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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Carbon-Supported Iron Phosphides: Highest Intrinsic Oxygen Evolution Activity of the Iron Triad Daniel P. Leonard,† William F. Stickle,‡ and Xiulei Ji*,† †
Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States Hewlett-Packard, Inc., Corvallis, Oregon 97330, United States
‡
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S Supporting Information *
ABSTRACT: We compared the oxygen evolution reaction (OER) activity of carbon-supported iron triad metal phosphide nanoparticles using Fe-purified KOH electrolyte. We prepared these catalysts with nearly identical structural properties using an ammonia-assisted ambient hydrolysis deposition (AHD) method allowing us to explore how their compositions affect their OER activities. We discovered that iron phosphide is far superior to its counterparts with an onset potential lower by 150 mV. Furthermore, we found that the anodic voltammetric scan with associated oxidation of Fe(II) in the phosphide generates OER active sites; however, ambient oxidation of iron phosphide prior to that anodic scan deactivates the catalyst.
KEYWORDS: water splitting, ambient hydrolysis deposition, oxygen evolution reaction, transition metal phosphide
T
To date, there are two primary unsolved questions associated with the catalytic behavior of TMP: (1) Which iron triad metal is the most active? (2) What is the impact of iron impurities from the KOH electrolyte on the activity of non-iron catalysts? In 2014, Boettcher et al. reported that iron impurities in KOH electrolytes significantly enhance the activity of nickel oxides. 18 Nevertheless, to date this phenomenon has yet to be investigated for TMP OER catalysts. Herein we provide the answers to these imperative questions by investigating TMP nanoparticles supported on a nanoporous carbon. We compare Fe, Co, and Ni TMP composite constructs under nearly equivalent conditions and reveal that the iron TMP exhibits the highest intrinsic OER activity without the influence of the electrolyte-born iron impurity. The supported TMP catalysts were fabricated by a unique ammonia-assisted AHD method (Figure S1).19,20 In this new method nanoparticles of transition metal oxides are deposited within a nanoporous carbon before phosphidization. In the conventional AHD method water vapor is infiltrated into the nanopores of a porous carbon substrate. That water-loaded carbon is then soaked in a nonaqueous solution containing solvated metal alkoxide precursors. Hydrolysis reactions occur between the adsorbed water and the metal alkoxides resulting in the formation of metal oxides entirely within the nanoporous carbon supports. In this study we saturated the water trapped inside nanoporous carbon with NH3 gas, to
he nature of the current electrical grid requires that nearly all electricity generated must be simultaneously consumed. Such a system is incongruous with intermittent renewable energy where times of peak generation do not necessarily coincide with times of peak usage;1 therefore, energy storage technologies are required to bridge the gap between generation and consumption.2−4 Electrical energy can be converted to and stored as chemical energy in electrochemical devices (e.g., batteries) or in chemical bonds of molecules (e.g., H2). With respect to the latter strategy, electrolytic water splitting as a means of hydrogen production could potentially fulfill the storage role.5−7 Water splitting comprises two half-reactions: the cathodic hydrogen evolution reaction (HER) and the anodic OER. To date, most efficient water-splitting systems require the use of acidic electrolytes and precious metal catalysts, such as Pt for the HER and IrO2 or RuO2 for the OER. Alternatively, alkaline conditions allow for the use of non-noble metal catalysts, usually Ni for HER and Ni alloys for OER; the active species of Ni alloy OER catalysts are, de facto, oxides of these alloys gradually formed during anodic oxidation. OER is a major contributor to the overpotential of water splitting and the associated low energy efficiency.6,7 Recently transition metal phosphides (TMP) have emerged as promising OER catalysts in alkaline conditions.8−13 Progress has been made in preparing unsupported TMP nanoparticles by reacting metal oxides with the P-containing species released from hypophosphite salt decomposition.14−17 However, to be more practically relevant TMP nanoparticles should be supported on durable, cost-effective substrates. © XXXX American Chemical Society
Received: May 30, 2018 Accepted: July 16, 2018 Published: July 16, 2018 A
DOI: 10.1021/acsaem.8b00861 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
Figure 1. HAADF-STEM images and EDX elemental maps of CoOx (column A), NiOx (column B), and FeOx (column C) materials before phosphidization. HAADF-STEM images and EDX elemental maps of CoPx (column D), NiPx (column E), and FePx (column F) after phosphidization.
