16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Review www.advenergymat.de Progress on Emerging Ferroelectric Materials for Energy Harvesting, Storage and Conversion Xian-Kui Wei,* Neus Domingo, Young Sun, Nina Balke, Rafal E. Dunin-Borkowski, and Joachim Mayer created his famous “sel polychreste.”[2] The Since the discovery of Rochelle salt a century ago, ferroelectric materials Rochelle salt is highly soluble in water and have been investigated extensively due to their robust responses to elec- is very complex in crystal structure. About tric, mechanical, thermal, magnetic, and optical fields. These features give 160 years later, follow-up studies about pyroe- lectric, dielectric and piezoelectric properties rise to a series of ferroelectric-based modern device applications such as finally establish the existence of ferroelec- piezoelectric transducers, memories, infrared detectors, nonlinear optical tricity in Rochelle salt. After the finding of devices, etc. On the way to broaden the material systems, for example, from KH2PO4 in 1935,[3] the first non-hydrogen- three to two dimensions, new phenomena of topological polarity, improper bonded FE BaTiO3, with a simple perovskite ferroelectricity, magnetoelectric effects, and domain wall nanoelectronics bear structure, appears in the public vision in the the hope for next-generation electronic devices. In the meantime, ferroelectric early 1940s,[4] which indicates that the spring of FE research comes. To date, more than research has been aggressively extended to more diverse applications such as 700 pure FE materials have been uncovered, solar cells, water splitting, and CO2 reduction. In this review, the most recent including inorganic oxides, organic-inor- research progress on newly emerging ferroelectric states and phenomena in ganic hybrids, organic compounds and poly- insulators, ionic conductors, and metals are summarized, which have been mers, liquid crystals, and so on.[5–7] used for energy storage, energy harvesting, and electrochemical energy con- From the viewpoint of crystallography, an FE compound must adopt one of the ten version. Along with the intricate coupling between polarization, coordination, polar point groups, that is, C1, Cs, C2, C2v, defect, and spin state, the exploration of transient ferroelectric behavior, ionic C3, C3v, C4, C4v, C6 and C6v, out of the total migration, polarization switching dynamics, and topological ferroelectricity, 32 point groups.[8] Considering the sym- sets up the physical foundation ferroelectric energy research. Accordingly, metry of all point groups, the belonging the progress in understanding of ferroelectric physics is expected to provide relationship classifies the dielectric mate- insightful guidance on the design of advanced energy materials. rials, that is, ferroelectrics ⊆ pyroelectrics ⊂ piezoelectrics ⊂ dielectrics (Figure 1a). With the integration of pyroelectric, piezoelectric 1. Introduction and dielectric properties, the ferroelectrics naturally secures a core position in the study of dielectrics.[9–14] Benefitting from the It is well known that the study of ferroelectric (FE) materials starts robust responses to electric, mechanical, thermal, and optical from Rochelle salt, [KNaC4H4O6]3⋅4H2O (potassium sodium tar- fields, FE materials have been widely used in various modern trate tetrahydrate),[1] which is the first compound discovered by electronic devices, such as varactors and memories,[15,16] trans- Valasek in 1921. Looking back at history, we find that the time of ducers and transistors,[17] infrared detectors,[18] microwave phase exploring Rochelle salt may date back to 1665, when Seignette filters,[19] piezoelectric microsensors and micromotors,[20–22] X.-K. Wei, R. E. Dunin-Borkowski, J. Mayer N. Domingo Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons Catalan Institute of Nanoscience and Nanotechnology (ICN2) Forschungszentrum Jülich GmbH CSIC and The Barcelona Institute of Science and Technology 52425 Jülich, Germany Campus UAB, Bellaterra, Barcelona 08193, Spain E-mail: x.wei@fz-juelich.de Y. Sun Center of Quantum Materials and Devices and The ORCID identification number(s) for the author(s) of this article Department of Applied Physics can be found under https://doi.org/10.1002/aenm.202201199. Chongqing University Chongqing 401331, China N. Balke © 2022 The Authors. Advanced Energy Materials published by Wiley- Materials Science and Engineering VCH GmbH. This is an open access article under the terms of the North Carolina State University ­Creative Commons Attribution-NonCommercial License, which permits 911 Partners Way, Raleigh, NC 27695, USA use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. J. Mayer Central Facility for Electron Microscopy RWTH Aachen University DOI: 10.1002/aenm.202201199 Ahornstr. 55, 52074 Aachen, Germany Adv. Energy Mater. 2022, 12, 2201199 2201199 (1 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 1. Expansion of ferroelectric phenomenon to energy materials. a) Belonging relationships of point groups and of ferroelectrics with pyroelec- trics, piezoelectrics, and second-harmonic generation (SHG) based on the symmetry considerations. Reproduced with permission.[8] Copyright 2012, American Chemical Society. b) Expansion of ferroelectric materials and their applications in various energy systems. nonlinear optical devices,[23] and capacitors.[24] In the past decade, These findings highlight that entanglement of FE polarization significant advances have been achieved in all aspects of mate- with various electronic states is becoming more and more promi- rial design and growth, structural characterization, theoretical nent in the newly emerging FE materials. With broadening of calculation and property measurement. As underscored by the the material systems, another indisputable fact is that the FE following findings, this thrived a new tide of FE research. research has been largely extended to energy-related applica- tions (Figure 1b), for example, solar cells,[42] electrostatic energy I) By constructing delicate boundary conditions, for example, storage,[43] water splitting,[44,45] and CO2 reduction.[46] Motivated ultrathin dielectric-layer-based (PbTiO3)m/(SrTiO3)n super- by urgent demand on clean and renewable energies, we there- lattices,[25] a number of emergent topological states such as fore summarize the latest research progress about novel FE phe- polar vortices, polar skyrmions and dipole waves are dis- nomena, their origin and correlation with enhanced performance covered. Through breaking the restriction of polarization in energy harvesting, storage and conversion. Instead of trying to orientation confined by lattice symmetry, this promotes the be exhaustive in every direction, this review aims to offer fresh FE research to the regime of intriguing topology. insights to comprehend the role of FE polarization in energy- II) Many intriguing physical phenomena are observed at related processes and activities. Based on mechanistic under- homogeneous and heterogeneous 2D boundaries of polar standing, we believe that this review will help to attract more and nonpolar materials, for example, charged domain walls attention and inspire more research efforts in the fast-evolving and SrTiO3/LaAlO3 interfaces. With involvement of polar joint fields of FE and energy materials. catastrophe or discontinuity, interfacial 2D electron gas, magnetism, and superconductivity offer a great potential for design of miniaturized electronic devices.[26–30] 2. Ferroelectric Origins and Phase Transitions III) Other than insulators, FE metals characterized by coexist- ing ferroelectricity with metallicity emerge one after anoth- FE materials have a spontaneous polarization (P) that can be er. Alongside single-phase multiferroics, for example, FE switched by an applied electric field (E), and breaking of spatial ferromagnets or magnetic ferroelectrics,[31,32] they provide inversion symmetry—allowing unit-cell-wise separation of posi- a fertile platform for investigation of FE origin, interplay of tive and negative charge centers—is a prerequisite condition polarity with intricate electronic and spin states[33–35] as well for emergence of FE polarization. For an ordinary (or proper) as novel device applications. FE, for example, BaTiO3, Pb(Zr1-xTix)O3 and BiFeO3, the mate- IV) Electrically switchable valley polarization[36,37] and Moiré fer- rial undergoes a structural phase transition from a paraelectric roelectricity[38,39] reported in 2D layered van der Waals (vdW) phase to an FE phase at the Curie temperature TC (Figure 2a). materials delineate unsketched spaces for exploration of In compliance with lattice distortion, spontaneous polarization emerging FE physics at confined dimensionss. The unprec- describes the symmetry change, thus the polarization vector edented electron-lattice-polarity correlation opens up a new era is termed as a primary order parameter. Depending on sym- for study and design of low-dimensional electronic devices.