Iridium-Based Nanowires as Highly Active, Oxygen Evolution Reaction

Subscriber access provided by READING UNIV

Article

Iridium-based Nanowires as Highly Active, Oxygen Evolution Reaction Electrocatalysts Shaun M. Alia, Sarah Shulda, Chilan Ngo, Svitlana Pylypenko, and Bryan S. Pivovar ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03787 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Iridium-based Nanowires as Highly Active, Oxygen Evolution Reaction Electrocatalysts Shaun M. Alia*, Sarah Shulda, Chilan Ngo, Svitlana Pylypenko, and Bryan S. Pivovar Dr. S. M. Alia, Dr. B. S. Pivovar Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, United States of America

S. Shulda, Dr. C. Ngo, Prof. S. Pylypenko Department of Chemistry and Geochemistry, Colorado School of Mines, 1012 14th Street, Golden, CO 80401, United States of America

ABSTRACT: Iridium-nickel (Ir-Ni) and iridium-cobalt (Ir-Co) nanowires have been synthesized by galvanic displacement and studied for their potential to increase the performance and durability of electrolysis systems. Performances of Ir-Ni and Ir-Co nanowires for the oxygen evolution reaction (OER) have been measured in rotating disk electrode half-cells and single-cell electrolyzers, and compared to commercial baselines and literature references. The nanowire catalysts showed improved mass activity, by more than an order of magnitude compared to commercial Ir nanoparticles in half cell tests. The nanowire catalysts also showed greatly improved durability, when acid leached to remove excess Ni and Co. Both Ni and Co templates were found to have similarly positive impacts, although specific differences between the two

ACS Paragon Plus Environment

1

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

systems are revealed. In single-cell electrolysis testing, nanowires exceeded the performance of Ir nanoparticles by 4‒5 times, suggesting that significant reductions in catalyst loading are possible without compromising performance.

KEYWORDS: electrolysis, electrochemistry, nanostructures, iridium, oxygen evolution

Introduction Within the United States, approximately 2% of used energy goes through the hydrogen pathway, to produce ammonia in agriculture and to upgrade crude oil in transportation.1-2 With the emergence of hydrogen fuel cell vehicles and increased agricultural demands, the size of the hydrogen market will likely continue to grow. The increasing use of renewables (solar, wind) and reductions in their cost may further allow for expanded hydrogen use as an energy carrying intermediate, connecting the electric grid to transportation and industrial processes.1, 3-4 Although the majority of hydrogen is currently produced in the United States by steam methane reformation, electrolysis is expected to become more competitive as we shift towards increasingly available low-cost, intermittent renewable sources of energy, and as societal and policy drivers for sustainable energy systems become more prevalent. Today’s electrolyzers are typically used under continuous operation at constant generation rates.1 Under these conditions, electricity cost is the primary driver (up to 85% of produced hydrogen cost). Electrolyzers are therefore operated at high catalyst loading to maximize performance and avoid durability losses.5 As electrolysis shifts to lower-cost, variable power input, advanced catalysts will provide a pathway to enhanced performance at lower loading, and to improved durability at low loading and during intermittent operation.6 Catalyst development in proton exchange membrane (PEM) electrolyzers typically focuses on the oxygen evolution


2

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reaction (OER), since the half-cell reaction is 5 orders of magnitude slower than hydrogen evolution.6-7 PEM electrolyzers commonly use iridium (Ir) in OER as it is more stable than ruthenium (Ru) and requires a lower overpotential than platinum (Pt).8-12 Early efforts in electrolysis catalyst and electrode development have included efforts to improve Ir electrode performance by combining Ir and Pt for enhanced electrical connectivity, Ru to lower the OER onset, and titanium to improve durability.13-25 More recent studies have established baselines for Ir performance and made improvements to the activities of conventional catalysts.6,

8, 26

As a guide to further catalyst development, modeling has also correlated OER

activity to oxygen binding strength, finding that Ir-O chemisorption is too strong for optimal OER activity.27 Various alloying elements, including nickel (Ni) and cobalt (Co), have been studied in an attempt to improve Ir- and Ru-based OER performance.13, 28-37 A major challenge exists for evaluating electrolysis catalysts because established baseline performance and testing protocols do not exist and/or are not applied uniformly within the limited community of researchers working on these systems. This is an area where our team has tried to make progress within the literature, suggesting specific test conditions and baselining the performance of several commercially available materials.6 Such standardized performances are necessary to gauge the potential performance improvement benefits of novel catalysts and will be presented and referenced later in this paper. This study examines Ir-Ni and Ir-Co nanowires for OER activity in PEM electrolysis. Previous comparisons between the specific activities of polycrystalline Ir and Ir nanoparticles suggested that there is a potential benefit to these and other extended Ir surfaces in OER.6 Within novel extended surface catalysts, 3M developed nanostructured thin films (NSTF) for electrolysis applications, which have shown promising performance and durability in


3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

electrolyzers.38-39 Studies on nanodendrites and nanoflowers also found a potential benefit in rotating disk electrode (RDE) half-cell testing.40-41 The Ir-based nanowires included here use similar structures, templates, and routes previously used in the development of Pt-based oxygen reduction electrocatalysts in PEM fuel cells, and build off of an existing program in fuel cell catalyst development.42

Results The results of this paper have been divided into “As-synthesized” and “Acid leached” subsections. The as-synthesized nanowires are investigated for electrochemical properties, Ir surface area (ECA) and specific activity, with comparisons being made between Ni and Co nanowire templates. Acid leaching of the as-synthesized materials, however, is necessary to improve catalyst durability, including minimizing Ni/Co content and ionic contamination concerns required for electrolyzer implementation.

As-synthesized Ir-Ni and Ir-Co nanowires were synthesized by the galvanic displacement of Ni and Co nanowires with Ir (Figure 1, S1, and S2). Although modifying the amount of Ir precursor in the displacement reaction affected the overall composition, the nanowire morphology was generally preserved with clear delineation between the Ni/Co “core” and Ir “shell” (Figure 1 and S2). X-ray diffraction (XRD) patterns, reported as a function of Ir displacement extent, revealed differences between the Ni and Co nanowire templates (Figure 2 and S3). At high Ir composition (full displacement), the Ir-Ni and Ir-Co nanowires displayed nearly identical spectra, consistent with bulk Ir (Ir lattice constant of 3.84 Å). For Ir-Ni nanowires, a clear


4

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

separation of Ir and Ni peaks was found at all displacement levels, with lattice constants that were consistent with bulk Ir and Ni. This data suggests that Ir and Ni phases have relatively little interaction. In contrast, Ir lattice compression was observed in all Ir-Co nanowire samples except for those with high (>50%) Ir content. Ir was the most compressed (3‒4%) at low Ir composition (< 15 wt. % Ir), and somewhat compressed (2%) at intermediate Ir compositions (15‒50 wt. % Ir). This observed change in Ir compression correlated with increasing specific activity presented later in the paper. X-ray photoelectron spectroscopy (XPS) was also run on these samples to complement other characterization (Figure 3). The observed spectra showed results consistent with other data, including decreasing Ni and Co signals at higher displacement levels.43 Tracking of oxide and hydroxide species further showed that for all Ir-Ni and Ir-Co materials, Ir hydroxide was found to be the dominate species near the nanowire surface.24, 44-45 Mercury underpotential deposition was used to evaluate the ECAs of the nanowires (Figure S4).46 At high levels of displacement, the nanowire surface areas were approximately 15 m2 gIr‒1 (Figure 4a and 4b), about half that of commercial Ir nanoparticles. Lower displacement levels, however, yielded higher Ir ECAs likely due to thinner Ir layers forming on the nanowire surface, decreasing the percentage of subsurface Ir. In general, Ir-Ni exhibited higher ECA than Ir-Co. This may be due to several factors related to differences between the template morphologies and their surfaces, including oxide layers that may alter the displacement rate (Ir-Ni limited to 22 wt. % Ir displacement without the addition of acid, Ir-Co capable of >90% displacement without acid) and the behavior of Co and Ni in the displacement process. At high Ir compositions, the OER specific activities of the nanowires were comparable to polycrystalline Ir and 4‒5 times greater than Ir nanoparticles, suggesting a performance benefit due to the extended nanowire surface (Figure 4c and 4d). As mentioned previously, XPS


