Additively Manufactured 316L Stainless Steel: an Efﬁcient Electrocatalyst

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Short Communication Additively manufactured 316L stainless steel: An efﬁcient electrocatalyst

M.J.K. Lodhi a, K.M. Deen b, Waseem Haider a,* a School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI, 48859, USA b Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada highlights

Etched AM 316L stainless steel manifested enhanced catalytic activity for OER. The etched AM 316L manifested 310 mV overpotential at 10 mA/cm2. The effective surface area of the AM sample was increased after etching. The etched AM presented stable OER potential for 100 h at 10 mA/cm2. article info abstract

Article history: In the quest of finding an economical, yet efficient material, the idea of fabricating 316L Received 15 April 2019 stainless steel using additive manufacturing technology was explored to produce material Received in revised form with refined sub-granular structure. The surface of the stainless steel was further chemi- 18 July 2019 cally treated with an etching solution to expose the grain boundaries. The grain boundary Accepted 29 July 2019 enriched surface resulted in more active sites for the oxygen evolution reaction (OER) in Available online xxx additively manufactured treated (AM-T) 316L stainless steel. AM-T sample manifests enhanced catalytic activity for OER with an overpotential of 310 mV to draw a 10 mA/cm2 Keywords: current density, along with a lower Tafel slope of 42 mV/dec compared to AM and wrought Additive manufacturing samples. These features were validated from the increased double-layer capacitance of Stainless steel AM-T and approximately 1.5 times larger electrochemically effective surface area of AM-T OER due to etching treatment compared to the wrought sample. Furthermore, AM-T also pos- Overpotential sesses stable activity retention for 100 h at a current density of 10 mA/cm2. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

fossil fuels e.g. natural gas, coal, and oil [1]. The replacement Introduction of these existing resources and environment-friendly fuel is tantalizing searched. Among the various candidates, The global demand for energy is increasing due to rapid hydrogen as a fuel has been the center of attention, because it industrialization and population growth. The resources is environment-friendly and the resources for its production available for meeting the energy requirements are mostly

* Corresponding author. E-mail address: [email protected] (W. Haider). https://doi.org/10.1016/j.ijhydene.2019.07.217 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: Lodhi MJK et al., Additively manufactured 316L stainless steel: An efﬁcient electrocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.217 2 international journal of hydrogen energy xxx (xxxx) xxx

are virtually inexhaustible [2]. Out of the different methods for further chemically treated by etching in an acidic solution the production of hydrogen, electrochemical water splitting is containing 15 ml hydrochloric acid, 10 ml nitric acid, 10 ml considered as the most impressive [3,4]. Hydrogen evolution acetic acid and 3 drops of glycerol for 35 s followed by rinsing reaction (HER) and oxygen evolution reaction (OER) are the in deionized water. Due to chemical treatment of the AM two primary half-cell reactions occur during electrochemical samples, the grain and subgrain boundaries were revealed on water splitting [5,6]. OER is more sluggish in nature, compare the surface and that was believed to endow the sample with to HER due to the requirement of four protons reduction by the useful functionalities in terms of OER activity. This chemically transfer of four electrons from the catalytic surface [7,8]. Thus, treated AM sample will be referred as AM-T in further the search for efficient, stable and economical OER catalyst is discussion. the pursuit of water splitting reaction. Recent studies have shown the focus on 3d-transition metal i.e. Fe and Ni-based Microstructural characterization catalysts with high efficiency and catalytic stability [9e11]. When there is the use of nickel and iron-based catalysts, it is Microstructural analysis for both the wrought and AM sam- straightforward to think about stainless steel as a candidate ples were carried out via scanning electron microscope (SEM) material for this purpose. (Hitachi S-3400-II). The samples were ground till 1200 grit size, Stainless steel is a low-cost alloy containing iron and followed by fine polishing using 3 and 1 mm diamond sus- nickel, thus possessing intrinsic OER activity, but with medi- pension, sequentially. The samples were etched in a similar ocre catalytic performance. Literature suggested the use of manner as mentioned in the above section. For electron different chemical, electrochemical and hydrothermal treat- backscatter diffraction (EBSD) the samples were ion milled. ments to enhance the catalytic efficiency of stainless steel [12e15]. Various reports also suggest the use of stainless steel Electrochemical characterization foam, as an electrocatalyst for OER, due to its large electrochemically active area [16,17]. However, stainless steel foam A three-electrode electrochemical cell was used for electro- produced by conventional processes have limitations of non- chemical characterization in 1 M KOH solution. Wrought, AM uniform fiber discontinuity and the presence of large cavities. and AM-T samples were used as working electrodes with an Furthermore, the mechanical properties of stainless steel exposed surface area of 1.26 cm2. Graphite rod and Hg/HgO foams are also very inferior that restrict its use in extreme and were used as counter and reference electrodes, respectively. harsh environments [17]. All the potential values reported in this study are with respect Additive manufacturing of metals is a very nascent process to the reversible hydrogen electrode (RHE). Linear sweep vol- that provides the flexibility of design and allows the tammetry curves were recorded between 0 and 1 V vs. refer- manufacturing of controlled porous structures. In addition to ence electrode at a scan speed of 5 mV/s, and a positive the design flexibility, the very high cooling rate intrinsic to this feedback mode was applied for I-R compensation. Electro- process develops a very non-conventional microstructure chemical impedance spectra were recorded at the onset po- with very refined sub-grains confined within the larger grains tential of OER (constant DC potential) as evaluated from the [18,19]. In this work, we employed additive manufacturing to linear sweep voltammetry of all the samples within 100 kHz to fabricate 316L stainless steel electrodes and chemically 1 Hz frequency range. The chronopotentiometry curve for AM- treated the surface to use as an OER catalyst. T was obtained at a current density of 10 mA/cm2. The cyclic galvanostatic test was performed at a current density of 10 mA/cm2 for 15 min followed by 15 min delay at OCP. This Experimental details was one cycle (15 min galvanostatic followed by 15 min OCP) and 100 cycles were performed. Materials

