Hydrogen Production By Water Splitting With Synergetic Composite Metals Under Hydrothermal Conditions

Hydrogen has been regarded as one green and clean energy,and its utilization is of great importance to mitigate the energy crisis and for a sustainable development of human society.Dissociation of water,one of the most abundant resources on earth,to hydrogen has been regarded as one of the most promising ways to provide the clean energy.At present,methods of hydrogen production from water mainly include photocatalytic way and electrochemical way.The electrochemical water splitting method has problems of high energy consumption,low efficiency and high cost.And photocatalytic water splitting has disadvantages such as low reaction efficiency,low energy conversion efficiency and immature technology,which limit its practical application.Therefore,developing a new,efficient and low-cost method to dissociate water for hydrogen production is of great significance to mitigate the fossil fuel depletion and to achieve a sustainable development.In our previous studies,it has been found that earth-abundant metals（such as iron,manganese,zinc,aluminium,etc.）can in situ produce hydrogen from high-temperature water（HTW）to reduce carbon dioxide.However,previous studies were focused on carbon dioxide reduction.More imporantly,these studies could only use highly active metals（such as iron,manganese,zinc and aluminum）to produce hydrogen,and the oxides of these metals were difficult to be reduced to elemental metals,which could not achieve metal recycling.Therefore,this paper carried out a study on the feasibility and effectiveness of using the less active metal（Ni and Co）to produce hydrogen by water splitting,and studied the feasibility of using biomass to restore metal oxide to achieve metal recycling and regeneration.Meanwhile,the reaction mechanism was studied.First,the feasibility and reaction characteristics of hydrogen production by high-temperature water splitting with Ni as a reducing agent were studied,and the promotion effect of the addition of copper on hydrogen production was studied.Meanwhile,the effects of sodium chloride and other inorganic salts on hydrogen production were also studied.The results showed that few hydrogen was produced when only Ni was added into water.However,when Cu was added to the Ni-water system,especially when inorganic salt（NaCl）was added,Ni could obviously decompose water,and Ni was oxidized into NiO.After optimizing the experimental conditions,a 26.7%hydrogen yield,which is defined as the ratio of the amount of hydrogen produced to the amount of metal added,was finally obtained with 9 mmol Ni,9 mmol Cu,0.4 mol/L NaCl,30%water fiiling rate at 300°C for 3 h.Then,the feasibility of hydrogen production by high-temperature water splitting with Co as a reducing agent were studied,and the effect of the addition of copper,sodium chloride and other inorganic salts on hydrogen production was studied.The results showed that few hydrogen was produced when only Co was added into water.However,when Ni was added to the Co-water system,especially when inorganic salt（NaCl）was added,Ni could obviously decompose water,and Co was oxidized into CoO.Results showed that,an optimum hydrogen yield of 74.3%was successfully obtained with 12 mmol Co,12 mmol Ni,0.5 mol/L NaCl,40%water filling rate at 300 ^oC for 3 h.As a synergetic metal,Ni can provide free active sites on its surface to generate reaction intermediate of NiH_ads,which contributed to hydrogen production.Additionally,a synergetic metal with low redox potential can promote the hydrogen production rate.Moreover,in the case of adding NaCl or KCl,Cl^-ions can replace OH^-in the solvent to form CoCl₂ with Co,thus preventing metal hydroxide from forming a passivation layer on the metal surface to inhibit the hydrogen production.Then,the regeneration of metal Co from CoO was studied by reacting of CoO with glucose as a reducing agent under alkaline hydrothermal conditions.Results showed that an increase in temperature and alkali concentration can promote the the reduction of CoO to Co.CoO can be completely reduced to Co under the conditions of 12.6 mmol CoO,4 mol/L glucose,6 mol/L NaOH,30%water filling rate,8 h and 300 ^oC.Then,the reaction pathway and mechanism of reducing CoO by glucose were studied by decreasing the reaction time and examing the varieties of intermediates.Finally,hydrogen production from water with regenerated Co was studied,and a 73%hydrogen yield was successfully achieved,which is nearly the same as that obtained with fresh Co.Thus,the Co can be recycled for the hydrogen production from HTW.In this study,a new method of hydrogen production by water splitting with synergetic composite metals under hydrothermal conditions was firstly proposed.By reacting reductant,synergetic metal,inorganic salt with HTW,a 74.3%hydrogen yield and metal oxide were obtained.This research achieved hydrogen production from water with less active metals under mild hydrothermal conditions,on which the effect of synergetic metal was important.Meanwhile,the regeneration of metal Co from CoO was achieved by reacting of CoO with biomass as a reducing agent.