5

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

revealed that the nanowires contained high amounts of Ir hydroxide species at the surface; those species may have benefitted nanowire activity, as Ir hydroxide was previously found to be more active than Ir metal or oxide (IrO2).47 For the Ir-Co nanowires, increasing site-specific activity was also observed at lower levels of displacement, particularly at less than 15 wt. % Ir. The increasing specific activity corresponded to Ir lattice compression and was rationalized as a potential alloying effect, where Ir lattice compression weakened Ir-O chemisorption (Figure S5).27 Although the Ir-Co specific activity was higher with a compressed lattice, this benefit is somewhat complicated since: modeling and defined surface catalysis work is unavailable to confirm that lattice compression improves Ir activity; and the other sample subsets (acid leached Ir-Co, as-synthesized Ir-Ni, acid leached Ir-Ni) produced a range of specific activities with a characteristically Ir lattice. The addition of non-Pt group metals (PGMs) may further be undesirable in electrolysis due to dissolution concerns at elevated operating potentials, confirmed in later durability experiments. In contrast to Ir-Co, the Ir-Ni samples did not exhibit lattice compression and had nearly constant specific activity regardless of composition. It is worth noting that at very low Ir compositions (< 9 wt. % Ir, Ir-Ni), both the specific activity and ECA dropped. This trend is also similar to one previously found in Pt-Ni nanowire catalysts developed for PEM fuel cells and suggests that a minimum layer thickness (or continuity) may be required to achieve performance improvements.48 The highest performing as-synthesized Ir-Ni and Ir-Co nanowires exceeded the mass activity of Ir nanoparticles by 10 and 9 times when measured in RDE, respectively. The properties that resulted in increased mass activity, however, differed in that Ir-Ni produced higher ECA (70 vs. 40 m2 gIr‒1, Figure 4a and 4b) while Ir-Co produced higher specific activity (7 vs. 5 mA cmIr‒2 at 1.55 V, Figure 4c and 4d). This trend is also similar (although more


6

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

moderate) to one previously found for Pt-Ni and Pt-Co nanowire catalysts developed for PEM fuel cells.48-49 Durability tests were completed in half-cells by applying a 1.6 V hold over 13.5 h (Figure 4e, 4f, S6, and S7). When performed on Ir nanoparticles, these tests were previously found to result in comparable losses to single-cell electrolyzer tests at low catalyst loadings.6 Although the as-synthesized catalysts produced high initial OER activity, durability losses were a concern, particularly at high Ni and Co content. At high Ir content, half-cell durability tests resulted in nanowire mass activity losses around 50%, a relative loss comparable to that observed for Ir nanoparticles. At low Ir content, however, the losses were higher. For initially high performing as-synthesized nanowires, these losses were roughly 70% (Ir-Ni 12.5, Ir-Co 4.2), although in a few cases durability testing led to complete performance loss (Ir-Ni, ≤ 11.0 wt. % Ir). Mass activity losses resulted from a combination of both lost ECA and specific activity; the ECA losses, however, tended to be larger. Inductively coupled plasma-mass spectrometry (ICP-MS) experiments were also completed on electrolytes to evaluate dissolution during electrochemical conditioning and durability (Figure 4g, 4h, S8, and S9). For Ir-Co, as expected, large amounts of Co dissolution (> 40%) were observed at low displacement. For Ir-Ni, however, much smaller Ni losses were found, potentially due to a thicker or more dissolution resistant oxide layer. Although the Ni dissolution rates in RDE half-cells (< 1 wt. %) were relatively low, Ni dissolution could still be an issue in electrolysis cells with different electrode fabrication conditions and longer operating time requirements.

Acid leached


7

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

Due to the durability concerns of transition metal use in electrolysis applications, the assynthesized catalysts were exposed to sulfuric acid in an ex-situ step (acid leaching) to preferentially remove Ni and Co. Exposure to sulfuric acid removed the majority of the template material, and all samples became greater than 90 wt. % Ir (Figure 5a and 5b) post-leaching. Although acid leaching dramatically changed the overall elemental composition of many of the as-synthesized catalysts, we use their as-synthesized displacement level to more clearly differentiate the samples in the following figures and discussion. Many performance features, most notably ECA, remain after acid washing and presenting data in this way is helpful for comparison purposes. Specific catalysts are referenced throughout the remainder of the paper, by both their as-synthesized and acid leached compositions. The nomenclatures of these references are two numeric values, with the as-synthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (9.3) 90.5 was 9.3 wt. % Ir assynthesized and 90.5 wt. % Ir following acid leaching with the remaining balance representing Ni. Acid leaching produced Ir-Ni and Ir-Co nanowire specific activities comparable to, but slightly higher than polycrystalline Ir, regardless of the initial displacement level (Figure 5c, 5d, and S10). While the high specific activity could be attributed to the extended nature of the nanowires, surface Ir species may also have contributed in a positive way. Acid leaching removed Ni, Co, and decreased the relative prevalence of Ir oxide (IrO2), which resulted in an Ir hydroxide-dominant surface, potentially more active for OER (Figure 3).47 For the assynthesized Ir-Co nanowires, low displacement produced Ir lattice compression and increased specific activity. Following acid leaching, which removed the majority of Co, the specific


8

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

activity decreased and Ir lattice compression was eliminated, resulting in a lattice constant consistent with bulk Ir (Figure S11). For Ir-Ni, no alloying benefit was found in the as-synthesized nanowires and specific activity was essentially unchanged following acid leaching. An exception, however, was found at low displacement levels where the Ir-Ni specific activity slightly increased after acid leaching to reach levels comparable to polycrystalline Ir. While acid leaching leveled all nanowire specific activities, many of the ECA trends remained. Specifically, Ir-Ni produced higher ECAs than IrCo, and the trend of higher ECAs at low as-synthesized displacement were maintained following acid leaching. In terms of mass activity, acid leached Ir-Ni (9.3) 90.5 and Ir-Co nanowires (4.2) 93.6 produced 11 and 8 times the activity of Ir nanoparticles, respectively (Figure 4c and 4d). Overall, the generally positive initial electrochemical properties of low displacement samples were maintained even following acid leaching. While initial electrochemical properties were roughly maintained following acid leaching, a major improvement in durability was found. Following a 1.6 V hold for 13.5 h, the highest performing as-synthesized nanowires lost 70% of their mass activity; by comparison, each of the acid leached samples lost very little activity (< 10%, Figure 5e and 5f). The performance and durability of acid leached nanowires further compared favorably to Ir nanoparticles, which had lower initial performance and increased mass activity losses (50% loss, Figure 5e, 5f, S12, and S13). Additionally, low Ni and Co leaching rates were found in the acid leached samples (Figure 5g, 5h, S14, and S15). The decreased dissolution rate was the most pronounced for Ir-Co, where 40% Co dissolution was observed for the as-synthesized nanowires, and less than 1% Co dissolution was observed for the acid leached nanowires. The impacts of Ni and Co removal by acid leaching on observed durability are obvious. The high relative durability of these nanowires


