8,32]. In this research, the cathode side of the BPP was found to be at OCP (ca. þ0.5 V vs. RHE) and the anode side at ±0 V vs. RHE drifting towards OCP under airingress, e.g., during start up shut down (SUSD) cycles. In this contribution we take a systematic look at transient stainless-steel dissolution relevant for PEMFC operation. First, we investigate the situation when the BPP follows the electrode transients to fully comprehend multicomponent dissolution modes and, subsequently, the discussion is reduced to more realistic, decoupled BPPs as elucidated by Hinds et al. [29e31]. We will show that this drastically lowers the metal (cat)ion contamination “potential” of stainless steel in PEMFCs.
Experimental Atomic emission spectroelectrochemistry (AESEC): The AESEC technique has been well described in previous articles [33,34]. An inductively coupled plasma - atomic emission spectrometer (ICP-AES, HORIBA Jobin Yvon, Ultima 2C) is coupled with an electrochemical flow cell to measure the dissolution originating from the sample downstream. The sample was exposed as the working electrode (1.0 cm2) to a small volume of electrolyte in the flow cell. Immediately before the experiment the samples were cleaned ultrasonically in acetone, ethanol, and water for 10 min each, leaving the pristine surface oxide (bright anneal, see below) intact. The reference electrode (reversible hydrogen electrode, RHE, MiniHydrogen, Gaskatel, Germany) and the counter electrode (platinum foil) were placed in a secondary compartment separated from the reaction compartment by a cellulose membrane (Zellu Trans/Roth) which allowed ionic conduction but prevented bulk mixing of the electrolytes. The electrolyte was introduced into the reaction compartment by a capillary from the bottom of the reaction compartment at a flow rate of about 3 mL/min using a peristaltic pump. The wavelengths used for ICP-AES on Fe, Cr, Ni, Mn, and Mo (not shown since only present in 904L) as well as the detection limits of those are shown in Table 3. The detection limits C3s are calculated as follows C3s ¼ 3 sblank=a with sblank a standard deviation of the background and a as the sensitivity factor calculated from the calibration curves of each element at their specific wavelengths. The Ultima 2C ICPAES is equipped with a polychromator and a monochromator. Normally, the element detected by the monochromator has a lower detection limit. For 304L and 316L, Ni was used in the monochromator and Mo for 904L. The AESEC measures the intensity of each element, Il, at the specific wavelength from Table 3 as a function of time. The
measured intensities were transformed into dissolution rates using the equation below n ¼ fðIl I+l Þ kl A with f as the flow rate, Il as the background intensity, kl as the sensitivity factor and A as the surface area exposed to the electrolyte. The electrolyte used in AESEC experiments was H2SO4 solution at pH 3.0. The temperature was kept at 343.15 K (70 C). All potentials are reported relative to RHE. Three potential cycles (see i, ii, and iii below) were applied to various steel grades (304L, 316L and 904L, see Table 1) and each step was held for 10 min: (i) OCP - 0.9 Ve1.5 V - 0.9 V - OCP (ii) OCP - 0.5 Ve1.5 V - 0.5 V - OCP (iii) OCP - 0 Ve1.5 V - 0 V e OCP. Thermodynamic calculations using FactSage: All stainlesssteel samples (see Table 1) are subjected to a bright annealing treatment in a continuous annealing furnace. To reduce the oxidation of the steel surface, a reducing gas atmosphere is created via an N2eH2 gas mixture. However, the reducing atmosphere is still insufficient to prevent the oxidation of strongly oxophilic elements such as Cr, Mn, and Fe. As a result, formation of complex spinels is inevitable. These oxides play an important role in controlling the dissolution of ions such as Fe, Cr, and Ni during AESEC experiments. Therefore, it is important to deduce the chemistry of these oxides as a function of the steel grade, reducing atmosphere, and furnace temperature. Thermodynamic calculations were carried out using FactSage thermochemical software [35]. FactPS, FTOxide, and FSteel databases were used for the calculations, as they contain critically evaluated and consistently assessed thermodynamic data for gases, oxides and steel, respectively. X-ray photoelectron spectroscopy (XPS): A Phi Quantera II instrument with an Al Ka monochromator was used for XPS analysis. The C1s CeC peak was calibrated to 284.8 eV. Survey scans were performed with 224 eV and the region scans with a pass energy of 26 eV. Peak fitting was performed with Casa XPS. The fitting parameters such a binding energy, percentage of total area and spectral component separation was taken from the work of Biesinger et al. [36].
Results and discussion Atomic-emission spectroelectrochemistry (AESEC) [33,34,37,38] The investigation of stainless-steel dissolution under conditions relevant to PEMFC operation requires a method that is
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
4
international journal of hydrogen energy xxx (xxxx) xxx
Scheme 1 e Overview of the atomic emission spectroelectrochemistry (AESEC), see text.
both time and element resolved due to the widely differing effects of dissolved ions on the fuel cell (Table 2). The US Department of Energy (DoE) suggests to simulate the fuel cell by a 49 ppm H2SO4 solution (pH 3) [39]. It is, furthermore, proposed to add 0.1 ppm of HF to mimic the degradation of the PEM [25], however, this has been omitted to keep the investigated system simple, as follows. Fluorides, in particular HF, are used as pickling solutions for stainless steels and are known to lead to potential-independent ion dissolution, e.g., HF/HNO3 leads to the formation of FeF3 and (CrFe)F3 phases in spent pickling solutions at low pH, following treatment of 316 [40,41]. Moreover, negative effects of fluorides on stainlesssteel orthodontic wires are discussed [42]. Potentialdependent fluoride-assisted dissolution is also a possibility as found, e.g., for Ta and Nb [43]. A further concern is that the presence of fluoride might change ion speciation by the formation of anions (e.g., FeF3 6 ), which cannot enter the membrane, or more specific, the ionomer phase and, thus, have no consequences for membrane degradation. We believe that potential-dependently released cations are the true species responsible for membrane degradation, however, we are not aware of any previous studies that investigated the effect of ion speciation on the chemical destruction of the PEM/ ionomer. The AESEC method is presented in Scheme 1. An electrochemical flow cell is attached to an electrolyte reservoir that is analyzed either directly by ICP-AES (blank) or after it has passed the flow cell so that it contains solutes from the sample. The ICP analysis allows the investigation of the
dissolved elements and, thus, their effect on PEMFC durability can be judged (see Table 2). Moreover, fast electronics permit recording of element concentrations in a time-resolved fashion, which is important since stainless steels are highly transient materials when considering dissolution and oxidation (passive film formation) [16]. This is due to their multicomponent nature and is explained in more detail below.
Bright annealed stainless steels and their surfaces All steels from Table 1 were bright annealed, which is a typical step in the manufacturing of stainless steel, to reduce the oxide layer thickness and give it a bright and attractive finish. Bright annealing describes the exposure of the steel to high temperatures in a reducing atmosphere to obtain the pure face-centered cubic (fcc) phase. The exact conditions of this treatment are unknown to us and, thus, we have evaluated a range of conditions by thermodynamic calculations using FactSage [35]. The oxygen partial pressure in an annealing furnace is controlled by the H2 concentration and the dew points of the N2eH2 mixture in the furnace. The chemical reaction that controls the oxygen partial pressure is given as: H2 (g) þ 0.5 O2 / H2O (g)
(1)
As seen from eq. (1), the oxygen partial pressure can be decreased with increasing H2 concentration and decreasing dew point. The calculated oxygen partial pressure with varying concentration of H2/N2 and corresponding dew points are
Fig. 1 e Calculated oxygen partial pressures in N2eH2 atmospheres and the corresponding dew points. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 2 e Phase diagrams of 304L, 316L, and 904L under typical bright annealing conditions calculated using thermochemical software FactSage [35].
shown in Fig. 1. The figure shows that the oxygen partial pressure obtained at the lowest dew point, 50 C, is log10(pO2) ¼ 27 at 800 C. Since, as stated above, the exact operating conditions of the annealing furnace are unknown, O2 partial pressure variation between 20 and 40 (log10[pO2]) and furnace temperatures between 700 and 1000 C are assumed. This variation encompasses the whole range of operating conditions in the bright annealing furnace. Although this is a very
conservative range, the practical operating conditions target temperatures around 800 C and O2 partial pressures between 25 and 30 (log10[pO2]). The oxidation phase diagrams for the 304L, 316L, and 904L steel grades are presented in Fig. 2 and, in all three cases, a mixture of SiO2, Cr2O3, and complex spinel phases are predicted in the temperature range between 700 and 800 C and log10(pO2) ¼ 26 to 30. In addition to FactSage calculations an XPS study of the pristine materials has been carried out and is presented in
Fig. 3 e XPS study of pristine, bright-annealed 304L, 316L, and 904L. Peak fitting has been carried out using Casa XPS and fit parameters were obtained from Biesinger et al. [36]. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e Surface models of pristine, bright-annealed stainless steels as obtained from XPS. Please note that this surface structure is simplified and does not consider other elements. For a more in-depth look at stainless-steel surfaces readers are referred to the publication by Olsson as an overview [15].
