Heat pump (HP) is one of the most energy efficient tools for address heating and possibly cooling needs in buildings. Growing environmental concerns over conventional HP refrigerants, chlorofluorocarbons (CFCs), and hydrofluorocarbons (HFCs) have forced legislators and researchers to look for alternatives. As such, carbon dioxide (R744/CO2) has come to light due to its low global warming potential (GWP) and zero ozone depleting characteristics. Even though CO2 is environmentally benign, the performance of CO2 HP has been of concern since its inception. To improve the performance of CO2 HP, research has been playing a pivotal role in developing functional designs of heat exchangers, expansion devices, and compressors to suit the CO2 transcritical cycle. Different CO2 HP cycles coupled with auxiliary components, hybrid systems, and refrigerant mixtures along with advanced control strategies have been applied and tested. This paper presents a complete overview of the most recent developments of transcritical CO2 HPs, their components, and applications.

A novel porous three dimensional (3D) hierarchical graphene-beaded carbon nanofibers with incorporated Ni nanoparticles (G/CNFs-Ni) were used for the first time as cost-effective counter electrode for dye-sensitized solar cells (DSCs). G/CNFs-Ni was synthesized by electrospinning G/PAN/Ni(AcAc)2 precursor nanofibers, followed by carbonization and activation. The introduction of graphene nanosheets and Ni nanoparticles in CNF networks significantly increased the cells' stability and decreased the charge-transfer resistance at the interface between electrolyte and counter electrode, leading to the high electrocatalytic activity/efficiency for triiodide reduction. The G/CNFs-Ni composite counter electrodes possessed larger capacitance than that of Pt counter electrodes due to larger specific surface area, leading to significantly higher electrocatalytic activity/efficiency for triiodide reduction at the interface between electrolyte and counter electrode. The dye-sensitized solar cells (DSCs) fabricated using G/CNF-Ni composite as counter electrodes were tested at 100 mW/cm 2 AM 1.5 illumination. The G/CNFs-Ni composite exhibited an overall power conversion efficiency of 7.14 % as compared to 7.59 % for reference platinum (Pt) counter electrodes.

Activated graphene nanoplatelets (aGNPs) prepared by a hydrothermal method using KOH as activating agent were used as counter electrode for high efficiency dye-sensitized solar cells (DSSCs). After the KOH activation, the scanning electron microscopy image shows that aGNPs demonstrate a more curled, rough, and porous morphology which could contain both micro- and mesopores. The KOH activation changed the stacked layers of GNPs to a more crumpled and curved morphology. The microstructure of large pores significantly increased the electrode surface area and roughness, leading to the high electrocatalytic activity for triiodide reduction at the counter electrode. The DSSCs fabricated using aGNP as counter electrodes were tested under standard AM 1.5 illumination with an intensity of 91.5 mW/cm2. The device achieved an overall power conversion efficiency of 7.7%, which is comparable to the conventional platinum counter electrode (8%). Therefore, the low cost and high performance aGNP based counter electrode is a promising alternative to conventional Pt counter electrode in DSSCs.

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