Developments of Metal Phosphides as Efficient OER Precatalysts

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Developments on Metal Phosphides as Efficient OER Pre-catalysts Anirban Dutta, and Narayan Pradhan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02249 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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The Journal of Physical Chemistry Letters

Developments of Metal Phosphides as Efficient OER Pre-catalysts Anirban Dutta and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032 India

Abstract: Over last two years, it has been observed that metal phosphides are emerged as efficient electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER). However, while HER has been immensely studied, OER is limited. The chemistry in OER is more complicated which involves irreversible surface oxidations of these materials and transforms to their corresponding oxide/oxy-hydroxide. Interestingly this insitu changes have been widely observed generating more active catalysts with superior performance. Phosphides of Fe, Co and Ni with different compositions have been proved as efficient catalysts for water oxidations. Considering their importance, structures, compositions, surface modifications and also insitu transformation during electrolysis, this perspective provides state-of-art views of their current developments and future prospects.

TOC:

Keywords: HER, OER, Metal Phosphides, Pre-catalysts, Electrocatalysts

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Development of electrocatalysts for efficient anodic oxygen evolution reactions (OER) in electrochemical water splitting is a major concern of current energy research. This involves 4e multiproton-coupled electron transfers (PCET) and causes significant efficiency loss of the electrolyzer.1-3 In addition, the core fundamental insights in the chemical processes involved in bond breaking and bond making on the surface of electrocatalysts during oxidation of H2O to O2 have versatile reported mechanisms.1, 4 Hence, even though OER for water oxidation is widely reported, but designing highly efficient and durable OER catalysts still remain challenging. Literature reports reveal that Iridium, Ruthenium and their corresponding oxides are well-established state-of-art OER catalysts.5-6 However, the scarcity and the consequent high cost limits their wide spread applications. Hence, intensive research is focused in finding non noble metal based earth abundant and efficient water oxidation catalysts. Recently, 3d metal based compounds have been found as promising alternatives and immense efforts have been put forwarded for developing these metal-based catalysts including their oxides, hydroxide, oxy-hydroxide, layer double hydroxide (LDH), chalcogenides, carbide and nitride.4, 7-21

Recently, metal phosphides are also emerged as a new class of active OER catalysts with

superior electrochemical activity. Though the activity is tested mostly for Iron triad based phosphide materials, still their promising results along with their established superior performance towards hydrogen evolution reaction (HER)22-23 provide possible signatures of active futuristic energy materials for a complete electrolyzer.24-30 Unfortunately, these catalysts show chemical instability during OER and this has been reported almost in all such cases. Under anodic oxidation reaction condition these undergo insitu transformation to their corresponding oxy-hydroxides. However, in spite of such


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chemical changes, the uniqueness here is the high activity which is comparable or even superior to several leading catalysts (Figure 1a).31-35 Further, inserting foreign impurities, increasing the metallic content and also both by cations as well as anion modulations, the catalytic activities were further tuned. Hence, even though these are not the true catalysts, these were observed as high performance anode materials for facilitating efficient water splitting. While in recent years these phosphide materials has proven their strong activity for hydrogen evolution reactions (HER),22-23 but extensive research on OER is limited. Also, several reviews and perspectives on HER of these materials were already documented;22-23, 36 but no such reports on OER is reported till date. Hence, keeping in mind the growing demand of these new classes of catalytic materials, in this perspective, the latest developments on different exciting OER results, the involved interface chemistry and the variation of activity parameters along with their insitu evolution of the true catalyst are summarized. In addition, possible future prospects of these materials are also discussed.

Figure 1. (a) Histogram showing activities of different metal phosphide OER catalysts.(b)

Histogram replicating timely evolution of metal phosphide OER catalysts.