sizes of NiPx and CoPx composites increase slightly to 40 and 38 nm from 38 and 36 nm, respectively, while it remains unchanged at 40 nm for the FePx composite (Figure S3B). The NiPx sample shows broad XRD peaks that index to Ni2P (ICSD 601077); the XRD results suggest that the FePx remains completely amorphous. The XRD pattern of CoPx appears to reveal a mixture of highly amorphous phases of CoP and Co2P (Figure S4A−F). During phosphidization, Fe(III) in the FeOx precursor is partially reduced to Fe(II) with a percentage of ∼23%, as shown by the presence of the XPS Fe 3p peak at a binding energy of 54.8 eV consistent with Fe(II) (Figure S5). HAADF-STEM and corresponding elemental mappings corroborate the amorphous phases of FePx and CoPx that are highly dispersed on the carbon substrate (Figure 1D,F), while the NiPx sample comprises spheroidal crystallites sized from 10 to 20 nm. We tested the electrochemical performance of the carbonsupported transition metal oxides (TMOs) and the TMP materials by cyclic voltammetry (CV), steady-state Tafel analysis, and chronoamperometric experiments in both standard 1 M KOH solution and Fe-purified 1 M KOH solution using the method described by Boettcher et al.22 The experimental details are in the Supporting Information, and all results reported are iRu-corrected. Electrochemical performance of the TMOs in 1 M KOH shows much activity consistent with previous studies (Figure 2A). However, in Fepurified 1 M KOH, OER activity drops substantially when compared to the standard 1 M KOH electrolyte, as expected (Figure 2B). The incorporation of phosphorus into the active material has a significant, positive impact on the overall capability of the catalyst. Additional comparisons between the CV polarization curves of TMPs and TMOs are shown in Figure S6A−F. The clearest example of the positive impact of the incorporation of
further expand the applicability of the AHD method to most metals. This creates an alkaline condition within the adsorbed water, thus precipitating any low-solubility metal hydroxides. This method negates the need for inert atmospheres required by the previous method’s reliance on unstable metal isopropoxides. This leads to a safer and potentially more scalable deposition method. With this highly generalized method our group has been able to deposit all first-row transition metals, and platinum, both singly and as binary oxide mixtures. Here, we infiltrated a nanoporous carbon with three metal oxides of iron triad. The nanoporous carbon support chosen for this study was resorcinol-formaldehyde-derived hard carbon prepared using previously described methods.21 Thermogravimetric analysis (TGA) in air reveals AHD deposition of CoOx, NiOx, and FeOx constituting 12.4, 12.4, and 9.4 wt % of the composites, respectively (Figure S2A−C), and high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM) and the corresponding energy dispersive X-ray spectroscopy (EDX) elemental mappings reveal their uniform distribution in the nanoporous carbon (Figure 1A−C). The deposition of metal oxides reduces the average pore size of the nanoporous carbon from 50 nm to 38, 36, and 40 nm for the NiOx, CoOx, and FeOx samples, respectively (Figure S3A). The X-ray diffraction (XRD) patterns in Figure S4 reveal the amorphous nature of the infiltrated metal oxides, where the peaks at 23° and 43° are attributed to the (002) and (101)/(010) planes of the graphitic nanodomains of nanoporous carbon. After phosphidization of the metal oxides, TGA in air shows the mass percentage of the supported phase increased to 23.6, 17.5, and 12.2 wt % for CoPx, NiPx, and FePx samples, respectively (Figure S2A−C). The increased mass loading after phosphidization is expected as heavier phosphorus replaces oxygen in the deposited material and is subsequently converted to phosphorus oxides upon calcination in air. The average pore B
DOI: 10.1021/acsaem.8b00861 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials
