Abstract A strong societal and political drive is motivating the development and optimization of novel energy conversion and storage systems for decarbonization. The successful implementation of solid state devices such as fuel cells and secondary batteries depends, however, on achieving ambitious targets in terms of performance, reliability and cost competitiveness. Research and technology are addressing these needs through a holistic approach including exploration of new materials and nanoarchitectures, as well as system engineering. These significant efforts require the support of appropriate characterization tools capable of assessing nanometer-scale phenomena such as concentration profiles of ionic and electronic charges, local chemical compositions and their evolution over time across interfaces. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real-time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2025 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications. This roadmap provides an overview of selected advanced characterization techniques for energy materials and devices. Specific focus is put on in situ/operando methods for probing electrochemical phenomena in real time under realistic working conditions. Experts in the field provide an extensive review of the current state of the art in 2024 and the current and future challenges for the characterization of local chemistry and kinetics in the bulk of the material, in nanoarchitectures (e.g. thin films) and at the interfaces (e.g. grain boundaries, phase contacts, solid/liquid and solid/gas interfaces) . The aim is to provide a detailed guide to the techniques, describing opportunities and bottlenecks for their practical deployment and examples of successful applications.

Electrochromic devices (ECDs) offer a compelling route toward low-power, non-emissive optical modulators with nonvolatile states. However, their widespread implementation is hindered by limitations in operating voltage, switching speed, color tunability, and long-term stability. Mixed ionic-electronic conductors (MIECs) provide a promising alternative platform, enabling optical modulation through ion-driven redox and structural transformations. Oxygen-based MIECs offer enhanced durability, environmental robustness, and compatibility with oxide electronics and silicon photonics, yet remain largely underexplored for electrochromic and photonic applications. Here, we demonstrate structure-driven analog optical control in an ion-pumped SrFeO_3-δ thin-film device by undergoing reversible oxygen-driven phase transitions between brownmillerite and perovskite structures. Phase transition is accompanied by pronounced changes in its electronic structure and optical constants. By harnessing these ion-induced structural transformations and integrating an optically passive Al_2O_3 interference layer, we achieve continuous and reversible modulation of optical transmittance and color. These results provide a general framework for ion-driven analog photonic and electrochromic devices and highlight the potential of oxygen-based MIECs for next-generation ionochromic systems compatible with silicon-based photonic platforms.

The development of materials with improved performance and stability relies on the analysis of local compositional inhomogeneities, which may occur at various stages from synthesis to application. This type of analysis benefits from rapid methods that provide high lateral and depth resolution, alongside broad elemental sensitivity. The latter is of paramount importance when considering devices such as lithium-ion batteries and solid oxide cells, whose operating principle depends on the transfer and accumulation of light elements. In this work, we validate glow discharge optical emission spectroscopy (GDOES) as an alternative to state-of-the-art techniques for the chemical analysis of complex oxides. We consider several systems of technological interest for materials used in energy storage and conversion-related applications, namely highly complex perovskite oxide thin films with formula ABO3 (A = La, Sr, and B = Fe, Co, Mn) and single-phase SrFeO3-delta (SFO). Quantitative, depth-resolved elemental maps of B-site cations in combinatorial films are generated and benchmarked against state-of-the-art methods. Additionally, an oxygen quantification was achieved on films subjected to different post-annealing treatments. This work demonstrates the potential of GDOES for fast analysis of complex oxide films and heterostructures, enabling both laterally and nanoscale depth-resolved elemental analysis, including difficult-to-quantify light elements.