[40,41] metry of the lattices, the polarization direction can be fixed by Adv. Energy Mater. 2022, 12, 2201199 2201199 (2 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 2. Ferroelectric phase transitions and multi-coupling functional dimension. a) Illustration of a paraelectric-to-ferroelectric phase transition via Curie temperature TC. b) A typical P–E hysteresis loop of ferroelectrics and associating domain polarization reversal. c,d) Displacive and order- disorder type phase transitions as exemplified in PbTiO3 and NbNO2, respectively. e,f) Creation of ferroelectricity by antisite defects and sliding layers as exemplified in SrTiO3 and hexagonal BN (h-BN), respectively. Reproduced with permission.[70] Copyright 2021, AAAS. g) An expanded functionality dimension of electric polarization (P) controlled by electric field (E) and their coupling with magnetism (M), strain (ε), concentration of ionic defects (c) alongside their conjugate fields of magnetic field (H), stress (σ) and chemical potential (µ). Reproduced with permission.[76] Copyright 2013, AAAS. a polar axis (e.g., tetragonal), a specific plane (monoclinic) or Among the FE oxides, it is noteworthy that materials like HfO2 even without any constraint (triclinic).[47–50] As a result of strain- show an opposite thickness scaling behavior, whose FE prop- polarization coupling, proper ferroelectrics exhibit large elec- erty disappears above a critical thickness.[60] tromechanical and pyroelectric effects.[9,51] If the polarization On the energy scale, to stabilize an FE state, a long- appears as a by-product of some other orders, for example, spin range electrostatic force should appear to compete with the or charge, the ferroelectrics are called improper and the polari- short-range repulsive force, which usually favors a nonpolar zation is termed as a secondary order parameter. A number of symmetric structure.[61] Corresponding to different origins and single-phase multiferroic materials such as RMnO3 (R = rare manifestations, the FE phase transitions can be classified into earth), LuFe2O4 and RMn2O5 belong to this category.[31] the following categories (Figure 2c–f). I) Displacive type phase For a single-phase FE, a uniform polarization leads to net transition. For instance in PbTiO3 and BaTiO3, the Ti-3d and charging at sample surfaces due to uncompensated bound O 2p hybridization at T < TC plays an essential role in driving charges. To lower the electrostatic energy, the ferroelectrics tend a collective off-center shift of Ti atoms away from centers of to split into domains, which are separated by domain walls and oxygen octahedra.[62] II) Order-disorder type phase transition. differ in polarization orientation. Macroscopically, ferroelectrics Being sensitive to temperature change, the transition is a col- are characteristic of a P–E hysteresis loop (Figure 2b), which lapse process for atomic occupation from all possible sites to results from switching of domain polarization and motion of specific ones, for example, in NaNO2. In fact, these two types domain walls.[52] The maximum polarization (Pmax), remnant of phase transitions are not mutually exclusive but coexisting polarization (Pr) and coercive field (EC) manifest the FE property, in many ferroelectrics.[63,64] III) Antisite defect driven phase and the loop area denotes the energy consumed in the polariza- transition. Although with inhomogeneous distribution, atomic tion-switching event. So far, a general consensus is that there antisites such as Ti on Sr site (TiSr) in SrTiO3[65,66] indeed give is no thickness limit on the existence of ferroelectricity,[53–55] rise to emergence of robust ferroelectricity in originally non- and the polarization may decouple with lattice tetrago- polar materials,[32,67] which may possibly associate with off nality in perovskites at ultrathin conditions.[56,57] At T > TC, stoichiometry in composition. IV) Layer sliding driven phase polar nanoclusters and precursor dynamics prevail in perov- transition. This adapts to stacking of homogeneous or het- skite ferroelectrics below a temperature of ≈TC + 75 K.[58,59] erogeneous 2D layered vdW materials.[68–71] Taking hexagonal Adv. Energy Mater. 2022, 12, 2201199 2201199 (3 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de BN (h-BN) as an example, a 180°-rotation between adjacent 3.2. Novel Ferrielectric Phases in PbZrO3-Based System layers (AA′ stacking) naturally restores the inversion symmetry broken in a monolayer. However, when BN monolayers slide Nonpolar PbZrO3 (space group Pbam), NaNbO3 (Pbcm), and into an AB or BA order, interlayer charge redistribution and AgNbO3 (Pbcm) are typical AFE materials,[87–89] which undergo associated ionic displacement result in an out-of-plane electric unit cell doubling from paraelectric cubic (C) phases to AFE polarization.[69,70] It is noteworthy that the FE origins are not orthorhombic (O) phases at TC. For practical device usage, the limited to the above-mentioned situations, octahedral rotation AFEs are usually chemically modified to optimize its electrical mismatch at interfaces and vacancy-ordering-induced polyhe- and mechanical properties (Figure 3b), for example, doping of dral distortion are other ways of generating ferroelectricity.[72,73] PbTiO3 and PbSnO3 into PbZrO3 (termed as PZST). The aim From the perspective of thermodynamic theory, the phase is to lower its critical field (EC) to an appropriate value below transitions can be classified into first-order and second-order the dielectric breakdown strength (EB).[90,91] On the other hand, transitions,[74] which is manifested by discontinuity and con- the energy levels of the FE phases, pertinent to vibrations of tinuity of derivatives of Gibbs free energy, for example, latent Pb ions and oxygen octahedral tiltings,[92–94] can be tuned with heat, volume change, and susceptibility, etc. Near a tricritical respect to the AFE ground state to adjust the energy-storage point, the first-order and second-order transitions can even performance (Figure 3c). Taking AFE PbZrO3 as an example, compete and give rise to a frustrated-order transition, which its structure is dominated by antiparallel Pb displacements leads to deviation from the classic mean-field theory and a (≈24 pm) along a axis and antiferrodistortive (AFD) order of number of anomalies in critical exponents and structure.[75] oxygen octahedra (a−a−c0 in Glazer’s notation), which rotate These features indicate that the ferroelectric properties show along [210]O and [210]O directions (Figure 3d). Compared with great sensitivity to chemical composition, defect structure, the ferroelectrics, concave curvatures of P–E hysteresis loops boundary condition, and stimulus of external field. As a result, endow AFEs a natural advantage in effectively storing the elec- the ferroelectric polarization shows coupling to many other trostatic energy. Specifically, an inherent coupling of AFE-to-FE physical quantities and even their conjugate fields, for example, transition with the energy storage makes AFE materials ideal magnetism versus magnetic field (M vs H), strain versus stress candidates for in situ biasing structural pathway studies.[95–97] (ε vs σ), defect concentration or conductivity versus chemical Recently, intriguing improper ferroelectricity has been potential (c vs μ) and beyond (Figure 2g). Therefore, ferroelec- reported in PbZrO3-based oxides. By taking illumination elec- trics offer a broad platform for exploration of intriguing physics tron beam as an imaging source and a stimulus field, Wei et al. and quite a few energy-related device applications. investigated time-dependent AFE-to-FE transitions using tem- poral- and atomic-resolution negative spherical-aberration (CS) imaging (NCSI) technique in a transmission electron micros- 3. Electrostatic Energy Storage Systems copy (TEM).[98,99] During dynamic structural changes, a tran- sient ferrielectric phase is discovered in [001]O-oriented PbZrO3 3.1. Energy Storage Mechanism between the AFE phase and FE monoclinic/rhombohedral phases. With preservation of antiparallel Pb displacements, fer- An electrostatic capacitor consists of a dielectric insulator layer rodistortive (FD) distortion[100] of oxygen octahedra gives rise sandwiched by two parallel electrodes. Under application of to a cycloidal order of polarization in the ferrielectric phase an electric field (E), the stored capacitance (C) and recoverable (Figure 3e,f). In ABO3 perovskites, a steric linkage of BO6 energy density (Ue) can be expressed by framework usually constrains octahedral rotations to have equal magnitudes but opposite sense in a plane, thereby pre- C = ε 0ε r A / d (1) serving inversion symmetry. Analogous to interfacial patterning of octahedral distortion,[72,101,102] an AFD-to-FD phase transition Pmax points out a new improper approach[103] to transform nonpolar Ue = ∫ EdP Pr (2) materials into polar ones and realize ferroelectricity with non- collinear polarization features. respectively (Figure 3a). Here, ε0, A, εr (>>1), d and dP are Associated with dilute La or Nb replacement at A site, Sn and permittivity in vacuum (≈8.85 × 10−12 F m−1), electrode over- Ti doping at B site of PbZrO3 leads to incommensurate modu- lapping area, relative permittivity, thickness and polarization 1 lations (ICMs),[104,105] which are manifested by {110}C (n is increment of the dielectric layer. The electrostatic capacitors n are capable of offering an ultrahigh power density (tens of a non-integer between 4 and 10) superstructure reflection in MW kg−1) at a time scale of ≈1 ms or less,[77] ultrahigh energy reciprocal space. In real space, the ICMs result from a mixture efficiency (≈97%),[78] and ultrahigh operating voltage (up to of stripes, whose thickness stochastically ranges from two to MV cm−1).