Al-Induced In Situ Formation of Highly Active Nanostructured Water

Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States. ACS Catal. , 2018, 8, pp 2595–2600. DOI: 10.1021/acscatal.7b03817. Publ...
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Al-induced In situ Formation of Highly Active Nanostructured Water-oxidation Electrocatalyst Based on Ni-phosphide Junyuan Xu, Juliana P. S. Sousa, Natalia Mordvinova, José Diogo Costa, Dmitri Y Petrovykh, Kirill Kovnir, Oleg Igorevich Lebedev, and Yury V. Kolen'ko ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03817 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Al-induced In situ Formation of Highly Active Nanostructured Water-oxidation Electrocatalyst Based on Ni-phosphide Junyuan Xu,† Juliana P. S. Sousa,† Natalia Mordvinova,‡ José Diogo Costa,† Dmitri Y. Petrovykh,† Kirill Kovnir,§,⊥ Oleg I. Lebedev,‡ and Yury V. Kolen’ko*,† †

International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal. Laboratoire CRISMAT, UMR 6508, CNRS-ENSICAEN, Caen 14050, France. § Department of Chemistry, Iowa State University, Ames, IA 50011, USA. ⊥ Ames Laboratory, U.S. Department of Energy, Ames, IA 50011, USA. ‡

ABSTRACT: We report a simple low-cost concept for the preparation of water-oxidation electrocatalyst via modification of selfsupported Ni5P4–Ni2P foam catalyst precursor with Al. As an anode for alkaline oxygen evolution reaction, this material exhibits an impressive Tafel slope of 27 mV dec–1, and offers anodic current densities of 10, 100, and 300 mA cm–2 at overpotentials of merely 180, 247, and 312 mV, respectively. Moreover, the anode demonstrates high operational stability, as reflected by a steady-state performance for more than eight days. Combining state-of-the-art electron microscopy and photoelectron spectroscopy we investigated the role of the Al dopant in the formation of active Ni(OH)2/NiO/Ni5P4–Ni2P nanocatalyst, which exhibits oxygen-evolving activity among the highest reported to-date. KEYWORDS: OER, sacrificial Al, nickel phosphide, in situ catalyst formation, PGM-free catalyst. Water oxidation, also known as the oxygen evolution reaction (OER), is an important electrochemical half-reaction of water electrolysis for generating hydrogen and oxygen fuels.1 Despite being studied for more than half a century, this sluggish multi-step reaction involving 4e– transfers still poses several unresolved challenges2 and suffers from poor catalytic efficiency. Hence, to achieve high current densities, j, required for efficient H2 production, one should apply a potential, E, that is higher than the standard equilibrium potential (+1.229 V) of the 2H2O ↔ O2 +4H+ + 4e– reaction with respect to the reference reversible hydrogen electrode (RHE). Furthermore, since the reaction is performed under highly oxidizing potentials, the anode material must be chosen from a limited group of materials that are chemically stable under these harsh reaction conditions. Platinum group metal (PGM) oxides, namely RuO2 and IrO2, remain the most active and stable OER electrocatalysts.3 Unfortunately, the status of PGMs as critical raw materials (CRMs) calls for developing alternative CRM-free electrocatalysts for water oxidation.4 Our effective solution for the development of such active, stable, and Earth-abundant OER electrocatalysts is based on nanostructured nickel phosphides, which serve as precursors for the in situ formation of the active electrocatalyst during water oxidation. Originally, while investigating the OER performance of Fe0.2Ni0.8P2 in alkaline electrolyte, we found the phosphide to undergo a rapid transformation into a mixed hydroxide, which can further catalyze OER and remain active and stable for at least 60 h.5 More recently, we improved the activity and stability of the in situ formed OER electrocatalyst by interfacing the nickel phosphide precursor with Mg2O(OH)2-like phase.6 The resultant electrocatalyst reaches j of 10 mA cm−2 at overpotential, η, of only 280 mV, while being stable over eight days. Taking into account that in situ electrocatalyst formation from Ni phosphide precursors is a viable concept towards