Fig. 5 e Typical potential profile used in the AESEC experiments shown below. The “working potential” is changed to mimic the potentials in an operando fuel cell, i.e., cathode operation at low or high load and anode operation. The potential steps called break in, transient event, and return to working potentials are color coded. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Survey scans of 304L and 316L indicate the presence of carbon, oxygen, chromium, iron, and manganese on the surface. In case of 904L, Cr and Fe cannot be detected but, exclusively, Mn. The Mn species are very likely the predicted
ones from the thermodynamic calculations (Fig. 2) since we know from experience that, e.g., CrxMnyO4 and SiO2 particles, are found on pristine, bright-annealed stainless-steel surfaces. The carbon signal in all steels can be interpreted as
Fig. 6 e Dissolution transients of 304L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). Appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e Dissolution transients of 316L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). Appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
adventitious carbon [44], which is present on almost all surfaces following exposure to air. Adventitious carbon is present in varying amounts and the 904L surface contains the highest amount which is also reflected in the O1s signal as follows. The O1s signal of all investigated steel surfaces contains two distinct contributions. The peak at low binding energies (z530 eV, red fitting curve) is attributed to metal oxides [36] while the contribution at high binding energies (z532 eV) was attributed to oxygen functionalities of adventitious carbon (see C1s region scans). 904L, for which high amounts of adventitious carbon are detected, also has the highest contribution of the high binding energy part of the O1s signal. The Cr2p signals on all steels can be fitted as Cr2O3 as predicted by the thermodynamic calculations (Fig. 2) using the
parameters from Biesinger et al. [36]. Moreover, Cr2O3 is known to be responsible for the stainless properties of steels [15]. The most problematic element in XPS is also the most interesting one, namely Fe. It was already stated by Biesinger et al. [36] that “with so many possible species having overlapping binding energies erroneous interpretation can result”. Fe3O4 and Fe2O3 are iron-oxides that are typically found on stainless steels, e.g., duplex steel [45], it is, however, very hard to distinguish between them using peak fitting and similarly satisfying fits could be obtained for both species. Due to the deviating behavior of 904L (see below) we assigned the Fe2p signal to Fe3O4. The surface models presented in Fig. 4 can be derived for the pristine, bright annealed samples (considering only Fe and Cr for clarity).
Fig. 8 e Dissolution transients of 904L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). 904L is, in contrast to 304L and 316L (see Figs. 6 and 7), not stable towards Fe dissolution at OCP (purple circles). Apart from that appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
8
international journal of hydrogen energy xxx (xxxx) xxx
Table 4 e Dissolution modes of base metals and their alloys as described by Landolt [17]. active metal dissolution cations are formed at the metal-electrolyte interface soluble ions are formed and dissolve into the electrolyte higher dissolution rate than passive dissolution
passive metal dissolution
transpassive metal dissolution
surface of the metal is covered with a thin oxide layer (1 dissolution of the passive film e3 nm for stainless steels [15]) surface state is not well-defined, and the surface cations are formed at the metal-film interface may or may not be covered by an oxide cations migrate across the film to the film-electrolyte interface where they dissolve in the electrolyte lower dissolution rate than active dissolution
AESEC transients of 304L, 316L, and 904L (BPP follows electrode transients) A typical potential profile of an AESEC experiment is shown in Fig. 5 and it was assumed that the BPP follows the potential transients of the fuel cell electrodes (catalyst layers) during transient events. The potential can be described in the following sequence, OPC / working potential (i.e., cathode low or high load, anode) / transient potential (þ1.5 V vs. RHE) / working potential / OCP. The first potential step from OCP to the working potential is called “break in” (magenta in Fig. 5), the second step from the working potential to the transient potential is termed “transient event” (golden in Fig. 5), and the
final step back to the working potential will be referred to as “return to working potential” (green in Fig. 5). The AESEC traces (dissolution transients) of 304L, 316L, and 904L in sulfuric acid at 343.15 K (70 C) are presented in Figs. 6e8 and it is apparent that measurable dissolution of metal ions, in particular Fe, is found, exclusively, upon potential changes (red circles). Holding the potential for more than about 100s leads to a return to very low, i.e., with AESEC (Scheme 1) immeasurable dissolution. We term this return to low dissolution rates within about 100s “transient transpassivity” and the mechanisms will be described in detail below. That behavior is found in all investigated steel grades which is due to the multicomponent nature of the stainless-
Fig. 9 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of þ0.9 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 10 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of þ0.5 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8. steel alloys which leads to multicomponent surface oxides (“passive” films). In the case of 904L (Fig. 8) the alloy is not stable towards Fe dissolution at OCP (purple circles) which is attributed to a different Fe oxide on the surface after bright anneal (Fig. 4). This will be explained in detail below. Interpretation of the AESEC transients in Figs. 6e8 requires a review of the dissolution modes of base metals and their alloys as discussed by Landolt [17], they encompass active, passive, and transpassive dissolution. Their respective properties are summarized in Table 4. Surface films consisting of multiple oxide species should be considered as transpassive rather than passive films since their components might show
different potential-dependent dissolution behavior. The features in the AESEC traces, denoted as “break in”, “transient event”, and “return to working potential” (see Fig. 5) can be described in terms of “triads”, i.e., passive-transpassivepassive transitions. In the following we will explore every such transition for 904L because this alloy is not stable at OCP, which grants a closer look (transitions are analogous for 304L and 316L). In the next section we will see that metal cation release simplifies to the “break in” potential step for the more realistic BPP transients as measured by Hinds et al. [29e31]. Mechanistic cartoons of the triads observed for 904L steel at a working potential of þ0.9 V vs. RHE are presented in Fig. 9.
Fig. 11 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of ±0 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
10
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 12 e Estimation of the metal ion contamination “potential” of bare (uncoated) 304L bipolar plates at the cathode and anode side in the absence of high transient potentials (decoupled bipolar plates [29e31]).