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Phosphides: The emerging OER catalysts Survey on the history of metal phosphides as OER electrocatalyst indicates that long back in 1989 these materials were first reported by Kupka and Budniok.37 Electrocatalytically Ni-Co-P was generated and used for OER. Interestingly, it was found that the activities of both crystalline and amorphous phases were almost similar and during the course of reaction these were also oxidized to their corresponding oxy-hydroxides. Unfortunately, after this report, more than two decades passed, but no progress was made further (Figure 1b). Again in 2015, inspired by this pioneer work, Yoo and co-workers (2015) investigated deeply the catalytic process. Importantly, analysis of post catalysis sample of CoP nanoparticles38 revealed that these phosphides were indeed transformed into new porous material consisting of phosphate-enriched cobalt-oxo/hydroxo molecular units. Hence, the true catalyst observed here was actually not the metal phosphide rather than their oxidized products. Soon after, the research on these materials intensified and several phosphide systems including Ni2P,32,

39

CoP,3,

40

Ni5P4,41 FeP42 etc. were explored with similar

observation. At the end of the 2015 to till now, researchers are more interested to optimizing the activity using additives, surface modifications, shape and varying compositions, which are discussed in this perspective. The Phosphide Electrodes for OER. Conductivity is an important parameter for electrocatalytic measurements and hence, proper dispersion matrix is always selected for embedding the catalyst material during fabrication of electrodes. This protocol was also adapted to all reported phosphide materials even though these were widely known as good electrical conductors. Different strategies were adopted such as; (a) drop casting the material on highly conducting electrode like gold titanium and other highly conductive


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material, (b) drop casting material with conductive carbon support, and (c) growing the material on a highly conductive material. Xing and co-workers shown that by simple mechanical mixing of the material with carbon enhances conductivity of the material and consequently activity is also enhanced.3 However, Chen and co-workers have proven that significant catalytic activity enhancement is possible if the material is grown on highly conducting carbon nanotube (CNT). They synthesized CoP-CNT through phosphidation of the Co3O4-CNT and further compared activity of the Co3O4-CNT, CoP and state-of-art catalyst RuO2, the activity of the CoP-CNT remain superior.40 Very recently in a similar approach highly active OER catalyst is obtained by growing FeP on CNT.43 In a different approach by Zheng et. al. shown that high current density can be achieved by low temperature electro deposition of the NiPx on carbon fiber.44 Materials grown on Ni foam also drawn attention of the community in this regard, Ledendeckeret. al. reported superior activity as well as stability of Ni5P4 grown on Ni foam and further the materials showing superior overall water splitting behavior.41 Hence, the preparation of electrode remains a key for studying electrocatalysis. However, as these methods vary from one system to other, it is indeed difficult bringing different results together in a common platform for comparison. For avoiding this complexity, in this perspective, results are correlated which are performed only under identical conditions. Factors for enhancing the catalytic activities Morphology of the Catalyst. Electrocatalysts are involved with electrons transfer and the morphology has key role in the optimization of their performances45-47 and this has also been observed for metal phosphides.48-50 It has been observed that various 1D rods or wires like structure are more efficient than other shapes due to their superior charge transfer ability.51 A direct comparison of particles and 1d nanorods for cobalt phosphide revealed that rods 5 ACS Paragon Plus Environment


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have lower charge transfer resistance than dot shaped structure.3 Figure 2a and 2b present the TEM images of these two different shapes of nanostructure and Figure 2c depicts their OER activities. While compared with and without mixing carbon, in both cases rods were shown superior activity. The best value here also compared to the state-of-art catalyst IrO2. Similar observation was also reported for Ni2P nanostructure catalysts.39

Figure 2. Morphology dependent change in OER activities. TEM images of (a) CoP particles

and (b) rods. (c) J-V plots of both shapes of particles along with control measurements. SEM images of CoP (d) particles and (e) hollow polyhedral shapes. (f) J-V plots for both shapes of CoP along with control experiments. (Adopted from ref 3 and 58 respectively).