The combinatorial approach applied to functional oxides has enabled the production of materials libraries that formally contain infinite compositions. A complete ternary diagram can be obtained by pulsed laser deposition (PLD) on 100 mm silicon wafers. However, interest in such materials libraries is only meaningful if high-throughput characterization enables information extraction from the as-deposited library in reasonable time. While much commercial equipment allows for XY-resolved characterization at room temperature, very few sample holders have been made available to investigate structural, chemical, and functional properties at high temperatures in controlled atmospheres. In the present work, we present a furnace that enables the study of 100 mm wafers as a function of temperature. This furnace has a dome to control the atmosphere, typically varying from nitrogen gas to a pure oxygen atmosphere with external control. We present the design of such a furnace and an example of X-ray diffraction (XRD) and fluorescence (XRF) measurements performed at the DiffAbs beamline of the SOLEIL synchrotron. We apply this high-throughput approach to a combinatorial library up to 735 degrees C in nitrogen and calculate the thermal expansion coefficients (TECs) of the La0.8Sr0.2Co1-x-yFexMnyO3-delta (LSCFM) ternary system using custom-made MATLAB codes. The TEC analysis revealed the potential limitations of Vegard's law in predicting lattice variations for high-entropy materials.

Lithium iron phosphate (LiFePO4, LFP) is one of the main cathode materials for lithium-ion batteries on the market; however, its implementation in all-solid-state thin-film batteries remains challenged by transport and interfacial limitations, as well as by compatibility and reactivity issues arising from thin film fabrication processes. In this work, carbon-free LiFePO4 thin films with thicknesses between 120 and 300 nm were deposited by pulsed laser deposition and investigated as cathodes in LFP/ lithium-phosphorus oxynitride (LiPON)/Li all-solid-state thin-film batteries. Structural and morphological analyses confirm the growth of phase-pure, crystalline LiFePO4 films without post deposition annealing. Electrochemical measurements reveal reversible lithium insertion and extraction, yet the theoretical capacity of LiFePO4 is not fully accessed under most operating conditions. When cycled at elevated temperature (50 degrees C), the full cells show a clear enhancement in capacity utilization, with the 200 nm-thick cathode delivering an areal capacity of 7.2 & micro;A & centerdot;h & centerdot;cm(-2) (100 mA & centerdot;h & centerdot;g(-1)) at 5 & micro;A & centerdot;h & centerdot;cm(-2). The limited utilization of the active material may originate, in part, from interfacial phenomena at the LFP/Pt current-collector interface. From an application perspective, the achieved areal capacities and current densities fall within the operational range required for low-power autonomous microsystems, highlighting the potential relevance of LFP/LiPON thin-film batteries for internet-of-things applications.

Thin-film microbatteries provide on-chip and surface-mount energy storage for Si-based microsystems, where device area is the primary constraint. Commercial implementations, available for more than 20 years, have largely relied on LiCoO2 cathodes because they are straightforward to process and package. LiMn2O4 offers a cobalt-free alternative; however, in conventional liquid-electrolyte Li-ion cells, its use is constrained by Mn dissolution and capacity fade, especially when the voltage window is widened to access its theoretical capacity of ∼119 μAh·cm-2·μm-1 (∼296 mAh·g-1). Thin-film solid-state architectures can mitigate these limitations and are naturally aligned with footprint-limited applications, where areal capacity and areal energy are the relevant figures of merit. The focus of this study is to examine the device behavior of LiMn2O4 thin-film microbatteries operated in a wider voltage window, using a LiPON solid electrolyte and a Li metal anode. Polycrystalline LiMn2O4 cathodes (∼850 nm) were grown by pulsed laser deposition with sequential Li2O enrichment during growth. X-ray diffraction, Raman features, and depth-profiling glow discharge optical emission spectroscopy are consistent with the presence of a Li-rich spinel component formed during deposition. The resulting LiMn2O4/LiPON/Li cells, cycled between 2.0 and 4.5 V, deliver up to ∼50 μAh·cm-2 at low rates; at higher rates, the wider window enables capacities up to ∼4 times those obtained on the same devices in the conventional 3.5-4.5 V window. Impedance measurements are used to track evolution during conditioning and operation. Finally, we provide an overview of relevant LiMn2O4 solid-state thin-film microbatteries and outline a tentative route to stabilize the LiMn2O4/LiPON interface under wider-window operation.