[79] Nowadays, dielectric capacitors are ubiquitously six layers of the (110)C plane. Previously, the Pb displacements used in electronic devices and pulsed power systems, for are assumed to arrange in a fully compensated antiparallel example, hybrid electric vehicles, smart grids, avionic indus- fashion. However, by combining atomic-scale scanning TEM tries, high power lasers for military usage and consumer elec- (STEM) analysis and density functional theory (DFT) calcula- tronics.[80] Specifically, antiferroelectric (AFE) and relaxor FE tions, Ma et al. found that the Pb displacements in Nb-doped materials[81,82] promise to deliver exceptional energy storage PZST[106,107] are either antiparallel with different magnitudes, performance due to their reducible Pr and energy loss,[83,84] or nearly orthogonal between adjacent stripes (Figure 3g,h). improvable Pmax[85] and energy efficiency.[78,86] The imbalanced cationic displacements evidence the presence Adv. Energy Mater. 2022, 12, 2201199 2201199 (4 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 3. Wavy polarization orders in PbZrO3-based system. a) Half hysteresis loop of AFEs at E ≥ 0 with annotation of EC1, EC2, Pmax, Pr and energy storage density Ue (green area). b) Ternary phase diagram of PbZrO3-PbTiO3-PbSnO3 (PZST). Reproduced with permission.[110] Copyright 1964, Elsevier. c) Calculated potential energy of unstable high-symmetry phonon modes (Γ-point FE distortion, M-point in-phase tilting, and R-point out-of-phase tilting) and energy levels of stable PbZrO3 phases. Reproduced with permission.[94] Copyright 2019, American Physical Society. d) Structural model of AFE PbZrO3. e) NCSI-TEM images and refined structural model (Pb-yellow, Zr-green, O-pink circles) for the ferrodistortive (FD) phase of PbZrO3 induced by electron beam irradiation along [001]O direction. Yellow arrows—antiparallel Pb shifts, yellow rectangle—FD unit cell. f) 2D cycloidal polarization order of the FD phase of PbZrO3.[98] g,h) High-angle annular-dark field (HAADF) STEM image overlaid by Pb displacements and polariza- tion profile of [001]C-oriented (Pb0.99Nb0.02)[(Zr0.57Sn0.43)1-yTiy]0.98O3 (PZ-100y, y = 0.05), respectively. Reproduced with permission.[106] Copyright 2019, American Physical Society. i–k) ABF-STEM image overlaid by Pb-[100]O displacements, tilting and distortion map of BO6 octahedra and y-direction rippling map of O sublattices in [001]C-oriented Pb0.97La0.02(Zr0.50SnxTi0.50-x)O3 (PLZST 50/45/5), respectively. Reproduced according to the terms of the CC-BY license.[108] Copyright 2020, The authors, published by Springer Nature. of ferrielectricity along the [110]C direction. By imaging oxygen Glazer’s notation).[111] At the mesoscopic scale, this leads to atoms using annular-bright field (ABF) technique,[108] Fu et al. formation of inherent ferroelastic domains. Despite being further unveiled coupling of cationic shift with oxygen dis- nonpolar, SrTiO3 receives enduring attention because of its placement along y direction in La-doped PZST (Figure 3i–k). abundant usage as substrate materials. In parallel, it also pos- Being consistent with results of synchrotron X-ray and neutron sesses many fascinating physical properties such as spin-charge diffraction,[109] the wavy polarization orders interpret origin of conversion,[112] superconductivity,[113] magnetism and polarity linear polarization response and non-zero Pr in the P–E loop at ferroelastic domain walls.[29,114] By combing through the lit- of PbZrO3-based systems.[99] Given the group-subgroup rela- erature, one can see that these fascinating properties correlate tionship between FD and FE phase transitions,[74] the findings intimately with one “hidden” physical quantity—FE polariza- broaden the avenue of seeking for candidate energy-storage sys- tion. In fact, its ferroelectricity (Pmax = 1.5 μC cm−2) has been tems through chemical, defect, and strain engineering.[72,85] proved by Weaver in 1959 below Tq ≈ 45 K,[115] which is a classic- to-quantum paraelectric transition temperature. Nowadays, it has been established that through strain engineering,[116,117] its 3.3. Nonpolar-to-Polar Transition in SrTiO3-Based System FE TC can be increased to room temperature (Figure 4a). Other than that, it is reported that excitation of optical pump and ter- SrTiO3 has a cubic structure (space group Pm3m) at room tem- ahertz electric field can also stabilize its metastable ferroelec- perature. With lowering of temperature to T < TC = 105 K, it tricity, with TC up to ≈290 K.[118,119] transforms to a tetragonal phase (I4/mcm), which is featured As for the abnormal ferroelectricity below Tq, one opinion by antiphase rotation of TiO6 octahedra along c axis (a0a0c− in argues that it arises from FE antiphase boundary, which is Adv. Energy Mater. 2022, 12, 2201199 2201199 (5 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 4. Ferroelectric origins and energy storage in SrTiO3-based system. a) Temperature-strain phase diagram of SrTiO3 (STO) pertinent to an AFD transition and then further to a ferroelectric phase. Reproduced with permission.[118] Copyright 2019, AAAS. b,c) Antisite TiSr atom induced polarization along the [001] direction, local 3D polarization profile obtained from the DFT calculations and polarization vector map estimated from atomic displace- ments in the ABF-STEM image. Reproduced with permission.[65] Copyright 2015, AAAS. d) Comparison of remnant polarization (Pr) and maximum polarization (Pmax) between Ti/O-deficient SrTiO3 and the data from literature cited therein. The inset is the calculated unit cell of Ti/O-deficient SrTiO3. Reproduced with permission.[85] Copyright 2021, Wiley-VCH. e,f) P–E loops of Au/STO/(La0.67Sr0.33)MnO3 (LSMO) capacitors under positive field and energy density of Au/STO/LSMO and Au/STO/Pt capacitors under positive and negative fields (UPmax, UNmax) as a function of film thickness at room temperature. Reproduced with permission.[79] Copyright 2017, American Chemical Society. supported by results of Landau theory, first principles calcula- E, the recoverable energy density swaps between 300 and tions and low-temperature scanning stress microscopy.[114,120,121] 150 J cm−3 (energy efficiency η > 85%), which are the highest Another opinion argues that the ferroelectricity is associated values reported so far.[82,125] When the bottom electrode LSMO with an order-disorder transition,[122,123] which relates to the is changed to Pt, a stable Ue with values around 150 J cm−3 emergence of polar rhombohedral clusters from a nonpolar can still be obtained (Figure 4f). In combination with atomic- tetragonal matrix below ≈70 K. Thus, the role of strain is to scale STEM study, Hou et al. argued that inter-diffusion of establish long-range correlation of preexisting polar nano- oxygen vacancies (VO) and electric field modulation near the clusters. Specifically, Jang et al. pointed out that the polar SrTiO3/LSMO interfaces are responsible for the exceptional nanoclusters likely originate from minute amounts of Sr defi- storage performance. However, a dramatic thickness differ- ciency.[124] Using first-principles calculations and ABF-STEM ence, interface (≈5 nm) versus SrTiO3 film (410–710 nm), ques- characterization, Lee et al. demonstrated that structural defects tions the decisive role of the interface only. On the contrary, with a form of atomic antisite,[66,65] that is, Ti on Sr site (TiSr) associated with a metal-to-insulator transition of the LSMO or vice versa, can give rise to a stable room-temperature ferro- electrode, that is, perovskite-to-brownmillerite phase trans- electricity in SrTiO3 even at reduced dimension (Figure 4b,c). formation,[126,127] electrochemical activities relating to migra- Through introducing Ti/O vacancies, Li et al. further showed tion of vacancies and cations via antisite defects and change of that defect-induced nanoregions can trigger an ultrahigh TC,[85] elemental valence states, seem to play an important role in the which is up to ≈1098 K for the cubic-to-tetragonal transition. energy storage and release processes. At room temperature, the tetragonal phase has a very large tetragonality value (c/a = 1.038) and the maximum polarization is up to Pmax = 41.6 μC cm−2 (Figure 4d). Yet, its Pr value is only 3.4. Na0.5Bi0.5TiO3-Based Ferroelectric Ionic Conductors 6.6 μC cm−2 owing to the relaxor nature of the polar clusters. These unique features indicate that defect-engineered Considering the harm of Pb to the environment and human health, SrTiO3 are very suitable for electrostatic energy storage. In lead-free ferroelectrics[128,129] such as BaTiO3 and (K1-xNax)NbO3 Au/SrTiO3/La0.67Sr0.33MnO3 (LSMO) multilayers,[79] Hou et al. are widely investigated to replace Pb-based ones. Because of found that the Pmax can astonishingly rise to ≈120 μC cm−2 having exceptional piezoelectricity and pyroelectricity, relaxor at E = 6.8 MV cm−1 (Figure 4e). Under positive and negative FE Na0.5Bi0.5TiO3 (NBT, Eg = 3.1 eV)[130] serves as an important Adv. Energy Mater. 