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The Role of Aluminum in Promoting Ni-Fe-OOH Electrocatalysts for the Oxygen Evolution Reaction Jon G. Baker, Joel R. Schneider, Jose Antonio Garrido Torres, Joseph A. Singh, Adriaan J. M. Mackus, Michal Bajdich, and Stacey F. Bent ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00265 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019
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ACS Applied Energy Materials
The Role of Aluminum in Promoting Ni-Fe-OOH Electrocatalysts for the Oxygen Evolution Reaction Jon G. Baker, Joel R. Schneider, Jose A. Garrido Torres, Joseph A. Singh, Adriaan J.M. Mackus, Michal Bajdich, Stacey F. Bent Department of Chemical Engineering, Department of Chemistry, Stanford University, Stanford, California , United States
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA , USA
Email:
[email protected] ABSTRACT: Nickel-iron oxyhydroxide (Ni-Fe-OOH) catalysts have been widely studied for their high activity for the oxygen evolution reaction (OER). Here we demonstrate improved OER activity through incorporation of a third cation, aluminum. Atomic layer deposition (ALD) was used to deposit thin films of nickel oxide (Ni-O) and nickel-aluminum oxide (Ni-Al-O) to measure activity for the OER. Electrochemical preconditioning of the oxide films led to the formation of the OER-active oxyhydroxide catalysts. For Ni-Al-O films, electrochemical preconditioning resulted in aluminum dissolution until a stable composition at a Ni:Al ratio of : was reached. Compositional effects of iron were studied by controlling the iron impurity level in the electrolyte. Turnover frequencies (TOF) were determined for each catalyst, and it was found that the highest performing electrocatalysts were the films containing nickel, aluminum and iron, confirming that aluminum exerts a promotion effect on nickel oxyhydroxide catalysts. Studies showed that unlike the Ni-Fe-OOH films, for which the TOF had very little thickness dependence, the activity of Ni-Al-Fe-OOH catalysts was dependent on thickness. This effect may arise from morphological changes in the catalyst film that modulate the density of the active site with thickness. For the thinnest films, aluminum doping improved the TOF of Ni-Fe-OOH catalysts by over -fold.
Keywords: oxygen evolution, nickel-aluminum oxyhydroxide, catalysis, atomic layer deposition, electrochemistry, nickel-iron oxyhydroxide
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Introduction: As intermittent energy sources such as solar and wind power become more prevalent, efficient methods must be developed to store excess electrical energy for use during periods of insufficient production. One method to store electrical energy is to convert it into chemical energy via the formation of fuels. Fuels offer the advantages of facile storage and transportation, and can easily be converted back to electrical energy when intermittent power sources are insufficient. Electrochemical water splitting is a potential means to produce hydrogen gas for fuel by coupling power sources to electrolyzers. Currently, a major drawback of producing hydrogen gas in this way is the low cycle efficiency of producing and consuming hydrogen; one major cause of this low efficiency is the slow kinetics of the oxygen evolution reaction (OER) involved in water splitting.– The oxides of transition metals such as Mn, Co, Ni, and Fe have emerged as potential electrocatalysts for OER due to their stability under alkaline conditions, their earthabundance, and their relatively high activities.– However, these metal oxides do not remain in their oxide form under OER conditions; instead, they undergo structural and chemical changes.– Due to the changing nature of the films under OER conditions, identification and quantification of the active sites becomes challenging. Other confounding effects such as low conductivity and high porosity can further complicate the measurement of intrinsic activity., Nickel oxide-based catalysts, which can also exist as hydroxides (denoted as -OH) or oxyhydroxides (-OOH) under reaction conditions, have been identified as some of the most active catalysts in alkaline electrolytes. In addition, their activity is strongly dependent on dopant cations, and previous work has used other metal cations as dopants to increase activity even further.– Iron in particular increases OER activity dramatically, improving the onset potential by over mV compared to pure NiOOH catalysts., In recent combinatorial studies, it was found that binary metal oxyhydroxides such as Ni-Fe-OOH can be further improved through the incorporation of a third cation. Aluminum was identified as such a cation to increase OER activity., This effect was an interesting observation because aluminum oxide itself is OER-inert. In studying complex multi-component systems such as Ni-Al-Fe-OOH, it is important to understand the structural and compositional changes that occur under OER conditions. The amphoteric nature of aluminum oxide makes the films potentially unstable which can lead to changes in porosity. However, aluminum is known to be stabilized in the Ni(OH)/Ni(OOH) lattice as a substitutional dopant,– so this effect may be less prevalent for aluminum in the NiO-system. Aluminum doping may also influence the atomic structure of Ni(OH), as aluminum has been reported to stabilize the α-Ni(OH) over the -Ni(OH) phase.– A difference in hydroxide structure may impact observed OER activity, since the hydroxide phases are the precursors of the OER-
