Porphyrin-based catalysts for water splitting

Abstract

The porphyrin-based catalysts for water splitting are composites of porphyrin or metalloporphyrin active ingredients, conductive carbon (e.g., graphene sheets, vapor grown carbon fiber, carbon black, etc.), and a polymer or binder that may be coated on a glassy carbon electrode. The polymer or binder may be Nafion oil or polyvinylidine difluoride. The porphyrin may be a porphyrin having a transition metal or hydrogen at its center, and may be halogenated and/or have a thiophene substituent.

Claims

1. A composite electrode, comprising: a glassy carbon electrode; and a porphyrin catalyst coated on the glassy carbon electrode; wherein the porphyrin catalyst comprises a composite having a mixture of: a porphyrin compound; conductive carbon; and a polymer binder selected from the group consisting of Nafion oil and polyvinylidene difluoride; and wherein the porphyrin compound has the formula: ##STR00004## wherein M is H.sub.2, Co, Ni, Cu, or Zn, and X is F, CI, or Br.

2. The composite electrode according to claim 1, wherein the catalyst is active under pH conditions between pH 1 and pH 13.

3. A method of making a composite electrode according to claim 1 with a porphyrin catalyst for water splitting, comprising the steps of: dispersing the conductive carbon in an organic solvent to form a first reaction mixture; adding the porphyrin compound to the first reaction mixture to produce a second reaction mixture and mixing the second reaction mixture; adding a polymer solution comprising the polymer binder to the second reaction mixture to produce a third reaction mixture and mixing the third reaction mixture; coating the third reaction mixture onto the surface of the glassy carbon electrode; and drying the glassy carbon electrode.

4. The method of making a composite electrode of claim 3, wherein the dispersing and mixing steps comprise ultrasonic mixing using an ultrasonic probe for one hour.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a scanning electron micrograph (SEM) of an exemplary porphyrin-based catalyst for water splitting comprising 50 wt % 5,10,15,20 tetrakis (5-bromothiophen-2-yl) porphyrin, 40 wt % vapor grown carbon fiber (VGCF) and 10 wt % Nafion oil.

(2) FIG. 2 depicts a comparison of linear sweep voltammograms of various electrodes in an aqueous alkaline media containing 1 mol. Liter of sodium hydroxide (NaOH), including a glassy carbon electrode (GC), a glassy carbon electrode coated with 90 wt % VGCF and 10 wt % Nafion (VGCF), a glassy carbon electrode coated with a cobalt-porphyrin slurry 90 wt % with 10 wt % Nafion (CoP), and a glassy carbon electrode coated with a composite including 50 wt % of a cobalt-porphyrin, 10 wt % Nafion, and 40 wt % vapor grown carbon fiber (CoP-VGCF), showing current density as a function of potential vs. a normal hydrogen electrode.

(3) FIG. 3 depicts Tafel plots corresponding to the linear sweep voltammograms of FIG. 2.

(4) FIG. 4 depicts an energy-dispersive X-ray spectrograph of a porphyrin-based catalyst for water splitting.

(5) FIG. 5 depicts a graph of current density versus potential curves recorded from a porphyrin-based catalyst for water splitting coated electrode in solutions with varying pH.

(6) Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) The porphyrin-based catalysts for water splitting are composites of porphyrin or metalloporphyrin active ingredients, conductive carbon (e.g., graphene sheets, vapor grown carbon fiber, carbon nanotube, carbon black, etc.), and a polymer or binder that may be coated on a glassy carbon electrode. The polymer binder may be Nafion oil or polyvinylidine difluoride (PVDF). The porphyrin may be a porphyrin having a transition metal or hydrogen at its center, and may be halogenated and/or have a thiophene substituent. In a preferred embodiment, the porphyrin has the structure:

(8) ##STR00002##
wherein M is H.sub.2, Co, Ni, Cu, or Zn, and X is F, Cl, or Br.

(9) The catalysts are composites comprising porphyrin powder, carbon conductive materials, and polymer solution. The porphyrin has extraordinary catalytic activity towards the oxygen evolution reaction, while the conductive carbon additives provide an enhanced electrical conductivity, which is essentially required to catalyze the water splitting reaction. Additionally, the polymer will contribute in improving the mechanical integrity of the catalyst. Combining the unique properties of the components of the composite catalyst has significantly reduced the energy required to perform the water splitting process, and hence, improved the efficiency of oxygen gas production. Also, the porphyrin-based catalyst is very stable and uniquely catalyzes the oxygen evolution reaction in acidic, neutral and basic media. Additionally, flexibility of the organic porphyrin molecules and carbon network has contributed in improving the stability of the catalyst. Moreover, the catalysts are based on low cost carbon materials and scalable, synthesizable porphyrins, which make the catalysts a very cost effective technology for producing clean, renewable energy.