2022, 12, 2201199 2201199 (6 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 5. Phase diagram, topological structures and energy storage in Na0.5Bi0.5TiO3 (NBT)-based ionic conductors. a) Phase diagram of unpoled (1-x) NBT-xBT with annotation of Vogel–Fulcher freezing temperature (TVF), rhombohedral-to-tetragonal transition (T2), maximum dielectric permittivity (Tm), tetragonal-to-cubic transitions (T1, T″) and onset temperature of in-phase octahedral tilt (T2′). Reproduced with permission.[137] Copyright 2021, American Physical Society. b–d) PFM phase images of chemically (K, Li, Sr, Ta doping) driven domain evolution, flux-closure circinate and hedgehog-like skyrmions and 3D representation of a polar skyrmions in bulk NBT-based ferroelectrics, respectively. Reproduced according to the terms of the CC-BY license.[141] Copyright 2021, The authors, published by Springer Nature. e,f) P–E loops of Au/Mn:NBT-BT-BFO/Pt capacitors on mica substrates and the recoverable energy storage (Ue), efficiency (η), and energy loss (Uloss) under compressive and tensile bending states (r, bent radius), respectively. Reproduced with permission.[143] Copyright 2019, Wiley-VCH. g) Comparison of ionic conductivity of Na0.5Bi0.5TiO2.965 with different kinds of chemical doping in the range of 400–700 °C. Reproduced with permission.[146] Copyright 2018, Elsevier. lead-free system, which also exhibits decent photovoltaic effect classic polarization rotation mechanism,[11,51] which flattens the and photocatalytic activity in water splitting.[131,132] As for its energy profile at the MPB, this provides another insight to under- ground-state structure, the decades-long issue[133] reaches a gen- stand the origin of ultrahigh piezoelectricity in perovskite oxides. eral consensus until affirmation of a monoclinic (Cc) phase,[134,135] Interestingly, by chemically tuning the structural phases, NBT- which arises from Na/Bi positional disorder. By suppressing based ceramics show controllable domain morphologies, ranging this disorder via poling, the average Cc phase irreversibly trans- from labyrinthine, refined stripe to bubble domains (Figure 5b–d). forms into the R3c phase.[136] Apart from the Cc phase, unpoled Using atomic-scale STEM, Yin et al. observed intriguing polar skyr- NBT also experiences several other characteristic phases, for mions structures with either flux-closure circinate or hedgehog- example, coexisting rhombohedral (R3c, a−a−a−) and ordered like geometries in K-Li-Sr-Ta doped NBT bulks.[75,141] Similar tetragonal (P4bm, a0a0c+) phases at T2’ < T < T2, ordered P4bm to topological configurations reported in (PbTiO3)m/(SrTiO3)n phase at T2 < T < T1 (Figure 5a). Interestingly, associated with superlattices,[25] the interplay of bulk, elastic and electrostatic ener- phase coexistence, NBT traverses two characteristic relaxor tem- gies should be responsible for their formation. peratures, a Vogel–Fulcher freezing temperature at TVF ≈ 200 °C It has been known that the pure NBT ceramics have a and a maximum dielectric permittivity at Tm ≈ 350 °C. square-shape P-E hysteresis loop[131] with Pmax ≈ 38 μC cm−2, Through solid solution with other systems, for example, BaTiO3 which can be doubled when grown into thin films.[142] By means (BT), the transition lines further extend in the phase diagram of of chemical modification, for example, dilute Mn doping, (1-x)NBT-xBT, in which a morphotropic phase boundary (MPB) Yang et al., reported that the Pmax and EB can be increased is identified at x ≈ 0.06 compositional point.[137–139] By examining to 113 μC cm−2 and 2.3 MV cm−1 in Mn:NBT-BT-BiFeO3 atomic pair distribution functions and Raman scattering data at (Mn:NBT-BT-BFO) thin films.[143] This gives rise to a very high ambient conditions, Data et al. pointed out that the easy switch- recoverable energy density (Ue = 81.9 J cm−3) with η at a value ability of electric dipoles driven by polarization-strain decoupling of 64.4%. Although being smaller than its record-high values accounts for the enhanced piezoelectricity.[75,140] Apart from the (Ue = 154 J cm−3, η ≈ 97%),[125] the modified NBT-BT system Adv. Energy Mater. 2022, 12, 2201199 2201199 (7 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de exhibits superior frequency stability, broad operating window factors: 1) High absorption coefficient (α > 105 cm−1), 2) ultra- and anti-fatigue property (≈1 × 109 cycles)[143] even under long diffusion length and lifetime of carriers, 3) moderate carrier bending states (Figure 5e,f ). Some figures of merit are valid mobility, 4) high defect tolerance and slow carrier recombina- in NBT-based bulk capacitors as well.[144,145] Transport property tion.[154] By means of interface engineering, the performance of study indicates that the excellent electrical performances cor- HOIP solar cells has achieved a PCE of 25.6% and an opera- relate intimately with the conductivity of NBT. On one hand, tional stability up to 1400 h.[161,162] In spite of this, debates about being sensitive to BiO vacancies, Bhattacharyya et al. showed origin of the exceptional photovoltaic effect and nature of the that vacancy-doped NBT delivers an ultrahigh ionic conduc- charge carriers remain, which call for an in-depth mechanistic tivity ≈14.3 S cm−1 at 600 °C in grains (Figure 5g), which understanding about the carrier-lattice interaction,[163,164] polaron is three times higher than that of 8 mol% Y2O3 stabilized formation,[165,166] role of lattice polarization in photo-excited elec- ZrO2 (8YSZ) and is similar to that of 1CeO2-9Sc2O3-90ZrO2 tron–hole pairs and their reduced recombination, etc.[160,167–169] (1Ce9ScSZ).[146] On the other hand, the polar topological struc- Methylammonium lead halide MAPbI3 (MA = CH3NH3+, tures offer a pathway of enhancing its electronic conductivity, bandgap Eg ≈ 1.6 eV) is one of the representatives in the family as evidenced in ferroelectrics like BiFeO3.[147,148] Therefore, of HOIPs. With lowering of temperature,[170,171] X-ray diffraction referring to the superior anti-fatigue performance, one can study reveals that it undergoes a cubic-to-tetragonal (Pm3m see that the synergistic ionic and electronic conductivity yields to I4cm) transition at 327 K and a tetragonal-to-orthorhombic close-to-ideal interfacial polarization screening.[149–151] Similar (I4cm to Pna21) transition at 165 K (Figure 6a). With lattices to Au/SrTiO3/LSMO capacitors, this further implies that the elongated by 0.5% along c axis, the non-centrosymmetric improved energy storage performances are assisted by elec- tetragonal phase is characteristic of slightly twisted PbI6 octa- tric-field driven electrochemical activities. hedrons.[172] Specifically, the Rayleigh response persisting up to ≈348 K implies that the MAPbI3 is a relaxor FE.[173] Associated with orienting polarization into large domains by large poling 4. Solar Energy Harvesting field (>16 V cm−1), results of scanning microwave impedance microscopy (sMIM) measured at 200 kHz and contact Kelvin 4.1. Energy Harvesting Mechanism probe force microscopy (cKPFM)[173] further verify its FE polari- zation and switchable nature (Figure 6b). This is supported Solar energy is the most abundant energy resource among var- by observation of frequency dependent polarization behavior ious ones and its power that continuously strikes the Earth is in PFM and P–E loop measurements.[174] Single-crystal X-ray more than 10 000 times of the world’s total energy use. A solar cell diffraction reveals that polar distortion of the PbI6 octahedra directly converts the energy of visible light into electricity through (Figure 6c), with off-center Pb shift, is responsible for presence a photovoltaic effect, where charge carriers are excited to higher of the intrinsic ferroelectricity.