Computational Screening for ORR Activity of 3d Transition Metal

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A Computational Screening for ORR Activity of 3d Transition Metal Based M@Pt Core-Shell Clusters Akhil S Nair, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11483 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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The Journal of Physical Chemistry

A Computational Screening for ORR Activity of 3d Transition Metal Based M@Pt Core-Shell Clusters Akhil S Nair,† Biswarup Pathak†,#,* †Discipline

of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552, India

#Discipline

of Metallurgy Engineering and Materials Science, Indian Institute of Technology

Indore, Simrol, Indore 453552, India Email: [email protected] Abstract: Core-shell nanoparticles are widely recognized as potential catalysts for oxygen reduction reaction (ORR) occurring at the cathode of proton exchange membrane (PEM) fuel cells. A comprehensive analysis of ORR activity of low-cost core-shell nanoparticles is still lacking from previous screening studies. To address this, a complete series of 3d metal based platinum coreshell nanoclusters are designed and scrutinized for ORR activity as well as stability. The adsorption behavior of ORR intermediates is observed to highly depend on the core-shell combination. The analysis of ORR energetics along the associative pathway shows a non-uniform trend in free energy changes and rate determining steps. As compared to earlier reports, we show that a single intermediate binding energy is not enough for interpreting the ORR activity trends. Ti, Ni and Cu based core-shell clusters are observed to have elevated activity as compared to bare platinum nanocluster and periodic platinum (111) surface. The origin of activity differences is explained via structural, charge transfer and electronic structure analyses.

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1. Introduction Developing efficient catalysts for oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells is an important area of renewable energy research scenario to cope up with the burgeoning energy demand.1–3 In spite of the broad category of materials proposed theoretically as well as experimentally, majority of them have not been implemented as potential catalysts from industrial perspectives which has directed the research front towards attempts to accelerate the activity of well known platinum based catalysts.4,5 In this context, reducing platinum loading by alloying with non-platinum metals without affecting the activity has been an objective of immense research interest and has resulted in important outcomes including platinum based binary or ternary alloys, supported Pt monolayer, Pt sandwich structures and core shell nanoparticles.6–9 Amid these alternatives, core shell nanostructures have gained profound attention as they have founded a platform for optimizing catalytic activity by tuning the selection of component metals as well as the surface state properties. The core-shell catalysts are characterized by the presence of a core, generally constituted by relatively low-cost metals and an outer shell often composed of noble metals to sustain the electrochemical conditions. The versatility of these structures lies in the broad spectra of thermodynamically allowed combinations of different metals constituting the core as well as shell. Onset attempts to study the core-shell catalysts for electrocatalytic reactions were majorly focused on extended surface alloys (ESAs) where ‘Pt skin surface’ formed by leaching and ‘Pt skeleton surface’ formed by vacuum annealing were found to be highly active for ORR.10–12 Nørskov and coworkers have demonstrated that ORR activity was profoundly increased on Pt3M (M = Ti, Fe, Co, Ni) alloys with a Pt overlayer as compared to periodic Pt (111) surface resulting from the weakening of Pt-O bond and hence reducing the overpotential caused by surface site blocking of 2