Not only is the shape, catalyst functionality also dependent on the microscopic local environments such as dispersity, porosity etc. Porous materials always remain more active due to their wide exposed area with improved mass/charge transport.52-56 Hence, designing strategies for porous phosphide materials drawn attention to researchers.57 In a recent development, Li and Liu derived porous cobalt phosphide polyhedrons by phosphorization calcinations of Co-centered Metal−organic frameworks and while compared with CoP 6 ACS Paragon Plus Environment

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nanoparticle, the porous polyhedrons showed superior activity.58 Figure 2d and 2f show the SEM images of CoP particles and hollow polyhedrals respectively and Figure 2f presents the voltamograms of both materials under identical conditions. In a similar approach Chen and co-workers derived highly active Porous cobalt phosphide/graphitic carbon polyhedral hybrid composites.57 Following different strategy Yuan and co-workers designed mesoporous CoP nanorod array on conductive Ni foam following electrodeposition technique.33 These were observed with high catalytic activities which were attributed to their high specific surface area and excellent electric interconnection with improved mass transport. Composition of the Catalyst and The activity trend in transition metal phosphides. According to Sabiter Principle the activity of a catalyst depends on the interaction between the catalyst and the substrate. This interaction should be "just perfect" i.e. neither too strong nor too weak. In case of OER, though various mechanisms were proposed on different catalysts; but the key issue remained with the Metal-Oxygen (M-O) bond strength. For different systems this binding strength is different and this observed controlling the activity of the catalyst material. This has been already proved with several metal based catalysts.1 In a very initial theoretical work reported by Rüetschi and Delahay on OER59 in correlation with the experimental results of Hickltng and Hill60 on the bond energy, stated that "Despite the uncertainty in the values of bond energies, the foregoing considerations show that differences in the energy of the bond M-OH essentially account for variations of oxygen overvoltage from one metal to another under given conditions of electrolysis." The activity trend of the metals for OER was shown as Co>Fe>Cu>Ni. Jasemet. al. were one of the early groups who proposed the criteria for oxide semiconductor OER anodes.61 Further, inspired by the reported M-O bond strength as an activity descriptor for OER, Trasatti empirically



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predicted the activity trend in oxide materials as RuO2> IrO2> MnO2>NiOx> Co3O4> Fe3O4.62-63 However, all these oxides were unstable under practical OER condition and during electrolysis these were transformed into their amorphous hydrous oxide or oxy-hydroxides.64 Next, Markovican co-workers determined the trend for the 3d metal oxy-hydroxide by growing them on Pt(111) and correlated their results with the computationally calculated M-OH interaction. The results showed their activities following the order Ni > Co > Fe > Mn.65 Unfortunately, no such theoretical study has yet been proposed on the activity of phosphide materials till date and this is also difficult as there are not the true catalysts. During electrolysis these materials are oxidized to oxide or oxy-hydroxide species. However, interestingly the activity showed quit similar to the trend with the oxide and oxy-hydroxide materials. A direct comparison of the metal phosphide by Schaak and co-workers further suggested that the activity of Ni > Co > Fe which also retained the same trend. 66 Hence, with the it could be concluded here that the activity trend in transition metal phosphides could be same as those of corresponding oxides/oxy-hydroxides. These studies were mostly with binary metal phosphides; but the activity and its trend were also further optimized through several compositional variations which were discussed in next sections in detail.