[100,175,176] energy states of the materials. To realize this, three key physical The octahedral distortion driven FE origin is consistent steps should take place: 1) Generating electron–hole pairs or exci- with the robust ferroelectricity (PS = 16.13 µC cm−2) identified tons through absorption of light; 2) separation of charge carriers in N(CH3)4SnI3, in which the N(CH3)4+ cation is nonpolar,[177] of opposite types; 3) extraction of separated carriers to an external and absence of long-lived (A)FE domains in HOIPs with polar circuit. Ideally, a key functional material should have a bandgap but dynamically disordered A-site cations[178,179] (2.29 Debye for of Eg = 1–2 eV to maximize the photon absorption. For this the MA+ cation[167]). One should note that the out-of-phase octa- reason, crystalline and amorphous Si, GaAs, and metal halide hedral rotations couple with the orientation of A-site cations, perovskites[152,153] have been investigated for device industrializa- leading to instantaneous symmetry-broken nanostructures tion. For a solar cell, power conversion efficiency (PCE) is a core (1–3 nm) at room temperature and above.[180] By strengthening parameter to evaluate its performance, which is affected by fac- the corporative coupling, the ferroelectricity is further enhanced tors like[154] reflectance, quantum efficiency, separation, and col- in 2D Ruddlesden–Popper-type HOIPs,[181,182] for example, lection efficiency of charge carriers. Given the natural attributes BA2PbCl4 (space group Pb21m, Eg = 3.32 eV), which has vdW in separating electrons and holes, ferroelectrics are constantly interlayer bonding and its polarization is ≈8 μC cm−2 along b investigated to improve the PCE and open-circuit voltage (VOC) axis (Figure 6d). Similar to FE BaTiO3 and LiTaO3,[64] the 2D in the past decade.[155–157] Nowadays, hybrid organic–inorganic FE HOIPs also exhibit coexisting order-disorder and displacive perovskites (HOIPs) have become the research focus due to their features owing to ordering of organic dipoles and Pb off-center ideal optical property, ease of fabrication and low cost. displacements. Although the poor charge transport and narrow absorption band[183] constrain the 2D HOIPs (PCEmax = 19.3%)[184] from being ideal solar cells, their superior stability in air and 4.2. Hybrid Organic–Inorganic Perovskites for Solar Cells light and strengthened ferroelectricity present more insights to comprehend performance of the 3D ones and design of more Ever since 2009, lead halide perovskites have become a forefront stable solar cells via 2D/3D material integration.[185] of photovoltaics and optoelectronics research due to their superb Coupled with structural asymmetry, the Rashba splitting optoelectronic properties.[158,159] Accordingly, interaction of lat- further supports the role of FE polarization in sunlight har- tice instabilities pertinent to the emergence of ferroelectricity vesting. Under excitation of polarized light, Liu et al. observed has been hotly debated owing to their structural similarity with spin-dependent photocurrent in 2D-PEPI single crystal using other FE perovskites.[74,160] So far, it is generally accepted that the terahertz transient emission spectroscopy.[186,187] The circular exceptional PCE and high VOC are attributed to the following photogalvanic effect not only confirms the interband excitation, Adv. Energy Mater. 2022, 12, 2201199 2201199 (8 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 6. Ferroelectric polarization in HOIPs and device application. a) Crystal structures, temperature dependent phase transitions and Rayleigh response in MAPbI3. b) sMIM (dC/dV) amplitude (left) and phase (right) images of MAPbI3 showing increased and decreased signals within the posi- tive domain. Reproduced with permission.[173] Copyright 2019, AAAS. c) Illustration of the highest residual electron density peaks in the 2 I-site model (dark blue dots) and the PbI6 octahedron of MAPbI3.[175] d) Out-of-phase rotation of the BA = C6H5CH2NH3+ cation and PbCl6 octahedra as well as side view of the parallel aligned BA cations in 2D BA2PbI4. Reproduced with permission.[181] Copyright 2021, American Chemical Society. e) Schematic Rashba splitting induced by spin-orbit coupling and related optical transitions with circularly polarized light. Reproduced according to the terms of the CC-BY license.[186] Copyright 2020, The authors, published by Springer Nature. f) Ferroelectric photovoltaic measurement with different electrode and polari- zation configurations for the 2D (4,4-difluorocyclohexylammonium)2PbI4 (DFCHA2PbI4). Reproduced with permission.[193] Copyright 2019, Wiley-VCH. but also inversion symmetry breaking along out-of-plane direc- 4.3. Ferrielectric Ionic Conductor CuInP2S6 tion of the 2D HOIP (Figure 6e). This implies that the carrier lifetime is enhanced by Rashba spin-orbit coupling in the 3D The study of 2D FE materials starts from exploring the thick- HOIPs.[188–190] To a large degree, these structural and elec- ness limit of ordinary ferroelectrics, for example, BaTiO3 and trical features resemble that of relaxor FE SrTiO3.[112,119,124,191] PbTiO3.[197–199] Owing to the intrinsic size, surface and boundary To examine the role of polarization in enhancing the PCE, effects, properties of 2D ferroelectrics can differ significantly Xu et al. presented a typical example through introducing from that of bulks.[56,149,200] Stimulated by discovery of mono- homochical FE molecular into (FA0.95MA0.05)Pb(Br0.1I0.9)3 layer graphene, investigation of vdW-type 2D ferroelectrics[201] (FA = [CH(NH2)2]+). Associated with enlarging the built-in such as SnTe, d1T-MoS2, WO2Cl2, ReWCl6, and BA2PbCl4 has field, effective separation and transportation of charge car- become a hot research topic[38] due to their diverse proper- riers, they found that the PCE can be increased from 18.28% ties and potential device applications.[193,202,203] This includes to 21.78%.[192] Although the 2D HOIPs are non-ideal candidates piezoelectric and photovoltaic effects, field-effect transistors, for solar energy harvesting,[193–195] their decent optoelectronic photocatalysis, etc. One should bear in mind that owing to responses make them usable in other electronic devices. Taking the presence of vdW interfaces, “quantized” electric dipoles a fluorinated layered hybrid perovskite (4,4-difluorocyclohex emerge within the isolated 2D FE layers. Thus, synergistic con- ylammonium)2PbI4 (space group Cmc21, Eg = 2.38 eV) as an tributions of local charge redistribution, ionic displacement example, Sha et al. showed that depending on the polarization- and even vacancy location play important roles in generating electrode configuration, the collected photocurrent exhibits ferroelectricity.[69,70,204,205] polarization-dependent anisotropy (Figure 6f). This indicates Ferrielectric CuInP2S6, with TC = 315 K and Eg = 1.503 eV,[206] that the 2D FE HOIPs can act as an anisotropic photovoltaic is a representative 2D vdW material for exploration of unusual absorber and be used for optoelectronic sensors. Triggered by ferroelectricity and multi-functional device applications.[207,208] interfacial in-plane polarization, similar effect is also observed In structure, Cu and In atoms undergo opposite displacements in heterogeneous WSe2/black phosphorous bilayer.[196] within each layer along z direction and paired P–P atoms fill in Adv. Energy Mater. 2022, 12, 2201199 2201199 (9 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 7. Crystal structure, energy landscape, and device application of CuInP2S6 (CIPS). a) AFM image of the CIPS flakes with different thicknesses. Reproduced according to the terms of the CC-BY license.[209] Copyright 2016, The authors, published by Springer Nature. b) DFT calculated structure of CIPS corresponding to the first and second local energy minimum. c) Energy versus polarization relation as a function of Cu displacement per formula unit (f.u.) at c = 13.09 Å. d,e) Piezoelectric constant map and hysteresis loop as function of d.c. bias voltage averaged over a 3 × 3-point grid within the two domains (CIPS 1 and CIPS 2) measured by PFM. Reproduced with permission.[210] Copyright 2020, Springer Nature. f) AFM topography of CIPS flakes ranging from 6 to 25 nm on doped Si substrate and corresponding potential map change by heating the sample from 298 to 318 K. Reproduced with permission.[222] Copyright 2019, Elsevier. g,h) Schematic 2D device structure and the characteristic output I–V curves under different light illumination power (Pin). Reproduced according to the terms of the CC-BY license.[216] Copyright 2021, The authors, published by Springer Nature. the sulfur cages (Figure 7a,b). Using PFM and second-harmonic is two orders of magnitude higher than that of its bulk, FE generation, Liu et al. demonstrated a switchable polarization BiFeO3 and BaTiO3.[216,217] Furthermore, the photocurrent direc- in ≈4 nm thick CuInP2S6 with an on/off ratio of ≈100 memory tion, up to 9.6 mA cm−2 at a light illumination power of 2 µW, behavior.