Even at OCP preferential iron dissolution (red curve) can be observed and is attributed to the presence of Fe3O4, which is unstable towards dissolution in acidic media [46,47], rather than Fe2O3 in the other alloys (see Fig. 4). Cr2O3 seems to be stable towards dissolution at higher temperatures and pressures [48] and in our investigation it does not leach up to at least þ0.9 V vs. RHE for all steel grades (Figs. 6e8). This means, once a Cr2O3 film is formed at the outer surface (Fig. 9 OCP left) further Fe dissolution is stopped since the whole system is passive. Fe dissolution can only be triggered again when the Cr2O3 film becomes soluble (transpassive) which requires potentials in excess of þ0.9 V vs. RHE. In the “break in” step (OCP / þ0.9 V vs. RHE) not much is happening since soluble Fe has already been removed at OCP although a small amount can still be detected. 304L and 316L are stable at OCP and þ0.9 V vs. RHE and, thus, preferential Fe dissolution is observed only in the “break in” step. In the following “transpassive event” a potential is reached at which Cr2O3 becomes soluble as Cr6þ species [48] and, therefore, the whole system becomes transpassive again. However, at þ1.5 V vs. RHE iron is oxidized to yield Fe2O3, which is stable, and, hence, forms a new “passive” film consisting of iron oxide instead of chromium oxide (Fig. 9 transient event). This passive film “exchange” is known as secondary passivation of stainless steels [49]. In the “return to the working potential” step nothing happens since Fe2O3 is stable at þ0.9 V vs. RHE. This also suggests that Fe3O4 is not formed since this species was soluble at þ0.9 V vs. RHE in the “break in” step. Moreover, 904L, which was special in the pristine case, becomes “normalized”, i.e.,
it behaves like 304L and 316L after being subject to the potential steps in Fig. 5. Triads of 904L at a working potential of þ0.5 V vs. RHE are shown in Fig. 10. A minor difference compared to Fig. 9 can be found in the “break in” step. In the former all soluble Fe has been leached already at OCP so that nothing happens during “break in”. The major difference is that Fe2O3 becomes soluble at þ0.5 V vs. RHE while at þ0.9 V vs. RHE it is insoluble. This shows the impact of the transient event. When the passive chromium oxide film is “exchanged” for an iron oxide film this Fe2O3 film is soluble at low potentials and the Cr2O3 film must be reestablished (Fig. 10 return to working potential). A transient event will always trigger Fe dissolution by passive film exchange and should, therefore, be avoided. Fig. 11 shows the triads for a working potential of ±0 V vs. RHE and it is very similar to Fig. 10 since Fe2O3 is also soluble at the working potential here. The critical issue in Figs. 10 and 11 is the transient event. In the OCP/break in step all soluble Fe is leached and a stable Cr2O3 film is formed which suppresses further Fe dissolution (below measurable amounts). However, the “vicious” cycle of preferential Fe release is restarted by passive film “exchange” at high potential (secondary passivation [49]). It might be argued that metal ion dissolution depends strongly on the nature of the passive film and, in turn, on bright annealing or other treatment conditions. However, Figs. 6e8 also show the integrated AESEC traces, i.e., the total amount of dissolved metals in the respective potential step. The total mass of Fe in a monolayer (ML) of Fe2O3 or Fe3O4 can be estimated as 75 ngFe∙cm2 (assuming bulk density, i.e.,
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
5 g cm3 and a monolayer thickness of 0.2 nm). The total dissolved amounts in the OCP/break in step are similar for all stainless-steel grades with approximately 200 ngFe∙cm2 geo (>2 ML). This is also true for the “transient event” with about 300 ngFe∙cm2 geo (>3 ML) and the “return to working potential” with ca. 400 ngFe∙cm2 geo (>4 ML) when going from þ1.5 to ±0 V vs. RHE. In summary, stainless-steel dissolution can be described by passive-transpassive-passive transitions caused by potential changes. These transitions are on the order of 100s (transient transpassivity) and outside those transitions metal ion release cannot be detected by ICP-AES (passive state). Stainless steel dissolution can be imagined as playing the piano, i.e., a string of chords (triads) causes dissolution or music to happen. Holding the keys for too long will make the sound linger but it will quickly fade away just like transpassive dissolution returns to passive state. If the bipolar plate potentials follow the electrode (catalyst layer) transients Fe dissolution will be observed every time a transient event with excursion to high potentials takes place. The reason is the exchange of the Cr2O3 passive film by an Fe2O3 passive film which is dissolved when going back the normal working potential of the electrode (see Figs. 10 and 11).
Decoupled (shielded) bipolar plates Measurements of the mixed BPP/GDL corrosion potential in the presence of an MPL by Castanheira et al. [31] showed potentials of ±0 V vs. RHE for the anode side of the BPP and about þ0.5 V vs. RHE for the cathode side. Even under SUSD cycling (local hydrogen starvation) when the electrodes experienced high transient potentials the BPP was never above þ0.7 V vs. RHE. These results suggest that in an operando fuel cell the potential is unlikely to be in the range where Cr2O3 is dissolved (at least up to þ0.9 V vs. RHE, see Figs. 6e8). In turn, a passive film “exchange” (secondary passivation [49]) would not take place and, therefore, metal dissolution of the steel can be described by the OCP/break in potential steps. The Fe contamination “potential” of 304L is shown in Fig. 12 as an example. Even though, perfect coatings do not exist they will still attenuate the dissolved amount from uncoated steels to low double to single digit ngFe∙cm2 geo. In this ultra-low dissolution range classic potentiostatic trace-metal analysis is likely to fail, especially, considering that longer measuring times will only probe the passive state with low or no dissolution.
Conclusions Time-resolved measurements of stainless-steel dissolution using atomic emission spectroelectrochemistry (AESEC) suggest that ions are dissolved upon potential changes via passive-transpassive-passive transitions (transient transpassivity, ca. 100s). Continued Fe dissolution can only be observed in the case of transient events with potentials at which Cr2O3 can be dissolved. This leads to a passive film “exchange” (Cr2O3 vs. Fe2O3) which is known as secondary passivation and subsequent return to the working potential will dissolve this film. In the more realistic case of shielded
11
bipolar plates such potentials are unlikely to be observed at the BPP and, thus, continued Fe dissolution cannot take place. In combination with a coating this leads to ultra-low amounts of Fe that will be released into an operando fuel cell from stainless steel bipolar plates.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.191.
references
[1] Gasteiger HA, Garche J. Fuel cells. Handbook of heterogeneous catalysis. Wiley-VCH Verlag GmbH & Co. KGaA; 2008. [2] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [3] Bard AJ, Parsons R, Jordan J. Standard potentials in aqueous solutions. New York: Marcel Dekker, Inc.; 1985. [4] Bard AJ, Faulkner LR. Electrochemical methods fundamentals and applications. 2nd ed. New York: John Wiley & Sons, Inc.; 2001. NM. Just a dreamdor future reality? [5] Gasteiger HA, Markovic Science 2009;324:48. [6] Shao M, Chang Q, Dodelet J-P, Chenitz R. Recent advances in electrocatalysts for oxygen reduction reaction. Chem Rev 2016;116:3594e657. [7] Larminie J, Dicks A, Larminie J, Dicks A. Introduction. Fuel cell systems explained. John Wiley & Sons, Ltd; 2013. p. 1e24. [8] Buechi FN, Schmidt TJ. Polymer electrolyte fuel cell durability. New York: Springer Science þ Business Media, LLC.; 2009. [9] Lohse-Busch H, Duoba M, Stutenberg K, Iliev S, Kern M, Richards B, et al. Technology assessment of a fuel cell vehicle: 2017 Toyota Mirai. U.S. Department of Energy, Fuel Cell Technologies Office; 2018. [10] Wang H, Sweikart MA, Turner JA. Stainless steel as bipolar plate material for polymer electrolyte membrane fuel cells. J Power Sources 2003;115:243e51. [11] Statista. Number of cars sold worldwide from 1990 to 2018 (in million units). 2018. € m M, Eriksson L, Andersson J, Sjo € blom G, [12] Proch S, Stenstro Jansson A, et al. Coated stainless steel as bipolar plate material for anion exchange membrane fuel cells (AEMFCs). Int J Hydrogen Energy 2019. € m N, Bode n A, Randstro € m S, Lindbergh G. [13] Oyarce A, Holmstro In-situ measurements of contact resistance and in-situ durability studies of steels and coatings to be used as bipolar plates in PEMFCs. ECS Trans 2009;25:1791e801. € m N, Bode n A, Lagergren C, Lindbergh G. [14] Oyarce A, Holmstro Operating conditions affecting the contact resistance of bipolar plates in proton exchange membrane fuel cells. J Power Sources 2013;231:246e55. [15] Olsson COA. Passivation of stainless steels and other chromium bearing alloys. Encycl Interfacial Chem Surf Sci Electrochem 2017. https://doi.org/10.1016/B978-0-12-4095472.13585-1. Elsevier. [16] Olsson C-OA, Landolt D. Film growth during anodic polarization in the passive region on 304 stainless steels with Cr, Mo, or W additions studied with EQCM and XPS. J Electrochem Soc 2001;148:B438e49.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
12
international journal of hydrogen energy xxx (xxxx) xxx
[17] Landolt D. Corrosion and surface chemistry of metals. 1 ed. Lausanne, Switzerland: CRC Press; 2007. € ss H. Compositional [18] Kirchheim R, Heine B, Hofmann S, Hofsa changes of passive films due to different transport rates and preferential dissolution. Corros Sci 1990;31:573e8. [19] Herting G, Odnevall Wallinder I, Leygraf C. A comparison of release rates of Cr, Ni, and Fe from stainless steel alloys and the pure metals exposed to simulated rain events. J Electrochem Soc 2005;152:B23e9. [20] Gasteiger HA, Kocha SS, Sompalli B, Wagner FT. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl Catal, B 2005;56:9e35. [21] Fenton HJH. LXXIII.-Oxidation of tartaric acid in presence of iron. J Chem Soc Trans 1894;65:899e910. [22] Pozio A, Silva RF, De Francesco M, Giorgi L. Nafion degradation in PEFCs from end plate iron contamination. Electrochim Acta 2003;48:1543e9. [23] LaConti AB, Hamdan M, McDonald RC. Mechanisms of membrane degradation. Handbook of fuel cells. John Wiley & Sons, Ltd; 2010. [24] Wong KH, Kjeang E. Mitigation of chemical membrane degradation in fuel cells: understanding the effect of cell voltage and iron ion redox cycle. ChemSusChem 2015;8:1072e82. [25] Zaton M, Roziere J, Jones DJ. Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review. Sustainable Energy & Fuels 2017;1:409e38. [26] Coms FD, Liu H, Owejan JE. Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions. ECS Trans 2008;16:1735e47. [27] Reiser CA, Bregoli L, Patterson TW, Yi JS, Yang JD, Perry ML, et al. A reverse-current decay mechanism for fuel cells. Electrochem Solid State Lett 2005;8:A273e6. [28] Qin C, Wang J, Yang D, Li B, Zhang C. Proton exchange membrane fuel cell reversal: a review. Catalysts 2016;6. [29] Brightman E, Hinds G. In situ mapping of potential transients during start-up and shut-down of a polymer electrolyte membrane fuel cell. J Power Sources 2014;267:160e70. [30] Hinds G, Brightman E. Towards more representative test methods for corrosion resistance of PEMFC metallic bipolar plates. Int J Hydrogen Energy 2015;40:2785e91. [31] Castanheira L, Bedouet M, Kucernak A, Hinds G. Influence of microporous layer on corrosion of metallic bipolar plates in fuel cells. J Power Sources 2019;418:147e51. € pler J, Lehmann J. Hydrogen and fuel cell - technologies [32] To and market perspectives. Berlin Heidelberg: Springer-Verlag Berlin Heidelberg; 2014. [33] Ogle K. Atomic Emission Spectroelectrochemistry: a new look at the corrosion, dissolution and passivation of complex materials. Mater Corros 2012;37:58e65.