The influence of Fe impurities present in KOH electrolytes on the enhancement of metal oxide catalysts was reported by Boettcher et al.23−25 Therein, nickel oxyhydroxides, long thought to gain activity over time,26 instead achieve higher activity due to the incorporation of Fe impurities into the NiOOH lattice.18 Here, we examine TMPs in the absence of iron impurities to identify if the observed activity in the standard 1 M KOH electrolyte is inherent to the deposited TMPs or due to the incidental incorporation of iron. It turns out that NiPx is the most sensitive to the influence of iron from the electrolyte. NiPx’s required overpotential to reach 10 mA/ cm2 dramatically increases from 377 to 460 mV without electrolyte iron impurity, and its Tafel slope nearly doubles to 79 mV/dec (Figure 3B). This phenomenon is similar to what has been observed in nickel oxides.16 Analogous to the metal oxides, the first step prior to the onset of oxygen evolution is the adsorption of OH− ions and structural reorganization as the ions are incorporated into the lattice of the active material.27−29 This occurs simultaneously with the electrochemical oxidation of the active material. In the case of the TMPs the metal phosphides are oxidized to create an as yet unidentified metal oxo-phosphide species that is responsible for the OER activity of the TMP catalyst. Future studies of in situ monitoring of the catalyst structure and elemental oxidation states are necessary in order to identify the active sites. The evolution of the TMPs upon oxidation is observed in the elemental mapping and quantitative EDX images (Supporting Information Figures S8−S10). These maps show an increase in oxygen-to-metal ratio from NiPx and CoPx and a loss of phosphorus by FePx. This is best explained by the oxidation of the catalyst during operation. The impact of Fe-impurity on the CoPx catalyst is less significant; however, the overpotential still increases from 427 to 460 mV while the Tafel slope remains unchanged. Perhaps unsurprisingly, the FePx catalyst is the most insensitive to the presence of iron impurities with an overpotential of 310 mV and a Tafel slope of 42 mV/dec, nearly identical to the values with the standard KOH electrolyte. The iron TMP in the Fepurified electrolyte exhibits more resolved redox activity in Figure 3D. The FePx polarization curve displays a more pronounced oxidation peak as a shoulder immediately prior to the onset of OER at 0.58 V vs RHE, and the corresponding reduction peak is still present at 0.50 V vs RHE. To our knowledge, this is the first report of the impact of iron impurities on the performance of TMPs. Interestingly, the FePx catalyst loses its activity within 24 h when left at ambient conditions (Figure 4A). The zoomed-in view of the polarization curve shows the depression of the redox behavior after the ambient storage (Figure 4B). This is
Figure 2. CV polarization curves of TMOs in (A) standard 1 M KOH and (B) Fe-purified 1 M KOH.
phosphorus can be seen in the decrease in the charge transfer resistance from the electrochemical impedance spectroscopy experiments conducted at 300 mV overpotential (see Figure S7). This decrease suggests that the phosphorus present could be serving two possible roles. First, the phosphorus could be increasing the conductivity of the catalyst material itself, or second, it could be enhancing the interaction between the catalyst and the conductive support. Both of these roles would lead to a decrease in charge transfer resistance. In standard KOH electrolyte with iron impurities, FePx presents the best performance, where the overpotentials necessary to achieve a current density of 10 mA/cm2 were 320, 377, and 427 mV, and the Tafel slopes are 42, 47, and 63 mV/dec for FePx, NiPx, and CoPx materials, respectively (Figure 3A). Figure 3C shows the enlarged views of the CV
Figure 3. (A) CV polarization curves of the third of three consecutive polarization sweeps for each TMP after the Tafel analysis in the standard 1 M KOH electrolyte. Inset: Tafel plot. (B) CV polarization curves from the third of three consecutive polarization sweeps for each TMP after the Tafel analysis in the Fe-purified 1 M KOH electrolyte (inset Tafel plot). (C) A zoomed-in view of the reversible redox region of polarization curves in the standard KOH electrolyte. (D) A zoomed-in view of the reversible redox region of polarization curves in the Fe-purified electrolyte.