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Figure 3. Cation and anion modulation for enhancing OER activities. (a) Atomic model showing Co2P and CoMnP. (b) LSVs of Co2P, CoMnP and CoMnO2 modified electrodes (Adopted from reference 34). (c) LSVs of composition variations CoFeP in modified electrodes (Adopted from reference 35). (d) LSVs of NiCoP, Ni2P and Co2P grown on Ni foam. (Adopted from reference 67). (e) Schematic atomic models showing oxygen incorporation in Ni2P. (f) LSVs of Ni2P and oxygen incorporated Ni2P modified electrodes (Adopted from reference 72). Incorporation of Second Metal Ions. Incorporation of a suitable second element sometimes helps for enhancing the catalytic activity. In a recent study, Brock and co-workers showed that incorporation of Mn in Co2P lattice significantly decreased the overpotential.34 Figure 3a presents atomic models of Co2P and Mn incorporated Co2P nanocrystal and Figure 3b depicts their cyclic voltamograms. The activity was also further compared with their oxide nanostructures. The enhanced activity was related to the M-O bond strengths. For Co=O, while the bond formation was energetically demanding, but the product remained unstable. However, Mn=O species were relatively facile and facilitated the PCET step. Not only Mn, Fe was also found enhancing the activity of Co2P. This was carried out by Sun and co-workers and for Fe in Co2P ie Co(2-x)FexP was observed more efficient than Co2P, Fe2P and their corresponding ternary oxides (Figure 3c).35 The activity was also compared with Ir for comparison. In another case, the ternary NiCoP synthesized by Duan and co-workers on a 3D nickel foam also showed higher activity than their binary counter parts (Figure 3d).67 The effect was so prominent that any dopant amount of incorporation of a secondary metal was drastically changed the activity of the system. Further, Hu and co-workers had observed similar effects where even the trace amount of Fe incorporation on the oxidized Ni2P surface with using commercial KOH enhanced the OER activity.32 While tried to understand this impurity or dopant induced change in catalytic activity, it was observed that the concept was already established previously in oxide and oxy-hydroxide 9 ACS Paragon Plus Environment


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systems.68-69 This enhancement might be attributed due to following reasons. (a) It is believed that these foreign ions help in lowering the thermodynamic barrier of a PCET step and facilitates the O-O bond formation. (b) Incorporation of these secondary ions enhances the electroconductibility of the material which helps in decreasing the charge transfer resistance and facilitates electron transfer. However, certainly more experimental as well theoretical studies warranted for establishing more mechanistic concept for these newly emerged phosphide OER materials. Anion incorporation and surface modification. Being electrocatalysis is a surface phenomenon; surface states of electrocatalysts play a major role in controlling the activity. Not only with cationic but also anions played vital roles in altering the activity. For HER, this has been established by Jin and co-workers considering both experimental as well as theoretical supports.70-71 However, Wu and co-workers showed this for OER by adopting phosphidation of Ni(OH)2 which helped the oxygen incorporation in Ni2P surface.72 This modified Ni2P showed superior activity than pure Ni2P. Figure 3e shows the schematic atomic model of surface oxygen incorporation and Figure 3f depicts the voltammograms of Ni2P, Ni(OH)2 and the oxygenated Ni2P where the later one superseded all in the OER activity. Very recently Qiao and co-workers reported similar observation, where O doping enhances the activity for Co2P and even simultaneous doping of Fe and O led to further enhancement.73 This enhancement of efficiency in oxygen incorporation was attributed due to following reasons. (a) it is observed that incorporation of

oxygen reduces the charge transfer

resistance, hence facilitates the charge transfer. (b) Incorporation of oxygen enhances the electrochemically active surface area.


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Figure 4. TEM images of (a) Co2P and (b) CoP nanowires. (c) Histogram showing different

activities at overpotential 10 mAcm-2 (Adopted from reference 75). Metallic Density. Electroconductibility of materials is also an important parameter for any electrocatalytic activity. Blanchard through X-ray photoelectron and absorption spectroscopy (XPS) along with a charge potential model showed that metal rich phosphides were less ionic character and more metallic character.74 Due to this high metallic character, the metal rich phosphides became more catalytic active than corresponding monometalic phosphide. This was observed in case of cobalt phosphide, where the bimetalic phosphide reported showing superior activity compared to CoP with same morphology.75 Figure 4a and 4b present the TEM images of CoP and Co2P respectively and their OER activities are shown in Figure 4c. Results suggested Co2P is better OER catalyst that CoP. Hence, metallic character here played the significant role for influencing the OER activity. On the other hand similar reports for NiP and Ni2P, FeP and Fe2P were reported; but those could not be compared as reports were not followed identical condition.