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oxygenated species.13 Similarly, another screening study for Pt monolayers (MLs) supported on noble metal substrates (Pd, Au, Rh, Ir and Ru) was carried out by Stamenkovic et al. which has illustrated a highest activity for Pt ML/Pd (111) system where the authors attribute the enhanced activity to elevated OH hydrogenation rates.14 All these reports were well supported with the first principle studies on slab models with a platinum overlayer and Pt/non-Pt alloy subsurface layers where Pt-O binding energy and the d-band center position of Pt skin surface were adapted as important activity descriptors. Later, Pt3Ni, which was observed as the top candidate in many of these screening studies on ESAs, was synthesized in the nanoparticle (NP) form (M@Pt) with well defined facets and identified with a four to ten fold increment in activity in comparison with the Pt/C catalysts.15,16 The surprising enhancement in activity is brought about by modification in the interaction between platinum shell and adsorbates imparted by geometric effects as well as changes in the chemical nature of constituent atoms. Very recently, Wang et al. synthesized cuboctahedral Co@Pt core shell NPs of various Pt shell thickness and reported Pt skin (2 ML) covered NPs with ten-fold increment in ORR activity and high durability under potential cycling.17 A truncated octahedral Fe@Pt core-shell nanoparticle with high Fe content (55%) was reported by Jang et al. where the authors observed the 1ML Pt containing NPs resulting in superior ORR activity compared to conventional Pt/C catalysts.18 The majority of 3d series based core-shell structure studies were focused on late metals Fe, Co, Ni, Cu on par with studies on Pt-skin surfaces with diminished interaction of adsorbed O and OH intermediates ascribed as the origin of increased activity.19–21 Small sized nanocluster modeling has been highly useful for interpreting the activity as well as stability of these nanoparticles since they can simulate the dimensional derived structural and electronic effects much effectively than periodic slab models employed in extended surface

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studies.22 Moreover, the effect of chemical composition on structure-activity correlation can be much realistically figured out from nanocluster model studies. Wang et al. investigated the core shell preferences of late transition metals (Fe, Co, Ni, Cu, Au, Ru, Pd, Pt etc.) to form 55-atom cluster and reported that Pt prefers to stay in the shell for 3d metal cores whereas it segregates to core with noble metals Au, Pd and Ag.23 Henkelman and coworkers have done a series of significant screening studies on nanoclusters of ~ 1.5 nm diameter and established a correlation between O binding energy and catalytic activity.24 Some of the observations in these studies such as improved catalytic activity limited to only 4d and 5d metal cores of 9-11 groups and unstable nature predicted for structures with Cu, Co core metals were contradicted by later experimental investigations aforementioned which have found enhanced durability as well as activity for the less preferred combinations in these screening studies. Furthermore, the theoretical and/or experimental reports on highly active Ti@Pt,25,26 Cu@Pt27 nanoparticles of diverse geometry which were identified belonging to the low activity regime in the previous screening studies suggest that it is necessary to study ORR activity on non-noble transition metal based platinum core shell nanoparticles to obtain a comprehensive picture of origin of different trends in activity. Moreover the identification of binary or ternary platinum alloys of early transition metals such as Ti28,29 and Cr30,31 as efficient ORR catalysts prompts to investigate the activity of their core-shell analogues as well. In fact, the search for an ideal core-shell combination has covered a large number of metals and interestingly many of them have been experimentally observed to show an improved activity in comparison to conventional Pt/C catalyst. Nevertheless, a complete analysis of ORR activity from thermodynamic perspective is not reported for 3d transition metal based Pt core-shell clusters. Since it is known that the conventional scaling relations break for many of the core-shell structures, a detailed scrutiny of energetics of elementary

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steps is required for gaining a comprehensive picture of ORR activity rather than a single intermediate binding energy.32 In addition to that, an important activity descriptor, overpotential associated with ORR mechanism is not sufficiently addressed in the previous 3d series based M@Pt cluster studies. Above all, core-shell catalysts composed of scarce and expensive noble metal cores which are being widely studied in recent days contradicts the idea of economical viability expected out of core shell catalysts in fuel cell. In the current study, we investigate the ORR activity of platinum based core shell nanoclusters (CSNCs) with complete series of first row transition metal (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) cores by modeling a cuboctahedral cluster (M19@Pt60) of 79 atoms similar to Henkelman’s study. Cuboctahedal geometry with (111) and (100) facets has been used in many theoretical studies and has been identified as a standard model system for mimicking experimental scenario. Our objective is to understand the ORR activity trend across the 3d series of metal cores grounded on structural as well as thermodynamic aspects along with a detailed investigation of stability and a comparison with bare Pt79 nanocluster (Pt-NC) as well as periodic Pt (111) surface. Previously small sized core-shell nanoclusters of 13 atoms with first row transition metal cores were studied by Cheng et al. and made an observation that Pt12Mn and Pt12Fe clusters owned optimum activity.33 Similarly 55-atom cuboctahedral core-shell clusters with late transition metal cores have been studied by Shin et al. and reported an improvement in catalytic activity for 3d transition metal cores.34 However, our study covers the entire 3d series and is more emphasized on a well defined nanocluster correlated with experimental interest and analysis of complete ORR pathway from free energy considerations rather than mere oxygen binding energy. Overpotential is considered as the activity determining parameter since it is the most important activity descriptor from experimental interest and the major obstacle in attaining expected fuel cell efficiency. The results

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are explained from electronic structure, charge transfer and binding energy analyses along with a comparison of activity trend observed across all the clusters considered.

