## 1 Introduction  
Thermal ice storage is a very important kind of phase change cold energy storage, which can be used in the solar and wind energy system to reduce the fluctuations in the energy flow. The ice storage system not only can run fluently under a decoupled source-load condition, but can reduce maximum output power, average power and energy consumption by 20-40% in comparison with conventional systems in centralized and decentralized environments [1]. The thermal ice storage is also employed in heating, ventilation and air conditioning (HVAC) cooling, food processing, chemical reactions or pharmaceutical processing, inlet air cooling of turbine and district cooling plant [2]. As one type of phase change material (PCM), ice storage, however, has higher supercooling degree and lower thermal conductivity that reduce heat transfer performance. A high supercooling degree leads to reduced performance of the PCM thermal energy storage system because of the requirement of a large operating temperature range [3]. Fortunately, many researchers found that addition of nanoparticles not only enhance the thermal conductivity, but also reduce the supercooling degree, and improve the heat transfer performance greatly [4]. Nonetheless, the stability of the nanoparticles suspended in base fluids during the thermal cycling is becoming a key problem. Colloidal suspensions containing dispersed metal or nonmetal nanoparticles are called nanofluids, which are widely used in several heat transfer or performance improvement areas such as solar energy collectors [5; 6; 7; 8; 9; 10; 11; 12; 13], phase change energy storage [14; 15], micro- and power electronics [16], magnetic refrigeration [17; 18], diesel or biodiesel fuels [19; 20], ventilation and air conditioning systems [21], reactors in power generating plants, cancer treatment therapy, metallurgical operations and combustion engines [22; 23; 24]. Nanofluids can be divided into two categories according to the types of nanoparticles. One is a single nanofluid formed by a single nanoparticle and base liquids, and the other is a hybrid nanofluid formed by mixing multiple nanoparticles and base liquids. Hybrid nanofluid is a solid-liquid mixture formed by dispersing two or more different nanoparticles suspended in a liquid, which displays enhanced thermophysical properties and rheological characteristics compared to mono nanoparticles nanofluid due to a synergistic effect [25]. Properly selected hybrid nanofluids can not only have higher thermal conductivity than single-phase nanofluids, but also have better properties such as less mechanical resistance, better stability and lower cost. These superior characteristics make them particularly attractive for applications in many heat transfer areas [22; 23]. However, there are still several challenges that have impeded the development of this field as concluded in many review articles [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34], such as disagreement in results of various researchers on the same types of nanofluids; preparation of long-term, stable and homogeneous nanofluids; a lack of theoretical understanding of the mechanisms accountable for variety in properties and high cost of the hybrid nanoparticles. In particular, stability of nanofluids is an essential characteristic that impacts greatly the thermophysical properties and rheological characteristics as well as their field applications. Surfactants have been used to keep good dispersibility of nanofluids. However, surfactants may impact the thermal conductivity and viscosity of the hybrid nanofluid as well [35]. Among various nanomaterials used in nanofluids, graphene oxide (GO) is hydrophilic, has a superior dispersion stability and high thermal conductivity and can significantly reduce the degree of supercooling of water in phase change thermal storage [36; 37; 38; 39; 40; 41], but the preparation technology is complex and expensive.