Phosphorization For Transition Metal Molybdate Electrodes Of Water Electrolysis

It would be of great significance for human's life to develop hydrogen energy which is considered as the clean, efficient and sustainable new energy. Water electrolysis shows much advantage over other methods for its highly efficient gas productivity and high hydrogen purity, and thus be widely applicated. Water electrolysis process could be divided into oxygen evolution on anode and hydrogen evolution on cathode. Excessive overpotential on two aforementioned electrodes for gas evolution results in huge consumption of electric energy and high cost in water electrolysis industries. At present, noble metals materials with best hydrogen and oxygen evolution activities are difficult to be commercialized due to their limited storages and exorbitant price. Hence, it is especially important to develop highly efficient non-noble metals catalysts for water electrolysis. Based on this situation, we have prepared a bifunctional non-noble metals electrode that could catalyze hydrogen and oxygen evolution simultaneously, and explored the physical and electrochemical properties of it as well in this work. Details are listed as follows:(1) Utilizing in-situ hydrothermal method to grow nickel molybdate precursors on the surface of Ni foam, and this precursors were phosphorized at low-temporeture. The phosphorized nickel molybdate with unique semi-spherical flowers structures consisted of nanocuboids was produced by the optimum preparation conditions explored on this paper. The result came from XRD indicates that the crystal state of phosphorized nickel molybdate had be poor after phosphorization, but Ni2P4O12 appeared in phosphorized nickelmolybdate, the characteristic diffraction peaks of nickel molybdate precursors disappeared and no obvious diffraction peaks about molybdenum species were found. Maybe amorphous low-valence molybdenum oxide formed in phosphorized nickel molybdate. Analysis of XPS certifies that phosphorus species in the phosphorized nickel molybdate includes phosphides, phosphatesand oxides of phosphorus. The further research found that electrochemical activation could(be affected) impact morphology, composition and electrochemical activity of the phosphorized nickel molybdate. The results of phosphorized electrode after electrochemical activation as following: 1) the surface of nanocuboids is crude, which prompts the exposure of internal active sites and boosts(13.6%) the active specific surface area of catalysts. 2) Ni2+ came from surface of the activated phosphorized nickel molybdate partly transformed into Ni3+, and formed hydroxide of nickel. 3) Electrochemical activation could enhance the conductivity of electrodes. It's worth noting that appropriate phosphates on the surface of electrode after activation could be regarded as protective agent to enhance the stability of electrode. Based on the above advantages, we confirm that the activated phosphorized nickel molybdate electrode possesses great activity and stability for HER.(2) Utilizing the optimum preparation conditions in this paper to produce phosphorized transition metal molybdate electrodes and research their HER and OER catalystic activity. The electrochemical test indicates that HER activity of the phosphorized nickel molybdate electrode is the best, the next is phosphorized cobalt molybdate electrode, and phosphorized ferrous and manganese molybdate electrode are the last. And OER activity of the phosphorized nickel molybdate electrode is the best, the next is phosphorized cobalt molybdate electrode. The good HER and OER activities of phosphorized nickel molybdate attribute to its large active specific surface area and small charge-transfer resistance. In addition, we also explored the relationship between the morphology(nanocuboids vs. nanowires) of phosphorized nickel molybdate and HER activity based on excellent HER activity of phosphorized nickel molybdate nanocuboids. The result indicates that HER activity of the nanocuboids catalyst is superior to the nanowires. To investigate the potential industrial application of phosphorized nickel molybdate electrode, we further test its HER activity in 6.0 M NaOH. The results show that this electrode only needs 94 mV at a current density of 100 mA/cm2 for HER and has obvious advantages in the industrial application compared with the other electrodes. In order to better accommodate industrial conditions, we used Ni mesh as substrate to produce phosphorized nickel molybdate/Ni mesh electrode under the same preparation conditions. The results show that this electrode still has good activity for HER, which further demonstrates that prospects of applying phosphorized nickel molybdate to water electrolysis industry are broad.