Abstract:
This invention provides a simple approach for the straightforward and direct preparation of iron-oxide based electrocatalytic materials film (FeO x —Ci) on a simple Fe substrate by controlled surface-anodization and/or self-deposition in simple and low-cost carbonate buffer. The FeO x —Ci based electrocatalysts may advantageously be employed as electrode and as anode material in water oxidation, water conversion systems and fuel generation assemblies. The FeO x —Ci exhibits remarkably low over potential (η≈360) for anodic oxygen evolution relative to other Fe-oxide based catalysts, and show very high activity and stability for long-term water electrolysis operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention describes a process for forming and electro-assembling of metal oxide electrocatalytic material, a process to prepare an anode material, an electrochemical cell, and a process to convert water by electrochemical technology into oxygen and protons, via controlled surface-anodization and/or self-deposition of a simple iron surface and/or iron-derived substrates and alloys. 
         [0002]    Water, using renewable electricity and/or solar energy, can be converted into oxygen and protons via processes referred to as catalytic water oxidation or water splitting in an electrochemical system or via photo-electrochemical (PEC) methods. This is a promising technology and a feasible process for the direct conversion of light energy into renewable fuels and cheap energy carriers using simple water. The beauty of water splititng is the release of four electrons and four protons per O 2  trunover, that can be used either to make hydrogen as clean and high energy density fuel or in combination of CO 2  to direclty reduce it, and to convert it into useul nonfossil feuls and chemical energy carriers ( FIG. 1 ). This scheme looks promissing and provides a route to renewable and alternative energy carriers obtain from abundant water and enormous sun light. 
         [0003]    Many millions of years ago, nature has devised an efficient system to convert water and CO 2  into energy storable substances using sun light. In natural photosynthesis, the catalytic oxidation of water in photosystem II (PS-II) is facilitated by the presence of MnCaO based water oxidation material/complex that splits water with high efficiency and at a tremendous rate. Scientists are trying hard to mimic this state-of-the-art material in labs using both material-science and molecular approaches to be obtained from cheap sources and earth-abundant elements. 
         [0004]    Ru-oxide and Ir-oxide are established and benchmarking materials for electrochemical water splitting. But they are too expensive to be employed on large scale application. Recently, catalytic materials based on oxides of the abundant first row transition metals such as Ni, Co, Mn and Cu have been emerged as substitute of the noble metals based electrocatalysts. 
         [0005]    These transition metal-oxide electrocatalysts were developed by conducting substrates from metal ions solutions under electrochemical conditions. The presence of metal ions is a prerequisite for their activity and long-term water electrolysis performance and metal ions may possibly interact to contaminate and poison the cathode for the reduction reaction. In order to avoid the metal ions interaction, membranes or separators are usually employed, which make the system more complex and introduce resistance and diffusion limitations. Thus, new materials and methods are required to develop high activity water oxidation electrocatalysts. At the same time, there is a need to develop easily accessible and robust water oxidation catalytic systems operating at low overpotential with high rate turnover for anodic oxygen evolution and performing with high stability for long-term application. 
         [0006]    Applicant discovered a simple method for the formation of nanoscale metals-based and metal-oxides based electrocatalysts to be advantageously employed as electrode and as anode materials in water oxidation, water conversion systems and fuel generation assemblies. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Iron is interesting metal and it is the most abundant element among transition metals in the earth&#39;s crust. Iron is also the main component of many biological systems and enzymes for oxygen activation. Iron-oxide (Fe 2 O 3 ) is a very good candidate for photocatalytic water oxidation, however iron or iron-oxide based materials have been scarcely explored for anodic oxygen evolution reactions. 
         [0008]    It is difficult to prepare iron-oxide layer via electrodeposition as it requires Fe II /Fe III  ions which easily precipitates out from water under near-neutral conditions and Iron-oxide is not stable in low pH solutions. 
         [0009]    The present invention is a process for the direct preparation, electrodeposition and surface-assembling of iron-based and/or iron-oxide based electrocatalysts and/or anode materials by surface-anodization and/or self-deposition of an amorphous iron and/or iron-derived substrates and alloys in simple but not limiting to bicarbonate/carbonate (HCO 3   − /CO 3   2− ) buffer system. 
