Electrochemical process for water splitting using porous Co3O4 nanorods

Abstract

The present invention discloses an electrochemical process for water splitting for production of oxygen using porous Co.sub.3O.sub.4 nanorods with a considerably low overpotential and high exchange current density. The present invention further discloses a simple, industrially feasible process of for preparation of said nanostructured porous cobalt oxide catalyst thereof.

Claims

1. An electrochemical process for water splitting using porous Co.sub.3O.sub.4 nanorods as anode material for production of oxygen comprising placing an anode and a cathode in an electrolyte solution comprising a mixture of water and an acid salt that is present in a concentration between 0.05 and 0.15 M at pH ranging between 4-14, applying a potential across the anode and cathode sufficient to produce oxygen; wherein the Co.sub.3O.sub.4 nanorods has an overpotential in the range of 385 mV to 400 mV at 1 mA/cm.sup.2 and exchange current density in the order of 10.sup.6 A/cm.sup.2 (5-810.sup.6 A/cm.sup.2).

2. The electrochemical process as claimed in claim 1, wherein external source of energy used is electrical energy.

3. The electrochemical process as claimed in claim 1, wherein porous Co.sub.3O.sub.4 nanorods is coated on stainless steel or glassy carbon electrode as anode material.

4. The electrochemical process as claimed in claim 3, wherein porous Co.sub.3O.sub.4 nanorods used as catalyst have pore size ranging from 2 nm to 10 nm and BET surface area in the range of 145-155 m.sup.2/g.

5. The electrochemical process as claimed in claim 1, wherein the anode material used optionally comprises vulcanized carbon and Nafion solution having the ratio of Co.sub.3O.sub.4:vulcanized carbon:Nafion as 7.5:2:0.5.

6. The electrochemical process as claimed in claim 1, wherein the anode material is characterized by having faradaic efficiency in the range of 85% to 95%, preferably 90%.

7. The electrochemical process as claimed in claim 1, wherein the acid salts used are selected from potassium di-hydrogen phosphate (KH.sub.2PO.sup.4), di-potassium hydrogen phosphate (K.sub.2HPO.sub.4) either alone or in combinations thereof.

8. The electrochemical process as claimed in claim 1, wherein cathode is selected from platinum or platinized graphitic electrodes.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 depicts comparison of tafel plots

(2) FIG. 2 depicts comparison of the amount of evolved oxygen gas (in mol) with the theoretical value and the faradaic efficiency.

(3) FIG. 3 depicts the stability of the catalyst where the oxygen evolution measurement is done up to 5 cycles.

(4) FIG. 4 depicts BET data and pore size distribution of Co.sub.3O.sub.4.

DETAILED DESCRIPTION OF THE INVENTION

(5) The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

(6) Overpotential in electrolysis refers to the extra energy required than thermodynamically expected to drive a reaction. It is commonly needed when two or more molecules take part in a reaction at an electrode. Production of oxygen by oxidation of water is a complex reaction which requires a four-electron oxidation of two water molecules coupled with the removal of four protons to form a relatively weak oxygen-oxygen bond. In addition to controlling this proton-coupled electron transfer (PCET), a catalyst must tolerate prolonged exposure to oxidizing conditions. Even at the thermodynamic limit, water oxidation requires an oxidizing power that causes most chemical functional groups to degrade. Accordingly, the generation of oxygen from water presents a substantial challenge toward realizing artificial photosynthesis.

(7) To make the process commercially practical it is important to reduce the overpotential losses during electrolysis of water and improve upon the exchange current density which is a measure of rate of reaction at equilibrium potential. Accordingly, the present invention is directed to cost effective nanostructured porous Co.sub.3O.sub.4 nanorods as anode material to produce oxygen from water efficiently at a considerable low overpotential and at a high exchange current density. Since oxygen evolution is the rate determining step in water splitting, increasing the rate of oxygen evolution ultimately increases the rate of overall water splitting. In the process, hydrogen is evolved with less amount of energy supply.

