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WIREs Energy Environ.
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Stabilization of non‐native polymorphs for electrocatalysis and energy storage systems

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Abstract Evolution in material centric devices like batteries and electrocatalytic reactors have predominantly been made possible via the exploitation of the thermodynamic ground state of pristine or defective bulk crystal, referred to as the “Native polymorph” (NP) here. A significant increase in the material search space is possible by utilizing “Non‐Native polymorphs (NNP),” which are materials that have different translational symmetry with respect to NP. As the NNP have a distinct coordination structure from that of the NP, critical material properties can be anticipated to be different, making NNP a potential substitute material for the aforementioned applications, which are the focus of this review. To obtain a structure–function relationship, systematic approaches to the synthesis of NNP has been demonstrated. Following certain generalities behind NNP, we classify synthesis techniques into few categories with the hope of rationalizing the underlying mechanism of these synthesis and stabilization strategies. We discuss the utility of NNPs in the context of electrochemical water electrocatalytic reactions. Typically, the NNPs have more open volume space enabling lower lithium‐ion diffusion barrier, higher lithium‐ion binding energies, thereby making NNP efficient in the context of energy storage material. However, NNP have lesser stability than the NP and methods to calibrate and improve the stability of NNP are important. Overall, the discussion of polymorphic materials by demarcating them as NP and NNP provides a systematic approach towards modulating material properties as a trade‐off between thermodynamics and kinetics of physicochemical processes. Finally, the challenges and perspectives in this emerging field are discussed. This article is categorized under: Fuel Cells and Hydrogen > Science and Materials Energy Research & Innovation > Science and Materials Energy and Development > Science and Materials
(i) Crystal polymorphs of MnO2 polymorphs: (a) β/N‐MnO2, (b) r/NN1‐MnO2, (c) α/NN2‐MnO2, and (d) δ/NN3‐MnO2. Antiferromagnetic (AFM) ordering of Mn is shown with purple for spin up and orange for spin down. (ii) The density of states plot (DOS) for different polymorphs on MnO2 is shown. The figures are adapted from Gupta, Bhandari, Bhattacharya, and Pala (2019)
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(i) The octahedral coordination structure of the active sites (5‐cus) of most stable surfaces of anatase/NN, brookite/NN, columbite/NN, pyrite/NN, and rutile/N phases. The red, blue, and gray balls correspond to oxygen, surface active sites, and metal atoms, respectively. (ii) (a) Correlation between the adsorption and vacancy formation energy of all polymorphs, (b) Scaling relationships between adsorption energies on O, OH, and OOH for all systems, (c) Theoretical overpotential towards OER for all native and non‐native phases. Rutile/N is given by ○ open and the dashed lines connect rutile polymorphs with different amounts of isotropic strain. Anatase/NN, brookite/NN, columbite/NN, and pyrite/NN are given by □, ▽, Δ, and ◇ markers, respectively. RuO2, RhO2, IrO2, and PtO2 are given by gray, red, blue, and black colored markers, respectively. (iii) Density of states (DOS) of rutile/N, rutile/N with maximum tensile and compressive strain, and columbite/NN of IrO2 is compared. The green area represents the eg states, and the cyan area represents the t2g states. Filled areas represent the occupied electronic states below the Fermi level. The figures are adapted from Z. Xu and Kitchin (2015)
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(i) Effect of particle size and calcination temperature in stabilizing γ/NN3‐, ε/NN2‐, β/NN1‐, and α/N‐Fe2O3. The picture is reproduced from Sakurai, Namai, Hashimoto, and Ohkoshi (2009). (ii) Representative HRTEM image of with annealed arrested‐phase‐transition Rutile/N/core‐Anatase/NN/shell TiO2 heterostructures (Saha, Gyanprakash, Khan, Sivakumar, & Pala, 2017). (iii) Dopant induced stabilization of tetragonal/NN of LaVO4. The picture is reproduced from Rastogi et al. (2017), and (iv) Schematic of ligand steric hindrance mediated stabilization of cubic/NN‐NaYF4 wherein ligand modification and higher surface coverage results in lowering of phase transformation to 48°C through increased ligands/adsorbents surface coverage by ligand modification which increases Na/Y clusterization, thus reducing the stability of cubic/NN phase. The picture is reproduced from Saha, Pala, and Sivakumar (2018). (v) Different methods for the stabilization of NNP
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Electrochemical methods for renewable energy conversion and storage
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(i) Graphical representations of crystal polymorphs of Fe2O3: (a) α‐Fe2O3, (b) β‐Fe2O3, (c) γ‐Fe2O3, and (d) ε‐Fe2O3. The figure is published after taking permission from Machala and Zboil (2011). (ii) Potentiostatic cycling curves for α/N‐Fe2O3 and ϒ/NN‐Fe2O3 (Abraham, Pasquariello, & Willstaedt, 1990). (iii) First discharge graphs of cubic/N‐CoO and hexagonal/NN‐CoO nanocrystals (Nam, Seo, Song, & Park, 2017). (iv) Li‐amount insertion in N‐ and NN‐phases of MnO2 and TiO2. The NNP allows more open space in general, leading to greater Li‐insertion in crystal structure
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Tuning of material property by changing the ratio of native (N) and non‐native (NN1) via thermal treatment. The figure is adapted from Gupta, Bhandari, Saha, et al. (2019)
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(i) Schematic of lithium‐ion battery working principle. (ii) Summary of relationship among capacity retention, thermal stability, and the discharge capacity with variation in Co, Ni, and Mn in Li/Li‐rich NMC oxides. The figure is published after receiving permission from Rozier and Tarascon (2015)
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Band diagrams for different polymorphs of WO3 aligned with the water redox potential. The diagram of band‐edge positions of various polymorphs are reproduced on permission from the American Chemical Society (Ping & Galli, 2014)
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(i) The free energy to form adsorbed oxygen at 0 V versus RHE (ΔGO) and (ii) the dissociated adsorption energy of hydrogen (ΔGH) on the bare surfaces (blue) and oxygen‐passivated surfaces (brown) of fcc/NN‐ and hcp/N‐Ru slabs and amorphous Ru clusters. The insets of panel (i) show the top‐views of the supercells of Ru slabs and Ru50 amorphous clusters. The coverage of oxygen on slabs involved is 50%. (iii) Wulff polyhedrons of hcp/N‐phase (left) and the fcc/NN phase (middle) and an fcc/NN‐phase tetrahedron. Blue regions are the closest packed surface and brown regions are the open surface. (iv) Plots of ΔGO (upper) and ΔGH (lower) on the bare (111) slab of fcc/NN‐Ru depending on the lateral stretching ratio. The gray vertical line indicates the situation that the lattice parameter of the slab equals that of fcc/N‐Pt. The picture is reproduced after permission from Gu et al. (2015)
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