[34] Ogle K, Baeyens J, Swiatowska J, Volovitch P. Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel. Electrochim Acta 2009;54:5163e70. lisle E, Chartrand P, Decterov SA, Eriksson G, [35] Bale CW, Be Gheribi AE, et al. FactSage thermochemical software and databases, 2010e2016. Calphad 2016;54:35e53. [36] Biesinger MC, Payne BP, Grosvenor AP, Lau LWM, Gerson AR, Smart RSC. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl Surf Sci 2011;257:2717e30. [37] Hamm D, Ogle K, Olsson COA, Weber S, Landolt D. Passivation of FeeCr alloys studied with ICP-AES and EQCM. Corros Sci 2002;44:1443e56. [38] Swiatowska J, Volovitch P, Ogle K. The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry. Corros Sci 2010;52:2372e8. [39] DOE. DOE. Technical targets for polymer electrolyte membrane fuel cell components. 2015. [40] Covino BS, Scalera JV, Driscoll TJ, Carter JP. Dissolution behavior of 304 stainless steel in HNO3/HF mixtures. Metall Trans A 1986;17:137e49. [41] Reddy RG, Wang S, Chen B. Solubility of iron in spent pickling solutions. Mining, Metallurgy & Exploration 1993;10:102e7. [42] Castro SM, Ponces MJ, Lopes JD, Vasconcelos M, Pollmann MCF. Orthodontic wires and its corrosiondthe specific case of stainless steel and beta-titanium. J Dent Sci 2015;10:1e7. [43] Ranjith PM, Rao MT, Sapra S, Suni II, Srinivasan R. On the anodic dissolution of tantalum and niobium in hydrofluoric acid. J Electrochem Soc 2018;165:C258e69. [44] Barr TL, Seal S. Nature of the use of adventitious carbon as a binding energy standard. J Vac Sci Technol A 1995;13:1239e46. [45] Guo LQ, Qin SX, Yang BJ, Liang D, Qiao LJ. Effect of hydrogen on semiconductive properties of passive film on ferrite and austenite phases in a duplex stainless steel. Sci Rep 2017;7:3317. [46] Hickling A. The cathodic reduction potential of Fe3O4 and the flade potential of iron. Electrochim Acta 1973;18:635e7. [47] White AF, Peterson ML, Hochella MF. Electrochemistry and dissolution kinetics of magnetite and ilmenite. Geochem Cosmochim Acta 1994;58:1859e75. [48] Cook WG, Olive RP. Pourbaix diagrams for chromium, aluminum and titanium extended to high-subcritical and low-supercritical conditions. Corros Sci 2012;58:291e8. [49] Bojinov M, Betova I, Fabricius G, Laitinen T, d RR, Saario T. The stability of the passive state of ironechromium alloys in sulphuric acid solution. Corros Sci 1999;41:1557e84.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures Xuejie Li a, Peng Zhou a, Kevin Ogle a,**, Sebastian Proch b,*, €rgen Westlinder b Manas Paliwal b, Anna Jansson b, Jo a b
Chimie ParisTech, PSL University, CNRS, Institut de Recherche Chimie Paris (IRCP), F-75005, Paris, France Sandvik Materials Technology, Sandviken, Sweden
highlights Stainless-steels
graphical abstract
exhibit
highly
transient dissolution properties. Dissolution
via
passive-
transpassive-passive
transitions
(z100 s). Decoupled bipolar plate potentials will lead to ultra-low metal ion dissolution. Potentiostatic ex-situ screening of stainless-steel
bipolar
plates
seems unsuitable.
article info
abstract
Article history:
Stainless steels, due to their scalability, formability, and cost effectiveness, are, today, used
Received 16 August 2019
as bipolar plates in proton-exchange membrane fuel cells (PEMFCs). The release of cationic
Received in revised form
metal species during fuel cell operation may have an impact on fuel cell durability if
18 October 2019
accumulated in the ionomer phase (membrane electrode assembly). However, the mech-
Accepted 24 October 2019
anisms and time scales of this release from stainless-steel surfaces are not fully under-
Available online xxx
stood, hence, a systematic study was carried out. Time-resolved dissolution measurements of bright-annealed stainless steels in a simulated fuel cell show measurable ion dissolution
Keywords:
exclusively under changing potentials (passive-transpassive-passive transition, transient
Proton exchange membrane fuel
transpassivity, 100s). Together with shielded (decoupled) bipolar plates these findings
cells (PEMFCs)
suggest the absence of continued metal ion release from stainless-steel bipolar plates and,
Metallic bipolar plates (BPPs)
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Ogle), [email protected] (S. Proch). https://doi.org/10.1016/j.ijhydene.2019.10.191 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
2
international journal of hydrogen energy xxx (xxxx) xxx
Stainless steel
in turn, ultra-low Fe dissolution. Moreover, prolonged potentiostatic tests seem unsuitable
Atomic emission
since they will probe the passive state with low or absent dissolution for the most part.