One of the main challenges of the rising field of the Internet of Things (IoT) is the self-sustainable supply of energy to the sensors. Among the available environmental sources, heat can be harvested by means of thermoelectric devices. This work presents a new generation of densely packaged all-silicon micro-thermoelectric generators (mu TEGs) with planar architecture. Optimized boron-doped Si nanowires with 80 +/- 30nm in diameter are epitaxially integrated as dense arrays into these generators for an improved performance. A procedure to reliably place a heat sink on top of the devices, enlarging the fraction of external thermal gradient captured by the thermoelectrically active nanowires, is described. These improvements enhance the generated voltage up to eight times with respect to that of a bare mu TEG, leading to output powers well within the range of IoT needs (10 - 100 mu W cm-2). Specifically, the mu TEG on top of a heat source above 200 degrees C and under still air convection conditions generates more than 14 mu W cm-2. When exposed to the same temperatures and to an airflow of 1.3 m s-1 (equivalent to a light breeze) the power density increases above 150 mu W cm-2. Moreover, a long-term stability study running the device in load matching conditions for a period of 1000 h does not show degradation below 200 degrees C. Finally, the suitability of connecting the mu TEG with the current state of the art DC-DC converters is discussed, showing how eventual transients in real operation conditions can allow the device to reach the required cold start-up voltages. Overall, these results demonstrate the readiness of the presented mu TEG as a reliable power source for miniaturized IoT applications.

Operando TERS enables monitoring of electrochemical reactions at grain boundaries, revealing enhanced lithium diffusion and delayed phase transitions with nanometric and temporal resolution in lithium manganese oxide cathodes.

Electrochemical Impedance Spectroscopy (EIS) is the conventional technique for studying the electrical response of individual materials or complete energy devices such as batteries, fuel cells, and supercapacitors. However, EIS has several limitations, including its spatial resolution, the description of ion insertion phenomena (especially when multiple ion species are involved), and the presence of porous electrodes. In this paper, Generalized Ionic Impedance Spectroscopy (GIIS) is proposed to address these issues by complementing traditional EIS to analyze ionic concentration changes under an AC voltage stimulus. A broad range of characterization techniques can be employed to analyze such ionic concentration variations, as these significantly modify the functional properties of the material, such as optical, magnetic, and electrical behavior. Some of these techniques also offer high spatial resolution, enabling lateral and depth profiling analysis. This study provides a theoretical framework for the development of GIIS in the field of energy, analyzing battery-like and fuel cell-like devices while resolving the major limitations of EIS mentioned above. The proven versatility of GIIS opens new pathways for the detailed characterization of energy materials and devices, advancing the understanding of low-frequency fundamental electrochemical processes and broadening the scope of their applications. While many of the discussed cases are experimentally validated, others are presented as perspectives of GIIS applications.

A novel self-supported architecture, based on carbon nanofibers homogeneously coated by an ultrathin TiON layer and functionalized by a fine dispersion of Ir nanoparticles (NPs) with ultralow loading (60 μ g * cm ^−2 with particle size ≈1.5 nm), is presented as a potential anode for proton exchange membrane electrolysis cells. This structure serves as an ideal anode for rapid electrochemical oxygen evolution reaction in acidic conditions. It combines the high conductivity of the carbon-TiON core–shell structure, the chemical stability of the TiON thin layer to protect C against corrosion, and the enhanced mass activity of Ir NPs. Additionally, it offers excellent mechanical stability and a straightforward, scalable fabrication process. The electrochemical activity (80 mA cm ^−2 /1.2 mA * µ g _Ir ^−1 @ 1.6 V) outperforms C–Ir and Ir-black references. Preliminary long-term test are carried out and strategies for a potential improvement of the long-term stability are critically discussed.