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active NiOOH catalysts, with previous studies showing that α-Ni(OH) forms the -Ni(OOH) phase and the Ni(OH) forms -Ni(OOH)23 under OER relevant potentials. Unlike Ni-Fe-OOH catalysts, which tend to have a similar performance independent of the synthesis technique,– there has been a wide range of reported activity for Ni-Al-OOH OER catalysts, which to date have been tested only in iron-containing electrolytes. To better understand the factors which contribute to changes in observed OER activity, precise catalysts are thus required for study. In this work, atomic layer deposition (ALD) was used to prepare well-defined thin films of Ni-O and Ni-Al-O. ALD allows for deposition of conformal, uniform films with precise control over composition and thickness. Thin films are ideal for quantifying the intrinsic activity because they minimize the effects of low conductivity and high porosity which can impact the observed activity., In addition, an electrochemical step was performed to convert the as-deposited oxide films to the active oxyhydroxide catalysts (e.g. Ni-Al-OOH and NiOOH). Iron was incorporated into these films from the electrolyte (to form Ni-Al-Fe-OOH and Ni-Fe-OOH catalysts), and compositional effects of iron were studied as a function of iron content. By carefully controlling the components of the film, changes in the activity and in the rate determining step were ascertained as a function of composition. The results confirm that both iron and aluminum have beneficial effects on the OER activity of NiOOH electrocatalysts. In addition, the results provide insight into the catalytic mechanism and role of aluminum in promoting OER activity for Ni-OOH and Ni-Fe-OOH catalysts.
Experimental Section: . Film Deposition Deposition of ALD thin films was performed in a custom-built ALD reactor. Precursors were introduced through a showerhead gasket above a ’’ inch heated substrate holder. The substrate temperature was controlled at oC for all depositions. Aluminum oxide was deposited using trimethylaluminum (TMA, SigmaAldrich, %) and water, and nickel oxide was deposited using nickelocene (NiCp, Strem Chemicals, %) and ozone. Nickelocene was heated to oC to generate sufficient vapor pressure. Ozone was generated with an IN USA OG Series ozone generator, and ozone concentration was maintained between - g Nm-. Nitrogen was used as the purge gas. Metal oxide films were deposited on fluorine-doped tin oxide (FTO) coated glass (Hartford Glass, TEC ) for electrochemical characterization and on single-sidepolished, n-doped Si() with a native oxide layer for monitoring the deposition. The FTO substrates were rinsed with acetone and sonicated in both ethanol and MilliQ water (. MΩ-cm) for twenty minutes. Prior to ALD thin film deposition, the FTO glass slides and Si wafers were UV-Ozone treated for minutes. Film thicknesses on Si wafers were measured with a variable
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ACS Applied Energy Materials film to the electrolyte (approximately hours). After the redox feature stabilized in size, the catalysts were held at V vs. Ag/AgCl for hours to equilibrate with the electrolyte. Bubbles formed on the surface of the catalyst were mechanically dislodged, and the activity of the film was then measured. To accurately determine OER performance, the Ag/AgCl reference electrode was calibrated to the RHE at the end of each experiment, immediately following the CV collected to determine OER performance.
angle spectroscopic ellipsometer (Woolam alpha-SE) at o and o angles. Mixed metal oxide films were deposited using the supercycle approach. An ALD supercycle was composed of separate subcycles of the individual ALD chemistries, i.e. n cycles of NiO followed by m cycles of AlO, in which the ratio of the subcycles was varied to obtain different compositions of the mixed metal oxide film. A number of supercycles of this recipe was then repeated to obtain the desired overall thickness. Nickel oxide was deposited using a sub-saturating pulse of ozone. To achieve full saturation of ozone, long pulses were required ( seconds, see Figure S in Supporting Information for nickelocene saturation behavior and ALD Ni-O growth curve). Thus, depositions used a second pulse time of ozone with a saturating pulse of nickelocene of seconds. Under these reaction conditions, a steadystate growth rate of . Å/cycle for NiO was observed. The growth rate for AlO was found to be . Å/cycle.