9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

compared to nanoparticles at these test conditions can potentially be attributed to their extended surface nature. Potential holds at 1.6 V on Ir nanoparticles were previously found to produce durability losses driven by particle growth (ripening, sintering), not dissolution.6 At moderate potential, less loss may have been found with the nanowires since they were synthesized as nanostructures of aggregated particles, making them less prone to ripening and growth. These results are similar to those previously found for Pt-Ni and Pt-Co nanowires in fuel cell catalysis.48-49 Potential holds (1.6 V) were used to assess catalyst durability since this test on Ir nanoparticles previously produced similar loss to MEAs, although at a lower potential and over a shorter amount of time.6 Half-cell durability tests were also completed with potential cycles, and produced several trends (Figure S16). As with nanoparticles cycling resulted in less loss, possibly since the catalyst spent less time at higher potential.6 Although the nanowires were more durable at moderate potential ranges (1.4‒1.6 V), the benefit narrowed at elevated potential as the loss became dissolution-driven. While the nanowires are a promising class of material and show durability benefits under the specific test conditions probed, how well half-cell durability predicts MEAs, and to what extent catalyst development or system controls can limit MEA durability loss is still being investigated. In addition to the extended surface, other factors may influence durability. Both nanowires and nanoparticles contained Ir hydroxide near the surface, and Ir nanoparticles were used as the half-cell baseline since oxide surfaces can produce different performance and durability.6 Facets and crystallite sizes may also influence surface energies and modify catalyst reactivity and stability, but are still being investigated.


10

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The performances of Ir-Ni and Ir-Co nanowires were compared to the activities of commercial Ir catalysts, and of Ir and Ru catalysts available in literature (Table 1).6,33,35-37,39,56-74 Comparing data across the literature in this way was difficult because a lack of consensus exists on the potential where catalyst comparisons are made. This necessitated the inclusion of multiple potentials within the table, although approximations were needed in cases where mass activities were not reported or to group studies at more common evaluation potentials. We filled in all potential values for the data on our materials and took the data at various potentials available in the literature for other materials. For comparison purposes, we normalized mass activities at specified voltages using our acid leached Ir-Ni nanowires (highest performer) as a reference material.

This normalization allowed for us to show that the Ir-Ni and Ir-Co nanowires

outperform the highest reported Ir activities. The only literature reference with a comparable performance is for a Ru-based catalyst.33 Unfortunately, the durability of Ru in electrolysis is a concern that has limited the commercial market to Ir-based catalysts. This summary is not exhaustive and avoided studies where numerical values could not be reported (no reversible hydrogen electrode (RHE) correction or qualitative comparisons), nonstandard test conditions (flow cells or elevated temperature), and presents only the highest activities available. The baseline commercial activity presented here also outperforms many previously reported values. While this study primarily focused on ex-situ catalyst performance, the MEA performances of our nanowires and 3M’s NSTF are also included in Table 1. As noted previously, and also apparent from the data presented in Table 1, direct RDE/MEA comparisons can be difficult since MEAs typically outperform RDE activities.6 This observation is a significant deviation from fuel cells, where RDE activities typically outperform MEAs.50 Still, there is value in showing nanowire MEA performance and comparing it to another extended surface


11

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

electrocatalyst (NSTF) and Ir nanoparticles. A direct comparison between NSTF and nanowire MEAs, however, comes with caveats because loading (NSTF 0.25 mgIr cm‒2, nanowires 0.1 mgIr cm‒2) can significantly affect mass-normalized performance. Also, differences exist between the testing parameters of the studies, including the choice of membrane, internal resistance corrections (NSFT corrected, nanowires not), and electrode structure/ionomer content. Still, the relative MEA performance of both nanowires and NSTF are promising and highlight the potential benefit of extended surface catalysts in electrolysis cells compared to Ir nanoparticles. Polarization curves show that acid leached Ir-Ni ((9.3) 90.5) and Ir-Co nanowires ((4.2) 93.6) outperform nanoparticle catalysts by 4‒5 times in the kinetic region (Figure 6). Performance comparisons normalized to specific activity show a clear advantage, with nanowires outperforming nanoparticles. Comparison of mass activity demonstrate the impact of increasing ECA, with lower displaced samples (Ir-Ni (9.3) 90.5 and Ir-Co (4.2) 93.6) outperforming higher displaced samples. These comparisons assume that the RDE-ECA measurements were maintained in single-cell tests and unfortunately, the mercury underpotential deposition technique used to determine ECA in RDE is not practical (or perhaps possible) in single-cell electrolysis tests. In comparison to MEAs (4‒5 times higher nanowire performance), the nanowires outperformed Ir nanoparticles by 8‒11 times in RDE. This result is not surprising as the RDE and MEA data sets were obtained under different conditions, and could be influenced by a number of factors, including: temperature (RDE at room, MEA at 80°C), internal resistances (RDE data corrected, MEA not), the presence of a supporting electrolyte (RDE, not MEA), and catalyst loading (RDE 30.6 µg cm‒2, MEA 0.1 mg cm‒2). Additionally, MEAs were fabricated from catalyst batches with increased synthesis scale (flask volume increased from 80 ml to 1 L)


12

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

and concentration (0.5 mg ml‒1 to 1.0 mg ml‒1). The resultant nanowires had lower RDE performance (10‒15% lower mass activity), partially accounting for a portion of the observed RDE/MEA differences. The nanowire-based MEAs were also sprayed under a nanoparticleoptimized process and electrode composition (Nafion content). While the extent of these impacts is unclear, the nanowires clearly demonstrate significant promise in both RDE and MEA tests.

Conclusions The Ir-Ni and Ir-Co nanowires demonstrated a significant specific activity benefit over nanoparticle catalysts under all conditions studied, reaching or exceeding the specific activity of polycrystalline Ir. These materials also showed increasing ECA at low displacement, allowing the Ir-Ni and Ir-Co nanowires to achieve OER mass activities 10 and 9 times higher than Ir nanoparticles, respectively. These catalysts, however, suffered large performance and dissolution losses (in the case of Co) following half-cell durability tests. Ex-situ acid treatment of the nanowires was used to preferentially remove Ni and Co, and resulted in significantly improved catalyst durability. Following acid leaching, the Ir-Ni and Ir-Co nanowires were 11 and 8 times more active than Ir nanoparticles, with only minor changes in activity after durability testing for the conditions probed (potential hold 1.6 V). When compared to other efforts in literature, these nanowires exceeded the performance of other Ir-OER catalysts. Finally, the nanowire performance benefits were also demonstrated in single-cell electrolysis testing, where nanowires out-performed Ir and Ir oxide nanoparticles by a factor of 4‒5 times. Comparing the Ir-Ni and Ir-Co nanowires, as-synthesized Ir-Ni produced higher ECA while as-synthesized Ir-Co produced higher specific activity. Ex-situ acid leaching was necessary to improve catalyst durability, which eliminated the Ir-Co specific activity benefit but maintained


13

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

the higher Ir-Ni ECA. This resulted in acid leached Ir-Ni nanowires that outperformed Ir-Co by roughly 40%. The Ir-Ni nanowires therefore may be the more promising system. However, further increases to the ECA and specific activity of Ir-Ni and Ir-Co are possible and are being explored. The cost of electrolysis is limited today by the price of electricity input. As electrolysis shifts toward intermittent renewables, catalyst development and durability are expected to become critical in the progress of renewable hydrogen production. Extended Ir nanostructures have shown promise as OER electrocatalysts. They have demonstrated significant activity and durability benefits to conventional catalysts, in both half-cells and single-cell electrolyzers. The results of this study suggest that significant loading reductions (5‒10 times) are possible without sacrificing performance, while also potentially improving durability. Nanowires may therefore become enabling elements in the commercial progress of electrolysis, and in the emergence of a renewable-based hydrogen energy system.

