Developments of Metal Phosphides as Efficient OER Precatalysts

Dec 8, 2016 - Anirban Dutta received his B.Sc. degree from Siuri Vidyasagar College, Birbhum, WB, India and his M.Sc. from the Indian Institute of Tec...
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Developments of Metal Phosphides as Efficient OER Precatalysts Anirban Dutta and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India ABSTRACT: Over the last 2 years, it has been observed that metal phosphides have emerged as efficient electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER). However, while the HER has been immensely studied, the OER is limited. The chemistry in the OER is more complicated and involves irreversible surface oxidations of these materials and transforms them to their corresponding oxide/oxyhydroxide. Interestingly, these in situ 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 oxidation. Considering their importance, structures, compositions, surface modifications, and also in situ transformation during electrolysis, this Perspective provides state-of-the-art views of their current developments and future prospects.

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(HER)22,23 provide possible signatures of active futuristic energy materials for a complete electrolyzer.24−30 Unfortunately, these catalysts show chemical instability during OERs, and this has been reported almost in all such cases. Under anodic oxidation reaction condition, these undergo in situ transformation to their corresponding oxyhydroxides. However, despite such chemical changes, the uniqueness here is the high activity, which is comparable or even superior to that of several leading catalysts (Figure 1a).31−35 Further, by inserting foreign impurities, increasing the metallic content, and both cation and anion modulations, the catalytic activities were further tuned. Hence, even though these are not the true catalysts, these were observed as highperformance anode materials for facilitating efficient water splitting. While in recent years these phosphide materials has proven their strong activity for HERs,22,23 extensive research on OER is limited. Also, several reviews and perspectives on HERs of these materials were already documented,22,23,36 but no such reports on OERs is reported to 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 in situ evolution of the true catalyst are summarized. In addition, possible future prospects of these materials are also discussed. Phosphides: The Emerging OER Catalysts. A survey of the history of metal phosphides as OER electrocatalysts indicates that in 1989 these materials were first reported by Kupka and Budniok.37 Electrocatalytically, Ni−Co−P was generated and used for OERs. 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

evelopment of electrocatalysts for efficient anodic oxygen evolution reactions (OERs) in electrochemical water splitting is a major concern of current energy research. This involves 4e multiproton-coupled electron transfers (PCETs) and causes significant efficiency loss of the electrolyzer.1−3 In addition, 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 the OER for water oxidation is widely reported, designing highly efficient and durable OER catalysts still remains challenging.

Even though the OER for water oxidation is widely reported, designing highly efficient and durable OER catalysts still remains challenging. Literature reports reveal that iridium, ruthenium, and their corresponding oxides are well-established state-of-the-art OER catalysts.5,6 However, the scarcity and the consequential high cost limits their widespread applications. Hence, intensive research is focused on finding non-noble metal-based earthabundant 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, oxyhydroxide, layer double hydroxide (LDH), chalcogenides, carbide, and nitride.4,7−21 Recently, metal phosphides have 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, their promising results along with their established superior performance toward the hydrogen evolution reaction © 2016 American Chemical Society

Received: September 30, 2016 Accepted: December 8, 2016 Published: December 8, 2016 144

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

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 refs 3 and 58, respectively.)

corresponding oxyhydroxides. Unfortunately, after this report, more than 2 decades passed, but no progress was made further (Figure 1b). Again in 2015, inspired by this pioneering work, Yoo and co-workers (2015) investigated deeply the catalytic process. Importantly, analysis of a post-catalysis sample of CoP nanoparticles38 revealed that these phosphides were indeed transformed into new porous material consisting of phosphateenriched cobalt-oxo/hydroxo molecular units. Hence, the true catalyst observed here was actually not the metal phosphide but rather its oxidized products. Soon after, research on these materials intensified, and several phosphide systems including Ni2P,32,39 CoP,3,40 Ni5P4,41 FeP,42 and so forth were explored with similar observation. From the end of 2015 until now, researchers have been more interested in 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, a 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 a highly conducting electrode like gold titanium and other highly conductive materials, (b) drop-casting material with a conductive carbon support, and (c) growing the material on a highly conductive material. Xing and co-workers show that simple mechanical mixing of the material with carbon enhances the conductivity of the material, and consequently, the 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 a highly conducting carbon nanotube (CNT). They synthesized CoPCNT through phosphidation of the Co3O4-CNT and further compared the activity of the Co3O4-CNT, CoP, and state-ofthe-art catalyst RuO2; the activity of the CoP-CNT remained superior.40 Very recently, in a similar approach, a highly active OER catalyst was obtained by growing FeP on CNT.43 A different approach by Zheng et al. showed that high current density can be achieved by low-temperature electrodeposition of the NiPx on carbon fiber.44 Materials grown on Ni foam also drew attention of the community. In this regard, Ledendecker 145

