Novel materials for high-performance water electrolyzer and ultrafast battery applications [electronic resource]
Keywords: Chemistry.

The gradually increasing demand for energy and diminishing fossil fuel resources that our world is currently relying on as well as the environment issues associated with fossil fuel combustion has posted the need for clean and sustainable energy. In recent years, renewable energy has come into the research and the industry communities to change the current energy structure into a cleaner manner. However, most of the renewable energy resources suffer from intermittency that cannot meet the need of continuous energy delivery. In consequence, the renewable energy needs to be stored when surplus and released when needed. Electrochemical energy storage (e.g. batteries) and electrochemical energy conversion (e.g. water splitting/fuel cell) are two prospective solutions of storing renewable energy that have their own advantages and disadvantages, but both of them are far from the ideal picture of storing renewable energy. Therefore, continuous efforts on improving the efficiency and capacity of energy storage and conversion devices are highly desired. In my PhD study with Prof. Dai, we aim to develop novel materials for both water splitting (energy conversion) and ultrafast battery applications (energy storage). The approach is to incorporate active materials with highly conductive nano-carbon materials (e.g. carbon nanotube) that our group specializes in to facilitate the electron transport. However, during the studies, these nano-carbon materials have been discovered to not only boost the performance of the existing materials but also help the formation and discovery of new active materials for various applications. Interestingly, these carbon-based materials themselves can also be utilized as the active materials in novel applications, such as rechargeable aluminum ion batteries. The discovery of these novel structures holds great promise of efficiently storing renewable energy with low cost. We first incorporated iron-doped nickel hydroxide with mildly oxidized carbon nanotubes by direct growth. This led to the discovery of a highly active NiFe layered double hydroxide (LDH) phase for water oxidation in alkaline condition. The LDH phase is the main contributor of the superior activity (better than the Ir-based benchmark). Intimate coupling with CNT for facilitated electron transport and nanoplate morphology for increased surface area optimize the activity towards water oxidation. The same strategy was applied to a hybrid material consisting of NiAlCo LDH interconnected with few-walled carbon nanotubes. The Al and Co co-doping was found to greatly stabilize the α-Ni hydroxide phase in the LDH form, affording significantly improved durability. The chemical coupling with highly conductive CNT along with the ultrathin nanoplate morphology could lead to a nickel-zinc battery delivering an energy density of 274 Wh/kg and a power density of 16.6 kW/kg with ultrafast charge/discharge times down to 41 seconds. To achieve low-cost alkaline electrolyzers with high performance, we further utilized oxidized carbon nanotube precursors for the discovery of nanoscale NiO/Ni hetero-structure for hydrogen evolution catalysis. The oxidized carbon nanotubes played an important role of impeding Ni reduction and aggregation for the formation of the desired structure. The high activity towards hydrogen evolution was attributed to the nanoscale NiO/Ni interface that can both stabilize H atom intermediate and efficiently remove the generated OH-. An efficient electrolyzer using NiO/Ni-CNT and NiFe LDH to achieve ~20 mA/cm2 at a voltage of 1.5 V was demonstrated. The insufficient long-term stability of NiO/Ni hetero-structure was further solved by chromium oxide (Cr2O3) blending. The Cr2O3 was discovered to be important to the durability by preventing the core NiO/Ni sites from oxidation and phase separation. The Cr2O3 blended NiO-Ni hetero-structure could lead to an efficient electrolyzer with a lifetime of > 500 hour and also unassisted water splitting with a solar-to-hydrogen efficiency of ~15% by pairing up the electrolyzer with GaAs solar cells. Moreover, we have taken efforts on developing Fe0.9Co0.1S2 nanosheets-CNT hybrid catalyst for hydrogen evolution catalysis in acidic solutions. The moderate Co doping ratio and electrical coupling to the carbon nanotube benefited the hydrogen evolution catalytic activity to achieve a low overpotential of ~0.12 V at 20 mA/cm2 with long-term durability. Co doping into FeS2 catalyst was discovered to reduce the energy barrier for transition state 1 of hydrogen atom adsorption. At last, we utilized pure graphitic materials for novel rechargeable aluminum ion battery. The rechargeable aluminum ion battery operates through the electrochemical deposition and dissolution of aluminum at the anode, and the intercalation/de-intercalation of chloroaluminate anions in the graphitic materials at the cathode. An unprecedented high-voltage plateau of ~2 volts with a capacity of ~70 mAh/g was achieved using pyrolytic graphite as cathode, and ultrafast capability was further enabled by 3D graphitic foam cathode with fast anion diffusion and intercalation, affording 1 minute charge/discharge up to ~7500 cycles. Overall, these efforts on discovering novel materials for water splitting and ultrafast battery applications may open up efficient ways of storing renewable energy as well as extend our understandings in material science, catalysis and electrochemistry.