Experimental Ir-Ni and Ir-Co nanowires were synthesized by the spontaneous galvanic displacement of Ni and Co nanowires (PlasmaChem GmbH) with Ir. Nanowires (40 mg) were dispersed by horn sonication in 80 ml of water, heated to 90°C, and stirred by a Teflon paddle at approximately 500 rpm in a 250 ml round bottom flask. A variable amount of Ir (IV) chloride hydrate was dispersed in 50 ml of water and added drop-wise to the flask over a period of 15 min. The flask contents proceeded at 90°C for 2 h prior to being cooled in an ice bath. The formed nanowires were washed by centrifugation, three times in water and once in 2-propanol prior to being dried in a vacuum oven at 40°C. This method is similar to one previously used in the synthesis of Pt-Ni


14

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

and Pt-Co nanowires, differing in the metal precursor and the quantity of that precursor added.4849

The nanowires were acid treated to remove excess Ni and Co, by dispersing 30 mg of

nanowire in 50 ml of 3

M

sulfuric acid at room temperature for 16 h. The product was then

washed by centrifugation three times in water and once in 2-propanol prior to being dried in a vacuum oven at 40°C. Scanning electron microscopy (SEM) was taken with a JEOL JSM-7000F field emission microscope. SEM samples were prepared by pipetting sonicated catalyst dispersions onto silica substrates. Bright field transmission electron microscopy (TEM) was taken with a Philips CM200 TEM and FEI Talos TEM, both operated at 200kV with samples dispersed onto copper grids with holey carbon support films. Scanning TEM (STEM) imaging and energy dispersive spectroscopy (EDS) measurements were also performed on the Talos. XRD data was collected at the Stanford Synchrotron Radiation Lightsource in the Stanford Linear Accelerator Center, using beamline 2-1 at 11.15 keV. Dry powders were pressed onto double-sided carbon tape, fixed to silica slides. XPS data was acquired on a Kratos Nova X-ray photoelectron spectrometer with a monochromatic Al Kα source operated at 300 W. CasaXPS software was used for data analysis with all spectra calibrated to adventitious carbon at 284.8 eV. Shirley background was used for Ir 4f and Co 2p, and linear background was employed for the C 1s and O 1s spectra. The O1s spectra were fit with a Gaussian (70%)–Lorentzian (30%) peak shape and the FWHM of the oxide species were constrained to 1.1 eV, the peaks assigned to hydroxyl species were confined to 1.3 eV, and the water/organics peak FWHM was constrained to 1.6 eV to account for the various organic species and water at adsorbed to the sample surface. ICP-MS experiments were completed with a Thermoscientific iCAP Q, calibrated to a blank, four Ir-Ni-Co standards (2, 10, 20, 200 ppb), and an internal standard. Experiments were


15

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

checked against the internal standard every unknown and an Ir-Ni-Co standard (20 ppb) every 5 unknowns. Nanowires (approximately 1 mg of each sample, weighed on a microbalance) were prepared for ICP-MS by ashing in sodium peroxide, at a sodium peroxide to sample ratio in excess of 4:1. Samples were ashed by heating from a Meker-Fisher burner in a zirconium crucible, rated to 200 exposures; full ICP-MS spectra were monitored to ensure zirconium did not appear. Following ashing, samples were dissolved in a 1:1 volumetric mixture of water and nitric acid. Samples were diluted to three concentrations aimed for 2, 20, and 200 ppb, and all ICP-MS samples (nanowires and electrolytes) were filtered at 0.4 µm. ICP-MS measurements were taken in standard and kinetic energy discrimination (KED) mode to remove Co interferences. The differences in Co concentration during standard and KED modes were less than 5%. Each sample was measured three times, with a standard deviation of less than 2% between measurements. Electrochemical characterization was completed in a three-electrode RDE half-cell containing 0.1

M

perchloric acid and equipped with a 5 mm diameter gold working electrode, a

gold mesh counter electrode, and a RHE reference. The reference electrode contained hydrogensaturated 0.1

M

perchloric acid and was connected to the main cell by a Luggin capillary. Gold

working electrodes were used since carbon electrodes corroded at elevated potential. Gold working and counter electrodes were also beneficial since gold was not particularly active for OER. Rotation of the working electrode was controlled with a modulated speed rotator (Pine Instrument Company) and measurements were taken with an Autolab PGSTAT302N potentiostat (Eco Chemie, Metrohm Autolab B.V.). Nanowire inks were made to a concentration of 0.2 mg ml‒1, with a water to 2-propanol ratio of 7.6:2.4; Nafion (5 wt. %, Sigma Aldrich) was added at a concentration of 4 µlNafion mlInk‒


16

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1

. Inks were chilled in ice, horn sonicated for 30 s, bath sonicated for 20 min, and horn sonicated

for 30 s prior to 10 µl being dispensed onto the gold electrode. The electrodes were dried at 40°C for 20 min and this process was repeated until the electrodes reached loadings of 30.6 µg cmelec‒ 2

. Inks of Ir nanoparticles contained 3.5 mg Ir (Ir black, Johnson Matthey, product number

160000), 7.6 ml water, 2.4 ml 2-propanol, and 40 µl Nafion. Chilled Ir nanoparticle inks were bath sonicated for 20 min; 10 µl of ink was then dispensed onto a gold electrode. Following coating, electrodes were dried for 20 min at 40°C. Electrochemical conditioning consisted of 20 cycles at 2500 rpm and 100 mV s‒1 in the potential range 1.2‒1.8 V vs. RHE. OER activities were evaluated at 1.55 V vs. RHE during anodic polarization scans at 2500 rpm and 20 mV s‒1. The potential (1.55 V) was used since it was low enough to be within the kinetic region, but high enough to give highly reproducible currents. Successive chronoamperometry experiments were completed every 25 mV for 10 s in the potential range 1.4‒1.6 V vs. RHE at 2500 rpm. All electrochemical experiments were corrected for internal resistance (25‒28 Ω), which was determined with a built-in current interrupter at 1.6 V vs. RHE. Identical resistance values were also found at 1.4 and 1.8 V vs. RHE. The resistances reported here were similar to those of Pt catalysts in oxygen reduction studies using 0.1

M

perchloric acid (21‒23 Ω). The values in this study were slightly larger and

may be due to test-to-test changes in cell configuration or to differences in the catalyst layer.51 ECAs were determined by mercury underpotential deposition in a 0.1

M

perchloric acid

electrolyte containing 1 mM mercury nitrate.46, 52 Working electrodes were rotated at 1500 rpm and cyclic voltammograms were completed at 50 mV s‒1 in the potential range 0.025‒0.55 V vs RHE. The ECA calculation was completed assuming a Coulombic charge of 138.6 µC cmIr‒2; this experiment on a polycrystalline Ir electrode produced a roughness factor of 1.43. The gold


17

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

working electrode was also active for mercury adsorption/desorption.53-55 Current responses from the gold electrode were subtracted from the data presented for mercury. Repeats of these experiments on carbon working electrodes produced minimal changes. Accelerated stress tests were completed by 13.5 h potential holds at 1.5 and 1.6 V vs. RHE, at a rotation speed of 2500 rpm in a 0.1

M

perchloric acid electrolyte. Catalysts were tested for

OER activity and ECAs following the accelerated stress tests. Samples of the electrolytes were also taken to evaluate catalyst dissolution. Although a consensus on RDE durability testing protocols for OER electrocatalysts has not been reached, these conditions were previous found to be similar to electrolyzer losses (with low-loaded anodes).6 Loss in RDE half-cells, however, occurred on a shorter time frame and at lower potential.