polarization curves around the redox potentials of the deposited TMPs in the standard 1 M KOH electrolyte. The NiPx sample shows a set of peaks of the Ni3+/Ni2+ redox couple prior to the onset potential of the OER, which is commonly observed on nickel oxide catalysts.23 The CoPx catalyst does not exhibit any redox behavior prior to the onset of OER in the standard KOH electrolyte.
Figure 4. (A) CV polarization curve of the pristine FePx and ambient oxidized FePx (FePx-ox). (B) A zoomed-in view of the reversible redox region of the CV polarization curves. C
DOI: 10.1021/acsaem.8b00861 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
ACS Applied Energy Materials
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ACKNOWLEDGMENTS X. Ji is thankful to Oregon State University for the support of this work. We are also grateful to Ms. Teresa Sawyer and Dr. Peter Eschbach for their help in STEM and EDX measurements at the OSU EM Facility, funded by the National Science Foundation, Murdock Charitable Trust, and Oregon Nanoscience and Microtechnologies Institute.
likely due to the fact that the oxidative environment of the catalyst in the electrochemical cell is markedly different than the lab bench. The electrochemical cell environment is rich in reduced oxygen species, such as OH− and H2O, which may not oxidize the catalyst. The OH− is incorporated into the active material simultaneously with the electrochemical oxidation of that material leading to significant structural rearrangement that favors the catalysis.27−29 While under ambient conditions on the lab bench, the atmosphere is rich with O2 and humidity that can oxidize Fe(II) to Fe(III) leaving behind an inactive oxide. As a result the active structure formed during the Fe2+/ Fe3+ transition can no longer be obtained when beginning from the higher oxidation state present in the ambient oxidized material. Nevertheless, if the FeP x catalyst is tested immediately following the phoshidization reaction, FePx exhibits high activity, although, prior to OER, Fe(II) is certainly oxidized to Fe(III) during the anodic CV scan, and the oxidized FePx is able to continuously catalyze OER reactions with progressively enhanced activity over a period of 1 h (Figure S11). From the above observations we hope to propose three insights: (1) The oxidized species of FePx after the ambient storage and after the OER CV scan differ in their associated activity which possibly originated from disparate active sites. (2) The storage-generated oxidized species of FePx cannot be converted to catalytically active species by OER operation. (3) The electrochemical oxidation of FePx in CV is a uniquely favorable process to generate active sites for OER catalysis. In summary, we have demonstrated a versatile new method for preparing highly dispersed phosphide nanoparticles of iron triad elements supported in nanoporous carbon substrates. We discover that the electrolyte-born iron impurities have a large impact on the catalytic performance of the NiPx catalyst while CoPx remains a poor catalyst in either electrolyte. The FePx catalyst exhibits the lowest overpotential of 320 mV for 10 mA/cm2 and a Tafel slope of 42 mV/dec with or without iron impurities. However, that activity is tempered by its instability under ambient storage conditions. The activity of these TMP OER catalysts is dependent on two factors: the first is the presence of phosphorus in the deposited material, and the second is the presence of Fe(II) in the catalyst material prior to OER oxidation. Since iron in some form is a requirement for significant OER activity in all of our catalyst materials, we conclude that iron itself possesses the highest intrinsic activity among iron triad for alkaline OER. These promising results show a new avenue of investigation for alkaline OER catalysts.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00861.
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Letter
Synthetic details, electrochemical analysis details, and supplementary figures (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiulei Ji: 0000-0002-4649-9594 Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acsaem.8b00861 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acsaem.8b00861 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
