Graphene-based technologies for energy applications, challenges and perspectives

Abstract: Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

• The paper's main contribution is demonstrating how graphene and its composites enhance energy device performance in photovoltaics, fuel cells, and storage systems.
• It details methodologies such as advanced doping, nanocomposite engineering, and DFT simulations to optimize electrodes, catalysts, and storage architectures.
• Results reveal challenges in scalability and interface stability, highlighting the need for improved production methods and industrial applicability.

Graphene-Based Technologies for Energy Applications: Advances, Challenges, and Perspectives

Introduction

The integration of graphene and graphene-related materials (GRMs) into device architectures represents a major strategic focus within the Graphene Flagship European initiative for the energy sector. Exploiting the extraordinary electrical conductivity, chemical versatility, high surface area, and scalable processability of GRMs, this review delineates technological developments and implementation strategies for graphene-enabled energy devices, including photovoltaics (PVs), fuel cells, hydrogen storage, and electrochemical storage systems. Emphasis is placed on functional nanocomposites engineered for performance gains in key device metrics, the leveraging of novel surface and interface design concepts, and rigorous assessment through experimental and theoretical methods.

Photovoltaic Applications of Graphene and GRMs

Graphene integration into PV devices primarily aims to address the limitations of second-generation thin-film solar cells and emerging roll-to-roll processed technologies such as organic photovoltaics (OPV), perovskite solar cells (PeSCs), and quantum-dot-based architectures. The investigation highlights several roles for GRMs:

• Transparent and Conductive (TC) Electrodes: Single-layer graphene (SLG) exhibits superior transmittance in the short wavelength region, yielding higher internal quantum efficiency (IQE) in a-Si:H solar cells versus ZnO:Al electrodes. Yet, the sheet resistance of graphene is orders of magnitude higher, curtailing photocurrent collection under terrestrial illumination. The technological bottleneck is to simultaneously achieve $T > 85\%$ and $R < 50\ \Omega/\square$; while achievable at the lab scale, industrial scalability remains problematic.
• Adaptive Electronic Buffer Layers: Laser-based photochemical doping (e.g., Cl, Li) of graphene oxide (GO) enables large and tunable shifts in work function, confirmed by DFT calculations, facilitating optimal band alignment at PV interfaces. Such functionalized GO buffer layers outperform PEDOT:PSS in durability and efficiency, and are compatible with roll-to-roll device processing.
• Graphene-Quantum Dot (QD) Hybrids: Covalent functionalization of reduced graphene oxide (rGO) with linker molecules provides a platform for solution-processed integration with semiconductor nanocrystals. This architecture leverages high carrier mobility in graphene to compensate for poor transport in close-packed QD films, targeting improved collection in near-IR-extended PV devices. Ongoing work includes the development of QD/rGO composites with controlled band alignment and interface optimization.

Graphene-Enabled Electrocatalysis for Fuel Cells

A central impediment for automotive and stationary fuel cell deployment is the cost and utilization efficiency of platinum group metals (PGMs) and the durability of electrocatalysts. The Flagship approach employs GRMs to address several axes:

• Pt/Graphene Supports: Methods including supercritical CO$_2$ deposition and impregnation-reduction yield highly dispersed Pt nanoparticles (NPs) on graphene nanoplatelets (GNPs) or rGO. Compared to conventional carbon supports (Vulcan XC-72), these composites provide increased electrochemically active surface area (ECSA), enhanced corrosion resistance, and higher cell performance at low current densities, thus supporting the reduction of catalyst loading towards the 10 kW g$_\text{Pt}^{-1}$ target.
• Carbon Nitride (CN)-Coated Graphene: A core–shell strategy wherein GRMs are cloaked with CN layers embedding active ORR sites (e.g., PtNi$_x$ alloys) yields electrocatalysts with improved stability under oxidative conditions and selectivity comparable to, or exceeding, state-of-the-art Pt/C. Synthesis protocols offer tunability in site composition and nitrogen content with the flexibility to support non-Pt catalysts for AEMFCs.