Figure 1. Microscopy and spectroscopy of Ir-Ni nanowires, including (a-d) the as-synthesized nanowires (9.3 wt. % Ir) and (e-f) the acid leached nanowires (9.3 wt. % Ir as-synthesized, 90.5 wt. % Ir acid leached, (9.3) 90.5). EDS mapping shows elemental distribution for Ir (red) and Ni (green).


18

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

a)

b)

Ir-Co 99.8 42.1 17.2 10.7 4.2

Ir-Ni 82.0 52.3 11.0 9.3 4.3

20

25

30 35 2θ [Degrees]

20

40

c)

25

30 35 2θ [Degrees]

40

d)

Ir-Co (99.8) 100.0 (42.1) 94.5 (17.2) 96.3 (10.7) 97.7 (4.2) 93.6

Ir-Ni (82.0) 97.5 (52.3) 98.3 (11.0) 91.7 (9.3) 90.5 (4.3) 97.3

20

25

30 35 2θ [Degrees]

40

20

25

30 35 2θ [Degrees]

40

Figure 2. XRD patterns of as-synthesized (a) Ir-Ni and (b) Ir-Co nanowires with all Ni (Ni fcc) and Co (Co fcc, Co3O4 monoclinic, Co hcp) signals removed. XRD patterns of acid leached (c) Ir-Ni and (d) Ir-Co nanowires. Vertical red lines (a‒d) correspond to Ir fcc (111, solid red line) and Ir fcc (200, dashed red line). The Ir content of the catalysts was included in the figure legends, by both the as-synthesized and acid leached compositions. The nomenclatures for catalyst references are (a-b) a single numeric value for the as-synthesized composition, or (c-d) two numeric values, the as-synthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (4.3) 97.3 was 4.3 wt. % Ir assynthesized and 97.3 wt. % Ir following acid leaching. More information and complete XRD


19

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

patterns (all catalysts, not background-subtracted, and full 2θ) are available in the Supporting Information section.

Figure 3. XPS spectra with of Ir-Ni, as-synthesized and acid leached (Acid). XPS includes spectra of (a) Ir 4f (as-synthesized), (b) Ir 4f (acid leached), (c) Ni 2p (as-synthesized), (d) O 1s (as-synthesized), and (e) O 1s (acid leached). The nomenclatures for catalyst references are a single numeric value for the as-synthesized composition and two numeric values, the assynthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (9.3) 90.5 was 9.3 wt. % Ir as-synthesized and 90.5 wt. % Ir following acid leaching.


20

Page 21 of 33

b) 90

75

75

ECA [m2 gIr‒1]

ECA [m2 gIr‒1]

a) 90

60

60

45

45

30

30

15

15

0

0

4

8 7

3

6

2.5

5

2

4

1.5

3

1

2

0.5

1 0

0

0

im1.55V [A mgIr‒1]

d)

4

8 7

3

6

2.5

5

2

4

1.5

3

1

2

0.5

1

3.5

20 40 60 80 100 Ir-Co, Ir Displacement [wt % Ir]

4

f)

Initial

3.5


1.6 V Hold

3

0 0

Initial


3.5

20 40 60 80 100 Ir-Ni, Ir Displacement [wt % Ir]

4

2.5

1.6 V Hold

3

2.5

2

1.5

2

1.5

1

0.5

1

0.5

0

0 0

20 40 60 80 Ir-Ni, Ir Displacement [wt % Ir]

g) 5

100


h) 50

Co Dissolution [%]

1.6 V Hold Acid Exposure

4

0

3 2 1

30 20 10

Ir-Ni, Composition [wt % Ir]

99.8

95.3

67.6

42.1

21.1

19.4

17.2

12.2

5.9

10.7

5.5

82.0

52.3

24.7

20.0

12.5

11.0

9.3

8.3

4.3

3.7

0

2.2

0

1.6 V Hold Acid Exposure

40

4.2


0

3.5

0

e)

100

is1.55V [mA cmIr‒2]


c)

20 40 60 80 Ir-Ni, Ir Displacement [wt % Ir]


0

Ni Dissolution [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Ir-Co, Composition [wt % Ir]

Figure 4. ECAs of as-synthesized (a) Ir-Ni and (b) Ir-Co nanowires as a function of Ir displacement. OER mass (red) and specific (blue) activities of as-synthesized (c) Ir-Ni and (d) IrCo nanowires as a function of Ir displacement. Activities were corrected for internal resistance


21

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

and taken with a rotation speed of 2500 rpm and at a scan rate of 20 mV s‒1 in a 0.1 M perchloric acid electrolyte. The mass and specific activities, and ECA of Ir nanoparticles (a‒d) were included as solid lines. The specific activity of polycrystalline Ir (c‒d) was included as a dashed blue line. Mass OER activities of as-synthesized (e) Ir-Ni and (f) Ir-Co nanowires prior to (Initial) and following (1.6 V Hold) durability testing, a 1.6 V hold for 13.5 h. The activity of Ir nanoparticles prior to and following durability testing (e‒f) was included as a dashed and solid red line, respectively. Loss of catalyst mass due to Ni/Co dissolution following electrochemical conditioning (Acid Exposure) and durability (1.6 V Hold) of as-synthesized (g) Ir-Ni and (h) IrCo nanowires. The Ir content of the catalysts was included on the x-axis.


22

Page 23 of 33

98

98

Leached [wt % Ir]

b) 100

96 94 92

96 94 92

90

90

4

8

3.5

7

3

6

2.5

5

2

4

1.5

3

1

2

0.5

1 0

0

0 0 20 40 60 80 100 Ir-Co, Initial Ir Displacement [wt % Ir]

0

4

8

3.5

7


d)


3

6

2.5

5

2

4

1.5

3

1

2

0.5

1

0 20 40 60 80 100 Ir-Ni, Initial Ir Displacement [wt % Ir] 4 3

Initial

3.5

1.6 V Hold

2.5

1.6 V Hold

3

2.5

2

1.5

2

1.5

1

0.5

1

0.5 0 20 40 60 80 100 Ir-Ni, Initial Ir Displacement [wt % Ir] h) 5

Ir-Ni, Composition [wt % Ir]

(95.3) 97.9

(99.8) 100.0

0

(67.6) 96.3

(82.0) 97.5

(52.3) 98.3

(24.7) 97.2

(20.0) 95.6

(12.5) 95.5

(9.3) 90.5

(11.0) 91.7

(8.3) 92.6

(4.3) 97.3

(3.7) 96.1

(2.2) 97.1

0

1

(42.1) 94.5

1

2

(19.4) 95.5

2

Acid Exposure

3

(17.2) 96.3

Acid Exposure

3

1.6 V Hold

4

(12.2) 96.1

1.6 V Hold

4


(10.7) 97.7

g) 5

0

Co Dissolution [%]

0

(21.1) 94.9

0

(5.9) 94.3


3.5

4

f)

Initial


e)

(5.5) 94.2


c)

0 20 40 60 80 100 Ir-Co, Initial Ir Displacement [wt % Ir]


0 20 40 60 80 100 Ir-Ni, Initial Ir Displacement [wt % Ir]

(4.2) 93.6

Leached [wt % Ir]

a) 100

Ni Dissolution [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Composition [wt % Ir]

Figure 5. (a) Ir-Ni and (b) Ir-Co nanowire composition, as-synthesized (x-axis) and following ex-situ acid leaching (y-axis). OER mass (red) and specific (blue) activities of acid leached (c) Ir-Ni and (d) Ir-Co nanowires, as a function of the as-synthesized Ir displacement level.