high-performing oxygen-evolving electrocatalysts, we envisioned that Al-modified Ni phosphide is a good precursor for OER electrocatalyst. Notably, we recently developed a selfsupported Al-doped Ni phosphide cathode as a highly active material for hydrogen evolution reaction (HER),7 where Al atoms substitute Ni in the crystal structure of Ni phosphide, favorably changing the electronic structure of the resultant cathode. In sharp contrast to the chemical doping approach, in this work, we deduced that good physical mixing of Al and Ni at the Ni foam surface followed by sacrificial leaching of the Al phase under alkaline OER conditions would produce a high-surface-area electrocatalyst, somewhat similar to Raney Ni.8 Here, we report the excellent OER performance of the resulting electrocatalyst and describe its comprehensive structural characterization. To prepare Al-modified nickel phosphide (for details, see the Supporting Information (SI)), commercial Ni foam was first cleaned by aqueous HCl solution to remove the surface oxide layer. Next, 500 nm of Al followed by 50 nm of Ni were sputtered on both sides of the Ni foam. To ensure good physical mixing between Al and Ni as well as good anchoring of Al to the Ni foam surface, the resultant Ni/Al/Ni composite was annealed at 600 °C for 2 h under Ar, and then subjected to gas-transport phosphorization. The product of a typical synthesis is a 2.4 × 5 cm2 piece of self-supported Al-modified Ni−P foam anode, hereafter abbreviated as AlNiP. For comparison, a reference foam anode, abbreviated as NiP, was prepared under identical conditions without Al. We used previously developed by us phosphorization method to generate nickel phosphide,9a and we experimentally observed that the method does not allow for the synthesis of the phase-pure Ni5P4 due to kinetic limitations of the gas-transport reaction. Single phase samples of Ni5P4 can be synthesized by other methods summarized elsewhere.9b

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Figure 1. Water oxidation performance of Al-modified nickel phosphide AlNiP in 1 M KOH, compared with the Al-free NiP, and benchmark RuO2, IrO2, and NiO electrocatalysts supported on Ni foam: (a) the reduction branches of the CV curves after 40 cycles recorded at a scan rate of 5 mV s−1 (inset is enlargement of the potential window); (b) Tafel plots; (c) Nyquist plots of AlNiP and NiP anodes at the applied η = 260 mV (inset is equivalent electrical circuit model); (d) O2 TOFs of AlNiP and benchmark Ni-foam-supported RuO2 electrocatalysts as a function of overpotential η, TOFs for RuO2 are calculated using mass loading of the electrocatalyst. (e) Chronopotentiometric measurements of the OER under a constant current density of 10 mA cm−2 for the AlNiP and NiP anodes.

Specifically, the lamellas of Ni foam phosphorization product always consisted of Ni5P4 thick shell with Ni2P core underneath.9a Interestingly, we observed that the phase-pure Ni2P can be synthesized by phosphorization of Ni foam at T ≥ 800 °C, since at this temperature Ni5P4 phase is not stable and decomposes to provide Ni2P. But we were not able to employ the phosphorization at 800 °C to generate phase-pure anode, since this temperature is significantly higher than the melting point of Al (660 °C). Therefore, in the current study, the phosphorization was conducted at the established conditions of 500 °C for 6 h under Ar.9a Powder X-ray diffraction (XRD), showed both AlNiP and NiP anodes to contain ≈80% of Ni5P4 as the major phase and ≈20% of Ni2P phase (Figure S1 in the SI). Notably, no Al-containing secondary phases were detected in AlNiP. Inductively coupled plasma–optical emission spectrometry (ICP–OES) confirms the assynthesized AlNiP to contain Al with estimated Al/Ni/P molar ratios of about 1:89:47. We use the as-prepared Al-modified self-supported Ni5P4– Ni2P foam as a precursor to explore the in situ formation of the

electrocatalyst during the initial cyclic voltammetry (CV) testing. Such in situ development of the active material via oxidation of the nickel phosphides under OER conditions has been reported in our previous studies5-6 as well as observed by other groups.10 First, we found that the stable CV curves can be obtained after testing for ≥40 cycles (Figure S2 in the SI). Additionally, we observed that the final CV curve features quasi-reversible oxidation and reduction peaks at 1.41 V and 1.25 V, respectively, which are the characteristic peaks of the Ni2+/Ni3+ redox couple in Ni-based OER catalysts.10a Thus, our results demonstrate that the stable and active electrocatalyst is indeed forming in situ, recapitulating previous studies,5-6, 10 and that the oxidation of the surface of Al-modified selfsupported Ni5P4–Ni2P anode is quite fast and already completed after 40 cycles. Our further electrochemical investigation of the Almodified OER electrocatalyst revealed that the AlNiP electrode exhibits a superior performance in 1 M KOH electrolyte in comparison to the control Al-free NiP and reference RuO2, IrO2, and NiO electrocatalysts supported on Ni foam (Figure

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1a). Remarkably, AlNiP reaches anodic j of 10, 100, and 300 mA cm–2 at the very

Figure 2. Representative low-magnification HAADF−STEM images of the as-synthesized AlNiP (a), together with the simultaneously collected EDX maps of Ni, P, O, Al, and their superposition (b, c). The inset in (a) is the corresponding ring electron diffraction pattern indexed based on Al2O3 hexagonal structure.