         [0010]    Next, the present invention comprising the steps of: (1) surface cleaning of simple amorphous iron and/or iron-derived substrates and alloys with neat water, following cleaning with dilute acid and washing with water, and (2) immersing the clean amorphous iron and/or iron-derived substrates and alloys as an anode in an aqueous bicarbonate/carbonate (HCO 3   − /CO 3   2− ) buffer system at a pH in the range from 8.5 to 13.5, and (3) applying a current over the anode and cathode suitable for electrolytically surface-anodizng and/or self-depositing the iron-based and/or iron-oxide based electrocatalysts and/or anode materials, and (4) using the thus obtained surface-assembled electrocatalytic material in a suitable electrolyte systems of water electrolysis and its conversion into fuel. 
         [0011]    Further, the present invention relates to the use of the iron-derived material as an electrolysis catalyst applicable to a wide range of pH and variety of electrolyte systems. 
         [0012]    Further again, the present invention relates to an iron-based and/or iron-oxide based catalytic electrode material having moderate water electrolysis overpotential (η) from 300 to 500 mV. 
         [0013]    Further again, the present invention relates to an iron-based and/or iron-oxide based catalytic electrode material having very high activity and stability for long-term water electrolysis systems. 
         [0014]    Further again, the present invention relates to simple and direct formation of an iron-based and/or iron-oxide based catalytic material thus avoiding the difficulties during electrodeposition from metal ions in the neutral and above neutral pH system that cause the precipitation and reduce catalytic formation and electro-activity. 
         [0015]    Further again, the present invention relates to an iron-based and/or iron-oxide based material catalytically active in metal-ions free phosphate, borate, carbonate, hydroxide or other aqueous electrolytes. 
         [0016]    Further, the present invention relates to an iron-based and/or iron-oxide based electrocatalytic materials with nanosclae surface morphology with an average particle size in the range of from 25 nm to 250 nm or more as determined by SEM microscopy. 
         [0017]    Further again, the present invention relates to an iron-based and/or iron-oxide based materials whereby the surface nanoparticles have an average thickness of from 10 to 500 nm or more as determined by SEM microscopy. 
         [0018]    The present invention is an electrochemical cell comprising an anode comprising the iron and/or iron-oxide nanoparticulate material according to the invention. 
         [0019]    Further, the present invention relates to a process to convert water into oxygen, and releasing electrons and protons, comprising an electrochemical cell according to the invention, and applying a suitable voltage to the iron-based and/or iron-oxide derived anode and a cathode, using a power source. 
         [0020]    Further, the present invention relates to a process of water conversion in an electrochemical cell according to the invention, using a suitable power source from renewable sources, hydel power, wind, and from solar energy. 
         [0021]    Further, the present invention relates to a process of oxidation, catalysis, splitting, oxidation and conversion of water and for the fuel generation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  depicts a schematic representation of the electrochemical formation of iron-based and/or iron-oxide derived electrocatalytic material for water oxidation in-line with a cathodic hydrogen evolution module. 
           [0023]      FIG. 2  depicts scanning electron microscopy (SEM) images for starting iron-substrate and for iron-oxide derived electrocatalytic materials. 
           [0024]      FIG. 3  depicts an enlarged view of the SEM (scanning electron microscopy) image of iron-oxide based catalytic material, at 250×10 3  magnifications. 
           [0025]      FIG. 4  depicts EDX (energy dispersive X-ray) spectrum of the electro-generated iron-oxide derived electrocatalytic material. 
           [0026]      FIG. 5  depicts the XPS (X-ray photoelectron spectroscopy) survey spectrum of electrochemically generated iron-oxide derived electrocatalytic sample. 
           [0027]      FIG. 6  depicts the XPS (X-ray photoelectron spectroscopy) spectrum of Fe (2p) and O (1s) in the electrochemically generated iron-oxide derived electrocatalytic sample. 
           [0028]      FIG. 7  depicts a forward current—potential sweep for oxygen evolution durong water oxidation catalysis on surface-assembled iron-oxide derived electrocatalyst in carbonate buffer (pH˜11.1), at a scan rate of 25 mV sec −1 . 
           [0029]      FIG. 8  depicts an enlarged view of the CV curve for the iron-oxide derived electrocatalyst on a concise potential window under the same conditions as for  FIG. 7 , at a scan rate of 25 mV sec −1 . 
           [0030]      FIG. 9  depicts the 1 st  and 100 th  consecutive forward potential sweeps for iron-oxide derived electrocatalyst at a rate of 25 mV sec −1 . 
           [0031]      FIG. 10  depicts a Tafel plot or log i vs overpotential (η) curve obtained for electrocatalytic oxide derived catalyst material while oxygen evolution in carbonate buffer. 