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Figure 5. Evolution of true catalysts. (a) Irreversible oxidation in CoP modified electrode in

initial scan, (b) corresponding tafel slop. (Adopted from reference 38). Step wise evolution of catalyst from Ni2P. (c) nanowires and (d) particle. (Adopted from reference 39).

Insitu Evolution of Catalysts. The catalyst activation is the most important aspect for metal phosphides. Close look of the first cycle revels that in the anodic sweep of almost all such cases showed a broad irreversible per oxidation peak before the onset potential.76 This peak disappeared in consecutive scans along with decrease in the overpotential and tafel slope, indicating the oxidative transformation of the catalyst, frequently termed as catalyst activation (Figure 5a and 5b).38 In an exciting study, Du and co-workers showed the enhancement of the activity in Ni2P continued even upto 500 cycles irrespective of the catalyst morphologies (Figure 5c and 5d).39 This suggested that after more electroactive species were formed during successive scans. From the general electrochemical knowledge, it could be speculated that the oxidation led to higher valent metal species during the transformation which might be acted as true catalyst. This prompted researchers for extensive ex-situ investigation of the transformed catalyst. For 12 ACS Paragon Plus Environment

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Ni2P, Hu and co-workers through extensive HRTEM analysis along with elemental mapping showed the presence of secondary layer of materials on catalysts after electrolysis.32 Figure 6a presents post catalysis HRTEM analysis along with elemental mapping of Ni2P electrocatalyst. The results suggested the presence of a shell or over-layer composed of Nioxide/hydroxides. Figure 6b depicted corresponding XPS spectra and interestingly this shows successively lowering of full width at half maxima indicating presence of strong metallic nickel content. Correlating with elemental mapping data, it was confirmed that Ni2P stil retained in the core and only surface was oxidized. Similarly, for CoP, Xing and co-workers reported the surface oxidations after catalysisdation.3 Figure 6c presents high angle annular dark field-scanning tunneling microscope (HAADF-STM) image of a post catalysis nanorod and elemental mapping of Co, P and O. The correlation area of these elements suggested that oxygen were mostly on the surface indicating oxidative transformation of Co on the surface. Further, analysis of XPS spectra, shown in Figure 6d and Figure 6e for Co and P respectively, indicated that not only Co, but also P was oxidized. From all these results, it was concluded that on the surface, Co was was oxidized to Co-oxide/hydroxide and P mostly to phosphate.3 In addition, as stated earlier, Yoo and co-workers also had performed the surface analysis of post CoP catalyst by XPS and X-ray absorption near edge structure (XANES), and confirmed the surface contained porous, amorphous and nanoweb-like dispersed morphologies.38 This unique microscopic structure mainly contained phosphate-enriched Co-oxo/hydroxo molecular units. Hence, phosphide materials undergo oxidative transformation during electrolysis and these are only the pre-catalysts.



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Figure 6. (a) HRTEM image of post catalysis sample of Ni2P showing core/shell structure of

Ni2P/NiOx and elemental mapping for Ni, O and P. (b) XPS of Ni 2p. (c) HAADF-STEM image of post catalyzed sample of CoP nanorod and the elemental mapping of Co, P and O. XPS of (d) Co (2p) and (e) P (2p) of post catalyst CoP nanoparticles and nanorods respectively. (Figure 6a-6b were adopted from reference 32 and Figure 6c-6d from reference 3.)

Figure 7. Activity comparison between oxy-hydroxide and phosphide. (a) Digital image

showing FeOOH and FeP modified carbon paper electrodes, (b) corresponding LSVs. (Adopted from reference 77).