(8) The nanostructured porous Co.sub.3O.sub.4 nanorods used, due to increase in the surface area minimizes the overpotential losses thereby enhancing the efficiency of the cell during the electrochemical splitting of water to produce oxygen.

(9) In an embodiment, the present invention provides cost effective Co.sub.3O.sub.4 nanorods catalyst with a pore size ranging from 2 nm to 10 nm and with a BET surface area in the range of 145-155 m.sup.2/g for production of oxygen by electrochemical splitting of water at low overpotential.

(10) The presence of the high surface area of the catalyst improves its performance and lifetime in the electrolysis reaction. Moreover, Co.sub.3O.sub.4 catalyst is inert, has very less solubility in water under experimental condition.

(11) The anode is made from Co.sub.3O.sub.4 nanorods catalyst coated on stainless steel (SS 316) or glassy carbon electrode. The cathode is selected from platinum or platinized graphitic electrodes.

(12) The electrolyte solution used in the instant invention is a mixture of water and an acid salt that is readily soluble in water thus improving the conductivity of water and such that the negative and the positive ions have low standard electrode potential, also acts as a buffer. The acid salt is preferably selected from potassium dihydrogen phosphate (KH.sub.2PO.sub.4), dipotassium hydrogen phosphate (K.sub.2HPO.sub.4) either alone or in combination thereof. The pH of the reaction is adjusted by adding KH.sub.2PO.sub.4, K.sub.2HPO.sub.4 or their combinations.

(13) For pH 4, 0.1M KH.sub.2PO.sub.4 is used, for pH 7 combination of KH.sub.2PO.sub.4 and K.sub.2HPO.sub.4 (21.1 ml of 0.1M KH.sub.2PO.sub.4 and 28.9 ml of K.sub.2HPO.sub.4 diluted to 100 ml) is used.

(14) In an embodiment, the overpotential at 1 mA using Co.sub.3O.sub.4 nanorods with phosphate buffer as electrolyte and with a pH 4 is reduced to 389 mV and with the exchange current density of 6.510.sup.6 A/cm.sup.2.

(15) Exchange current density is a measure of rate of reaction at equilibrium potential. In the present invention, the exchange current density observed is 10.sup.5 times higher the rate known in the art indicating the high reaction rate of oxygen production using nanostructured porous Co.sub.3O.sub.4 nanorods as anodic catalyst.

(16) In a preferred embodiment, the present invention disclose a catalyst composition comprising porous cobalt oxide nanorods as anode material for production of oxygen by electrochemical splitting of water with overpotential in the range of 385 mV to 400 mV at 1 mA/cm.sup.2 and exchange current density in the range of 5810.sup.6 A/cm.sup.2 at a pH in the range of 4 to 14.

(17) In a preferred embodiment, the present invention discloses a catalyst composition comprising porous cobalt oxide nanorods as anode material for production of oxygen by electrochemical splitting of water with overpotential in the range of 385 mV to 400 mV at 1 mA/cm.sup.2 and exchange current density in the order of 10.sup.6 A/cm.sup.2 at a pH in the range of 4 to 14.

(18) The invention uses an external source of energy. A steady state galvanostatic method is used to construct the Tafel plot. A-constant stirring (400 rpm) is maintained throughout the experiment to avoid the contribution of current from mass transfer. IR correction is carried out manually by measuring the cell resistance before applying the current.

(19) In another embodiment, the present invention provides a method for production of oxygen by electrochemical splitting of water characterized with overpotential in the range of 385 mV to 400 mV at 1 mA/cm.sup.2 and exchange current density in the order of 10.sup.6 A/cm.sup.2 at a pH in the range of 4 to 14 comprising placing an anode and electrode in an electrolyte solution comprising a mixture of water and an acid salt that is readily soluble in water, wherein the negative and the positive ions have low standard electrode potential and also acts as a buffer and wherein the anode comprises a substrate coated with porous Co.sub.3O.sub.4 nanorods that act as catalyst; and electrochemical splitting of water for production of oxygen using an external source of energy. The faradaic efficiency of said catalyst is in the range of 85% to 95%, preferably 90%.