           [0032]      FIG. 11  depicts the extended period constant-current water electrolysis (CCE) on the iron oxide derived electrocatalyst in carbonate buffer (pH˜11.1) for current dnesities 15 mA cm −2  and 50 mA cm −2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The nanoscale iron-based and/or iron-oxide based electrocatalyst is generated in a metal-ions free solution during constant-current electrolysis (CCE) at a current density of 5.0 mA cm −2  in carbonate buffer (pH≈11). The surface-assembling of iron-oxide (FeO x —Ci) electrocatalyst on simple iron substrate can be ascribed to the surface electrochemical process involving the surface oxidation to from Fe n+  type surface species that quickly turned into metal hydroxide/oxide type composition on Fe surface. These initial nano-assemblies act as nuclei for the generation and growth of nano-structured FeO x —Ci electrocatalyst on simple iron substrate. (Studies are in progress to explore more insight into the mechanism of FeO x —Ci generation on Fe surface in carbonate buffer). Scanning electron microscope image shows nicely distributed nano-structures of FeO x —Ci on the entire surface of the anodized Fe substrate ( FIG. 2 a   ). These nanoscale surface structures appear to be amalgamation of nanoparticles on the flat Fe surface ( FIG. 2 b   ). The enlarged view of the SEM picture clearly reveals nano particulates type structures with spongy texture ( FIG. 3 ). The electro generated nano particulate iron-oxide looks fairly uniform in size representing their controlled oxidative electrochemical generation. 
         [0034]    EDX (energy dispersive X-ray) measurements for the elemental composition show Fe and O in the FeO x —Ci electrocatalyst sample ( FIG. 4 ). There is a minor contribution from Na and about 7% carbon contents are also present in the catalytic deposit. Previously, we noticed that the electro-induced deposition of Co—Ci and Ni—Ci based water oxidation catalytic materials in carbonate/bicarbonate systems incorporated carbon assimilation in the catalytic deposits that was ascribed to their enhanced catalytic efficiency. Carbon based materials are thought to support electron transfer while introducing superior physical properties such as high surface areas and electronic communication, structural flexibility and enhanced mechanical strength. 
         [0035]    The surface composition of the nano particulate FeO x —Ci is examined by X-ray photoelectron spectroscopy (XPS). The elemental detection on the XPS survey for electro generated iron-oxide layer indicates the presence of iron, oxygen and carbon in the catalytic film ( FIG. 5 ). XPS spectral signatures for the Fe2p core levels are present in the XPS region from 708 eV to 738 eV ( FIG. 6 a   ) showing the characteristic peaks of Fe(III) in the deposited layer as the energy separation for high binding energy Fe 2p3/2 between the main envelope (710.8 eV) and the satellite peak (719.1 eV) is 8.5 eV. The other lower energy level Fe 2p1/2 exhibits 724.6 eV as envelop with a satellite peak at 733 eV. The binding energy data for Fe 2p3/2 is consistent with the presence of only Fe(III) oxidation state, and the presence of Fe(II) can be excluded as it has 5 eV binding energy difference between the main envelope and the satellite. Further, the O 1 s binding energy region on the XPS spectrum clearly shows the lower-binding-energy peaks at 530.0 eV and 531.4 eV ( FIG. 6 ). This suggests that there is no presence of amorphous FeOOH based material in the catalytic layer. The binding-energy peak at 530.0 eV is assigned to the oxygen atoms of oxide ions (in metal oxide) whereas the 531.4 eV signal is ascribed to the hydroxy groups in the FeO x —Ci layer. 
         [0036]    For water oxidation catalysis using FeO x —Ci electrocatalyst, voltammetry and long-term water electrolysis experiments are undertaken in clean carbonate buffer solutions. The forward sweep voltammetry for the iron-oxide shows onset of the catalytic current for oxygen evolution at ˜1.59 V vs RHE (ηon=360 mV), following a sharp rise in the current density ( FIG. 7 ). The current density further grows rapidly reaching 10 mA cm −2  and 20 mA cm −2  at 1.71 V vs RHE (η=470 mV) and 1.74 V vs RHE (η=510 mV), respectively. The 10 mA cm −2  current density of is an optimal requisite to achieve 10% efficiency for the solar-to-fuel conversion system. For FeO x —Ci electrocatalyst, the oxygen onset potential of E on =1.59 V; η on =360 mV ( FIG. 8 ), is much less than for recently reported Fe-oxide based catalysts showing much higher E on  (η on &gt;500 mV). The O 2  onset potential for FeO x —Ci is also much lower compared to other metal oxides electro catalysts such as Co—Pi (1.67 V vs. RHE), Ni—Bi (1.71 V vs. RHE). This makes FeO x —Ci electrocatalyst a new electrodeposited Fe-based benchmark material for anodic water oxidation. 
         [0037]    The repetitive potential sweeps for FeO x —Ci electrocatalyst sample reproduce the similar current density signatures for the 1st and 100th scan suggesting no noticeable degradation of FeO x —Ci system and representing remarkable stability and long-time activity of the new Fe-based electro catalyst ( FIG. 9 ). 
         [0038]    Current—over potential (η vs log i) plot of the FeO x —Ci electrocatalyst during oxygen generation produces a Tafel slope of 47 mV dec −1  ( FIG. 10 ). A Tafel slope of 47 mV dec −1  is very impressive and unique for the iron-oxide based electro catalyst, as other Fe-oxide based electro catalytic systems show much higher Tafel slopes (Table 1). A small Tafel slope in important as water oxidation electro catalyst is desired to operate over a narrow potential window for high performance, and this small current-voltage window for FeO x —Ci is attractive for the integration with photo-responsive materials. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Electrochemical and catalytic water oxidation data for different 
               