EXAMPLE 1

Synthesis of Porphyrin Catalysts

(10) The targeted-porphyrins may be prepared using a one-pot, multi-step approach. Generally, a mixture of equimolar quantity of pyrrole and selected aldehydes in dimethylformamide (DMF) was heated at 100° C. under argon atmosphere. Then, a similar molar quantity of p-toluenesulfonic acid (PTSA) was added, and the reaction mixture was heated up gradually to 140° C. and kept at this temperature for one hour. The obtained dark violet solution was left to cool to room temperature. Then the dark violet solution was poured into cooled water/triethylamine (TEA) (5%) and left for 30 min. with stirring. The resulting solid precipitate was filtered, washed several times with water, air dried and purified by column chromatography (CHCl.sub.3/MeOH 99:1 v/v) and crystallization from CHCl.sub.3/MeOH. The resulting porphyrins may then be reacted with a metal acetate to produce a desired metallo-porphyrin complex as follows: a mixture of the free base porphyrin (1 mole) and metal acetate (3 moles of cobalt acetate, nickel acetate, copper acetate, or zinc acetate) is dissolved in dimethylformamide (DMF). The reaction mixture is heated and kept at 120° C. under argon atmosphere for 3 hours, left to cool, and poured into cool water. The solid product is collected by filtration, washed with water several times, air dried, and purified by column chromatography (CHCl.sub.3/MeOH 99:1 v/v) and crystallization from CHCl.sub.3/MeOH.

(11) A preferred structure of the porphyrins is:

(12) ##STR00003##
wherein M is H.sub.2, Co, Ni, Cu, or Zn, and X is F, Cl, or Br.

(13) Composite electrodes with the porphyrin-based catalysts may be made by dispersing a conductive carbon in an organic solvent (e.g., methanol, ethanol, propanol or mixtures thereof) to form a first reaction mixture, mixing the porphyrin complexes with the first reaction mixture to form a second reaction mixture, adding a polymer solution (e.g., Nafion oil, PVDF, etc.) to the second reaction mixture to produce a third reaction mixture (a thick, homogenous ink), and coating the third reaction mixture (between 100 and 200 μl) directly on a conducting substrate (e.g. a glassy carbon electrode), which is baked in a drying oven at 80° C. for ten minutes, thereby producing a composite electrode with a porphyrin-based catalyst. Each reaction mixture may be mixed using an ultrasonic probe for about an hour.

(14) The weight percentage of the porphyrin compound, conductive carbon, and polymer used to form the composite electrode may vary, exemplary weight percentages being listed in Table 1.

(15) TABLE-US-00001 TABLE 1 Exemplary Composition of Porphyrin- Based Catalyst by Weight Percent Porphyrin Conductive Carbon Polymer 80 10 10 70 20 10 60 30 10 50 40 10

(16) Exemplary porphyrin-based catalysts were made as described above. FIG. 1 shows a SEM micrograph of one such catalyst [50 wt % 5,10,15,20 tetrakis (5-bromothiophen-2-yl) porphyrin, 40 wt % vapor grown carbon fiber (VGCF) and 10 wt % Nafion oil]. In the micrograph, the carbon fibers can be easily recognized, while the porphyrin particles are visible as less dark particles physically attached to the fibers. The fibrous nature of the VGCF is also apparent, and the uniform distribution of the porphyrin-based catalyst throughout the VGCR carbon network is confirmed.

(17) Exemplary composite electrodes were made with the porphyrin-based catalysts and tested as follows. Performance of the electrocatalysts was evaluated by measuring overpotential (η), the difference between the applied potential (E) and the potential under equilibrium conditions (E.sub.eq). A low η was indicative of improved catalytic performance.

(18) The rate of oxygen evolution reaction was calculated using the Tafel equation, correlating the rate of the electrochemical reaction and the overpotential according to Equation 3:

(19) $\begin{array}{cc}\log \left(i\right)=\log \left(i_0\right)+\frac{\eta }{b},& \left(3\right)\end{array}$
where i is the current density, i.sub.o is the exchange current density (i.e., the current at zero potential), and b is the Tafel slope. The magnitude of the exchange current density (i.sub.o) reflects the intrinsic bonding/charge transferring interaction between the electrocatalyst and the reactant. A high exchange current density was used as an indicator of a suitable catalyst for the reaction being tested.

(20) Tafel slopes were also calculated by plotting log(i) versus η, producing lines with a slope of b. A small Tafel slope indicates that current density will increase significantly with little change in overpotential, thus indicating a faster reaction rate for oxygen evolution. Thus, small Tafel slopes were used as indicators of better performing electrocatalysts. Tafel slopes were calculated according to Equation 4:

(21) $\begin{array}{cc}b=\frac{\partial \eta }{\partial \log i}.& \left(4\right)\end{array}$

(22) For this example, four different composite electrodes were tested, including a bare glassy carbon electrode (GC), a GC electrode coated with 90 wt % vapor grown carbon fiber and 10 wt % Nafion (VGCF), a glassy carbon electrode coated with 90 wt % cobalt-porphyrin compound and 10 wt % Nafion (CoP) (but no conductive carbon), and a glassy carbon electrode coated with 50 wt % cobalt-porphyrin compound, 10 wt % Nafion, and 40 wt % VGCF (CoP-VGCF). The weight load of the coated electrodes was 1 mg/cm.sup.2.

