Proton‐exchange membrane water electrolyzers (PEMWEs) will play a key role in future sustainable hydrogen production for mobility, households or chemical industry. Yet, the anode in PEMWEs, where the pivotal oxygen evolution reaction takes place, needs further improvement in terms of performance and cost. Both catalyst materials and electrode structure have to be optimized in order to inhibit degradation and reduce noble metal loadings. This review focuses on a holistic approach, covering all catalyst material, electrode structure, and transport layers within the framework of an overall electrode design, which not only optimizes the catalyst but also, all components of the electrode in conjunction. This review defines the goals for performance metrics of future PEMWEs in terms of power density and durability of the anode. Moreover, it summarizes manufacturing techniques and approaches that have a chance to be upscaled to meet the megawatt deployment of PEMWEs. The different aspects described jointly in this review such as novel catalyst system with higher intrinsic and structural performance or graded porous transport layers shall help to advance a next generation of electrodes.

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene–ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide–ZnO (GO–ZnO) and thiol-functionalized reduced graphene oxide–ZnO (TrGO–ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO–ZnO and TrGO–ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (V_Zn ⁻) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively “heal” the oxygen vacancy (V_O ⁺) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO–ZnO and TrGO–ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO–ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Electron paramagnetic resonance (EPR) spectroscopy has been applied to different chemically treated reduced graphene oxide (rGO) and multiwalled carbon nanotubes (CNTs). A narrow EPR signal is visible at g = 2.0029 in both GO and CNT-Oxide from carbon-related dangling bonds. EPR signals became broader and of lower intensity after oxygen-containing functionalities were reduced and partially transformed into thiol groups to obtain thiol-functionalized reduced GO (TrGO) and thiol-functionalized CNT (CNT-SH), respectively. Additionally, EPR investigation of CdSe quantum dot-TrGO hybrid material reveals complete quenching of the TrGO EPR signal due to direct chemical attachment and electronic coupling. Our work confirms that EPR is a suitable tool to detect spin density changes in different functionalized nanocarbon materials and can contribute to improved understanding of electronic coupling effects in nanocarbon-nanoparticle hybrid nano-composites promising for various electronic and optoelectronic applications.