Solid Oxide Cells excel as energy conversion devices have demonstrated high efficiency in power generation as fuel cells and energy storage through electrolysis. One of the most concerning topics on SOC field on electrolyte supported cell (ESC) is reducing the thickness of the electrolyte to minimize and the associated serial resistance. Conventionally, the fabrication of this multi-layer ceramic device involves advanced manufacturing processes like tape casting and screen-printing. Recently, electrolyte-supported Solid Oxide Cells have been recently started to be manufactured using ceramic 3D printing techniques, enabling the creation of distinctive geometries with previously unexplored possibilities on functional ceramics. In this work, stereolithography 3D printing of yttria-stabilized zirconia (YSZ) was used to fabricate thin planar SOC electrolytes for electrolyte supported cells achieving thickness up to 30um on button cells. For their scaling up, nerve structured architectures were added to electrolyte membranes by the vast possibilities of SLA 3D printing. The Nerve interface engineering is used to thin some parts of the cell, reducing the average thickness, improving the electrochemical properties while improving the mechanical robustness of the cell (Fig1.). Full cell electrochemical characterization of the here developed electrolyte-supported solid oxide cells was carried out through I–V polarization curves, obtaining a maximum current density of 198mA/cm 2 in and 152mA/cm 2 in SOFC and SOEC mode respectively at 750ºC and electrochemical impedance spectroscopy (EIS), obtaining a minimum serial resistance (ASRs) of 1 Ωcm 2 at 750ºC Introduction Solid oxide fuel cells (SOFCs) are highly efficient, zero-emission power generators that convert hydrogen into electricity, making them one of the most effective energy generation technologies available today. When operated in reverse mode, these devices function as energy storage systems, producing storable hydrogen from electricity and water. Known as solid oxide electrolysis cells (SOECs), they offer a highly efficient method for energy conversion. Solid oxide cells (SOCs) are multilayer electrochemical devices with a ceramic-based structure. They feature a dense oxide-ion-conducting electrolyte sandwiched between two electrodes. The state-of-the-art materials for SOCs include yttria-stabilized zirconia (YSZ) as the electrolyte, with electrode compositions based on YSZ composites—lanthanum strontium manganite (LSM-YSZ) for the oxygen electrode and nickel–YSZ (Ni–YSZ) for the fuel electrode. Electrolyte-supported cells (ESC) and fuel electrode-supported cells (FESC) are classified based on their supporting layer. Regardless of the cell configuration, their fabrication follows a multi-step shaping process using conventional functional manufacturing techniques such as tape casting and screen printing. However, innovative methods incorporating alternative techniques such as spin coating, pulsed laser deposition, inkjet printing, infiltration, binder jetting among others have also been explored 4 . Notably, additive manufacturing and 3D printing of ceramics have recently demonstrated their potential for producing highly complex functional ceramic components with excellent mechanical and electrochemical properties 2 . It has been shown that intricate geometries and surface modifications of the electrolyte can enhance thermomechanical stability, enabling a reduction in membrane thickness. In addition to offering greater design flexibility, 3D printing significantly reduces material waste—an important factor when working with functional ceramic materials. It also provides a time- and cost-efficient alternative to traditional ceramic manufacturing, particularly for applications such as SOFCs and SOECs. Recent studies have further demonstrated the benefits of leveraging geometric patterns, made possible by the design freedom of additive manufacturing, to enhance volumetric current density, especially in critical areas. Among the various 3D printing techniques, stereolithography (SLA) has emerged as a promising approach for producing functional ceramic-based materials for energy applications. 5,6 Through additive manufacturing, the resulting electrolytes were thicker than those produced using conventional techniques. This has been one of the primary drawbacks of the technology, as a thinner electrolyte corresponds to lower serial resistance and higher peak power density. First, Masciandaro et al. printed 300μm-thick self-supported 3YSZ flat and complex shaped electrolytes of 1.54cm 2 area to demonstrate the suitability of SLA to generate electrolytes for SOC applications. The cells showed electrochemical serial resistance of 4Ωcm 2 and peak power density of 60mW at 800◦C. 1 In another study, Celika et al. achieved 75um 3YSZ electrolytes with a 40x40mm conventional tape casting technique and additionally 25um in a 24.15% of the active area by cutting patterns in the electrolyte active region. Resulting as serial resistance of 1.95Ωcm 2 and 2.25Ωcm 2 for patterned and planar electrolyte respectively. Nevertheless, SLA showed the potential to improve SOC performance by overcoming the geometrical constraints of conventional ceramic production processes. 3 In this work, stereolithography 3D printing of yttria-stabilized zirconia (3YSZ) was used to fabricate thin planar SOC electrolytes for electrolyte supported SOC cells with thickness of up to 30um with a serial resistance 0.45Ωcm 2 for 750ºC. For the scaling up of this technique, nerve structure architectures were added to the previous electrolyte thanks to the vast possibilities of 3D printing. The Nerve interface engineering is used to thin some parts of the cell keeping the electrochemical properties while keeping mechanical robustness of the cell. The resulting 3YSZ nerve patterned have less than 45um mean thickness. This 3D printed novel electrolyte design was scaled up for fuel cell measurements to 50x50mm active area, obtaining a less than 80um mean thickness electrolyte with a performance of 120mW at 750◦C. Demonstrating that by 3D Printing techniques electrolyte thickness similar to the State of the art can be achieved, and consequently comparable efficiencies to conventional techniques, while presenting the already commented advantages of 3D Printing. Methodology 