(23) FIG. 2 depicts linear sweep voltammograms recorded in 1 M NaOH solution at a scan rate of 100 mV/s for all four tested electrodes. Electrochemical measurements were performed using an Ezstat potentiostat/galvanostat supported with EZware software. Electrochemical measurements were carried out in a conventional three-electrode cell. A glassy carbon base electrode coated with 2 mg cm.sup.−2 of the porphyrin-based catalyst was used as a working electrode. The counter electrode was made of a platinum mesh (area 1 cm.sup.2). The reference electrode was Ag/AgCl/KCL (sat.). The potential was measured against the Ag/AgCl/KCl (Sat.) electrode, then converted to a normal hydrogen electrode by adding 0.197 V. The resulting voltammograms illustrate the low potential required for the CoP-VGCF composite catalyst to catalyze the oxygen evolution reaction. Tafel plots of the results shown in FIG. 2 are depicted in FIG. 3, demonstrating the significantly reduced Tafel slope of the CoP-VCGF electrode when compared with the other tested electrodes.

(24) The following examples illustrate the present teachings

EXAMPLE 2

Fabricating 5,10,15,20 Tetrakis (5-bromothiophen-2-yl) Cobalt Porphyrin Electrode

(25) A porphyrin-based catalyst for water splitting was fabricated having 50 wt % cobalt porphyrin complex, 40 wt % VGCF, and 10 wt % Nafion oil. First, 0.4 g of VGCF was dispersed in 2 ml methanol in an ultrasonic bath for an hour. Then 0.5 g cobalt-porphyrin complex were added in the ultrasonic bath for a further hour. A further 2 g of Nafion oil polymer solution (5 wt %) was added to the mixture, producing a thick homogenous ink. This mixture was then mixed in an ultrasonic bath for a further hour. The resulting ink was stable and homogenous. A volume of between 100 and 200 μl of the ink was coated directly onto the surface of a glass carbon electrode (2 mm diameter) and the electrode was transferred to a drying oven and baked at 80° C. for 10 minutes.

(26) A scanning electron micrograph of this composite electrode is depicted in FIG. 1, illustrating the fibrous nature of the VCGF and confirming that the cobalt porphyrin catalyst is uniformly distributed on the VGCF carbon network. These results were also confirmed by

(27) Energy-Dispersive X-ray Spectroscopy (EDX). (See FIG. 4 and Table 2)

(28) TABLE-US-00002 TABLE 2 EDX of cobalt-porphyrin composite Element Weight % Atomic % Net Int. Error % CK 89.2 93.05 187.29 6.06 OK 7.41 5.8 6.61 17.51 SK 2.54 0.99 60.09 5.03 CoK 0.49 0.1 4.37 32.8 BrK 0.36 0.06 0.76 72.57

EXAMPLE 3

Testing Effect of pH on Catalyst

(29) The porphyrin-based water splitting catalysts were also tested to determine their impact on the oxygen evolution reaction in acidic, neutral, and basic media. A glassy carbon electrode was prepared according to Examples 1-2. The weight loading of the catalyst on top of the base electrode was 2 mg cm.sup.−2. Phosphate buffer solution, an aqueous mixture of 0.5 M potassium dihydrogen phosphate (KH.sub.2PO.sub.4) and 0.5M potassium hydrogen phosphate (K.sub.2HPO.sub.4) was used as medium. A neutral medium of pH 7 was obtained by mixing 21.0 ml of KH.sub.2PO.sub.4 with 30.0 ml of K.sub.2HPO.sub.4. An acidic solution of pH 1 was obtained by mixing 49.3 ml of KH.sub.2PO.sub.4 with 0.7 ml of K.sub.2HPO.sub.4. A basic solution of pH 13 was obtained by mixing 0.6 ml of KH.sub.2PO.sub.4 with 49.4 ml of K.sub.2HPO.sub.4 to produce a pH 8.85 buffer and adding sufficient NaOH to reach pH 13. Electrochemical measurements were performed using a conventional three-electrode cell and an Ezstat potentiostat/galvanostat supported by Ezstat software. The composite electrode was used as a working electrode. The counter electrode was made of a platinum mesh (area 1 cm.sup.2). The reference electrode as Ag/AgCl/Kcl (sat.). Linear sweep voltammograms were recorded by scanning the potential of the working electrode at a scan rate of 100 mV s.sup.−1 in a buffer solution of the required pH. The potential was measured against the Ag/AgCl/KCl (sat.), then was converted to a normal hydrogen electrode by adding 0.197 V.

(30) FIG. 5 depicts a graph of current density versus potential curves recorded from solutions with pH=1, pH=7, and pH=13. The dashed line indicates the electrode potential corresponding to a current density of 1 mA cm.sup.−2. The oxygen evolution reaction region is also highlighted. It is generally understood that the potential required to perform the oxygen evolution reaction increases as pH decreases.

(31) It is to be understood that the porphyrin-based catalysts for water splitting is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.