23

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

Activities were corrected for internal resistance and taken with a rotation speed of 2500 rpm and at a scan rate of 20 mV s‒1 in a 0.1 M perchloric acid electrolyte. The mass and specific activities of Ir nanoparticles (a‒d) were included as solid red and blue lines. The specific activity of polycrystalline Ir (c‒d) was included as a dashed blue line. Mass OER activities of acid leached (e) Ir-Ni and (f) Ir-Co nanowires, prior to (Initial) and following (1.6 V Hold) durability testing, a 1.6 V hold for 13. 5 h. The activity of Ir nanoparticles prior to and following durability testing (e‒f) was included as a dashed and solid red line, respectively. Although (c-f) includes the activities of the acid leached Ir-Ni and Ir-Co nanowires, their performance is presented in terms of the as-synthesized composition (x-axis) to maintain a discussion of relevant trends. Loss of catalyst mass due to Ni/Co dissolution following electrochemical conditioning (Acid Exposure) and durability (1.6 V Hold) of acid leached (g) Ir-Ni and (h) Ir-Co nanowires. The Ir content of the catalysts was included on the x-axis, by both the as-synthesized and acid leached compositions. The nomenclatures of these references are two numeric values, the as-synthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (9.3) 90.5 was 9.3 wt. % Ir as-synthesized and 90.5 wt. % Ir following acid leaching.


24

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 1. Comparison of Ir nanowire activity to commercial baselines and previously studied OER catalysts in RDE half-cells.6, 33, 35-37, 39, 56-74 Activities in single cell electrolyzers were also included below the double line for Ir nanowires and NSTF. Membrane electrode assembly (MEA) performance was presented to ensure the inclusion of NSTF, and was not a comprehensive review. Activities were reported at specified potentials on an A gIr‒1 basis. Nanowire catalysts are referenced by both their as-synthesized and acid leached compositions. The nomenclatures of these references are two numeric values, the as-synthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (9.3) 90.5 was 9.3 wt. % Ir as-synthesized and 90.5 wt. % Ir following acid leaching. Ref

Composition

1.45 V

1.48 V

1.51 V

1.53 V

1.55 V

1.60 V

i/iIr-Co

[wt. % PGM] Ir-Ni (9.3) 90.5

This Work

‒

90.5

59

244

810

1650

3353

9666

1.00

Ir-Ni (52.3) 98.3

This Work

‒

98.3

10

40

151

355

767

2322

0.23

Ir-Co (4.2) 93.6

This Work

‒

93.6

44

174

594

1300

2327

9942

0.69

Ir-Co (42.1) 94.5

This Work

‒

94.5

14

55

251

550

964

2533

0.29

Johnson Matthey

6

‒

3

10

50

138

295

1030

0.09

IrO2

Alfa Aesar

6

‒

1

5

22

56

138

729

0.04

Ir-Cu

Bergens et al.

56

89

‒

142

‒

‒

‒

‒

0.58

Ir-Ni

Bergens et al.

57

89

‒

140

‒

‒

‒

‒

0.57

Ir-Sn-Sb

Cap et al.

58

33

‒

‒

‒

‒

‒

2400

0.25

Ir-Ni

Strasser et al.

35

79

‒

‒

‒

370

‒

‒

0.22

Ir-Ni

Strasser et al.

36

23.2

‒

38

‒

‒

‒

‒

0.16

Ir-Ni

Strasser et al.

37

23.2

‒

‒

88

‒

‒

‒

0.11

Ir

Strasser et al.

59

‒

‒

‒

70

‒

‒

‒

0.09

Ir

Chen et al.

60

‒

‒

‒

‒

‒

‒

572

0.06

Ir-Nb-Ti

Chen et al.

61

30.9

‒

‒

‒

‒

‒

537

0.06

Ir

Shao et al.

62

‒

‒

‒

‒

‒

‒

475

0.05

Ir-Ti

Yan et al.

63

‒

‒

‒

32

‒

‒

‒

0.04

Ir

Gago et al.

64

100

‒

8

‒

‒

‒

‒

0.03

Ir-C

Feng et al.

65

6

‒

‒

‒

‒

‒

180

0.02

Ir

Schmidt et al.

66

100

‒

‒

18

‒

‒

‒

0.02

Ir-La-Li

Grimaud et al.

67

40

‒

‒

17

‒

‒

‒

0.02

Ir-Y

Schmidt et al.

68

64

‒

‒

‒

33

‒

‒

0.02

Ir-Cu

Yang et al.

69

70

‒

‒

‒

21

‒

‒

0.01

Ir

Shao-Horn et al.

70

‒

‒

3

‒

‒

‒

‒

0.01

Ir-Bi

Walton et al.

71

42

‒

‒

‒

6

‒

‒

0.00

Ir


25

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

Ru-Co

Strasser et al.

33

60

‒

205

‒

‒

‒

‒

0.84

Ru-Ir

Gago et al.

72

100

‒

38

‒

‒

‒

‒

0.16

Ru

Shao-Horn et al.

70

‒

‒

14

‒

‒

‒

‒

0.06

Ru-Ir-W

Baumann et al.

73

10

‒

‒

‒

‒

‒

380

0.04

Ru-Ir

Mayousse et al.

74

‒

2

‒

‒

‒

‒

‒

0.03

Ir-Ni (9.3) 90.5

This Work

‒

90.5

689

1661

3317

4678

6127

9354

‒

Ir-Ni (9.3) 98.3

This Work

‒

98.3

240

571

1267

1913

2772

5168

‒

Ir-Co (4.2) 93.6

This Work

‒

93.6

466

1027

2277

3544

4900

8447

‒

Ir-Co (42.1) 94.5

215

517

1088

1720

2482

4804

‒ ‒

This Work

‒

94.5

Ir (NSTF)

Xu et al.

39

‒

192

520

1080

1760

3400

‒

Ir

Johnson Matthey

‒

‒

59

285

619

998

1530

3198

IrO2

Alfa Aesar

‒

‒

56

276

607

971

1450

2946


26

Page 27 of 33

2

b) 1.7

1.9

1.65

1.8

1.6

E [V]

E [V]

a)

1.7

1.5

1.5

1.45 1.4

0

c)

1.55

1.6

1.4 0.5

1 1.5 2 i [A cmelec‒2]

2.5

3

0.1

1 im [A mgIr‒1]

10

1.7

1.75 Ir-Ni (9.3) 90.5 1.7 1.65Ir-Ni (52.3) 98.3

1.65 1.6

E [V]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1.55

1.6Ir-Co (4.2) 93.6

1.5

1.55Ir-Co (42.1) 94.5 1.5Ir 1.45IrO O

1.45 1.4

2

0.5

5 is [mA cmIr‒2]

50

1.4

Figure 6. Single-cell electrolyzer data of acid leached Ir-Ni nanowires ( (9.3) 90.5 and (52.3) 98.3, red), acid leached Ir-Co nanowires ((4.2) 93.6 and (42.1) 94.5, blue), Ir nanoparticles (Ir, green), and Ir oxide nanoparticles (IrO2, yellow) at an anode loading of 0.1 mgIr cmelec‒2, presented as Tafel plots on an (a) electrode area, (b) mass activity and (c) specific activity basis. Specific activity evaluations were made based on ECAs determined during RDE half-cell testing. Nanowire catalysts are referenced by both their as-synthesized and acid leached compositions. The nomenclatures of these references are two numeric values, the as-synthesized composition in parentheses and the acid leached composition immediately following. For example, Ir-Ni (9.3) 90.5 was 9.3 wt. % Ir as-synthesized and 90.5 wt. % Ir following acid leaching.