3YSZ electrolytes were manufactured using stereolithography (SLA) with an industrial ceramic 3D printing system (CERAMAKER C900, 3DCERAM, FR). The parts were built by sequentially depositing and curing 25 μm-thick layers, defining the resolution in the z-direction, while the x-y resolution was determined by the laser spot size (~50 μm). A UV semiconductor laser with a characteristic wavelength of 355 nm was used for curing the slurries. The printed button-cell membranes had a circular shape with a diameter of 2.00 cm, an active area of 1.75 cm². To achieve these final dimensions after sintering, a rescaling process was applied to compensate for shrinkage during the sintering stage. For the fabrication of nerve-patterned solid oxide cells, a rib structure with a height of 30 μm and a width of 180 μm was incorporated into the electrolyte CAD design. Large-area samples featured a nerve-patterned structure, were square-shaped (8 × 8 cm), and had an active area of 25 cm². Sintered 3YSZ electrolytes have been functionalized for fuel cell application by painting commercial electrode materials (FuelCell Materials) of LSM-YSZ, for the oxygen electrode. And by a self-made thermo-curable ink with ceramic loading of NiO-YSZ for the fuel electrode. The NiO-YSZ painted electrode layer was sintered in air at 1450◦C for 2 h, while the YSZ-LSM (FuelCell Materials) layers were attached at 1200◦C for 2 h, at a heating and cooling rate of 2◦C/min. Symmetrical characterization of the electrolytes was carried out by the application of LSM-YSZ in both sides of the electrolyte and gold ink to ensure electrical contact. Button cells were electrochemically tested in both fuel cell and electrolysis modes using a ProboStat™ system (NOR-ECS, Norway) within a temperature range of 700–900 °C. Their electrochemical performance was assessed through I–V polarization curves and electrochemical impedance spectroscopy (EIS). Measurements were conducted using a potentiostat/galvanostat and a frequency response analyzer (Parstat 2273, PAR, USA) over a frequency range of 500 mHz to 1 MHz, with an amplitude of 100 mV under open-circuit voltage (OCV) and a bias of 0.7 V as operating conditions. Large-area solid oxide cell was characterized using a custom-built electrochemical test bench with controlled temperature and atmosphere. The setup included a DC power supply (EA-PSI 9080–60T) and electronic loads (TrueData Load – FuelCon), enabling cell testing under different operating modes (SOFC and SOEC). The performance was evaluated through I–V polarization curves. Morphological and microstructural characterization was carried out using a Scanning Electron Microscope (SEM) (AURIGA, ZEISS, Germany) and optical microscopy with a PLu Neox 3D Optical Profiler (SENSOFAR, Spain) confocal microscope. Results and discussion Flat and nerve-patterned 3YSZ freestanding membranes were successfully fabricated using SLA 3D printing, followed by high-temperature sintering. Although the nerve pattern is barely visible to the naked eye (Fig.2a), surface topography analysis using confocal imaging (Fig. 2b) provides a detailed view of the printed structure, revealing a nerve height of 20 μm, a width of 185 μm, and a pitch of 130 μm. SEM cross-sectional images (Fig. 2c) show that the thickness of 34 μm in the planar area, while the nerve area of the electrolyte has a thickness of 52 μm. The pitch between nerves is 135 μm, and the width is 190 μm, consistent with the confocal imaging results. The patterned electrolyte layer itself has a thickness of 34 μm, with the nerves reaching a height of 23 μm. For fuel cell measurements this design was thickened to a flat and nerved button cells of 60um electrolyte layer. Nerves were thickened to 30um approximately for the patterned design, resulting the mean thickness of the patterned electrolyte 80um. This electrolyte, was scaled up to a squared cell of 50x50mm of active area (Fig. 3) SOFC and SOEC mode results at 750ºC are shown in Fig4. IV polarization curves for flat cell design and nerve patterned design are shown in Fig. 4a and 4b. EIS results for the two button cell designs are shown in Fig. 4c and Fig.4d. SOFC IV (Fig4.a) show a similar current density for both designs achieving almost 200mA/cm 2 at 0.6V with a peak power of 120mW/cm 2 . In SOFC EIS (Fig4.c) it can be seen that serial resistance is lower for the flat design (1Ωcm 2 ) than the nerved (1,27Ωcm 2 ) as expected due to lower mean electrolyte thickness. This is consistent with SOEC EIS (Fig4. d) in which serial resistances present the same values. For the SOEC IV (Fig.4 b), flat cell has a higher current density being 152mA/cm 2 for the flat cell and 108mA/cm 2 for the nerve patterned. Regarding IV polarization curves and EIS of the large, area results will be shown in the near future. Conclusions Our findings confirm that SOC electrolyte membranes can be fabricated by SLA 3D printing achieving thicknesses comparable to those achieved using state-of-the-art techniques (75 μm), and importantly, this can be accomplished on large scale stackable cells. This progress highlights the potential of ceramic 3D printing to become an increasingly viable and competitive alternative to conventional manufacturing methods due to these advantages on complex shape and waste material among others. As this technology continues to evolve, improvements in precision, scalability, and material properties will further enhance its adoption in industrial and high-performance applications. References [1] Three-dimensional printed yttria-stabilized zirconia self-supported electrolytes for solid oxide fuel cell applications. S. Masciandaro, et al. Journal of the European Ceramic Society, 2019. [2] Large-area 3D printed electrolyte-supported reversible solid oxide cells. M. Lira, et.al. Electrochimica Acta 467 (2023) [3] Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells. Selahattin Celik, et al. Ceramics International41(2015)8785–8790 [4] 3D printing the next generation of enhanced sòlid oxide fuel and electrolysis cells. Arianna Pesce, et. al. J. Mater. Chem. A, 2020, 8,16926 [5] 3D printed electrolyte-supported solid oxide cells based on Ytterbium-doped scandia-stabilized zirconia. S. Márquez et. al. J. Phys. Energy 6 (2024) 015016 [6] 2022 roadmap on 3D printing for energy. A.Tarancón et. al. J. Phys. Energy 4 (2022) 011501 Figure 1