spectroelectrochemistry (AESEC)
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Transient transpassivity
Introduction Proton-exchange membrane fuel cells (PEMFCs) are imagined as a vital part of locally carbon emission-free automotive propulsion [1,2]. The reversible voltage of a single-cell PEMFC is only 1.23 V [3,4] which is, moreover, substantially reduced down to 0.5e0.9 V by the sluggishness of the oxygen reduction reaction (ORR) taking place at the PEMFC cathode [1,5e8]. In contrast, practical automotive applications require several hundreds of volts, e.g., the 2017 Toyota Mirai has an open circuit potential (OCP) of 315 V, which can be boosted up to 650 V. This is achieved by connecting 370 single cells in series resulting in a fuel cell stack [9]. A PEMFC stack is commonly constructed in a bipolar architecture [7] and the bipolar plates (BPPs), connecting the single cells, are desired to have the following properties. (a) gas tight (separation of H2 and O2) (b) high conductivity (low interfacial contact resistance [ICR] to minimize ohmic losses) (c) high stability against dissolution (no trace-metal contamination of the cell/stack) (d) high heat conductivity (to facilitate easy cooling) (e) light-weight and thin (high power density) (f) good mechanical and forming properties (stable stack, mass production). Metallic, especially stainless-steel, BPPs fulfill many of the above properties [10] and are, furthermore, scalable and massproduction compatible for a market encompassing millions of vehicles per year [11]. Bare, i.e., uncoated, stainless steels typically lack in properties (b) and/or (c) and, thus, they are usually coated to ensure low interfacial contact resistance and low metal (cat)ion dissolution [12e14]. Stainless steels are multicomponent iron alloys and their corrosion stability is brought about by a thin chromium oxide film (passive film) [15]. Common stainless-steel compositions are presented in Table 1. For uncoated stainless steels ICR and
Table 1 e Elemental compositions of common stainlesssteel grades. Given are the maximum allowed values while Fe is obtained from the balance. Elements in low concentration, e.g., Si, C, N, etc. have been neglected. elemental content
Cr (wt%) Mn (wt%) Fe (wt%) Ni (wt%) Cu (wt%) Mo (wt%)
dissolution are strongly potential dependent as follows. High positive potentials lead to anodic (oxide) film formation on stainless steels via migration and, hence, high contact resistance [15e18]. Moreover, the various alloy components in stainless steels exhibit individual, potential- and pHdependent dissolution patterns which cause time-dependent (transient) phenomena that are absent for pure metals [15,18,19]. Metal cations can replace membrane protons in the proton exchange membrane (PEM) and the ionomer which leads to higher proton resistance and, therefore, lower voltages [8,20]. More severely, some metal ions (Ti, Fe, Cu) are “fenton-active” [21] and create radicals that degrade the PEM and the ionomer chemically (membrane thinning) [22e25], Fe2þ is particularly dangerous in this respect. Membrane thinning can be mitigated by the addition of radical scavengers like Ce and Mn [26]. The effect of metal cations on PEMFCs is summarized in Table 2. A comparison between Tables 2 and 1 presents a significant overlap in relevant metal cations, especially fenton-active Fe and Cu, rendering stainless steels a potential threat to PEMFC durability [22]. However, a systematic, time-resolved study of stainless-steel dissolution taking “multicomponent effects” [15,18,19] into account is lacking so far. What would be the relevant potential range (transients) for such a study? One assumption is that the BPP experiences the same potentials (transients) as the fuel cell electrodes (catalyst layers). The anode potential is usually at ±0 V vs. reversible hydrogen electrode (RHE) while the cathode potential is load dependent and ranges between þ0.5 (high load) and þ0.9 V vs. RHE (low load) [1,8]. However, both anode and cathode can be at potentials above of þ1.5 V vs. RHE during transient events due to conditions called global [8] and local [27,28] hydrogen starvation. In contrast, research by Hinds et al. using an in-situ reference electrode during fuel cell operation has demonstrated that potentials experienced by the BPP are very mild due to a strong shielding of the BPP from the fuel cell electrodes [29,30]. The shielding effect is enhanced in the presence of microporous layers (MPLs) [31]. MPLs as part of the gas diffusion layer (GDL) are standard in
Table 2 e Overview of the effects of metal cations on PEMFCs. replacement of membrane protons
steel grade 304L
316L
904L
20 2 65 12 e e
18 2 61 15 e 3
23 2 39 28 2 5
Cr Ni Co Cu Fe Mn …
membrane degradation
mitigation
Ti Fe Cu
Mn Ce
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
3
international journal of hydrogen energy xxx (xxxx) xxx
Table 3 e Wavelengths used for ICP-AES and detection limits of the investigated elements.
Wavelength (nm) Detection limit (ppb)
Fe
Cr
Ni
Mn
Mo
259.940 3.3 ± 0.5
267.716 5.8 ± 1.0
231.604 12.4 ± 0.9(Poly)/3.7 ± 0.9(Mono)
257.610 0.7 ± 0.1
202.032 8.1 ± 0.4(Poly)/3.3 ± 1.0(Mono)
PEMFC technology [8,32]. In this research, the cathode side of the BPP was found to be at OCP (ca. þ0.5 V vs. RHE) and the anode side at ±0 V vs. RHE drifting towards OCP under airingress, e.g., during start up shut down (SUSD) cycles. In this contribution we take a systematic look at transient stainless-steel dissolution relevant for PEMFC operation. First, we investigate the situation when the BPP follows the electrode transients to fully comprehend multicomponent dissolution modes and, subsequently, the discussion is reduced to more realistic, decoupled BPPs as elucidated by Hinds et al. [29e31]. We will show that this drastically lowers the metal (cat)ion contamination “potential” of stainless steel in PEMFCs.
Experimental Atomic emission spectroelectrochemistry (AESEC): The AESEC technique has been well described in previous articles [33,34]. An inductively coupled plasma - atomic emission spectrometer (ICP-AES, HORIBA Jobin Yvon, Ultima 2C) is coupled with an electrochemical flow cell to measure the dissolution originating from the sample downstream. The sample was exposed as the working electrode (1.0 cm2) to a small volume of electrolyte in the flow cell. Immediately before the experiment the samples were cleaned ultrasonically in acetone, ethanol, and water for 10 min each, leaving the pristine surface oxide (bright anneal, see below) intact. The reference electrode (reversible hydrogen electrode, RHE, MiniHydrogen, Gaskatel, Germany) and the counter electrode (platinum foil) were placed in a secondary compartment separated from the reaction compartment by a cellulose membrane (Zellu Trans/Roth) which allowed ionic conduction but prevented bulk mixing of the electrolytes. The electrolyte was introduced into the reaction compartment by a capillary from the bottom of the reaction compartment at a flow rate of about 3 mL/min using a peristaltic pump. The wavelengths used for ICP-AES on Fe, Cr, Ni, Mn, and Mo (not shown since only present in 904L) as well as the detection limits of those are shown in Table 3. The detection limits C3s are calculated as follows C3s ¼ 3 sblank=a with sblank a standard deviation of the background and a as the sensitivity factor calculated from the calibration curves of each element at their specific wavelengths. The Ultima 2C ICPAES is equipped with a polychromator and a monochromator. Normally, the element detected by the monochromator has a lower detection limit. For 304L and 316L, Ni was used in the monochromator and Mo for 904L. The AESEC measures the intensity of each element, Il, at the specific wavelength from Table 3 as a function of time. The
measured intensities were transformed into dissolution rates using the equation below n ¼ fðIl I+l Þ kl A with f as the flow rate, Il as the background intensity, kl as the sensitivity factor and A as the surface area exposed to the electrolyte. The electrolyte used in AESEC experiments was H2SO4 solution at pH 3.0. The temperature was kept at 343.15 K (70 C). All potentials are reported relative to RHE. Three potential cycles (see i, ii, and iii below) were applied to various steel grades (304L, 316L and 904L, see Table 1) and each step was held for 10 min: (i) OCP - 0.9 Ve1.5 V - 0.9 V - OCP (ii) OCP - 0.5 Ve1.5 V - 0.5 V - OCP (iii) OCP - 0 Ve1.5 V - 0 V e OCP. Thermodynamic calculations using FactSage: All stainlesssteel samples (see Table 1) are subjected to a bright annealing treatment in a continuous annealing furnace. To reduce the oxidation of the steel surface, a reducing gas atmosphere is created via an N2eH2 gas mixture. However, the reducing atmosphere is still insufficient to prevent the oxidation of strongly oxophilic elements such as Cr, Mn, and Fe. As a result, formation of complex spinels is inevitable. These oxides play an important role in controlling the dissolution of ions such as Fe, Cr, and Ni during AESEC experiments. Therefore, it is important to deduce the chemistry of these oxides as a function of the steel grade, reducing atmosphere, and furnace temperature. Thermodynamic calculations were carried out using FactSage thermochemical software [35]. FactPS, FTOxide, and FSteel databases were used for the calculations, as they contain critically evaluated and consistently assessed thermodynamic data for gases, oxides and steel, respectively. X-ray photoelectron spectroscopy (XPS): A Phi Quantera II instrument with an Al Ka monochromator was used for XPS analysis. The C1s CeC peak was calibrated to 284.8 eV. Survey scans were performed with 224 eV and the region scans with a pass energy of 26 eV. Peak fitting was performed with Casa XPS. The fitting parameters such a binding energy, percentage of total area and spectral component separation was taken from the work of Biesinger et al. [36].