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voltammograms 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 a different strategy, Yuan and coworkers designed a mesoporous CoP nanorod array on conductive Ni foam following an 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 the Sabiter Principle, the activity of a catalyst depends on the interaction between the catalyst and the substrate. This interaction should be “just perfect”, that is, neither too strong nor too weak. In the case of the OER, though various mechanisms were proposed on different catalysts, the key issue remained with the metal−oxygen (M− O) bond strength. For different systems, this binding strength is different, and this observation controls the activity of the catalyst material. This has been already proved with several metal-based catalysts.1 A very initial theoretical work reported by Rüetschi and Delahay on the OER59 in correlation with the experimental results of Hickling 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 the OER was shown as Co > Fe > Cu > Ni. Jasem et al. were one of the early groups who proposed the criteria for oxide semiconductor

et al. reported superior activity as well as stability of Ni5P4 grown on Ni foam, and further, the materials showed superior overall water splitting behavior.41 Hence, preparation of the electrode remains a key for studying electrocatalysis. However, as these methods vary from one system to another, it is indeed difficult to bring different results together in a common platform for comparison. To avoid this complexity, in this Perspective, results are correlated that are performed only under identical conditions. Factors for Enhancing the Catalytic Activities. Morphology of the Catalyst. Electrocatalysts are involved with electron transfer, and the morphology has a key role in the optimization of their performance.45−47 This has also been observed for metal phosphides.48−50 It has been observed that various 1D rod- or wire- like structures 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 have lower charge transfer resistance than dot-shaped structures.3 Figure 2a,b presents TEM images of these two different shapes of nanostructures, and Figure 2c depicts their OER activities. While compared with and without mixing carbon, in both cases, rods were shown to have superior activity. The best value here also compared to that of the stateof-the-art catalyst IrO2. Similar observation was also reported for Ni2P nanostructure catalysts.39 Besides the shape, catalyst functionality is also dependent on the microscopic local environments, such as the dispersity, porosity, and so forth. 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 has drawn the attention of researchers.57 In a recent development, Li and Liu derived porous cobalt phosphide polyhedrons by phosphorization calcination of Cocentered metal−organic frameworks, and compared with the CoP nanoparticle, the porous polyhedrons showed superior activity.58 Figure 2d,f shows SEM images of CoP particles and hollow polyhedrals, respectively, and Figure 2f presents the

The activity trend in transition metal phosphides could be same as those of corresponding oxides/ oxyhydroxides.

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 ref 34). (c) LSVs of composition variations of CoFeP in modified electrodes. (Reprinted with permission from ref 35. Copyright 2015, John Wiley and Sons.) (d) LSVs of NiCoP, Ni2P, and Co2P grown on Ni foam. (Reprinted with permission from ref 67. Copyright 2016, Tsinghua University Press.) (e) Schematic atomic models showing oxygen incorporation in Ni2P. (f) LSVs of Ni2P and oxygen-incorporated Ni2P-modified electrodes. (Reprinted with permission from ref 72, Copyright 2016, Royal Society of Chemistry.) 146

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Figure 4. TEM images of (a) Co2P and (b) CoP nanowires. (c) Histogram showing different activities at an overpotential of 10 mA cm−2. (Reprinted with permission from ref 75, Copyright 2016, Royal Society of Chemistry.)

OER anodes.61 Further, inspired by the reported M−O bond strength as an activity descriptor for OER, Trasatti empirically predicted the activity trend in oxide materials as RuO2 > IrO2 > MnO2 > NiOx > Co3O4 > Fe3O4.62,63 However, all of these oxides were unstable under practical OER conditions, and during electrolysis, these were transformed into their amorphous hydrous oxide or oxyhydroxides.64 Next, Markovican and co-workers determined the trend for 3d metal oxyhydroxides 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 to date, and this is also difficult as they are not the true catalysts. During electrolysis, these materials are oxidized to oxide or oxyhydroxide species. However, interestingly, the activity was quit similar to the trend with the oxide and oxyhydroxide materials. A direct comparison of the metal phosphide by Schaak and co-workers further suggested that the activity of Ni > Co > Fe also retained the same trend.66 Hence, with this, it could be concluded here that the activity trend in transition metal phosphides could be same as those of corresponding oxides/oxyhydroxides. These studies were mostly with binary metal phosphides, but the activity and its trend were also further optimized through several compositional variations, which are discussed in next sections in detail. Incorporation of Second Metal Ions. Incorporation of a suitable second element sometimes helps enhance 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 Co2Pand Mn-incorporated Co2P nanocrystal, and Figure 3b depicts their cyclic voltammograms. 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, the product remained unstable. However, MnO species were relatively facile and facilitated the PCET step. Not only Mn but also Fe was also found to enhance the activity of Co2P. This was carried out by Sun and co-workers for Fe in Co2P, and Co(2−x)FexP was observed to be 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 its binary counterparts (Figure 3d).67 The effect was so prominent that any dopant amount of incorporation of a secondary metal drastically changed the activity of the system. Further, Hu and co-workers observed similar effects where even a trace amount