Additively manufactured (AM) 316L stainless steel samples Results & discussion were fabricated from gas atomized 316L stainless steel powder (particle size 15e45 mm) using selective laser melting process. Microstructure Renishaw AM 250 unit was used to fabricate the samples (circular disks of 15.2 mm diameter and 5 mm thickness) at a The grain distribution and grain orientation of the samples laser power of 200 W, keeping a layer thickness of 30 mm and a were assessed using electron backscatter diffraction (EBSD) hatch spacing of 100 mm. Wrought 316L stainless steel (circular using a scanning electron microscope (Fig. 1). Wrought sample rod of 15.2 mm diameter) was purchased from “Onlineme- (Fig. 1a) exhibited the typical faceted morphology (polygonal tals®” and cut into 5 mm thick disk samples, using a high grains), whereas the AM samples exhibited the elongated speed saw. grains in the build direction (Fig. 1b). The width of the elongated grains in the AM sample is its finest dimension that is Surface preparation comparatively finer than the wrought sample. The elongated structure of the grains in the AM sample is due to the heat The surface of both the wrought and AM samples were ground removal through the build plate during the solidification using silicon carbide (SiC) papers from 180 to 1200 grit size process. A continuous change in color within each grain was under running water stream, followed by ultrasonication in observed for the AM sample, depicting the continuous varia- ethanol and rinsing in deionized water. AM samples were tion in the orientation. Furthermore, the AM sample reveled a

Please cite this article as: Lodhi MJK et al., Additively manufactured 316L stainless steel: An efﬁcient electrocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.217 international journal of hydrogen energy xxx (xxxx) xxx 3

Fig. 1 e EBSD IPF color map with inverse pole ﬁgure (Red 001, Green 101 and Blue 111) for (a) wrought and (b) AM samples. Scale bars for ‘a’ is 200 mm and ‘b’ is 100 mm. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the Web version of this article.)