Results and discussion Atomic-emission spectroelectrochemistry (AESEC) [33,34,37,38] The investigation of stainless-steel dissolution under conditions relevant to PEMFC operation requires a method that is
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
4
international journal of hydrogen energy xxx (xxxx) xxx
Scheme 1 e Overview of the atomic emission spectroelectrochemistry (AESEC), see text.
both time and element resolved due to the widely differing effects of dissolved ions on the fuel cell (Table 2). The US Department of Energy (DoE) suggests to simulate the fuel cell by a 49 ppm H2SO4 solution (pH 3) [39]. It is, furthermore, proposed to add 0.1 ppm of HF to mimic the degradation of the PEM [25], however, this has been omitted to keep the investigated system simple, as follows. Fluorides, in particular HF, are used as pickling solutions for stainless steels and are known to lead to potential-independent ion dissolution, e.g., HF/HNO3 leads to the formation of FeF3 and (CrFe)F3 phases in spent pickling solutions at low pH, following treatment of 316 [40,41]. Moreover, negative effects of fluorides on stainlesssteel orthodontic wires are discussed [42]. Potentialdependent fluoride-assisted dissolution is also a possibility as found, e.g., for Ta and Nb [43]. A further concern is that the presence of fluoride might change ion speciation by the formation of anions (e.g., FeF3 6 ), which cannot enter the membrane, or more specific, the ionomer phase and, thus, have no consequences for membrane degradation. We believe that potential-dependently released cations are the true species responsible for membrane degradation, however, we are not aware of any previous studies that investigated the effect of ion speciation on the chemical destruction of the PEM/ ionomer. The AESEC method is presented in Scheme 1. An electrochemical flow cell is attached to an electrolyte reservoir that is analyzed either directly by ICP-AES (blank) or after it has passed the flow cell so that it contains solutes from the sample. The ICP analysis allows the investigation of the
dissolved elements and, thus, their effect on PEMFC durability can be judged (see Table 2). Moreover, fast electronics permit recording of element concentrations in a time-resolved fashion, which is important since stainless steels are highly transient materials when considering dissolution and oxidation (passive film formation) [16]. This is due to their multicomponent nature and is explained in more detail below.
Bright annealed stainless steels and their surfaces All steels from Table 1 were bright annealed, which is a typical step in the manufacturing of stainless steel, to reduce the oxide layer thickness and give it a bright and attractive finish. Bright annealing describes the exposure of the steel to high temperatures in a reducing atmosphere to obtain the pure face-centered cubic (fcc) phase. The exact conditions of this treatment are unknown to us and, thus, we have evaluated a range of conditions by thermodynamic calculations using FactSage [35]. The oxygen partial pressure in an annealing furnace is controlled by the H2 concentration and the dew points of the N2eH2 mixture in the furnace. The chemical reaction that controls the oxygen partial pressure is given as: H2 (g) þ 0.5 O2 / H2O (g)
(1)
As seen from eq. (1), the oxygen partial pressure can be decreased with increasing H2 concentration and decreasing dew point. The calculated oxygen partial pressure with varying concentration of H2/N2 and corresponding dew points are
Fig. 1 e Calculated oxygen partial pressures in N2eH2 atmospheres and the corresponding dew points. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 2 e Phase diagrams of 304L, 316L, and 904L under typical bright annealing conditions calculated using thermochemical software FactSage [35].
shown in Fig. 1. The figure shows that the oxygen partial pressure obtained at the lowest dew point, 50 C, is log10(pO2) ¼ 27 at 800 C. Since, as stated above, the exact operating conditions of the annealing furnace are unknown, O2 partial pressure variation between 20 and 40 (log10[pO2]) and furnace temperatures between 700 and 1000 C are assumed. This variation encompasses the whole range of operating conditions in the bright annealing furnace. Although this is a very
conservative range, the practical operating conditions target temperatures around 800 C and O2 partial pressures between 25 and 30 (log10[pO2]). The oxidation phase diagrams for the 304L, 316L, and 904L steel grades are presented in Fig. 2 and, in all three cases, a mixture of SiO2, Cr2O3, and complex spinel phases are predicted in the temperature range between 700 and 800 C and log10(pO2) ¼ 26 to 30. In addition to FactSage calculations an XPS study of the pristine materials has been carried out and is presented in
Fig. 3 e XPS study of pristine, bright-annealed 304L, 316L, and 904L. Peak fitting has been carried out using Casa XPS and fit parameters were obtained from Biesinger et al. [36]. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e Surface models of pristine, bright-annealed stainless steels as obtained from XPS. Please note that this surface structure is simplified and does not consider other elements. For a more in-depth look at stainless-steel surfaces readers are referred to the publication by Olsson as an overview [15].
Fig. 5 e Typical potential profile used in the AESEC experiments shown below. The “working potential” is changed to mimic the potentials in an operando fuel cell, i.e., cathode operation at low or high load and anode operation. The potential steps called break in, transient event, and return to working potentials are color coded. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. Survey scans of 304L and 316L indicate the presence of carbon, oxygen, chromium, iron, and manganese on the surface. In case of 904L, Cr and Fe cannot be detected but, exclusively, Mn. The Mn species are very likely the predicted
ones from the thermodynamic calculations (Fig. 2) since we know from experience that, e.g., CrxMnyO4 and SiO2 particles, are found on pristine, bright-annealed stainless-steel surfaces. The carbon signal in all steels can be interpreted as
Fig. 6 e Dissolution transients of 304L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). Appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e Dissolution transients of 316L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). Appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
adventitious carbon [44], which is present on almost all surfaces following exposure to air. Adventitious carbon is present in varying amounts and the 904L surface contains the highest amount which is also reflected in the O1s signal as follows. The O1s signal of all investigated steel surfaces contains two distinct contributions. The peak at low binding energies (z530 eV, red fitting curve) is attributed to metal oxides [36] while the contribution at high binding energies (z532 eV) was attributed to oxygen functionalities of adventitious carbon (see C1s region scans). 904L, for which high amounts of adventitious carbon are detected, also has the highest contribution of the high binding energy part of the O1s signal. The Cr2p signals on all steels can be fitted as Cr2O3 as predicted by the thermodynamic calculations (Fig. 2) using the
parameters from Biesinger et al. [36]. Moreover, Cr2O3 is known to be responsible for the stainless properties of steels [15]. The most problematic element in XPS is also the most interesting one, namely Fe. It was already stated by Biesinger et al. [36] that “with so many possible species having overlapping binding energies erroneous interpretation can result”. Fe3O4 and Fe2O3 are iron-oxides that are typically found on stainless steels, e.g., duplex steel [45], it is, however, very hard to distinguish between them using peak fitting and similarly satisfying fits could be obtained for both species. Due to the deviating behavior of 904L (see below) we assigned the Fe2p signal to Fe3O4. The surface models presented in Fig. 4 can be derived for the pristine, bright annealed samples (considering only Fe and Cr for clarity).
Fig. 8 e Dissolution transients of 904L in H2SO4 (pH 3) at 343.15 K (70 C) recorded by AESEC (atomic emission spectroelectrochemistry). Shown is the AESEC trace (top) with the dissolution rates and the total dissolution amount during the respective potential step (bottom, obtained by integration of the AESEC trace). 904L is, in contrast to 304L and 316L (see Figs. 6 and 7), not stable towards Fe dissolution at OCP (purple circles). Apart from that appreciable dissolution is, exclusively, observed upon potential changes (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
8
international journal of hydrogen energy xxx (xxxx) xxx
Table 4 e Dissolution modes of base metals and their alloys as described by Landolt [17]. active metal dissolution cations are formed at the metal-electrolyte interface soluble ions are formed and dissolve into the electrolyte higher dissolution rate than passive dissolution
passive metal dissolution
transpassive metal dissolution
surface of the metal is covered with a thin oxide layer (1 dissolution of the passive film e3 nm for stainless steels [15]) surface state is not well-defined, and the surface cations are formed at the metal-film interface may or may not be covered by an oxide cations migrate across the film to the film-electrolyte interface where they dissolve in the electrolyte lower dissolution rate than active dissolution
AESEC transients of 304L, 316L, and 904L (BPP follows electrode transients) A typical potential profile of an AESEC experiment is shown in Fig. 5 and it was assumed that the BPP follows the potential transients of the fuel cell electrodes (catalyst layers) during transient events. The potential can be described in the following sequence, OPC / working potential (i.e., cathode low or high load, anode) / transient potential (þ1.5 V vs. RHE) / working potential / OCP. The first potential step from OCP to the working potential is called “break in” (magenta in Fig. 5), the second step from the working potential to the transient potential is termed “transient event” (golden in Fig. 5), and the
final step back to the working potential will be referred to as “return to working potential” (green in Fig. 5). The AESEC traces (dissolution transients) of 304L, 316L, and 904L in sulfuric acid at 343.15 K (70 C) are presented in Figs. 6e8 and it is apparent that measurable dissolution of metal ions, in particular Fe, is found, exclusively, upon potential changes (red circles). Holding the potential for more than about 100s leads to a return to very low, i.e., with AESEC (Scheme 1) immeasurable dissolution. We term this return to low dissolution rates within about 100s “transient transpassivity” and the mechanisms will be described in detail below. That behavior is found in all investigated steel grades which is due to the multicomponent nature of the stainless-
Fig. 9 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of þ0.9 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 10 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of þ0.5 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8. steel alloys which leads to multicomponent surface oxides (“passive” films). In the case of 904L (Fig. 8) the alloy is not stable towards Fe dissolution at OCP (purple circles) which is attributed to a different Fe oxide on the surface after bright anneal (Fig. 4). This will be explained in detail below. Interpretation of the AESEC transients in Figs. 6e8 requires a review of the dissolution modes of base metals and their alloys as discussed by Landolt [17], they encompass active, passive, and transpassive dissolution. Their respective properties are summarized in Table 4. Surface films consisting of multiple oxide species should be considered as transpassive rather than passive films since their components might show
different potential-dependent dissolution behavior. The features in the AESEC traces, denoted as “break in”, “transient event”, and “return to working potential” (see Fig. 5) can be described in terms of “triads”, i.e., passive-transpassivepassive transitions. In the following we will explore every such transition for 904L because this alloy is not stable at OCP, which grants a closer look (transitions are analogous for 304L and 316L). In the next section we will see that metal cation release simplifies to the “break in” potential step for the more realistic BPP transients as measured by Hinds et al. [29e31]. Mechanistic cartoons of the triads observed for 904L steel at a working potential of þ0.9 V vs. RHE are presented in Fig. 9.