(20) The electrochemical properties of the catalyst for oxygen evolution are determined from Tafel plot. The overpotential and the exchange current density are tabulated in Table 1 below:

(21) TABLE-US-00001 at low overpotential Overpotential At high overpotential Exchange at Exchange Tafel current Tafel 1 mA/cm.sup.2 current slope density(A/ slope Material Electrolyte pH (mV) density(A/cm.sup.2) (mV) cm.sup.2) (mV) Co.sub.3O.sub.4 0.1M 4 389 6.5 10.sup.7 122 6.55 10.sup.6 245 nanorods KH.sub.2PO.sub.4 Co.sub.3O.sub.4 0.1M 7 385 4.7 10.sup.9 72 7.6 10.sup.6 254 nanorods (KH.sub.2PO.sub.4 + K.sub.2HPO.sub.4) Co.sub.3O.sub.4 1M KOH 14 399 .sup.3.62 10.sup.13 42 5.4 10.sup.6 249 nanorods Co.sub.3O.sub.4 0.1M 4 1470 1.25 10.sup.8 305 Bulk KH.sub.2PO.sub.4

(22) A comparison of the exchange current density and the overpotential w.r.t the prior arts are given below in Table 2:

(23) TABLE-US-00002 Exchange current Overpotential Material density (mV) Reference Co.sub.3O.sub.4 4.7 10.sup.9 The particular overpotential International nanowires A/cm.sup.2 value is not mentioned in the Journal of (1M publication. Only the exchange hydrogen NaOH) current density values are energy, compared. Moreover, it is very 36 (2011) 72 difficult to compare the overpotential values of a catalyst under different experimental conditions and different substrates. In the above mentioned journal report, the substrate used is Ni foam which can go to higher current density region. The substrate used in the present work is glassy carbon electrode. Cobalt 4-6 10.sup.11 410 Nocera et al, based A/cm.sup.2 Science 321 phosphate (2008) 1072 compound (pH 7)

(24) Quantitative oxygen evolution measurement has been done to check the faradaic efficiency of the catalyst in a homemade set up. 1 mA/m.sup.2 current is applied for long time. The evolved hydrogen and oxygen bubbles are separated in the home made set up. Only the oxygen bubbles are collected in the inverted measuring cylinder. The amount of evolved oxygen gas (in mol) is compared (FIG. 2) with the theoretical value and the faradaic efficiency is calculated as 90%.

(25) To check the stability of the catalyst, the oxygen evolution measurement is done up to 5 cycles (FIG. 3). Each cycle takes around 8 hours. The faradaic efficiency is relatively constant and dosen't change much. The stability of the catalyst is tested up to 40 hours which proves the catalyst has the capability to work in a wide range of pH (4-14) without degradation for long time.

(26) The BET data (FIG. 4) shows good adsorption and desorption behavior. The BET surface area calculated is 150 m.sup.2/g and the pore size in the range of 2 nm to 10 nm radius.

(27) In yet another embodiment, the present invention provides a simple process for the preparation of said nanostructured porous cobalt oxide, Co.sub.3O.sub.4. Accordingly, 0.0339M of Co(NO.sub.3)2.6H.sub.2O and 0.0627M of K.sub.2CO.sub.3 are added drop by drop simultaneously to the RBF which contain distill water at 70 C. The pH of the solution is maintained in the range of 7-8. After completion of precipitation, the obtained precipitate is digested in the same mother liquor for 8 hours at 70 C. The obtained precipitate is washed several times with distill water to remove K.sup.+ ions and dried at 70 C. overnight and calcined at 300 C. to obtain Co.sub.3O.sub.4 nanorods.

(28) It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative examples and that the present invention may be embodied in other specific forms without departing from the essential attributes thereof, and it is therefore desired that the present embodiments and examples be considered in all respects as illustrative and not restrictive.