State-of-the-art electrodes for the oxygen reduction/oxidation reaction typically present durability issues due to the appearance of (surface) precipitates during operation at high temperature. In this work, we employ a combinatorial approach to study the effect of B-site co-doping on the thermal degradation of the La0.8Sr0.2Mn x Co y Fe1-x-yO3 +/-delta family. A continuous library of materials was fabricated in a single process by means of combinatorial pulsed-laser deposition, followed by an annealing at 800 degrees C for a period of 100 h. The library was then characterized by advanced techniques, involving surface microstructural and chemical analysis and cation profiling throughout the range of compositions. Remarkable stability of Mn-doped materials (and of the parent La0.8Sr0.2MnO3-delta compound) was observed regarding the appearance of segregated surface strontium species, particularly sulfates. This result was correlated with a drastic reduction of the degradation of the Mn-containing La0.8Sr0.2Mn x Co y Fe1-x-yO3 +/- delta films during midterm electrochemical performance studies. Complementary density functional theory calculations reveal a direct correlation between cation reducibility (i.e., the O 2p band center position) and surface Sr enrichment. These results indicate that the addition of Mn to La0.8Sr0.2Mn x Co y Fe1-x-yO3 electrodes plays a substantial role in the stabilization of strontium segregation phenomena and suggest a general strategy for enhancing perovskite stability based on co-doping and band engineering.