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Since oxide materials are not stable under practical alkaline OER condition, they typically transformed into hydrated oxides or oxy-hydroxide which are till date known as the best OER catalysts. Further, in a twisting experiment, Xiong et. al compared the activity of phosphides in a reverse process.77 They converted Iron oxy-hydroxides to Iron phosphides in hydrothermal method and compared their catalytic activity under identical condition. Figure 7a and 7b present the photograph of the carbon paper electrodes of FeOOH and FeP and their corresponding voltammograms. The post-catalysis study of the FeP catalyst reveals the surface getting oxidized to its corresponding oxide or oxy-hydroxide form but still the activity is superior than the FeOOH. All these results suggests phosphidation enhances the OER activity. This superior activity of the phosphide may be summarized as: (a) Efficient carrier transfer via phosphide-oxide/hydroxide interface: Phosphide materials are good electrical conductor which remains at the core and during catalytic transformation, an oxide/hydroxide over-layer formed on their surfaces. This MPx-MOx interface helps for better carrier transportation from the core MPx to the MOx. (b) The role of Phosphate: During surface oxidation reactions phosphide is mostly converted to phosphate which might have played a major role in the superior activity of the catalysts. Though the role of phosphate is still unclear, but from different literature reports we assume here these help in the PCET step. The phosphate residue on the surface possess the ability acting as a labile ligand which can vary its co-ordination or chelating modes during redox switching process of the metal ion and helps in facilitating OER.



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In a particular case, Brock and co-workers also further studied the decrease in activity over the time for CoMnP. They reported the fast leaching of the phosphate ion from the catalyst compared to the metallic leaching. Hence, this also suggests about the involvement of phosphate in these insitu oxidation process of phosphide materials. In summary, metal phosphides as efficient OER pre-electrocatalysts is discussed. These phosphides undergo insitu chemical transformation to oxides/oxy-hydroxides during anodic potential sweep which enhances the catalytic activities. All possible attempts made for optimizing the efficiency were also discussed. In addition, several further investigations are also required for understanding these superior activities. These can be as follows: (1) Current developments are mostly confined to Iron triad and hence this needs to be explored further to other group of phosphide materials. (2) The major obstacle remains here is the synthetic developments of these materials. Unlike oxides and chalcogenides, advances in the synthesis of phosphides are limited. Hence, understanding the underlying chemistry of activity enhancement in OER, designing synthesis protocols of these materials with morphology variation material is highly essential. (3) Further, theoretical study along with more advance techniques for justifying the intrinsic activities of these materials for the electron transfer process needs in-depth investigations. (4) For enhancing the conductivity of the material study of mixing different additives with the embedded substrate is also required. (5) Metal-semiconductor heterostructures with conductive metal might also enhance the activities by enhancing electrical connectivity between nanostructure and bare electrode.


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Hence, designing appropriate coupled system where the metal part is not electrochemically active but high conductivity might help in enhancing the electrical interconnection.

AUTHOR INFORMATION Corresponding Author Email: [email protected] Notes The authors declare no competing financial interest. Anirban Dutta received B. Sc. degree from Siuri Vidyasagar College, Birbhum, WB, India and M.Sc. from Indian Institute of Technology, Delhi; currently, he is a Ph.D. student in the Department of Materials Science, IACS, Kolkata and working on synthesis and photo/electrochemical properties of metal phosphides. Narayan Pradhan is Professor in the Department of Materials Science, IACS, Kolkata. He has obtained his PhD degree from IIT Kharagpur and carried out his post-doctoral research work in Israel and USA. He joined in IACS in 2007. His research area is investigating the chemistry and physics of nanomaterials from synthesis to applications. ACKNOWLEDGMENTS DST of India (SR/NM/NS-1383/2014(G)) is acknowledged for funding. AD acknowledges CSIR and NP to DST Swarnajayanti project for fellowships.

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Developments of Metal Phosphides as Efficient OER Precatalysts

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