2. Computational Details 2.1. DFT Calculations: The spin polarized DFT calculations are performed by using the Vienna Ab initio Simulation Package (VASP).35 Generalized gradient approximation of Perdew-BurkeErnzerhof (GGA-PBE) is employed to describe the exchange-correlation potential.36 The ionelectron interactions are described by projector augmented wave (PAW) method.37 Kohn-Sham electronic wavefunctions are expanded in a plane wave basis set with a kinetic energy cutoff of 470 eV. The cuboctahedral clusters are optimized in a 22 × 22 × 22 Å3 supercell by using conjugate gradient (CG) algorithm and minimizing the Hellman-Feynman forces on atoms Ni>Cu =Pt(111)>Pt79>Co>Fe>V>Mn>Cr>Sc. The lack of periodic trend in the activity in the 3d series can be understood from the structural changes, adsorption site differences and the composition dependent activity across the series. From our study, Ti, Fe, Co, Ni, and Cu are identified as suitable cores for improved ORR activity which validates the considerable number of experiments and theoretical reports based on their Pt based core-shell structures. The emergence of Ti19@Pt60 to the high activity scenario suggests that apart from late transition metals, early metals can also be employed as core species for CSNCs. Here, it is important to note that the direct correlation between O* binding energy and activity is not adequate for a complete description of activity trends as seen in the free energy analysis since the scaling behavior of ORR intermediates is not well protected over the M19@Pt60 series. For example, the Mn19@Pt60 cluster showing the strongest O* binding energy is not the least active candidate identified from the overpotential based activity analysis. Hence a complete description of activity trends over these clusters requires a systematic analysis of adsorption behavior of each intermediate over the catalyst surface and their mutually dependent energetics. Also, the periodic changes in chemical, structural properties control the surface state properties of these CSNCs

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which induces a characteristic interaction with the intermediates and hence determining activity trends. To understand the reason behind the variation in the free energy changes among the CSNCs considered, we attempt to scrutinize the adsorption of ORR intermediates from the structural as well as electronic effects. In this context, we have calculated d-band centers of all the CSNCs which corresponds to the average d-band energies and has been adapted as an important tool for explaining the interaction of adsorbates with catalyst surfaces. The calculated d-band centers are plotted with O* binding energy as it is the most strongly adsorbed intermediate in the ORR mechanism. The results are shown in Figure 3.

Figure 3: O* binding energy plotted against d-band center of Pt shell in M19@Pt60 clusters.

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From the figure, it is evident that the O* binding energy and the d-band center of Pt shell of M19@Pt60 clusters show a reasonable correlation. It fairly explains the overall trend of binding energy values where Pt79, Cu19@Pt60 and Ni19@Pt60 belonging to the strong binding regime with d-band centers positioned relatively closer to the Fermi level whereas V19@Pt60, Ti19@Pt60 and Cr19@Pt60 belonging to the weak binding regime with the most downshifted band centers. The large deviations observed for Ti19@Pt60 and Mn19@Pt60 can be understood from their weakest and strongest O* binding, respectively. It is found that for the CSNCs, there is no regular increment in d-band center energy along the 3d series as observed for Pt-rich Pt–3d alloy surfaces studied by Kitchin et al.58 The reasonable correlation of d-band center with O* binding energy prompts to investigate the origin of d-band center shifts on the shell layer Pt atoms of CSNCs. In this context, we have analyzed the strain present on the Pt (111) facet of the CSNCs and the charge transfer between the core metal and Pt atoms to analyze the strain effects and ligand effects, respectively. These two effects are known to play important role in determining the d-state filling of the surface platinum atoms.59–61 The calculated strain effect and charge transfer effect are plotted in figure 4 with reference to O* binding energy. Bare Pt79 NC where there is no charge transfer and Sc19@Pt60 where strain cannot be determined as O* adsorption is accompanied by distortion of the (111) facet are not included in this analysis.