low η of only 180, 247, and 312 mV, respectively. Moreover, AlNiP exhibits a smaller Tafel slope, TS, of 27 mV dec–1 (Figure 1b), suggesting that the chemical step of MOOH + OH– ↔ MOO– + H2O is the rate determining step for this electrocatalyst.11 Interestingly, AlNiP also shows faster charge-transfer kinetics over anode/electrolyte interface (Figure 1c) as compared to NiP. This indicates that Almodification significantly decreases resistance of the anode, thus resulting in an over threefold higher activity compared to the control Al-free NiP electrocatalyst. The O2 turnover frequencies (TOFs),10a, 12 shown in Figure 1d as a function of η, evidence that AlNiP exhibits significantly higher efficiency towards water oxidation than the benchmark RuO2 electrocatalyst, offering a high TOF of 0.08 s−1 at η = 300 mV. To verify that all of the current goes to OER, we have also estimated Faradaic efficiency towards electrochemical water oxidation over AlNiP. Figure S3 in the SI shows that the anode splits water with Faradaic efficiency of nearly 100%, matching fairly well the calculated theoretical value of generated O2. Finally, we investigated the stability of the Almodified electrocatalyst under galvanostatic water oxidation. AlNiP exhibits high resistance to alkaline solution while providing steady j = 10 mA cm−2 at very low η ≈ 185 mV for over eight days (Figure 1e). We also analyzed the structure of the resultant anode after such a long-term durability testing. As shown in Figure S4 in the SI, the AlNiP largely preserved its bulk phosphide structure, while showing the presence of a tiny amount of nickel hydroxide in the end product.

To understand the origins of the observed high OER activity with η10 = 180 mV and TS = 27 mV dec−1 of our Al-modified nickel phosphide foam anode, we performed detailed transmission electron microscopy (TEM), electron diffraction (ED), high-angle annular dark-field scanning TEM (HAADF−STEM), and energy-dispersive X-ray spectroscopy (EDX) analyses of the as-synthesized AlNiP (Figure 2) and of the same sample after CV testing for 40 cycles (Figure 3). The surface of the as-synthesized Al-modified Ni5P4–Ni2P foam anode (Figure 2) consists of Ni5P4 crystallites (ca. 0.5– 1 µm) with the expected inclusion of an Al2O3 phase. X-ray photoelectron spectroscopy (XPS) characterization of the assynthesized AlNiP anode (Figure 4a) indicates surface chemistry similar to that previously reported for Ni phosphidebased electrocatalysts supported on carbon paper.13 Specifically, the composition of AlNiP is consistent with Ni phosphides undergoing surface oxidation.13-14 The P 2p spectrum includes signatures of a well-defined Ni phosphide (resolved doublet with 2p3/2 component at 129.0 eV) and phosphates (a broad peak at 133.9 eV);13-14 the latter assignment supported the total O to phosphate-P ratio of ca. 4:1. The interpretation of the P data is complemented by the Ni 2p3/2 spectrum of AlNiP, which includes signatures of a NiO and Ni phosphide mixture (Ni 2p3/2 at 853.0 eV and satellites around 860–865 eV)13, 15 and of Ni phosphates (Ni 2p3/2 at 856.5 eV).13 The high P/Ni ratio of ca. 3:1 further identifies phosphates as the dominant surface functionality and indicates that the phosphorization step efficiently deposits a few-nm-thick layer of excess amorphous P on the surface. Aluminum is not detected by XPS, as expected from its low concentration in AlNiP. The detailed investigation of the AlNiP surface after short CV testing confirms that the anode undergoes a rapid oxidation under OER conditions. Specifically, in HAADF−STEM images, apart from the presence of pristine Ni5P4 on the anode surface (Figure 3a), we observed a large number of interlocked nanocrystals (1−3 nm) of cubic NiO (Figure 3d and 3f). This observation is well supported by the XRD analysis of the CV-tested AlNiP, which shows a decreased content of Ni5P4 phase due to its oxidation under OER conditions. Additionally, metallic Al nanocrystals (10−20 nm) were found in the sample (Figure 3e). This is somewhat surprising result, since one would expect a rapid oxidation of such small Al particles. We hypothesized that very thin oxidized surface layer of Al nanocrystals could be reduced during electron microscopy investigation by e-beam under high vacuum. Overall, the existence of Al nanocrystals in the CV-tested sample confirms the partial dissolution of the Al2O3 phase in alkaline electrolyte. This partial etching as well as rapid oxidation of the CVtested AlNiP is also supported by the ICP–OES analysis, which shows reduced quantities of Al and P in the sample, with a bulk molar ratio of Al/Ni/P ≈ 1:102:44. XPS data of CV-tested AlNiP confirm that a dramatic change takes place (Figure 4b), similarly as seen for other Ni phosphide electrocatalysts after OER,13 and the surface is transformed into Ni hydroxide/oxide.13-14, 15 This is evidenced by the main components of Ni 2p3/2 (shifted to 855.3 eV with a single satellite around 861 eV) and O 1s (shifted to 530.7 eV) combined with the disappearance of P and of Ni 2p3/2 at 853.0 eV. A small fraction of the O contribution is associated with the minor amount (ca. 3 at.%) of potassium carbonate (spectra not shown) deposited during CV testing. Based on the STEM and XPS data, the as-synthesized selfsupported AlNiP foam anode consists of a Ni5P4 (sur-