EXAMPLES

(29) The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

Example 1

Preparation of Catalyst

(30) Co.sub.3O.sub.4 nanorods were synthesized by simple co-precipitation method. 0.0339M of Co(NO.sub.3)2.6H.sub.2O and 0.0627M of K.sub.2CO.sub.3 were added simultaneously drop by drop to the water (70 ml) in the RBF (round bottom flask) at 70 C. The pH of the solution was maintained at 7.5. The precipitate obtained after completion of addition was digested within the mother liquid for 8 hours. The precipitate so obtained was washed several times to remove K.sup.+ ions and dried at 70 C. followed by calcination of the precursor at 300 C. (3 Hrs) to obtain Co.sub.3O.sub.4 nanorods. The process for the preparation of Co.sub.3O.sub.4 nanorods is already disclosed in Talanta, 2010, 81, 37-43.

Example 2

Electrochemical Measurements

(31) For all electrochemical measurements, a simple three electrode system was used. Glassy carbon was used as a working electrode. Standard calomel electrode and platinum were used as a reference and counter electrode respectively. 0.1M KH.sub.2PO.sub.4 solution was used as electrolyte for pH 4. A 0.1M mixture of KH.sub.2PO.sub.4 and K.sub.2HPO.sub.4 solution was used as electrolyte for pH 7, and 1M KOH solution was used as electrolyte for pH 14. To improve the conductivity, vulcanized carbon was added to the Co3O4 and nafion solution was added for binding purpose. The ratio of Co3O4:vulcanized carbon:nation is 7.5:2:0.5. The mixture was dispersed in isopropyl alcohol and coated on the glassy carbon electrode for electrochemical analysis.

(32) Addition of vulcanized carbon and nafion solution (very dilute solution and not membrane) to the catalyst is the usual procedure for coating the catalyst on the electrode. It doesn't have much importance and need not be shown in the table.

(33) The electrochemical properties of the catalyst for oxygen evolution are given below:

(34) TABLE-US-00003 at high at low overpotential overpotential Exchange Exchange Overpotential current Tafel current Tafel at 1 mA/cm.sup.2 density(A/ slope density slope Material Electrolyte pH (mV) cm.sup.2) (mV) (A/cm.sup.2) (mV) Co.sub.3O.sub.4 0.1M 4 389 6.5 10.sup.7 122 6.55 10.sup.6 245 nanorods KH.sub.2PO.sub.4 Co.sub.3O.sub.4 0.1M 7 385 4.7 10.sup.9 72 7.6 10.sup.6 254 nanorods (KH.sub.2PO.sub.4 + K.sub.2HPO.sub.4) Co.sub.3O.sub.4 1M KOH 14 399 3.62 10.sup.13 42 5.4 10.sup.6 249 nanorods Co.sub.3O.sub.4 0.1M 4 1470 1.25 10.sup.8 305 mV Bulk KH.sub.2PO.sub.4

Advantages of the Invention

(35) 1. The present invention provides an improved nanostructured porous cobalt oxide (Co.sub.3O.sub.4) as catalyst in electrochemical splitting of water to produce oxygen at considerable reduced overpotential and high exchange current density. 2. Increasing exchange current density. 3. The nanostructured porous cobalt oxide (Co.sub.3O.sub.4) of the instant invention can be used as anode material in electrolyzers to produce oxygen efficiently at low energy supply compared to the conventional precious metal oxides. 4. Hydrogen and oxygen obtained are separate and this is an advantage over fuel cell and can decrease global warming. 5. The catalyst is prepared externally and is crystalline in nature with high surface area, BET150 m.sup.2/g and the pore size in the range of 2 nm to 10 nm radius. 6. The present method for synthesis of catalyst is simple co-precipitation at 70 C. form cobalt hydroxy carbonate nanorods followed by calcination at 300 C. to obtain Co.sub.3O.sub.4 nanorods.