varying grain size distribution. Therefore, the EBSD data sug- conversion devices with 10% efficiency [23]. AM-T required an gests that AM sample showed a very non-conventional grain overpotential (ɳ) of only 310 mV to achieve the current density shape, orientation, and size. of 10 mA/cm2, however, wrought and AM samples demon- Furthermore, the SEM micrographs of the mechanically strated a comparatively higher overpotential of 367 and polished and chemically etched surfaces of both the wrought and AM samples are presented in Fig. 2. The wrought sample showed sharp grain boundaries with an equiaxed grain structure (Fig. 2a) [20]. Twin bands were also evident in the wrought sample, possibly associated with prior mechanical processing. However, AM sample revealed a very non- conventional and heterogeneous microstructure due to rapid solidification, under anisotropic heat removal (Fig. 2b and c). Of all the different features, cellular structure (Fig. 2c) present in the AM samples is of particular interest. The average size of the cellular structure present in the AM sample was approximately 800 nm. The development and size of these cellular structures are associated with the rapid solidification (inherited to additive manufacturing process). Therefore, the geometry of this cellular subgranular structure is related to the laser processing parameters. The earlier report proved the boundaries of these cellular structures to be concentrated with dislocations [21]. The development of these high dislocation densities in AM sample is because of the thermal contraction stresses, because of rapid solidification. The dislocation densities, corresponds to large stored energies, resulting in active areas in the material [22].

Electrocatalytic properties

The electrocatalytic activity of the chemically treated additively manufactured (AM-T) 316L stainless steel was investi- gated in 1 M KOH solution. For comparison purpose, AM and wrought 316L stainless steel (mechanically polished up to 1200 grit size) were also studied. Fig. 3a shows the linear sweep voltammetry (LSV) curves of the samples for OER. In comparison with wrought (1550 mV vs. RHE) and AM (1540 mV vs. RHE), AM-T exhibited a noticeable lower onset potential (1490 mV vs. RHE), suggesting its remarkable electrocatalytic activity and being more desirable for OER. The current density of 10 mA/cm2 has been envisioned as a standard to study the electrocatalytic efﬁciency of the electrocatalyst, as this is the Fig. 2 e SEM image for etched (a) wrought and (b, c) AM amount of current density expected for solar to fuel samples.

Please cite this article as: Lodhi MJK et al., Additively manufactured 316L stainless steel: An efﬁcient electrocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.217 4 international journal of hydrogen energy xxx (xxxx) xxx

Fig. 3 e (a) Linear sweep voltammetry (Polarization curves) of AM-T, AM and wrought samples (b) Tafel plots for OER over AM-T, AM and wrought samples, (c) Electrochemical impedance spectroscopy curves for AM-T, AM and wrought samples, (d) Chronopotentiometric curve for AM-T at a current density of 10 mA/cm2.

340 mV respectively, to draw the same amount of current current density with a small change in the overpotential that density. The value of overpotential for wrought 316L stainless proves it to be more electrochemically active. In AM and steel conﬁrmed the values reported in the literature [24]. This wrought stainless steel the OER is considered to be controlled overpotential needed for AM-T is also comparable or lower by electron/proton reaction, i.e. adsorption and surmount of than the NiFeOx (310 mV), CoFeOx (370 mV), NiCoOx (380 mV) activation energy by the OH species to interact with the active þ / e þ and CuxCoyO4 (390 mV) multi-metal electrocatalysts under sites on the catalyst surface, (S OH S OH e together similar conditions [23,25]. However, the overpotential for AM- with S OH / S OH*, where S represents the catalytic active

T is comparable to IrO2 based catalysts (310 mV) [26], which is sites). This can be estimated from the Tafel slope of around 60 considered as a benchmark electrocatalyst for OER in alkaline mV/dec [27]. The relatively high Tafel slopes of AM and solution. This clearly confirmed the value-added trans- wrought samples indicate the slow kinetics of OH species formation to the stainless steel using additive manufacturing adsorption on the electrocatalytic active sites. In case of AM-T, approach and subsequent chemical treatment, making it the electrocatalytic behavior towards oxygen evolution can be comparable to precious metal-based electrocatalysts. It is determined from the first and second electron/proton transfer þ / þ þ worth mentioning here that the perspective of revealing the reaction (S OH OH SO H2O e ), which is highlighted grain and subgranular structure in AM material through by the typical Tafel slop of nearly 40 mV/dec [28]. The above- chemical etching phenomenon, proved worthy of enhanced mentioned reactions on the surface of wrought, AM and AM- catalytic activity. The superior catalytic activity of the AM-T T samples, during OER, suggested that the kinetics of OER is sample has been attributed to its microstructure, consti- enhanced on the surface of AM-T, because of the more grain tuting the subgranular structure. Such subgrain boundaries boundaries. These grain boundaries helps in the activation of are concentrated with dislocations, which provides more the reactants and adsorption process [29,30]. This, once again active sites for OER to happen. reflects the enhanced electrocatalytic efficiency of AM-T as In order to further evaluate the kinetics of OER on AM-T that of wrought stainless steel. surface, Tafel plots were also obtained, as shown in Fig. 3-b. To gain more insight into the electrocatalytic efficiency of AM-T showed a lower Tafel slope (42 mV/dec) than that of AM the AM-T, electrochemical impedance spectra were also ob- (51 mV/dec) and wrought (56 mV/dec) stainless steel. The tained as represented in Fig. 3-c. Charge transport resistance lower Tafel slope for AM-T indicated a rapidly increased (Rct) and adsorption resistance (Rads) were calculated from