               
                 electrochemically generated Fe-oxides based electrocatalysts. 
               
             
          
           
               
                   
                 O 2  onset 
                   
                   
                   
               
               
                   
                 Eon (vs 
                 η at J = 10 mA 
                 Tafel Slope 
                   
               
               
                 Catalyst/Substrate[a] 
                 RHE) [b]   
                 cm −2[c]   
                 (mV dec −1 ) 
                 Ref 
               
               
                   
               
               
                 FeO x —Ci/Fe 
                 1.59 V 
                   470 mV 
                 47 
                 This 
               
               
                   
                   
                   
                   
                 Work 
               
               
                 FeOx/ITO 
                 1.67 V 
                 ~730 mV 
                 52 
                 16 
               
               
                 FeOOH/FT O 
                 1.73 V 
                 ~560 mV 
                 — 
                 23 
               
               
                   
               
               
                   [a] FeO-based electro catalysts prepared by electrochemical methods. 
               
               
                   [b] Oxygen onset taken from the anodic current at J &gt;0.1 mA cm −2 . 
               
               
                   [c] Overpotential require to achieve a current density of 10 mA cm −2 . 
               
             
          
         
       
     
         [0039]    For long-term water electrolysis testing and stability performance of the FeO x —Ci based electro catalyst, electro catalytic experiments are conducted in clean metal ions free carbonate solution. We chose constant-current electrolysis (chronopotentiometry) experiments while preserving stable current densities of 15 mA cm −2  and 50 mA cm −2  and monitoring the potential response of the system at the same time. The FeO x —Ci electrocatalyst remains remarkably stable during high activity oxygen evolution at current densities of 15 mA cm −2 . To achieve 15 mA cm −2 , a very stable steady-state potential of ˜1.75 V (vs RHE) is preserved for 17 hours of the catalytic water electrolysis ( FIG. 11 ). 
         [0040]    Meanwhile, a rich stream of oxygen bubble is also coming out of FeO x —Ci surface as monitored by online GC. Further, the current density is switched to a very high magnitude of 50 mA cm −2  which is maintained at just ˜2.15 V (vs RHE) in clean carbonate system ( FIG. 11 ). Remarkably, the monitored potentials 1.75 V and 2.15 V (vs RHE) to get these high current densities for oxygen evolution are stable and sustained for long time. In both instances, there is no noticeable potential change or catalytic degradation during the water electrolysis test, which is a direct indication of the stability and superior catalytic performance of the FeO x —Ci electrocatalyst during extended-period water electro oxidation. The chronopotentiometry data is very impressive for electro catalytic FeO x —Ci system representing its remarkable activity and stability in clean electrolyte solution. For a recently reported FeOx derived catalytic film (electrodeposited from Fe(II) in acetate solution), controlled-potential water electrolysis at ˜1.76 V (vs RHE) in a phosphate buffer (1.35 V; pH=7) to maintain a very small current density of approximately 0.90 mA cm −2 . During 10 h water electrolysis, decrease in oxygen evolution current density is also observed indicating the catalytic degradation of the Fe-oxide material. This shows that FeO x —Ci is a new low over potential and high activity Fe-based water oxidation electrocatalyst. 
         [0041]    A comparative analysis of different Fe-oxide based water oxidation eletrocatalysts and their electrochemical performance for oxygen evolution is presented in Table 1. It is evident that FeO x —Ci exhibits the lowest onset potential of 1.59 V vs RHE (η=360 mV) relative to other Fe-based catalysts. FeOOH type Fe-catalyst exhibits the highest onset over potential, i.e &gt;1.70 V vs RHE. We show that the benchmark current density of 10 mA cm −2  is achieved at η≈470 mV for FeO x —Ci sample. Other Fe-oxide based catalysts exhibit much higher over potentials to reach 10 mA cm −2 . Surface-generated FeO x —Ci system also shows the smallest Tafel Slope 47 mV dec −1 , which is again lowest in the list of Fe-based eletro catalyst.