Conductivity measurements were carried out using a gold-printed, interdigitated array (IDA) electrode (ALS Co., pairs, mm length, μm interval and μm width). ALD films were deposited directly to the IDA electrode. The two working electrodes were then connected, and the films were preconditioned following the standard procedure. In situ conductivity was measured using a bipotentiostat (Biologic VMP-). A potential bias of ∆V= mV and mV was applied at each potential and the resulting current was measured through the two working electrodes. The method to calculate conductivity from this measurement was reported by Nishizawa et al. The conductivity was calculated independently for the two biases of ∆V= mV and mV to confirm that the OER current was properly accounted for.
Ni-Al-O films deposited for electrochemical analysis were prepared with a supercycle recipe that deposited no more than a full monolayer of NiO or AlO per supercycle to promote intermixing and avoid nanolaminate structures.
. Electrochemical Measurements
Turnover frequencies (TOF) were calculated by approximating the rate of oxygen evolved per electrochemically active nickel atom. The amount of evolved oxygen was estimated from the measured current density (assuming a e- process and % faradaic efficiency). To determine the number of electrochemically active nickel atoms, the reduction feature of Ni/Ni was numerically integrated assuming that each electricallyaccessible nickel atom contributed e- to this reduction feature. Previous work by others has shown that this is likely an underestimation and that the redox feature can contain up to . e- per active nickel atom; thus, the TOF calculated here can be considered to be a lower bound. The TOF was then calculated by dividing the rate of oxygen evolved per second by the number of electrochemically active nickel atoms.
All electrochemical measurements were made in a custom-made, polytetrafluoroethylene (PTFE) compression cell that exposed . cm of the catalyst samples. The catalytic activity of the films for the OER was measured using cyclic voltammetry (CV). A Bio-Logic SP potentiostat was used in three-electrode mode for the CV measurements. A coiled Pt wire was used as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. The Ag/AgCl reference electrode was measured vs. the reversible hydrogen electrode (RHE) by bubbling ultra high purity hydrogen (Praxair, .%) over a Pt wire freshly cleaned with nitric acid. The reference electrode was measured to be . V vs. RHE in . M KOH. Electrochemical data was corrected for uncompensated series resistance, RU. RU was measured experimentally with electrochemical impedance spectroscopy (EIS) in the frequency range of kHz to kHz. Nitrogen (Praxair, .%) was continuously bubbled in the electrolyte solutions prior to and throughout electrochemical testing. Films were cycled from -. V to . V vs. Ag/AgCl at a scan rate of mV/s. Prior to measuring catalytic activity, films were preconditioned to allow reconstruction of the asdeposited oxide films to the hydroxide species, which under OER conditions exist as oxyhydroxides. This preconditioning consisted of cycling the catalyst until the Ni/Ni reduction wave was stable in size. Due to the compact nature of the ALD-grown nickel oxide films, this step included as many as cycles to expose the entire
EIS was used to measure the double layer capacitance as a function of applied potential in the frequency range of kHz to Hz. The resulting data was modeled using an equivalent circuit shown in Figure Sa. The double layer capacitance measured by EIS was used to compare the electrochemically active surface area (ECSA) with that determined by the nickel redox feature (See Supporting Information for discussion and comparison of the two methods). The number of monolayers was calculated using the nickel redox feature and the measured surface area of the FTO substrate (determined by atomic force microscopy, Park XE-) to correct for the roughness of the FTO substrate. Assuming each nickel atom contributed e- and a lattice parameter of .Å, for α-Ni(OH), a
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plasma cleaner. Each sample was imaged in the region exposed to electrolyte (. M Reagent Grade KOH, i.e. NiFe-OOH and Ni-Al-Fe-OOH catalyst) during electrochemical testing and the region not exposed. The unexposed region is considered to have the same morphology as the initial film.
monolayer was estimated to consist of Ni atoms/cm.
. KOH Electrolyte Choice Compositional effects of iron on electrochemical activity were studied using four different electrolytes with varying iron concentrations: (i) . M TraceSelect KOH (Fluka Analytical TraceSelect, ≥%, diluted with . MΩ-cm water); (ii) . M Reagent Grade KOH (Macron Fine Chemicals, Pellets ACS); (iii) . M iron-saturated KOH; and (iv) iron-free . M KOH prepared from . M TraceSelect KOH. According to manufacturer lot analysis and the solubility of iron at pH=., the iron concentrations in these electrolytes are