of Fe incorporation on the oxidized Ni2P surface using commercial KOH enhanced the OER activity.32 While we tried to understand this impurity- or dopantinduced change in catalytic activity, it was observed that the concept was already established previously in oxide and oxyhydroxide 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 facilitate O−O bond formation. (b) Incorporation of these secondary ions enhances the electroconductibility of the material, which helps decrease the charge transfer resistance and facilitates electron transfer. However, certainly more experimental as well theoretical studies warranted establishing a more mechanistic concept for these newly emerging phosphide OER materials. Anion Incorporation and Surface Modif ication. Being that electrocatalysis is a surface phenomenon, surface states of electrocatalysts play a major role in controlling the activity. Not only cations but also anions played a vital role in altering the activity. For HERs, this has been established by Jin and coworkers considering both experimental as well as theoretical supports.70,71 However, Wu and co-workers showed this for the OER by adopting phosphidation of Ni(OH)2, which helped oxygen incorporation in the Ni2P surface.72 This modified Ni2P showed superior activity over 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 latter one superseded all in the OER activity. Very recently Qiao and co-workers reported a 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 to following reasons. (a) It is observed that incorporation of oxygen reduces the charge transfer resistance, hence facilitating charge transfer. (b) Incorporation of oxygen enhances the electrochemically active surface area. 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 metalrich phosphides had less ionic character and more metallic character.74 Due to this high metallic character, the metal-rich phosphides became more catalytic active than corresponding monometallic phosphides. This was observed in the case of cobalt phosphide, where the bimetallic phosphide reported showing superior activity compared to CoP with the same morphology.75 Figure 4a,b presents the TEM images of CoP and Co2P, respectively, and their OER activities are shown in 147

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Figure 5. Evolution of true catalysts. (a) Irreversible oxidation of a CoP-modified electrode in the initial scan and (b) the corresponding tafel slop (adopted from ref 38). Stepwise evolution of the catalyst from Ni2P, (c) nanowires, and (d) particles. (Reprinted with permission from ref 39, Copyright 2015, Royal Society of Chemistry.)

Figure 6. (a) HRTEM image of a postcatalysis sample of Ni2P showing the core/shell structure of Ni2P/NiOx and elemental mapping for Ni, O, and P. (b) XPS of Ni 2p. (c) HAADF-STEM image of a postcatalyzed sample of a CoP nanorod and the elemental mapping of Co, P, and O. XPS of (d) Co (2p) and (e) P (2p) of postcatalyst CoP nanoparticles and nanorods, respectively. (Panels a and b, Reprinted with permission from ref 32, Copyright 2015, Royal Society of Chemistry. Panels c and d are adopted from ref 3.)

In Situ Evolution of Catalysts. Catalyst activation is the most important aspect for metal phosphides. A close look at the first cycle reveals that in the anodic sweep almost all such cases showed a broad irreversible per oxidation peak before the onset potential.76 This peak disappeared in consecutive scans along with a decrease in the overpotential and tafel slope, indicating the oxidative transformation of the catalyst, frequently termed

Figure 4c. Results suggested that Co2P is a better OER catalyst that CoP. Hence, the metallic character here played a significant role in influencing the OER activity. On the other hand, similar reports for NiP, Ni2P, FeP, and Fe2P were reported, but those could not be compared as reports did not follow identical conditions. 148

