Rational Design of Trimetallic Electrocatalyst for Electrochemical Overall Water Splitting

Noble-metal-free bifunctional electrocatalysts for overall water splitting have attracted increasing attention due to their earth-abundancy and high efficiency. However, current bifunctional electrocatalysts suffer from the disadvantages of the complex synthetic process, low yield, or low energy conversion efficiency. Thus, it is highly desirable and significant to develop efficient electrocatalysts with high energy conversion efficiency through the facile process. The Co-Fe-W multi-metal oxides have been reported as one of the best OER catalysts, but their overall water splitting activity is not reported yet, which might be due to the poor HER activities. In Chapter 3, we develop a trimetallic CoFeW film on Ni foam by hydrothermal deposition and subsequent thermal annealing process. The trimetallic CoFeW exhibits great HER activity (ƞ10=147 mV) due to the improved conductivity and increased electrochemical active surface area after annealing. Benefitting from the enhanced HER and remained OER activities, the trimetallic CoFeW electrodes require a cell voltage of 1.57 V (10 mA∙cm-2) to drive overall water splitting. However, the overall performance is still limited and a more efficient trimetallic system needs to be developed. In Chapter 4, we have developed a trimetallic NiFeMo film on Ni foam via a similar process with different precursors. This electrode successfully integrates the benchmark HER (Ni-Mo) and OER (Ni-Fe) species into a single electrode. As a result of remarkable activities for both HER and OER, the NiFeMo electrode exhibits a low voltage of 1.45 V for overall water splitting, which outperforms the current reported bifunctional electrocatalysts. High-resolution transmission electron microscopy reveals that nanometer-sized single crystal domains of Ni, Fe, and Mo are intimately integrated, which enables a synergistic effect of metallic Ni, Fe, and Mo for efficient HER; while self-formed Ni-Fe-Mo (oxy)hydroxides on the surface of NiFeMo anode become active sites for OER. Such multi-metallic alloy and its (oxy)hydroxides represent a typical HER/OER catalyst couple, and our method provides a new route to develop efficient low-cost metallic alloys for overall water splitting.