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face)/Ni2P (core) foam structure with Al2O3/Al inclusions and is covered with a several nm thick overlayer of an amorphous

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P-rich Ni

Figure 3. Low-magnification HAADF−STEM image of the CV-tested AlNiP (a), together with the simultaneously collected EDX maps of Al, Ni, O, P, and their superposition (b, c). Representative high-resolution HAADF−STEM images of highly packed ultrasmall NiO nanocrystals (d) and Al nanocrystal (e). High-resolution HAADF-STEM image showing the complex intergrowth of NiO nanocrystals (f). Insets are the corresponding selected area electron diffraction patterns confirming the respective Ni5P4 and NiO structures.

Figure 4. XPS data for as-synthesized AlNiP (a) and CV-tested AlNiP (b). Spectra in Ni 2p3/2, O 1s, and P 2p regions are shown as raw intensities. Symbols = raw data, dashed lines = background, colored lines = highlighted fit components, thick black lines = overall fit.

Figure 5. Comparison between the Tafel slopes as well as overpotentials required for driving current densities of 10 mA cm−2 for

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the synthesized AlNiP and state-of-the-art PGM-free electrocatalysts for alkaline water oxidation.

phosphate. Under water-oxidation reaction conditions, the AlNiP anode is rapidly converted into NiO nanoparticles with a predominantly Ni hydroxide surface having Al inclusions. These in situ generated stable ultrasmall nanocrystals of Ni(OH)2/NiO are responsible for the high OER performance of the resulting electrocatalyst. It is noteworthy that the morphological appearance of the produced particles differ from the other morphologies generated by in situ oxidation of the nickel phosphides under OER conditions. Unlike previously reported core-shell nanoparticles6, 10c or poorly crystalline aggregates,5, 10a we observed a well-defined deposit of interlocked NiO nanocrystals on the surface of Ni5P4–Ni2P foam anode. Together, these results support the emerging concept of the in situ formation of active and stable hydroxide/oxide water oxidation electrocatalysts from metal phosphide precursors or scaffolds.10b We deduce that the Al modification plays several positive roles in AlNiP: (1) alkaline leaching of Al-containing phases produces the high surface area and porosity of the final electrocatalyst, i.e., Al2O3/Al are sacrificed to generate pores, similar to Raney Ni,8 and (2) the resultant porosity facilitates the surface formation/deposition of ultrasmall Ni(OH)2/NiO nanocrystals exhibiting accelerated water-oxidation kinetics (Figure 1c). The expected increase in the electrocatalyst surface area under OER conditions was confirmed by N2 physisorption measurements, with a detected increase in the surface area from 56 m2 g–1 for as-synthesized AlNiP to 73 m2 g–1 for CVtested AlNiP. In summary, we have developed a novel Earth-abundant efficient and stable electrocatalyst for alkaline water oxidation. Modification of pristine Ni foam by Al followed by phosphorization yields a self-supported Al-containing Ni5P4–Ni2P foam anode. Characterization by electron microscopy and XPS indicates that under OER conditions the initial Ni phosphide material serves as a precursor for the efficient in situ formed nanocrystalline Ni hydroxide/oxide electrocatalyst anchored on the Ni5P4–Ni2P foam. The Ni(OH)2/NiO formation is assisted and directed by leaching of the Al phases in the highly alkaline 1 M KOH electrolyte. To the best of our knowledge, this new catalytic system is one of the best performing PGM-free anodes reported for alkaline OER, as benchmarked against the state-of-the-art metal phosphide,16 metal hydroxide,17 metal oxide,18 metal selenide,19 metal sulphide,20 and metal-organic framework21 materials (Figure 5). Overall, this study demonstrates the potential of using surface Al sacrificial templating agent in producing an inexpensive anode for water electrolysis in alkaline media. Our future work will be focused on exploiting the promising properties of these materials as high-performance electrocatalysts via achieving a more refined control of their catalytic activity and stability.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website. Materials and methods, additional XRD, CV, and Faradaic efficiency data (PDF).

ACKNOWLEDGMENT We thank all members of the Nanochemistry Group at the INL for their fruitful scientific and technical input. This work was supported by the European Union’s Horizon 2020 research and innovation program through the CritCat Project under Grant Agreement No. 686053, and by FROnTHERA (NORTE-01-0145FEDER-000023) project co-financed by European Union Funds, through Portuguese NORTE 2020 programme. J.D.C. is thankful for the support to FCT grant SFRH/BD/7939/2011. K.K. is thankful to Iowa State University for the support.

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