[209] By combining theoretical calculations and bias- is switchable by reversing the polarization direction (Figure 7h). dependent PFM experiments,[210] Brehm et al. found that distinct These results exemplify that CuInP2S6-based vdW heterostruc- from conventional proper and improper ferroelectrics, the vdW tures show great potential to convert thermal and solar ener- gaps enable a uniaxial quadruple potential well, which corre- gies into electricity. These facts imply that analogous to the FE sponds to two pairs of stable polarization values with P3 = ± 4.93 Na0.5Bi0.5TiO3, the coupling of polarization with ionic conduc- and ± 11.26 μC cm−2 (Figure 7c). As illustrated in FE polymer tivity plays an important role in boosting the energy harvesting poly(vinylidene fluoride),[211] the unique potential energy land- and conversion. Specifically, given the inseparable FE and elec- scape for Cu displacements, strongly sensitive to changes of trochemical states on its surfaces, which are termed as a fer- lattice constant, is responsible for its negative piezoelectric coeffi- roionic state,[218–220] the diverse coupling effects show prospects cient.[212] Specifically, switching spectroscopy experiments clearly not only in suppressing recombination of photo-generated elec- demonstrate that the two high-polarization (HP) states can indi- tron–hole pairs, but also in spontaneous solar-to-fuel conver- vidually switch to the low-polarization (LP) states, that is, from sion via a photocatalytic CO2 reduction reaction (CO2RR).[221] +HP to -LP and from −HP to +LP as presented in Figure 7d,e. On the transport aspect, the ferrielectric CuInP2S6 also exhibits superior ionic conductivity[213] due to the anisotropic 5. Electrochemical Energy Conversion hopping of Cu ions. Based on scanning probe microscopy (SPM) measurements, Zhang et al. revealed that the Cu ions 5.1. Ferroelectric-Based Catalysis undergo a two-step transition process, from an in-plane intra- layer hopping to an out-of-plane interlayer hopping across the Given the challenge of supplying constant energy from fluc- vdW gaps.[214,215] As a result, it appears that the polarization tuating sources,[223,224] the use of renewable energy to power aligns against the applied electric field as Cu migrates along c the conversion of simple molecules (e.g., H2O and CO2) into axis. Using Kelvin probe force microscopy (KPFM), Niu et al. valuable fuels and chemicals (e.g., H2, O2, ethylene, ethanol) showed that ultrathin CuInP2S6 nanoflakes (even down to becomes a promising strategy toward a sustainable society.[225,226] bilayer) can efficiently convert surface pyroelectric charges into However, many technologies are not yet industrially feasible due electrical current as temperature changes (Figure 7f). When to the lack of robust catalysts with good performance and sta- sandwiched by graphene electrodes (Figure 7g), Li et al. found bility. Electrocatalysis is an energy conversion process between that the photovoltaic effect of 2D CuInP2S6 (thickness < 80 nm) electrical and chemical energies, which occurs in an electrolyzer Adv. Energy Mater. 2022, 12, 2201199 2201199 (10 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de consisting of a cathode, an anode and an acidic or alkaline elec- their counterpart, FE insulators. To remedy the property dis- trolyte solution. Driven by an external power supply, chemical parity, one primary consideration resides in removal of inver- reactions take place at catalyst/reactant interfaces, where the sion symmetry through displacements of atoms, whose elec- catalysts are connected with electrodes, electron transfer and tronic degrees of freedom are decoupled from the states at exchange participate in the energy conversion process.[227] Due the Fermi level. Guided by this strategy, Puggioni and Ron- to the strong coupling of polarization to external fields and sur- dinelli[246] generalized three routes to lift inversion symmetry face chemistry, FE surfaces offer exciting opportunities for the in metallic compounds: 1) Compositional order; 2) packing of study of electrochemistry and catalytic processes such as pyro- acentric polyhedra; 3) geometric-induced displacements. For catalysis, piezocatalysis or flexocatalysis and ferrocatalysis.[228,229] the 2D metal tellurides, its metallic ferroelectricity should arise At the catalyst/reactant reaction interface, a strong ion versus from the latter two routes. To be precise, first-principles calcula- solvent molecule interaction gives rise to a region called elec- tions show that its ferroelectricity stems from uncompensated trical double layer (EDL),[230] which is comprised of an array of interlayer charge transfer,[71,247] where its in-plane direction is charged species and/or oriented dipoles near the interface.[231] metallic but the electrons are confined vertically. Usually, the dipolar interfacial structure can be divided into five By sandwiching electrically contacted thin WTe2 flakes using distinct regions: 1) the electrode surface, 2) the inner Helm- dielectric BN sheets (Figure 8d,e), Fei et al. explicitly demon- holtz plane (IHP), 3) the outer Helmholtz plane (OHP), 4) the strated the FE switching via bistable conductance in bilayer and diffuse layer, and 5) the bulk solution. The total thickness is in trilayer WTe2.[248] Microscopically, the polarization reversal can the range of 0.1–10 nm. As is known, heterogeneous catalysis be attributed to re-distribution of electrons and holes and even on surfaces is limited by the Sabatier principle that entangles interlayer sliding.[69,249] The switchable polarization states are fur- the catalytic efficiency to the optimal strength of the adsorbate- ther confirmed by PFM experiments carried out on the 1T′-WTe2 surface interaction. That is to say, the interaction has to be thin films[250] (Figure 8f,g). Distinct from cycle-dependent strong enough to drive the reaction but weak enough to permit activity decay in ordinary catalysts,[251] the HER performance of desorption of the products.[232] Thus, the catalytic efficiency is 1T′-MoTe2 is dramatically improved as the electrode is held at a capped for each surface and reaction, and the search for a cata- cathodic bias,[252] where its over-potential decreases from 320 to lyst with the optimal active site for a given chemical reaction 178 mV at a current density of 10 mA cm−2 (Figure 8h). This has been the central topic of research, which lasts for almost a reveals that associated with adsorption of H onto surface of the century. Propelled by the limited reserves of noble metal cata- Te sites, the HER activity of 1T′-MoTe2[252] is greatly boosted by lysts, for example, Pt and Ir, investigation of their alternatives partial filling of d-bands near the EF and charge-to-spin conver- like carbides, sulfides, nitrides, and phosphides of transition sion.[253] Obviously, the FE metals with electrically-tunable elec- metals offers a plethora of opportunities to explore the impact tronic states serve as a novel template for probing efficient elec- of novel electronic states on the electrochemical process.[233] trocatalysis and electrochemical energy conversion.[202,254] 5.2. Ferroelectric Metal WTe2 5.3. Topological Ferroelectric Metal Ni2P In the family of metal chacolgenides, sulfides and selenides It is known that the performance of catalysts highly depends (e.g., MoS2 and WSe2) exhibit superior activity and efficiency on their morphology, for example, size, shape, crystal facet, and in hydrogen evolution reaction (HER).[233] Despite with infe- composition. At the same time, the types and attributes of the rior HER performance, metal tellurides such as 1T′-WTe2 and supporting materials also play important roles in influencing 1T′-MoTe2 broaden our vision to comprehend the intricate their activity and selectivity.[255] For an elemental catalyst, con- electrochemical process[234,235] due to their exotic nonlinear figuration of the reaction interface seems to be simple. Taking anomalous Hall effect[236] and unconventional charge-to-spin Au for HER as an example, the EDL geometry exhibits layer- conversion.[237] In structure, both 1T′-WTe2 and 1T′-MoTe2 dependent orientation discrepancy of dipolar H2O molecules (space group Pmn21)[238–240] have vdW-type interlayer bonding on the solution side (Figure 9a,b), which collectively vary from and belong to type-II Weyl semimetals. In momentum one-H-down to two-H-down configurations with increasing of space, the Weyl fermions emerge at the boundary between the electric potential.[256,257] While for a compound-type cata- tilted electron and hole pockets near the Fermi energy (EF) lyst, the dynamic charge transfer process is further complicated (Figure 8a,b).[241] Along c axis, breaking of structural inversion by physical property of the catalyst itself, intercalation versus symmetry simultaneously gives rise to an out-of-plane polari- de-intercalation of certain atomic species near the catalyst sur- zation. This tells that both of them are FE metals, which are face[258] and interfacial contact with the support.[259] a concept proposed by Anderson in 1965 and defined by coex- Transition-metal phosphides such as Ni5P4 and Fe0.5Co0.5P isting ferroelectricity with metallicity.