27

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

AUTHOR INFORMATION Corresponding Author Shaun M. Alia Chemistry and Nanoscience Center, National Renewable Energy Laboratory 15013 Denver West Parkway, Golden, CO 80401, United States of America E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Laboratory Directed Research and Development (LDRD) Program at the National Renewable Energy Laboratory. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

ASSOCIATED CONTENT Supporting Information. Electrochemical and microscopy results are included in the Supporting Information section. 2017_1231 SAlia IrNiCo SI.pdf

ACKNOWLEDGMENT The authors would like to thank Hui Xu and Brian Rasimick of Giner, Inc. for initial MEA testing of the Ir-Co nanowires, as well as Guido Bender and Fabrizio Ganci of the National


28

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Renewable Energy Laboratory for maintaining the MEA test stand. The authors would also like thank Michael Toney and Badri Shyam of the Stanford Linear Accelerator Center, Katherine Hurst of the National Renewable Energy Laboratory, and Alex Roman of the University of Colorado for their help in obtaining the XRD patterns presented in this study. The authors also acknowledge the Trefny Institute, and facilities support at Colorado School of Mines and the National Renewable Energy Laboratory.

REFERENCES 1. H2 at Scale: Deeply Decarbonizing our Energy System. Department of Energy, U. S., Ed. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf, 2016. 2. Milbrandt, A.; Mann, M., Department of Energy, U. S., Ed. http://www.nrel.gov/docs/fy09osti/42773.pdf, 2009. 3. Denholm, P.; Margolis, R. M.; Milford, J. M., Environ. Sci. Technol. 2009, 43, 226-232. 4. Denholm, P.; O'Connell, M.; Brinkman, G.; Jorgenson, J., Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart. Department of Energy, U. S., Ed. http://www.nrel.gov/docs/fy16osti/65023.pdf, 2015; Vol. NREL/TP-6A20-65023. 5. Levene, J. I.; Mann, M. K.; Margolis, R. M.; Milbrandt, A., J. Sol. Energy 2007, 81, 773780. 6. Alia, S. M.; Rasimick, B.; Ngo, C.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Xu, H.; Pivovar, B. S., J. Electrochem. Soc. 2016, 163, F3105-F3112. 7. Neyerlin, K. C.; Gu, W.; Jorne, J.; Gasteiger, H. A., J. Electrochem. Soc. 2007, 154, B631-B635. 8. Reier, T.; Oezaslan, M.; Strasser, P., ACS Catal. 2012, 2, 1765-1772. 9. Pourbaix, M., Atlas of electrochemical equilibria in aqueous solutions. 2nd Edition ed.; National Association of Corrosion Engineers: Houston, Texas, 1974. 10. Cherevko, S.; Zeradjanin, A. R.; Topalov, A. A.; Kulyk, N.; Katsounaros, I.; Mayrhofer, K. J. J., ChemCatChem 2014, 6, 2219-2223. 11. Pourbaix, M. J. N.; Van Muylder, J.; de Zoubov, N., Platinum Met. Rev. 1959, 3, 100106. 12. Park, S.; Shao, Y.; Liu, J.; Wang, Y., Energy Environ. Sci. 2012, 5, 9331-9344. 13. Kulandaisamy, S.; Rethinaraj, J. P.; Chockalingam, S. C.; Visvanathan, S.; Venkateswaran, K. V.; Ramachandran, P.; Nandakumar, V., J. Appl. Electrochem. 1997, 27, 579-583. 14. Kamachi Mudali, U.; Raju, V. R.; Dayal, R. K., J. Nucl. Mater. 2000, 277, 49-56. 15. Kamegaya, Y.; Sasaki, K.; Oguri, M.; Asaki, T.; Kobayashi, H.; Mitamura, T., Electrochim. Acta 1995, 40, 889-895. 16. de Oliveira-Sousa, A.; da Silva, M. A. S.; Machado, S. A. S.; Avaca, L. A.; de LimaNeto, P., Electrochim. Acta 2000, 45, 4467-4473.


29

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

17. Otogawa, R.; Morimitsu, M.; Matsunaga, M., Electrochim. Acta 1998, 44, 1509-1513. 18. Cheng, J.; Zhang, H.; Chen, G.; Zhang, Y., Electrochim. Acta 2009, 54, 6250-6256. 19. Mattos-Costa, F. I.; de Lima-Neto, P.; Machado, S. A. S.; Avaca, L. A., Electrochim. Acta 1998, 44, 1515-1523. 20. Kötz, R.; Stucki, S., Electrochim. Acta 1986, 31, 1311-1316. 21. Miles, M. H.; Klaus, E. A.; Gunn, B. P.; Locker, J. R.; Serafin, W. E.; Srinivasan, S., Electrochim. Acta 1978, 23, 521-526. 22. Viganoò, R.; Taraszewska, J.; Daghetti, A.; Trasatti, S., J. Electroanal. Chem. Interfacial Electrochem. 1985, 182, 203-209. 23. Kötz, R.; Stucki, S., J. Electrochem. Soc. 1985, 132, 103-107. 24. Kötz, R.; Neff, H.; Stucki, S., J. Electrochem. Soc. 1984, 131, 72-77. 25. Kötz, R.; Stucki, S., Electrochim. Acta 1986, 31, 1311-1316. 26. Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., Int. J. Hydrogen Energy 2013, 38, 49014934. 27. Man, I. C.; Su, H. Y.; Calle‐Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J., ChemCatChem 2011, 3, 1159-1165. 28. Yeo, R.; Orehotsky, J.; Visscher, W.; Srinivasan, S., J. Electrochem. Soc. 1981, 128, 1900-1904. 29. Hutchings, R.; Müller, K.; Kötz, R.; Stucki, S., J. Mater. Sci. 1984, 19, 3987-3994. 30. Balko, E.; Davidson, C., J. Inorg. Nucl. Chem. 1980, 42, 1778-1781. 31. Boodts, J.; Trasatti, S., J. Electrochem. Soc. 1990, 137, 3784-3789. 32. Neyerlin, K. C.; Bugosh, G.; Forgie, R.; Liu, Z.; Strasser, P., J. Electrochem. Soc. 2009, 156, B363-B369. 33. Forgie, R.; Bugosh, G.; Neyerlin, K.; Liu, Z.; Strasser, P., Electrochem. Solid-State Lett. 2010, 13, B36-B39. 34. Zhao, C.; Yifeng, E.; Fan, L., Microchim. Acta 2012, 178, 107-114. 35. Reier, T.; Pawolek, Z.; Cherevko, S.; Bruns, M.; Jones, T.; Teschner, D.; Selve, S. r.; Bergmann, A.; Nong, H. N.; Schlögl, R., J. Am. Chem. Soc. 2015, 137, 13031-13040. 36. Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P., Chem. Sci. 2014, 5, 29552963. 37. Nong, H. N.; Oh, H. S.; Reier, T.; Willinger, E.; Willinger, M. G.; Petkov, V.; Teschner, D.; Strasser, P., Angew. Chem., Int. Ed. 2015, 54, 2975-2979. 38. Xu, H., High-Performance, Long-Lifetime Catalysts for Proton Exchange Membrane Electrolysis. Department of Energy, U. S., Ed. http://www.hydrogen.energy.gov/pdfs/review14/pd103_xu_2014_o.pdf, 2014. 39. Xu, H., High-Performance, Long-Lifetime Catalysts for Proton Exchange Membrane Electrolysis. Department of Energy, U. S., Ed. http://www.hydrogen.energy.gov/pdfs/review15/pd103_xu_2015_o.pdf, 2015. 40. Lee, W. H.; Kim, H., Catal. Commun. 2011, 12, 408-411. 41. Zhao, C.; E, Y.; Fan, L., Microchim. Acta 2012, 178, 107-114. 42. Alia, S. M.; Yan, Y. S.; Pivovar, B. S., Catal. Sci. Technol. 2014, 4, 3589-3600. 43. Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W.; Gerson, A. R.; Smart, R. S. C., Appl. Surf. Sci. 2011, 257, 2717-2730. 44. Pfeifer, V.; Jones, T. E.; Velasco Vélez, J. J.; Massué, C.; Arrigo, R.; Teschner, D.; Girgsdies, F.; Scherzer, M.; Greiner, M. T.; Allan, J., Surf. Interface Anal. 2016, 48, 261-273.