Hydrogen Storage: Physisorption and Chemisorption on GRMs

Hydrogen storage via physisorption on high-SSA GRMs or chemisorption on functionalized graphene is a vibrant research domain:

• High SSA GRMs: Systematic hydrogen uptake studies on rGO and activated rGO illustrate a strong correlation with SSA, with gravimetric capacities of up to 5.5 wt% at 77 K for ~2900 m$^2$/g materials. Theoretical studies predict that nano-perforation (hole diameters <10 Å) could enable SSA approaching 5000 m$^2$/g and storage capacities up to 6.5 wt%.
• Metal-Decorated GRMs: DFT calculations show alkaline-earth metal atoms (e.g., Ca, Mg) chemisorbed on oxygen-passivated edges of GNRs can bind multiple H$_2$ molecules with increased adsorption energy (up to 0.25 eV/H$_2$), suggesting a possible pathway to high-capacity, room-temperature hydrogen storage.
• GRM-Mediated Nanoconfinement for Hydrides: Encapsulation of MgH$_2$ within multiwall CNTs restricts the growth of hydride nanoparticles to the desired 1–5 nm regime, dramatically lowering desorption temperatures to ~167 °C. The challenge remains in scalable synthesis and stability against NP aggregation.

Electrochemical Storage: Batteries and Supercapacitors

In energy storage, GRMs offer both morphological and electrochemical advantages:

• Supercapacitors: Hybridization of graphene and CNTs via dynamic spray deposition yields flexible electrodes with specific capacitance of 120 F/g and power densities up to 92.6 kW/kg, outperforming both graphite/CNT and GO/CNT systems. Process engineering towards water-based ink deposition is ongoing for safer and more scalable manufacture.
• Nanocrystal/Graphene Composites for Batteries: SnO$_2$ NPs grown in situ within rGO aerogels display reversible specific capacities >1000 mAh/g after 100 cycles, far exceeding bulk SnO$_2$ theoretical capacity (782 mAh/g). NMR studies point to a combination of conversion and alloying mechanisms for Li storage, linked to the particle size and nature of the graphene matrix. However, high irreversible capacity—attributable to the large SSA of the graphene—remains a significant barrier to practical full cell deployment.

Discussion: Implications, Challenges, and Future Directions

The cross-cutting implication of these results is the unique role of graphene scaffolding in determining the nucleation, growth, dispersion, and stability of nanoscale functional domains (catalysts, hydrides, Li hosts) within energy devices. Laser-based processing unlocks interface and surface optimization in OPVs and broader organic electronics. Notably, tradeoffs in SSA present both opportunities (capacity, reactivity) and challenges (irreversible capacity, stability) that necessitate continued advances in materials engineering, particularly for practical device architectures.

Achieving industrial applicability for graphene TC electrodes in PVs—specifically matching the stringent $T$ and $R$ targets—requires further progress in low-defect, doped CVD graphene sheets and contamination control. The future of hydrogen storage may hinge on scalable synthesis of atomically engineered, perforated, or functionalized graphene frameworks. In electrochemical storage, the mechanistic understanding of SEI formation, volume changes, and Li-alloying in nanoscale GRM composites must be deepened—synergizing advanced characterization with theoretical modeling.

Integration strategies that combine precise synthesis, in situ characterization, multi-scale modeling, and scalable fabrication will be essential for translating the promise of GRMs into the next generation of competitive energy technologies.

Conclusion

Graphene and GRMs emerge as versatile and enabling materials for diverse energy applications, providing unique solutions to challenges in photovoltaic, fuel cell, hydrogen storage, and electrochemical storage systems. The dual requirements of high performance and process compatibility demand continued innovation in materials chemistry and device engineering. Addressing the fundamental questions around interface control, stability, nanostructure assembly, and scalable processing remains paramount for realizing the full industrial impact of graphene-based energy technologies.

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Cited Paper: "Graphene-based technologies for energy applications, challenges and perspectives" (2601.15744)