[33–35] Albeit different in are promising catalysts owing to their high activity and multi- origin, the exotic electronic state is reminiscent of bent band functionality in catalyzing chemical reactions.[260,261] Taking Ni2P structure caused by charged domain walls in ordinary ferroelec- as an example, it is highly efficient for HER,[262] oxygen evolu- trics,[242–244] where the band bending across the EF leads to spa- tion reaction (OER),[263] CO2RR,[264] and hydrodesulrization.[265] tially separated 2D electron and hole gases (Figure 8c). The nonpolar Ni2P (space group P62m) has a non-centrosym- As elaborated before, ferroelectricity stems from long- metric structure due to the 6 rotational symmetry along c axis range electrostatic forces, which can be annihilated by free (Figure 9c). In combination with valence state measurement electrons.[245] Thus, FE metals are very rare compared with using electron energy loss spectroscopy (EELS), Wei et al. found Adv. Energy Mater. 2022, 12, 2201199 2201199 (11 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 8. Ferroelectric switching in semimetal 1T′-WTe2 and electron doping enhanced HER in 1T′-MoTe2. a) Structure of 1T′-WTe2 with annotation of the glide plane (G) and polarization (red, green arrows) along c axis (W—blue, Te—orange). b) Dispersions of Type-II Weyl fermion with electron and hole pockets touching at two different energies near the Fermi level (EF). Reproduced with permission.[241] Copyright 2016, Springer Nature. c) Charged planar domain walls (⊕/⊖: bound charges, +, −: free charges) and depolarization field (Edep) induced band structure bending in ordinary ferroelec- trics.[243] d) Optical image (up) and schematic cross section of the device geometry used to apply a vertical electric field E⊥ normal to the WTe2 flake. e) Temperature dependent conductance Ggr of bilayer WTe2 measured when a bias Vb is applied to the bottom gate and sweeping the E⊥. Reproduced with permission.[248] Copyright 2018, Spring Nature. f,g) PFM phase images of WTe2 before and after application of a bias pulse of −2.5 V, respectively. Reproduced according to the terms of the CC-BY license.[250] Copyright 2019, The authors, published by AAAS. h) Current densities achieved before and after 100-cycle HER tests in 1 m H2SO4 for nanocrystalline 1T′-MoTe2. Reproduced according to the terms of the CC-BY license.[252] Copyright 2019, The authors, published by Springer Nature. that the paramagnetic Ni2P belongs to topological FE metal.[266] of normal lattice vibration, that is., ≈1013 Hz, which should With a winding number n = 1, the novel topological state is char- be weakly dependent on the applied voltage. Such a dynamic acterized by coupling of switchable in-plane polyhedral polariza- boosting mechanism is supported by a number of theoretical tion, elemental valence states, Ni atomic sites and spin polari- and experimental findings. zation, where the spin splitting is in the range of 20–100 meV Based on first-principles calculations, Lan et al. predicted that (Figure 9d,e). Since a number of catalysts[267–269] (e.g., Fe2P and for clean (001) and OH-covered surfaces (Figure 9h,i), the over- Ni3S2) exhibit similar structural features, this implies that their potential (ηop) values of improper FE InSnO2N can drastically bulky physical properties can be further understood from the reduce from 0.58 V (downward P) and 0.77 V (upward P) to 0.20 viewpoint of polyhedral polar topology (Figure 9f,g). and 0.23 V, respectively, as the polarization is dynamically switched It is interesting to note that apart from the possible active during the OER reaction.[277] These values are far below the min- sites for OER, for example, hydroxide/oxyhydroxide spe- imum theoretical one of oxide catalysts (ηop = 0.37 V).[278] On the cies and eg-electron occupancy,[270–272] it is indicated that the experimental side, by implementing high-frequency mechanical ultrafast polarization switching dynamics and spin-polarized vibration,[279] the ultrafast polarization reversal successfully trans- kinetics[112,273–275] also contribute a lot to the water oxidation. forms the insulating BaTiO3 and BiFeO3 into excellent water split- For metallic Ni2P, only ≈5 pm Ni displacements can transform ting catalysts.[44,280] Using ambient pressure X-ray photoelectron a center-convergent topological geometry to a center-divergent spectroscopy,[281–283] Domingo et al. have demonstrated different one (Figure 9d). With consideration of atomic Debye-Waller water affinity, dynamics of water adsorption, dissociation and oxi- 8π 2 2 dation on surfaces of Pb(Zr0.2Ti0.8)O3, LiNbO3 and BaTiO3. There- factors,[276] B = u (〈u 〉 is mean squared displacement), 2 3 fore, as an external agent to modulate surface electrochemical the much larger Ni displacement in the range of 11–16 pm properties, the use of FE polarization opens a route to switchable implies that the topological transition takes place at a frequency catalysis and offers new opportunities to overcome the Sabatier Adv. Energy Mater. 2022, 12, 2201199 2201199 (12 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 9. Impact of switchable polarization on (photo)electrochemical water splitting. a) Illustration of an EDL model and associated outer Helmholtz plane (OHP) under electric field (E). b) Potential-dependent evolution of hydrogen-bonded network of interfacial H2O molecules from calculation (Ndonor, number of H-bond donors) and experimental Raman frequency νO-H (blue circle). Reproduced with permission.[256] Copyright 2019, Springer Nature. c,d) Tail-to-tail polyhedral polarity in Ni2P and in-plane topological polarization configuration (center-convergent vs center-divergent, n = 1) as a function of mean <δNi> and <δP> for two structural data sets, respectively. e) In-plane compressive strain (a/a0 = 1.0, 0.95) induced Fermi surface change on a plane in the Brillouin zone (Blue—spin up, red—spin down along z axis). Reproduced according to the terms of the CC-BY license.[266] Copyright 2020, The authors, published by Wiley-VCH. f,g) 2D topological point defects about polarization or spin with n = 1 and −1, respectively. Reproduced with permission.[286] Copyright 1979, American Physical Society. h,i) Polarization switching and Gibbs free energy diagram of OER steps for clean (001) surface of improper ferroelectric InSnO2N, respectively. Reproduced with permission.[277] Copyright 2021, American Chemical Society. limitations with consideration of issues about surface adsorption, and NiP2) for unprecedented selectivity of C3 and C4 oxyhydro- charge carrier separation, redox reactions, etc.[284,285] carbon products,[264] single-atom-catalyst (SAC) Zn for highly efficient CH4 conversion.[289] Apart from these, FE materials such as SrBi4Ti4O15, Bi2MoO6, and CuInP2S6 have also been 5.4. vdW Ferroelectric α-In2Se3 used for CO2 photocatalytic reduction given their tunable activity and selectivity.[221,290,291] Besides (photo)electrochemical water splitting,[287] electrochem- Among various layered vdW ferroelectrics,[38], for example, ical CO2RR provides another pathway to convert electric energy d1T-MoS2, GeSn, and CrN, a class of III2-VI3 semiconductors into chemical fuels, for example, methane (CH4), ethylene represented by In2Se3 have attracted a great deal of attention (C2H4), methanol (CH3OH), and ethanol (C2H5OH).[226,288] It is due to its exceptional optical properties.[292,293] The In2Se3 known that Cu-based materials are efficient catalysts for CO2RR. is polymorphic with many structural phases (α′, α, β′, β, γ, However, they suffer from issues like low activity and selec- δ, κ),[294] in which the α, β, and β′ phases are of vdW type and tivity, insufficient Faradaic efficiency and poor durability.[226] are differed by 2H, 3R, and 1T stacking on the [Se-In-Se-In-Se] For this reason, many efforts have been made to develop new quintuple layers (QLs).[295] Specifically, the α-In2Se3 phase catalysts such as Ag-based catalysts for high-selectivity conver- (space group R3m) with a ZB’ structure has aroused intensive sion of butanol and hexanol,[288] nickel phosphides (Ni2P, Ni5P4, discussions on its intriguing FE property[296–298] and potential Adv. Energy Mater. 2022, 12, 2201199 2201199 (13 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Figure 10. Ferroelectric switching and controllable CO2 reduction on single atom anchored α-In2Se3. a) Crystal structure of 2D In2Se3 quintuple layers (QLs) linked by van der Waals gaps along vertical z direction. Each type of atom arranges only in one of the triangular (A, B, or C) lattices. b) Effective kinetic pathway of polarization reversal in one QL In2Se3 (ZB′: stabilized zincblende (ZB) structure) through a metastable face-cantered cubic (fcc′) phase. Reproduced with permission.[301] Copyright 2018, American Chemical Society. c) Out-of-plane (OOP) and in-plane (IP) PFM phase images of 6 nm thick In2Se3 flake acquired consecutively at −7 and +6 V voltages. Reproduced with permission.