30

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

45. Casalongue, H. G. S.; Ng, M. L.; Kaya, S.; Friebel, D.; Ogasawara, H.; Nilsson, A., Angew. Chem., Int. Ed. 2014, 28, 7297-7300. 46. Alia, S. M.; Hurst, K. E.; Kocha, S. S.; Pivovar, B. S., J. Electrochem. Soc. 2016, 163, F3051-F3056. 47. Pfeifer, V.; Jones, T. E.; Vélez, J. J. V.; Arrigo, R.; Piccinin, S.; Hävecker, M.; KnopGericke, A.; Schlögl, R., Chem. Sci. 2017, 8, 2143-2149. 48. Alia, S. M.; Larsen, B. A.; Pylypenko, S.; Cullen, D. A.; Diercks, D. R.; Neyerlin, K. C.; Kocha, S. S.; Pivovar, B. S., ACS Catal. 2014, 4, 1114-1119. 49. Alia, S. M.; Pylypenko, S.; Neyerlin, K. C.; Cullen, D. A.; Kocha, S. S.; Pivovar, B. S., ACS Catal. 2014, 4, 2680-2686. 50. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T., Appl. Catal., B 2005, 56, 935. 51. Shinozaki, K.; Zack, J. W.; Richards, R. M.; Pivovar, B. S.; Kocha, S. S., J. Electrochem. Soc. 2015, 162, F1144-F1158. 52. Kounaves, S. P.; Buffle, J., J. Electrochem. Soc. 1986, 133, 2495-2498. 53. Herrero, E.; Abruña, H., Langmuir 1997, 13, 4446-4453. 54. Herrero, E.; Buller, L. J.; Abruna, H. D., Chem. Rev. 2001, 101, 1897-1930. 55. Herrero, E.; Abruna, H., J. Phys. Chem. B 1998, 102, 444-451. 56. Wang, C.; Moghaddam, R. B.; Bergens, S. H., J. Phys. Chem. C 2017, 121, 5480-5486. 57. Moghaddam, R. B.; Wang, C.; Sorge, J. B.; Brett, M. J.; Bergens, S. H., Electrochem. Commun. 2015, 60, 109-112. 58. Massué, C.; Pfeifer, V.; Huang, X.; Noack, J.; Tarasov, A.; Cap, S.; Schlögl, R., ChemSusChem 2017, 10, 1943-1957. 59. Oh, H.-S.; Nong, H. N.; Reier, T.; Gliech, M.; Strasser, P., Chem. Sci. 2015, 6, 33213328. 60. Hu, W.; Wang, Y.; Hu, X.; Zhou, Y.; Chen, S., J. Mater. Chem. 2012, 22, 6010-6016. 61. Hu, W.; Chen, S.; Xia, Q., Int. J. Hydrogen Energy 2014, 39, 6967-6976. 62. Zhang, G.; Shao, Z.-G.; Lu, W.; Li, G.; Liu, F.; Yi, B., Electrochem. Commun. 2012, 22, 145-148. 63. Lu, Z.-X.; Shi, Y.; Yan, C.-F.; Guo, C.-Q.; Wang, Z.-D., Int. J. Hydrogen Energy 2017, 42, 3572-3578. 64. Lettenmeier, P.; Wang, L.; Golla‐Schindler, U.; Gazdzicki, P.; Cañas, N. A.; Handl, M.; Hiesgen, R.; Hosseiny, S. S.; Gago, A. S.; Friedrich, K. A., Angew. Chem., Int. Ed. 2016, 55, 742-746. 65. Zhang, J.; Wang, G.; Liao, Z.; Zhang, P.; Wang, F.; Zhuang, X.; Zschech, E.; Feng, X., Nano Energy 2017, 40, 27-33. 66. Abbott, D. F.; Lebedev, D.; Waltar, K.; Povia, M.; Nachtegaal, M.; Fabbri, E.; Copéret, C.; Schmidt, T. J., Chem. Mater. 2016, 28, 6591-6604. 67. Grimaud, A.; Demortière, A.; Saubanère, M.; Dachraoui, W.; Duchamp, M.; Doublet, M.-L.; Tarascon, J.-M., Nat. Energy 2016, 2, 16189. 68. Lebedev, D.; Povia, M.; Waltar, K.; Abdala, P. M.; Castelli, I. E.; Fabbri, E.; Blanco, M. V.; Fedorov, A.; Copéret, C.; Marzari, N.; Schmidt, T. J., Chem. Mater. 2017, 29, 5182-5191. 69. Sun, W.; Song, Y.; Gong, X.-Q.; Cao, L.-m.; Yang, J., Chem. Sci. 2015, 6, 4993-4999. 70. Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y., J. Phys. Chem. Lett. 2012, 3, 399-404.


31

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

71. Sardar, K.; Ball, S. C.; Sharman, J. D. B.; Thompsett, D.; Fisher, J. M.; Smith, R. A. P.; Biswas, P. K.; Lees, M. R.; Kashtiban, R. J.; Sloan, J.; Walton, R. I., Chem. Mater. 2012, 24, 4192-4200. 72. Wang, L.; Saveleva, V. A.; Zafeiratos, S.; Savinova, E. R.; Lettenmeier, P.; Gazdzicki, P.; Gago, A. S.; Friedrich, K. A., Nano Energy 2017, 34, 385-391. 73. Baumann, N.; Cremers, C.; Pinkwart, K.; Tübke, J., Fuel Cells 2017, 17, 259-267. 74. Mayousse, E.; Maillard, F.; Fouda-Onana, F.; Sicardy, O.; Guillet, N., Int. J. Hydrogen Energy 2011, 36, 10474-10481.


32

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Entry for the Table of Contents


33

Iridium-Based Nanowires as Highly Active, Oxygen Evolution Reaction

Recommend Documents