Fig. 11 e Mechanistic triads (passive-transpassive-passive transitions) for dissolution of 904L with a working potential of ±0 V vs. RHE. Naming of the potential steps is shown in Fig. 5 and the complete AESEC transient is found in Fig. 8. Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
10
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 12 e Estimation of the metal ion contamination “potential” of bare (uncoated) 304L bipolar plates at the cathode and anode side in the absence of high transient potentials (decoupled bipolar plates [29e31]).
Even at OCP preferential iron dissolution (red curve) can be observed and is attributed to the presence of Fe3O4, which is unstable towards dissolution in acidic media [46,47], rather than Fe2O3 in the other alloys (see Fig. 4). Cr2O3 seems to be stable towards dissolution at higher temperatures and pressures [48] and in our investigation it does not leach up to at least þ0.9 V vs. RHE for all steel grades (Figs. 6e8). This means, once a Cr2O3 film is formed at the outer surface (Fig. 9 OCP left) further Fe dissolution is stopped since the whole system is passive. Fe dissolution can only be triggered again when the Cr2O3 film becomes soluble (transpassive) which requires potentials in excess of þ0.9 V vs. RHE. In the “break in” step (OCP / þ0.9 V vs. RHE) not much is happening since soluble Fe has already been removed at OCP although a small amount can still be detected. 304L and 316L are stable at OCP and þ0.9 V vs. RHE and, thus, preferential Fe dissolution is observed only in the “break in” step. In the following “transpassive event” a potential is reached at which Cr2O3 becomes soluble as Cr6þ species [48] and, therefore, the whole system becomes transpassive again. However, at þ1.5 V vs. RHE iron is oxidized to yield Fe2O3, which is stable, and, hence, forms a new “passive” film consisting of iron oxide instead of chromium oxide (Fig. 9 transient event). This passive film “exchange” is known as secondary passivation of stainless steels [49]. In the “return to the working potential” step nothing happens since Fe2O3 is stable at þ0.9 V vs. RHE. This also suggests that Fe3O4 is not formed since this species was soluble at þ0.9 V vs. RHE in the “break in” step. Moreover, 904L, which was special in the pristine case, becomes “normalized”, i.e.,
it behaves like 304L and 316L after being subject to the potential steps in Fig. 5. Triads of 904L at a working potential of þ0.5 V vs. RHE are shown in Fig. 10. A minor difference compared to Fig. 9 can be found in the “break in” step. In the former all soluble Fe has been leached already at OCP so that nothing happens during “break in”. The major difference is that Fe2O3 becomes soluble at þ0.5 V vs. RHE while at þ0.9 V vs. RHE it is insoluble. This shows the impact of the transient event. When the passive chromium oxide film is “exchanged” for an iron oxide film this Fe2O3 film is soluble at low potentials and the Cr2O3 film must be reestablished (Fig. 10 return to working potential). A transient event will always trigger Fe dissolution by passive film exchange and should, therefore, be avoided. Fig. 11 shows the triads for a working potential of ±0 V vs. RHE and it is very similar to Fig. 10 since Fe2O3 is also soluble at the working potential here. The critical issue in Figs. 10 and 11 is the transient event. In the OCP/break in step all soluble Fe is leached and a stable Cr2O3 film is formed which suppresses further Fe dissolution (below measurable amounts). However, the “vicious” cycle of preferential Fe release is restarted by passive film “exchange” at high potential (secondary passivation [49]). It might be argued that metal ion dissolution depends strongly on the nature of the passive film and, in turn, on bright annealing or other treatment conditions. However, Figs. 6e8 also show the integrated AESEC traces, i.e., the total amount of dissolved metals in the respective potential step. The total mass of Fe in a monolayer (ML) of Fe2O3 or Fe3O4 can be estimated as 75 ngFe∙cm2 (assuming bulk density, i.e.,
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
international journal of hydrogen energy xxx (xxxx) xxx
5 g cm3 and a monolayer thickness of 0.2 nm). The total dissolved amounts in the OCP/break in step are similar for all stainless-steel grades with approximately 200 ngFe∙cm2 geo (>2 ML). This is also true for the “transient event” with about 300 ngFe∙cm2 geo (>3 ML) and the “return to working potential” with ca. 400 ngFe∙cm2 geo (>4 ML) when going from þ1.5 to ±0 V vs. RHE. In summary, stainless-steel dissolution can be described by passive-transpassive-passive transitions caused by potential changes. These transitions are on the order of 100s (transient transpassivity) and outside those transitions metal ion release cannot be detected by ICP-AES (passive state). Stainless steel dissolution can be imagined as playing the piano, i.e., a string of chords (triads) causes dissolution or music to happen. Holding the keys for too long will make the sound linger but it will quickly fade away just like transpassive dissolution returns to passive state. If the bipolar plate potentials follow the electrode (catalyst layer) transients Fe dissolution will be observed every time a transient event with excursion to high potentials takes place. The reason is the exchange of the Cr2O3 passive film by an Fe2O3 passive film which is dissolved when going back the normal working potential of the electrode (see Figs. 10 and 11).
Decoupled (shielded) bipolar plates Measurements of the mixed BPP/GDL corrosion potential in the presence of an MPL by Castanheira et al. [31] showed potentials of ±0 V vs. RHE for the anode side of the BPP and about þ0.5 V vs. RHE for the cathode side. Even under SUSD cycling (local hydrogen starvation) when the electrodes experienced high transient potentials the BPP was never above þ0.7 V vs. RHE. These results suggest that in an operando fuel cell the potential is unlikely to be in the range where Cr2O3 is dissolved (at least up to þ0.9 V vs. RHE, see Figs. 6e8). In turn, a passive film “exchange” (secondary passivation [49]) would not take place and, therefore, metal dissolution of the steel can be described by the OCP/break in potential steps. The Fe contamination “potential” of 304L is shown in Fig. 12 as an example. Even though, perfect coatings do not exist they will still attenuate the dissolved amount from uncoated steels to low double to single digit ngFe∙cm2 geo. In this ultra-low dissolution range classic potentiostatic trace-metal analysis is likely to fail, especially, considering that longer measuring times will only probe the passive state with low or no dissolution.