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Figure 4: O* binding energy of M19@Pt60 clusters plotted against (a) compressive strain on the surface (b) Charge transfer from core to shell. From the figure, it is known that changes in the O* binding energy is majorly determined by the charge transfer from the core to the shell Pt atoms and not the strain effect. The strain analysis shows that although for all the CSNCs, the Pt shell is under compressive strain with reference to the periodic Pt (111) surface which is known to be a favourable characteristic for enhanced activity, discrepancies such as the weak binding Ti19@Pt60 associated with lowest compression and highest strain possessing Ni19@Pt60 with high binding energy reveals that the O* binding energy trend does not undergo significant correlation with strain effect. This is consistent with the previous theoretical studies where it is observed that the strain effect is less influential for 3d transition metal core based clusters. This is manifested in the lowest binding energy of Ti shell which has the surface Pt atoms with a charge (0.29|e|) transferred from core due to the differences in Fermi levels of two metals. The charge transfer creates an electrostatic repulsion scenario which causes the surface to be less interacting to O* and hence substantiates the weakest binding energy

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observed for Ti19@Pt60 cluster. On the contrast, it is noticed that the Mn19@Pt60 remains as the most deviated outlier in both the strain and charge transfer vs binding energy relationships owing to its highest binding energy. This can be explained from the hcp binding site for O* on the Mn19@Pt60 surface which is constituted by one of the edge atom with a different coordination number. In such case, correlating binding energy with mere d-band center position might not be adequate to have a comprehensive picture. To have a more comprehensive picture on the discrete binding strengths exhibited by Ti19@Pt60 and Mn19@Pt60 clusters, we have analyzed the partial density of states (PDOS) of Pt-d states of the outer shell Pt atoms which is given in Figure 5.

Figure 5 : Partial density of states (PDOS) of shell layer Pt atoms of Ti19@Pt60 and Mn19@Pt60 clusters. εd denotes the d-band center of the Pt shell atoms in the clusters. From the PDOS analysis, it is revealed that the differences in the binding properties and the large extent of deviation from d-band center correlation directly originates from the distribution of both the core and shell atoms in Ti19@Pt60 and Mn19@Pt60 clusters. In the case of Ti19@Pt60, there is a dominancy of Pt-d states below the Fermi level but the unoccupied states have major contribution from Ti-d states. This observation is very much consistent with that of Johnston and coworkers 21



62where

the authors report that the electron transfer from the electropositive Ti to relatively

electronegative Pt fills the higher Pt d-bands and thereby results in an upshift of the Fermi level or downshift of average d-band center. But, at the region closer to the Fermi energy, there are very less d-states available for bonding with the intermediates O*/OH* which again enables Ti19@Pt60 to show a weakened binding and higher activity. The unexpected deviation of Ti19@Pt60 from the d-band center correlation is caused by the largest d-band center downshift exhibited by V19@Pt60 cluster in spite of its higher binding energy (-3.89 eV) than Ti19@Pt60 (-3.28 eV). This can be ascribed to the largest structural rearrangement occurring for V19 core in V19@Pt60 among all the other CSNCs considered. This is evident from the bond length contraction from 2.94 Å in bulk to 2.59 Å in the core-shell cluster. Furthermore, we have calculated the distortion energy of the M19 core for all the CSNCs (Figure S3 in supporting information) where V19@Pt60 holds the peak position with highest distortion energy of 0.34 eV. Such a compressive distortion enforces orbital overlapping among the d-states leading to d-band widening or d-band center downshift which suggests that apart from strain and charge transfer effects, structural effects also play a decisive role in determining the adsorbate interaction and hence the activity for core-shell nanoclusters. The PDOS of Mn19@Pt60 shows a clearly different pattern from Ti19@Pt60 where the both the occupied and unoccupied states are dominated by outer Pt atoms. The Mn19@Pt60 cluster possesses an upshifted d-band center as compared to Ti19@Pt60. But the difference is very small (0.2 eV) whereas the corresponding binding energy difference is very large (2.4 eV). This is substantiated by the high density of states near the Fermi level observed for Mn19@Pt60 in comparison with Ti19@Pt60 which enables the former cluster to facilitate much enhanced electron transfer to adsorbed atomic oxygen and could deviate from the d-band center correlation with binding energy. This is consistent with the observation made by Gorzkowski et al. from an experimental

