具有单电子传递特征的低能障光电化学反应系统设计及制氢性能研究

Water splitting by utilizing solar energy is one of the most challenging reactions in chemistry, and is also the ideal way to solve the current energy and environment problems. However, the efficiency of solar energy converting to hydrogen is limited by the multi-election processes involved in the decomposition of water and the associated thermodynamics and kinetics barriers. In addition, the finite mobility and short excitation-state lifetimes for the photogenerated carriers also constrain the solar-to-hydrogen conversion efficiency. In this proposal, inspired by the work mode of nickel-hydrogen batteries, we design a sandwich-type photoelectrochemical cell for hydrogen production, employing hydrogen-storage alloy, Ni(OH)2, and TiO2 nanotubes as the cathode plate, anode plate, and photoeletrode, respectively. In the device, the photogenerated holes and electrons were transferred via the following single-electron ways: Ni(OH)2+h→NiOOH and M+H2O+e→MH, which is believed to simplify the hydrogen evolution mechanism and decrease the energy barriers. Meanwhile, the chemical rectification and the space charge layer developed in the devices can systemically strengthen the separation and migration of photogenerated carries. Furthermore, by switching the devices to nickel-hydrogen battery, hydrogen accumulated in the hydrogen-storage alloy electrode can be transformed in situ to electrical power, realizing the flow of the energy and the recovery of device conditions. Alternatively, the hydrogen accumulated in the alloy can also be released to give gaseous hydrogen as the feedstock of hydrogen-based industries. As a consequence, it is rational to expect that the proposed single-electron transfer mechanism and sandwich-type photoelectrochemical device will not only overcome the thermodynamic and kinetic barriers of solar-to-hydrogen conversion, but also allow the flexible consumption of hydrogen.

太阳能光解水制氢是最富挑战性的化学反应之一，也是解决当下能源与环境问题的最佳出路。制氢研究的瓶颈是水分解时的多电子过程及其诱发的动力学和热力学阻力，光生载流子在光电极中的有限迁移能力和短暂激发态寿命也是重要不利因素。本项目提出学习镍氢电池工作机制，采用储氢合金M（阴极）、Ni(OH)2(阳极)和掺杂TiO2纳米管（光电极）构建太阳能制氢器件，以单电子反应Ni(OH)2+h→NiOOH和M+H2O+e→MH作为光生载流子的传递路径，达到简化制氢反应历程，降低反应阻力，产生化学整流效应以及强化光生载流子的分离和迁移等一系列目的。制氢器件完成太阳能制氢后，蓄积在储氢电极中的氢可直接转化为电力，实现能量的流动和器件状态的自我恢复。而且蓄积在储氢电极中的氢也可变为H2逸出，为氢经济输送上游原料。因此，本项目设计的单电子传递机制和器件不仅有望克服太阳能制氢反应的主要障碍，还有望形成灵活的氢能利用方法。

项目背景：目前燃煤和燃油引起的能源与环境问题始终困扰人类社会的发展，太阳能光解水制氢是最富挑战性的化学反应之一，也是解决当下能源与环境问题的最佳出路。.主要研究内容：通过剖析当前太阳能制氢研究的瓶颈和镍氢电池的工作机制，本项目提出构建“三明治”形Ni(OH)2||TiO2||M太阳能制氢器件，以单电子反应Ni(OH)2+OH－+h →NOOH+H2O和M+H2O+e→MH0+OH－作为器件中光生空穴和光生电子的转移路径,简化制氢历程，同时系统强化光生载流子的分离、迁移和传递，从而大幅降低太阳能制氢反应的热力学和动力学阻力，最终实现太阳能向氢能的高速和高效转化。.科学意义：所构建的光电化学制氢器件不仅有望实现固态氢的供应（束缚在储氢合金中）和气态的氢，还能直接将氢转化为电能，多样化的氢以及灵活多变的氢能利用技术将为未来氢经济的发展奠定基础。.重要结果及关键数据：1．成功制备了Ni(OH)2修饰的TiO2纳米管阵列：采用水热法成功制备了以泡沫镍为基底具有层级多孔特征的TiO2纳米管阵列作为必需的阴极材料。.2.研究了Ni(OH)2修饰的TiO2纳米管阵列在电解水制氢中的应用单质镍具有较小的析氢过电势，研究表明，纳米管阵列增强了电子的转移能力，也有效减小了电荷转移的阻力。.3．研究了储氢合金电极的制备：采用电化学沉积法在泡沫镍表面沉积了一层LaNi储氢合金。.4.研究了原型制氢器件的组装：以上述的掺杂单质镍和Ni(OH)2的纳米管阵列以及电沉积形成的储氢合金电极为核心，初步装配了“三明治”形制氢器件及其它制氢电极材料。初步装配了类似于“三明治”形的CuO@TiO2制氢器件。.5. 验证器件的单电子转移机制：CuO@TiO2纳米线膜表现出了众多适合光电解水应用的优异物理特性。.6. 研究所组装器件的光电化学性能，研究电极反应动力学特征、过电位程度等性能。.7. 根据前期的研究成果，总结器件的太阳能制氢性能与其结构之间的内在联系的规律，确定更合理的器件结构和电荷、实现器件结构和性能的最佳匹配，形成制氢效率最高的光电化学器件；研究器件的太阳能充氢性能、电力输出特性、储氢电极的放氢能力。