Conclusions Time-resolved measurements of stainless-steel dissolution using atomic emission spectroelectrochemistry (AESEC) suggest that ions are dissolved upon potential changes via passive-transpassive-passive transitions (transient transpassivity, ca. 100s). Continued Fe dissolution can only be observed in the case of transient events with potentials at which Cr2O3 can be dissolved. This leads to a passive film “exchange” (Cr2O3 vs. Fe2O3) which is known as secondary passivation and subsequent return to the working potential will dissolve this film. In the more realistic case of shielded
11
bipolar plates such potentials are unlikely to be observed at the BPP and, thus, continued Fe dissolution cannot take place. In combination with a coating this leads to ultra-low amounts of Fe that will be released into an operando fuel cell from stainless steel bipolar plates.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.191.
references
[1] Gasteiger HA, Garche J. Fuel cells. Handbook of heterogeneous catalysis. Wiley-VCH Verlag GmbH & Co. KGaA; 2008. [2] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [3] Bard AJ, Parsons R, Jordan J. Standard potentials in aqueous solutions. New York: Marcel Dekker, Inc.; 1985. [4] Bard AJ, Faulkner LR. Electrochemical methods fundamentals and applications. 2nd ed. New York: John Wiley & Sons, Inc.; 2001. NM. Just a dreamdor future reality? [5] Gasteiger HA, Markovic Science 2009;324:48. [6] Shao M, Chang Q, Dodelet J-P, Chenitz R. Recent advances in electrocatalysts for oxygen reduction reaction. Chem Rev 2016;116:3594e657. [7] Larminie J, Dicks A, Larminie J, Dicks A. Introduction. Fuel cell systems explained. John Wiley & Sons, Ltd; 2013. p. 1e24. [8] Buechi FN, Schmidt TJ. Polymer electrolyte fuel cell durability. New York: Springer Science þ Business Media, LLC.; 2009. [9] Lohse-Busch H, Duoba M, Stutenberg K, Iliev S, Kern M, Richards B, et al. Technology assessment of a fuel cell vehicle: 2017 Toyota Mirai. U.S. Department of Energy, Fuel Cell Technologies Office; 2018. [10] Wang H, Sweikart MA, Turner JA. Stainless steel as bipolar plate material for polymer electrolyte membrane fuel cells. J Power Sources 2003;115:243e51. [11] Statista. Number of cars sold worldwide from 1990 to 2018 (in million units). 2018. € m M, Eriksson L, Andersson J, Sjo € blom G, [12] Proch S, Stenstro Jansson A, et al. Coated stainless steel as bipolar plate material for anion exchange membrane fuel cells (AEMFCs). Int J Hydrogen Energy 2019. € m N, Bode n A, Randstro € m S, Lindbergh G. [13] Oyarce A, Holmstro In-situ measurements of contact resistance and in-situ durability studies of steels and coatings to be used as bipolar plates in PEMFCs. ECS Trans 2009;25:1791e801. € m N, Bode n A, Lagergren C, Lindbergh G. [14] Oyarce A, Holmstro Operating conditions affecting the contact resistance of bipolar plates in proton exchange membrane fuel cells. J Power Sources 2013;231:246e55. [15] Olsson COA. Passivation of stainless steels and other chromium bearing alloys. Encycl Interfacial Chem Surf Sci Electrochem 2017. https://doi.org/10.1016/B978-0-12-4095472.13585-1. Elsevier. [16] Olsson C-OA, Landolt D. Film growth during anodic polarization in the passive region on 304 stainless steels with Cr, Mo, or W additions studied with EQCM and XPS. J Electrochem Soc 2001;148:B438e49.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191
12
international journal of hydrogen energy xxx (xxxx) xxx
[17] Landolt D. Corrosion and surface chemistry of metals. 1 ed. Lausanne, Switzerland: CRC Press; 2007. € ss H. Compositional [18] Kirchheim R, Heine B, Hofmann S, Hofsa changes of passive films due to different transport rates and preferential dissolution. Corros Sci 1990;31:573e8. [19] Herting G, Odnevall Wallinder I, Leygraf C. A comparison of release rates of Cr, Ni, and Fe from stainless steel alloys and the pure metals exposed to simulated rain events. J Electrochem Soc 2005;152:B23e9. [20] Gasteiger HA, Kocha SS, Sompalli B, Wagner FT. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl Catal, B 2005;56:9e35. [21] Fenton HJH. LXXIII.-Oxidation of tartaric acid in presence of iron. J Chem Soc Trans 1894;65:899e910. [22] Pozio A, Silva RF, De Francesco M, Giorgi L. Nafion degradation in PEFCs from end plate iron contamination. Electrochim Acta 2003;48:1543e9. [23] LaConti AB, Hamdan M, McDonald RC. Mechanisms of membrane degradation. Handbook of fuel cells. John Wiley & Sons, Ltd; 2010. [24] Wong KH, Kjeang E. Mitigation of chemical membrane degradation in fuel cells: understanding the effect of cell voltage and iron ion redox cycle. ChemSusChem 2015;8:1072e82. [25] Zaton M, Roziere J, Jones DJ. Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review. Sustainable Energy & Fuels 2017;1:409e38. [26] Coms FD, Liu H, Owejan JE. Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions. ECS Trans 2008;16:1735e47. [27] Reiser CA, Bregoli L, Patterson TW, Yi JS, Yang JD, Perry ML, et al. A reverse-current decay mechanism for fuel cells. Electrochem Solid State Lett 2005;8:A273e6. [28] Qin C, Wang J, Yang D, Li B, Zhang C. Proton exchange membrane fuel cell reversal: a review. Catalysts 2016;6. [29] Brightman E, Hinds G. In situ mapping of potential transients during start-up and shut-down of a polymer electrolyte membrane fuel cell. J Power Sources 2014;267:160e70. [30] Hinds G, Brightman E. Towards more representative test methods for corrosion resistance of PEMFC metallic bipolar plates. Int J Hydrogen Energy 2015;40:2785e91. [31] Castanheira L, Bedouet M, Kucernak A, Hinds G. Influence of microporous layer on corrosion of metallic bipolar plates in fuel cells. J Power Sources 2019;418:147e51. € pler J, Lehmann J. Hydrogen and fuel cell - technologies [32] To and market perspectives. Berlin Heidelberg: Springer-Verlag Berlin Heidelberg; 2014. [33] Ogle K. Atomic Emission Spectroelectrochemistry: a new look at the corrosion, dissolution and passivation of complex materials. Mater Corros 2012;37:58e65.
[34] Ogle K, Baeyens J, Swiatowska J, Volovitch P. Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel. Electrochim Acta 2009;54:5163e70. lisle E, Chartrand P, Decterov SA, Eriksson G, [35] Bale CW, Be Gheribi AE, et al. FactSage thermochemical software and databases, 2010e2016. Calphad 2016;54:35e53. [36] Biesinger MC, Payne BP, Grosvenor AP, Lau LWM, Gerson AR, Smart RSC. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl Surf Sci 2011;257:2717e30. [37] Hamm D, Ogle K, Olsson COA, Weber S, Landolt D. Passivation of FeeCr alloys studied with ICP-AES and EQCM. Corros Sci 2002;44:1443e56. [38] Swiatowska J, Volovitch P, Ogle K. The anodic dissolution of Mg in NaCl and Na2SO4 electrolytes by atomic emission spectroelectrochemistry. Corros Sci 2010;52:2372e8. [39] DOE. DOE. Technical targets for polymer electrolyte membrane fuel cell components. 2015. [40] Covino BS, Scalera JV, Driscoll TJ, Carter JP. Dissolution behavior of 304 stainless steel in HNO3/HF mixtures. Metall Trans A 1986;17:137e49. [41] Reddy RG, Wang S, Chen B. Solubility of iron in spent pickling solutions. Mining, Metallurgy & Exploration 1993;10:102e7. [42] Castro SM, Ponces MJ, Lopes JD, Vasconcelos M, Pollmann MCF. Orthodontic wires and its corrosiondthe specific case of stainless steel and beta-titanium. J Dent Sci 2015;10:1e7. [43] Ranjith PM, Rao MT, Sapra S, Suni II, Srinivasan R. On the anodic dissolution of tantalum and niobium in hydrofluoric acid. J Electrochem Soc 2018;165:C258e69. [44] Barr TL, Seal S. Nature of the use of adventitious carbon as a binding energy standard. J Vac Sci Technol A 1995;13:1239e46. [45] Guo LQ, Qin SX, Yang BJ, Liang D, Qiao LJ. Effect of hydrogen on semiconductive properties of passive film on ferrite and austenite phases in a duplex stainless steel. Sci Rep 2017;7:3317. [46] Hickling A. The cathodic reduction potential of Fe3O4 and the flade potential of iron. Electrochim Acta 1973;18:635e7. [47] White AF, Peterson ML, Hochella MF. Electrochemistry and dissolution kinetics of magnetite and ilmenite. Geochem Cosmochim Acta 1994;58:1859e75. [48] Cook WG, Olive RP. Pourbaix diagrams for chromium, aluminum and titanium extended to high-subcritical and low-supercritical conditions. Corros Sci 2012;58:291e8. [49] Bojinov M, Betova I, Fabricius G, Laitinen T, d RR, Saario T. The stability of the passive state of ironechromium alloys in sulphuric acid solution. Corros Sci 1999;41:1557e84.
Please cite this article as: Li X et al., Transient stainless-steel dissolution and its consequences on ex-situ bipolar plate testing procedures, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.191