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catalyst activation (Figure 5a,b).38 In an exciting study, Du and co-workers showed that enhancement of the activity in Ni2P continued even up to 500 cycles irrespective of the catalyst morphologies (Figure 5c,d).39 This suggested that more electroactive species were formed during successive scans. From general electrochemical knowledge, it could be speculated that the oxidation led to higher valent metal species during the transformation, which might have acted as true catalysts. This prompted extensive ex situ investigation of the transformed catalyst. For Ni2P, Hu and co-workers showed the presence of a secondary layer of materials on catalysts after electrolysis through extensive HRTEM analysis along with elemental mapping.32 Figure 6a presents postcatalysis HRTEM analysis along with elemental mapping of the Ni2P electrocatalyst. The results suggested the presence of a shell or overlayer composed of Ni-oxide/hydroxides. Figure 6b depicted corresponding XPS spectra, and interestingly, this shows successive lowering of the full width at half-maxima, indicating the presence of strong metallic nickel content. Correlating with elemental mapping data, it was confirmed that Ni2P still remained in the core and only the surface was oxidized. Similarly, for CoP, Xing and co-workers reported the surface oxidations after catalysis.3 Figure 6c presents a high-angle annular dark-field scanning tunneling microscope (HAADFSTM) image of a postcatalysis nanorod and elemental mapping of Co, P, and O. The correlation area of these elements suggested that oxygen was mostly on the surface, indicating oxidative transformation of Co on the surface. Further, analysis of XPS spectra, shown in Figure 6d,e for Co and P, respectively, indicated that not only Co but also P was oxidized. From all of 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 performed surface analysis of a post CoP catalyst by XPS and X-ray absorption near-edge structure (XANES) and confirmed that 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 precatalysts.

Figure 7. Activity comparison between oxyhydroxide and phosphide. (a) Digital image showing FeOOH- and FeP-modified carbon paper electrodes and (b) corresponding LSVs. (Reprinted with permission from ref 77, Copyright 2016, Royal Society of Chemistry.)

FeP catalyst reveals the surface getting oxidized to its corresponding oxide or oxyhydroxide form, but still, the activity is superior to that of FeOOH. All of these results suggest that phosphidation enhances the OER activity. This superior activity of the phosphide may be summarized as (a) Eff icient carrier transfer via a phosphide-oxide/hydroxide interface: Phosphide materials are good electrical conductors that remain at the core, and during catalytic transformation, an oxide/hydroxide overlayer formed on their surfaces. This MPx−MOx interface helps provide 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 the phosphate is still unclear, from different literature reports, we assume here that these help in the PCET step. The phosphate residue on the surface possesses the ability to act as a labile ligand that can vary its coordination or chelating modes during the redox switching process of the metal ion and helps facilitate the OER. In a particular case, Brock and co-workers also further studied the decrease in activity over time for CoMnP. They reported fast leaching of the phosphate ion from the catalyst compared to the metallic leaching. Hence, this also suggests the involvement of phosphate in these in situ oxidation processes of phosphide materials. In summary, metal phosphides as efficient OER preelectrocatalysts are discussed. These phosphides undergo in situ chemical transformation to oxides/oxyhydroxides during anodic potential sweep, which enhances the catalytic activities. All possible attempts made to optimize 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 the iron triad, and hence, this needs to be explored further for other groups of phosphide materials. (2) The major obstacle remaining here is the synthetic development of these materials. Unlike oxides and chalcogenides, advances in the synthesis of phosphides are limited. Hence, understanding the underlying chemistry of activity enhancement in OERs and designing synthesis protocols of these materials with morphology variation materials is highly essential.

Because oxide materials are not stable under practical alkaline OER conditions, they typically transformed into hydrated oxides or oxyhydroxides, which are to date known as the best OER catalysts. Because oxide materials are not stable under practical alkaline OER conditions, they typically transformed into hydrated oxides or oxyhydroxides, which are to 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 oxyhydroxides to iron phosphides in a hydrothermal method and compared their catalytic activity under identical conditions. Figure 7a,b presents a photograph of the carbon paper electrodes of FeOOH and FeP and their corresponding voltammograms. The postcatalysis study of the 149

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(3) Further, theoretical study along with more advanced techniques for justifying the intrinsic activities of these materials for the electron transfer process needs in-depth investigations. (4) 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 the nanostructure and bare electrode. Hence, designing an appropriate coupled system where the metal part is not electrochemically active but with high conductivity might help in enhancing the electrical interconnection.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Anirban Dutta received his B.Sc. degree from Siuri Vidyasagar College, Birbhum, WB, India and his M.Sc. from the 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 Ph.D. degree from IIT Kharagpur and carried out his postdoctoral research work in Israel and the U.S.A. He joined IACS in 2007. His research area is investigating the chemistry and physics of nanomaterials from synthesis to applications.

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ACKNOWLEDGMENTS DST of India (SR/NM/NS-1383/2014(G)) is acknowledged for funding. A.D. acknowledges CSIR, and N.P. acknowledges the DST Swarnajayanti project for fellowships.

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REFERENCES

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