Please cite this article as: Lodhi MJK et al., Additively manufactured 316L stainless steel: An efﬁcient electrocatalyst, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.217 international journal of hydrogen energy xxx (xxxx) xxx 5

Table 1 e Kinetic parameters and electrochemically active surface area obtained from the impedance spectra. U * 2 U * 2 U * 2 m 2 2 Samples R1 ( cm )R2 ( cm )R3 ( cm )Rct (R2 R1) Rads (R3 R1) Cdl ( F/cm ) Effective surface area (cm ) (U * cm2) (U * cm2) AM-T 1.27 18.30 15.72 17.03 14.45 425.7 30.1 AM 1.68 42.69 36.06 41.01 34.38 275.5 19.9 Wrought 1.82 66.10 60.26 64.28 58.44 273.1 19.8

impedance spectra and reported in Table 1. AM-T possess and double layer thickness (d) at the interface were assumed U 2 U 2 e smaller Rct (17.03 *cm ) compared to AM (41.01 *cm ) and to be 15.6 and 0.1 nm, respectively [9,24,31 33]. wrought (64.28 U*cm2) samples, that is indicative of faster Besides, electrocatalytic activity of the AM-T, the electro- charge transport characteristic during electrochemical reac- catalytic stability of the AM-T was also assessed using chro- tion for AM-T sample [31]. AM-T also showed lower Rads nopotentiometry as demonstrated in Fig. 3-d. AM-T showed (14.45 U*cm2) than that of AM (34.38 U*cm2) and wrought very good stability over 100 h for driving a current density of (58.44 U*cm2) samples, suggesting the enhanced surface ac- 10 mA/cm2. The variation in the potential to drive a constant tivity of AM-T sample. The lower Rct and Rads offered by AM-T current during this period was negligible which proved the also supports its facile kinetic behavior, as predicted from stable catalytic efﬁciency of the AM-T sample for extended use. lower Tafel slope. The double-layer capacitance Cdl and elec- To further estimate the cyclic stability of the AM-T sample at trochemically effective surface area (EESA) was measured a current density of 10 mA/cm2, the sequence of constant cur- from the Nyquist plots according to equation (C ¼ εεοA/d) and rent (galvanostaticpolarization)and a delay (15 min) atOCP were the calculated values are given in Table 1. The AM and repeated 100 times. The total time for the test was approxi-

Wrought samples represented almost similar Cdl values mately 50 h as shown in Fig. 4. The results showed that catalytic m 2 m (275 F/cm ). However, relatively large Cdl of AM-T (425.7 F/ performance did not deteriorate, as the fluctuation in the OER cm2) compared to AM and wrought stainless steel indicated potential during the repeated consecutive tests remained con- the effectively increased catalytic activity of AM-T for OER stant. Furthermore, the total change in the potential from the after etching treatment. Similarly, the AM-T sample presented first cycle until the last cycle at the aforementioned potential almost 1.5 times larger EESA compared to AM and Wrought was highly negligible (14 mV). These results again proved that samples. For the calculation of EESA from Cdl, the dielectric AM-T can retain its stable catalytic activity and does not undergo constant (Ɛ) of the passive film present on the stainless steel any degradation during repeated cycles.

Fig. 4 e Cyclic galvanostatic polarization of AM-T stainless steel sample to estimate the OER potential stability. Magniﬁed images (a) cycle 1e5, (b) cycle 21e25, (c) cycle 51e55, (d) cycle 96e100 were also presented.

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