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electrocatalytic study of Pd supported Pt monolayer that it is the d-state density near the Fermi level rather than the average d-band center position is the more decisive factor which is again sensitive to the system’s electronic structure.63 The high electron density received by O* is further supported by a charge transfer of 0.68|e| from the Pt surface as well as the occurrence of OH* formation as the potential determining step in the free energy analysis.

3.3 Core-shell structure stability analysis The stability of CSNCs, both from thermodynamic and electrochemical origin is investigated by formation energy, average binding energy, core-shell interaction energy and dissolution potential analysis. The calculation methods behind these parameters are described in detail in Text S1 in supporting information. The thermodynamic plausibility of formation of a core-shell cluster with respect to the bulk states of component metals is analyzed via formation energy values and given in Figure 6(a). From the figure, it is revealed that all the CSNCs possess a lower formation energy than the bare Pt79 NC and hence host a higher plausibility of formation. The observed formation energy trend for the core shell clusters follows significant similarity with the heat of alloy formation for the corresponding bulk precursors calculated by Norskov et al. where the lower value region is occupied by Sc and Ti and the upper value region by late transition metals such as Co, Ni.13 Furthermore, to investigate the preference of constituent metal atoms to form the cuboctahedral cluster in comparison to their respective isolated atomic states, we have calculated the average binding energy of M19@Pt60 clusters and represented in Figure 6(a). The average binding energy values are relatively higher for early metal based CSNCs as we move across the 3d series which can be attributed to the electronegativity increment towards that of Pt across the period. A higher electro negativity difference between the core and shell metal atoms facilitate strong bonding as reported in previous studies.64,65 This is manifested in the Ti19@Pt60 cluster 23



where Ti and Pt shares the maximum difference in electronegativity. Similarly Cu19@Pt60 cluster has the lowest binding energy which is also originating from the almost completely filled d-states of Cu and Pt that results in a weaker interaction in the cluster. However, it is noteworthy that all the CSNCs possess a binding energy fairly close to that of Pt79 cluster remarking that they owe a reasonable stability as that of Pt79 cluster.

Figure 6: (a) Formation and average binding energies (eV) of M19@Pt60 clusters (b) Dissolution potentials and core-shell interaction energies of M19@Pt60 clusters. The M19@Pt60 is abbreviated to M@Pt for visualization convenience. Apart from energetic stability analysis, the electrochemical stability of the CSNCs is investigated by using dissolution potential as the crucial parameter. The dissolution potential verifies the tendency of the atoms of the outer shell of NCs to dissolve into the acidic medium from the surface. The calculated dissolution potential values for all the CSNCs and Pt79 NC are represented along with core shell interaction energy in Figure 6(b). From the diagram, it is evident that all the CSNCs considered encompass a higher dissolution potential than pure Pt79 NC which suggests an enhanced electrochemical stability for the outer shell layer atoms to be adhered with the core. The higher 24


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dissolution potentials for M19@Pt60 clusters are attributed to the elevated interaction between the core and shell atoms which is obvious from the good correlation between the core shell interaction energy and dissolution potential. A strong interaction between the core and shell enables the outer shell atoms to be strongly bound with the cluster and thereby sustain the activity in fuel cell conditions. 3.4 Stability Vs activity analysis The stability Vs activity relationship of the CSNCs considered is subjected to a correlation study by adapting dissolution potential and over potential as important parameters (Figure 7). Since we have obtained significant correlation between dissolution potential with binding as well as coreshell interaction energy, we believe that it is adequate to take account of thermodynamic stability as well.