[302] Copyright 2018, American Chemical Society. d–g) Differential charge density plots (In-purple, Se-green) and partial density of states of CO2 adsorbed on Pd@P↓-In2Se3 (d,e) and Pd@P↑-In2Se3 (f,g), respectively. Reproduced according to the terms of the CC-BY license.[306] Copyright 2021, The authors, published by Springer Nature. device application.[299,300] Distinct from the displacive and used for energy harvesting, storage, and conversion. From the order-disorder ferroelectrics, the polarization in α-In2Se3 is typical perovskite oxides, one can see that the polarization-related reversed through synergistic lateral shifts of top three atomic phenomenon has conspicuously extended to HOIPs, 2D vdW- layers in one QL, which experiences a metastable face-centered layered materials, metals, ionic conductors, etc. The physics cubic (fcc’) phase[301] and the energy barrier is about 66 meV underpinning these phenomena involves the intricate interplay (Figure 10a–c). As corroborated by PFM experiments, the of polarization with geometric coordination, defect, electron cor- 2D α-In2Se3 exhibit inter-correlated in-plane and out-of-plane relation and spin state under either static or dynamic conditions. polarization components.[302,303] Towards building a clearer structure-property relationship and Because of having large surface-to-volume ratio, short carrier maximizing performance of the materials, several guidelines for diffusion length and unique electronic properties, 2D mate- carrying out FE energy research are generalized as follows. rials serve as promising substrates for SACs.[304,305] Inspired by the ideal surface geometry, Ju et al. predicted that the cata- 1) As exemplified by PbZrO3-, SrTiO3-, and NBT-BT-based sys- lytic activity and selectivity of transition-metal atoms anchored tems, exploring cross-scale characterization and transient α-In2Se3 (TM@In2Se3, TM = Ni, Pd, Rh, Nb, or Re) used for structural transitions under in-situ biasing conditions[100] CO2RR can be controlled by FE switching through adjusting the represent two significant directions for dielectric capacitor d-band center and occupation of supported metal atoms.[306,307] research. Although the physical processes dominate, electro- By forming bidentate C-TM-O species, the strong hybridization chemical activities relating to ionic migration via defects and of O p and TM d orbitals and charge transfer ensure chemical interfaces[126] should play important roles in improving the capture of the inert CO2 on the TM@In2Se3 surfaces. The acti- storage performance of the dielectric capacitors. vation degrees on opposite FE surfaces are manifested by dif- 2) Order-disorder transition of A-site organic cations,[308] built- ferent ∠OCO angles of the CO2 molecules (Figure 10d,e). Spe- in field effect[309] and charge-to-spin conversion in 3D HOIPs cifically, the switchable polarization effectively modulates the d play essential roles in affecting the electron-hole separation orbital occupation of the TM atoms, which further control the and carrier lifetime in solar cells. In conjunction with anni- reaction barrier, reaction path, and even the intermediate/final hilation of structural defects by material engineering,[310] it is product of CO2RR (Figure 10f,g). believed that unraveling the structural mysteries of HOIPs under working conditions using atomically-resolved cryo- TEM[311] can help to further improve their PEC and stability. 6. Summary and Outlook 3) Case studies about 1T′-WTe2 and Ni2P highlight that be- sides the critical surface structures, understanding the in- In conclusion, we reviewed the origin of ferroelectricity and sum- herent bulky attributes of the transition-metal catalysts[312] marized the latest research progress about novel FE materials also play important roles in probing their energy conversion Adv. Energy Mater. 2022, 12, 2201199 2201199 (14 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de mechanisms. Although aspects like spin-to-charge conversion Keywords can hardly be accessed during the reaction, adoption of oper- capacitors, CO2 reduction, ferroelectric energy materials, solar cells, ando scanning transmission X-ray microscopy (STXM)[258] water splitting and cryo-STEM promise to unveil more concrete in-situ in- terface information about crystal, chemical and electronic Received: April 8, 2022 structures. 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Meledin, J. Mayer, U. Englert, [319] F. Yang, M. Li, L. Li, P. Wu, E. Pradal-Velázquez, D. C. Sinclair, M. Wuttig, R. Dronskowski, Inorg. Chem. 2018, 57, 11775. J. Mater. Chem. A 2018, 6, 5243. Xian-Kui Wei is a staff scientist at Ernst Ruska-Centre (ER-C) for Microscopy and Spectroscopy with Electrons in Forschungszentrum Jülich, Germany. After receiving his Ph.D. degree from Institute of Physics, Chinese Academy of Sciences in 2011, he worked as a collaborator scientist/ postdoc in Swiss Federal Institute of Technology Lausanne (EPFL) until 2016. His research interest focuses on atomic-scale light-element imaging and polar ultramicroscopy structure of ferroelectric oxides, catalysts, batteries, topological insulators as well as structure–property relationship. Adv. Energy Mater. 2022, 12, 2201199 2201199 (20 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Neus Domingo has been CSIC Distinguished Researcher at the Catalan Institute of Nanoscience and Nanotechnology (ICN2) in Bellaterra (Spain), where she led the Advanced Atomic Force Microscopy Platform from 2016 to 2021, focusing her research on the physical chemistry of ferroelectric and polar surfaces. She received the Ph.D. degree from the Fundamental Physics Department of the University of Barcelona in 2005, and after a postdoctoral fellowship at the CNR in Italy and several research positions at CIN2 and ICN2 in Spain, she just joined the Oak Ridge National Laboratory (USA) in January 2022, where she is now the group leader of the Functional Atomic Force Microscopy group of the CNMS. Young Sun is a full professor at the Center of Quantum Materials and Devices and the Department of Applied Physics in Chongqing University, People’s Republic of China. He received Ph.D. degree in the Department of Physics, University of Science and Technology of China in 2001. After a postdoctoral research associate in University of Illinois at Urbana-Champaign and Rice University, he joined the Institute of Physics, Chinese Academy of Sciences in 2004 and moved to Chongqing University in 2021. His research interests focus on the physics and applications of multiferroic materials and magnetoelectric effects. Nina Balke is an associate professor in the Materials Science and Engineering Department and the Director of the Analytical Instrumentation Facility at North Carolina State University (USA). She received her Ph.D. in Materials Science from the Technological University in Darmstadt Germany in 2006. Her research interest focuses on nanoscale materials’ functionality driven by electric fields for information and energy technology. This includes the study of electromechanical, mechanical and transport phenomena in inorganic materials and across fluid-solid interfaces. Rafal E. Dunin-Borkowski is Director of the Ernst RuskaCentre for Microscopy and Spectroscopy with Electrons (ER-C) and Institute for Microstructure Research (PGI-5) in Forschungszentrum Jülich and professor of Experimental Physics in RWTH Aachen University. His research involves the development of quantitative techniques in electron microscopy and has recently focused on the use of electron holography to study electromagnetic fields in nanoscale materials, thin films and devices. In 2009 he was awarded the Ernst Ruska Prize of the German Society for Electron Microscopy. In 2012 he was awarded an Advanced Grant by the European Research Council. Adv. Energy Mater. 2022, 12, 2201199 2201199 (21 of 22) © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2022, 24, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202201199 by Spanish Cochrane National Provision (Ministerio de Sanidad), Wiley Online Library on [17/10/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advenergymat.de Joachim Mayer received his Ph.D. in Physics at the Max-Planck-Institut für Metallforschung, Stuttgart, Germany. In 1988 he joined the Materials Department at the University of California, Santa Barbara, as a postdoctoral research associate. In 1990 he moved back to the Max-Planck-Institut für Metallforschung, Stuttgart, where he worked as a research scientist and Group Leader “Analytical Electron Microscopy.” In 1999 he joined RWTH Aachen University to become professor and Head of the Central Facility for Electron Microscopy of RWTH Aachen. In 2004, he received a co-appointment as one of the two directors of the Ernst Ruska-Centre at Research Centre Juelich. Adv. Energy Mater. 2022, 12, 2201199 2201199 (22 of 22) © 2022 The Authors. 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