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Figure 7: Stability Vs activity relationship of M19@Pt60 clusters represented as colour gradient plot by correlating dissolution potential and negative ORR overpotential (ηORR). The activity is marked in ascending order while moving from red to green region. From the figure 7, Ti19@Pt60 is identified as the most favorable CSNC from both activity as well as stability perspectives. The other early transition metals possess high resistance against dissolution but are associated with poor activity as obtained from free energy analysis. Late transition metals possess considerable activity but with less electrochemical stability, out of which Ni19@Pt60 cluster shows the best correlation. Fe19@Pt60 and Mn19@Pt60 clusters belong to the intermediate level both in terms of activity as well as stability. Pt79 cluster makes a boundary the above which occupied by Cu19@Pt60, Ni19@Pt60 and Ti19@Pt60 clusters. The results suggest that the attempts to improve the activity of core-shell nanoclusters should be accompanied with the stability and durability enhancement which can be well provided by novel experimental strategies and techniques.

4.Conclusions The screening study of M19@Pt60 clusters of 3d transition metal series shows a highly composition dependent ORR activity. The energetics of adsorption as well as ORR elementary reaction is completely different from bare Pt79 cluster and periodic Pt (111) surface. Our study has shown that a single intermediate binding energy cannot explain the complete activity trend, as different from earlier reports, as the adsorption behavior is highly varied over the core-shell clusters but a scrutiny of complete energy profile is required. Ti19@Pt60 cluster is identified as top candidate in terms of ORR activity in our study which is majorly evolving from the ligand effects. The late transition metal core based structures are also identified to possess a reasonable activity. The considerable

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activity for Pt79 cluster in comparison with many of the other 3d metal Pt core-shell clusters suggest that efforts to improve the stability of purely Pt derived structures can bring out much fruitful outcomes as compared to searches for new core-shell combinations of these metals. We believe that our study provides significant insights to the activity analysis of 3d transition metal core based catalysts by providing a comprehensive picture of ORR energetics over the complete 3d series of CSNCs for the first time. Supporting Information The supporting information is available free of charge on the ACS Publications website. The spin density distribution over M19@Pt60 clusters, adsorption configuration of ORR intermediates, distortion energy profile of M19 core in CSNCs and the calculation methods behind Stability calculations of M19@Pt60 clusters are given in the supporting information. Acknowledgements: We thank IIT Indore for providing the lab/computational facilities and DST SERB (EMR/2015/002057) and CSIR (01(2886)/17/EMR-II) projects for funding. ASN thank Ministry of Human Resources and Development, India for research fellowship. Conflicts of Interest The authors declare no conflicts of interest. References: (1)

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Author Bibliography Dr. Biswarup Pathak did his Ph.D. studies under the supervision of Professor E. D. Jemmis from Indian Institute of Science, Bangalore. Soon after his PhD, did his postdoctoral studies from Prof. Jerzy Leszczynski group at Jackson State University, USA (January, 2008-July, 2009) followed by Prof. Rajeev Ahuja at Uppsala University, (September 2009May 2012) where he received Wenner-Gren postdoctoral fellowship (January 2010December 2011) to carry out the DNA sequencing studies through solid nanopores. In 2012, he joined IIT Indore as an Assistant professor in the Discipline of Chemistry and since 2016, he is an associate professor. His major areas of interest are of designing of nanomaterials for oxygen reduction reactions. Dr. Pathak has published over 100 papers in highly reputed international journals and some of his outstanding works are published in prominent journals such as Journal of American Chemical Society (JACS), Angew. Chem. Int. Ed, Nature Communications and so on. His works have been cited over 2000 times and Dr. Pathak received Early and Mid-Career Research Award from INSA in 2017 and IIT Indore Best Researcher Award in 2016.

Author photograph

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Computational Screening for ORR Activity of 3d Transition Metal

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