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In this paper, a novel three-dimentional electrode consisting of porous nano-Ni film supported on Ni foam substrate is successfully prepared by electrodeposition using hydrogen bubbles as the template. The electrode is characterized by scanning electron microscopy and X-ray diffractometer, and it shows a unique open structure allowing the full utilization of Ni surface active sites. H2O2 electrooxidation in KOH solution on the porous Ni electrode is studied by linear sweep voltammetry and chronoamperometry. The electrode exhibits significantly higher catalytic activity for H2O2 electrooxidation than the state-of-the-art precious metal electrode. An apparent activation energy of 21.2 kJ mol-1 has been calculated from current densities at different temperatures, which demonstrates that H2O2 is a promising fuel with high enery density. A direct peroxide-peroxide fuel cell (DPPFC) using the Ni/Ni-foam as the anode achieves a peak power density of 19.4 mW cm-2 which is much higher than that reported in previous literature. The easy preparation, low cost, high activity and stability of the electrode make it a very promising anode of direct peroxide-peroxide fuel cell.
Keyword: Porous nickel film; Electrodeposition; Hydrogen template; Hydrogen peroxide electrooxidation; Direct peroxide fuel cell.
Hydrogen peroxide (H2O2) can be used as both a carbon-free energy carrier and a strong oxidant in a novel fuel cell, namely direct peroxide-peroxide fuel cell (DPPFC) . H2O2 electrooxidation in alkaline medium (Eq.1) and electroreduction in acid solution (Eq.2) occur respectively at the anode and cathode, which can be described as following equations:
Anode: E0 = 0.146 V (1)
Cathode: E0 = 1.776 V (2)
Fuel cell: E0=1.630 V (3)
DPPFC has many advantages over other types of fuel cells , such as, low cost, compact, easy operation, workable without air, and providing both power and oxygen. So it is a very promising underwater and space power source. Besides, both the anode and the cathode reaction have fast kinetics and involve no poisoning species. Presently, two different H2O2 cell structure have been developed rapidly. Some researchers focused on H2O2 electrooxidation and electroreduction in a one-compartment cell (membraneless) by the selective anode and cathode catalysis . Shaegh and co-workers reported a one-compartment fuel cell running on H2O2 both as fuel and oxidant under acidic conditions. A high performance membraneless H2O2 fuel cell using Prussian Blue as cathode and silver and nickel as anode materials in an acidic medium was realized. A maximum power density of 1.55 mW cm-2 at 0.3V and a high open circuit potential of 0.6V were obtained with a nickel anode. Besides, Sanli et al. used 1.0 mol L-1 H2O2 in a 6.0 mol L-1 KOH solution as a fuel and 2 mol L-1 H2O2 in a 1.5 mol L-1 H2SO4 solution with Ni/C and Pt/C as catalysts at anode and cathode sides in a Nafion-membrane fuel cell, a power density of 3.75 m W cm-2 at a cell potential of 0.55 V was achieved.
The preparation of catalysts with high electrocatalytic activity is key issue for DPPFC to achieve high performance. In our previous investigation , we found that dendritic Pd vertically supported on carbon fiber cloth (Pd/CFC) exhibited high electrocatalytic activity and stability for H2O2 electroreduction and electrooxidation. Nevertheless, Pd is a precious metal and catalyze the chemical decomposition of H2O2, leading to the reduction of the utilization efficiency of H2O2 fuel. Ni as the low-cost non-metal has been demonstrated to have no catalysis to H2O2 decomposition, and it is also reported to be a prominsing anode in DPPFCs . Furthermore, the structure of electrode is also a key factor to influence the electrocatalytic activity to H2O2 elecctrooxidation which should have an open and porous structure making O2 diffuse away from the electrode quickly. In recent years, 3D porous nanometal films prepared by a hydrogen bubble template have elicited much attention due to their distinctive structural features and intriguing properties . The mechanism of depositing various morphology such as dendrites, foam, and honeycomb- and dish-like structures was studied intensively. In this paper, we report a novel three-dimensional (3D) porous Ni electrode (Ni/Ni-foam) for H2O2 electrooxidation. This Ni/Ni-foam electrode was prepared by a simple electrodeposition method using hydrogen bubble as the template. The 3D porous structure enables the electrode to have larger surface area, excellent mass transport property allowing the easy access of reactants to the catalytic surface sites and quick release of oxygen gaseous product. This electrode has been demonstrated higher catalytic activity than the state-of-the-art Pd/CFC electrode for H2O2 electrooxidation in KOH solution. Moreover, a high performance two-compartment DPPFC using the Ni/Ni-foam as the anode is demonstrated, which shows an OCV of 0.9 V and a maxinum power density as high as 19.4 mW cm-2 at 20°C. Such a high performance DPPFC has not been previously reported. So, this new type of electrode is a very promising anode for DPPFC.
The electrode of porous nano-Ni film electrodeposited on the commercial Ni foam substrate was prepared via a hydrogen bubble template method with an electrolyte consisting of 2.0 mol L-1 NH4Cl and 0.1 mol L-1 NiCl2 as the deposition solution (Scheme 1). Before the electrodeposition, a piece of nickel foam (10 mm - 10 mm- 1.1 mm, 110 PPI, 320 g m−2; Changsha Lyrun Material Co., Ltd. China) was degreased with acetone, etched with 6.0 mol L-1 HCl for 15 min, rinsed with water, soaked in 0.1 mmol L-1 NiCl2 for 4 h, and then rinsed with water extensively . The depositions were carried out in a three-electrode electrochemical cell controlled by computerized potentiostat (Autolab PGSTAT302, Eco Chemie). The nickel foam served as the working electrode, which was placed between two pieces of platinum foil in parallel as the counter electrodes. A saturated Ag/AgCl (3 mol L-1 KCl) electrode was used as the reference electrode. The electrodeposition was carried out at a constant current of -2.0 A cm-2 (-3.0 V±0.2V) for 100s. The electrode morphology was characterized by a scanning electron microscope (SEM, JEOL JSM-6480).The structure was analyzed using an X-ray diffractometer (Rigaku TTR III) with Cu Ka radiation (λ = 0.1514178 nm).
H2O2 electrooxidation was carried out in KOH solution in a three-electrode electrochemical cell. The Ni/Ni-foam acts as the working electrode. Pt foil placed behind D-porosity glass frits (to eliminate the interference of O2 from decomposition of H2O2 on Pt) served as the counter electrodes. The reported current densities were calculated using the geometrical area of the electrode. All solutions were made with analytical grade chemical reagents and ultra-pure water (Milli-Q 18 MW cm). All measurements were performed at ambient temperature (20±2°C) under N2 atmosphere.
Combination of Ni/Ni-foam electrode as the anode and Pd/CFC electrode as the cathode in a DPPFC was also examined. The two-compartment H2O2 fuel cell has the conventional fuel cell configuration. Nafion-115 (DuPont, USA) membrane was used to separate the anode and cathode compartments. The membrane was pretreated by boiling in 3% H2O2 for 1 h, in 0.5 mol L-1 H2SO4 for 2h and in ultrapure water for 2h prior to use. The membrane electrode assembly (MEA) was fabricated by hot-pressing the anode and cathode onto each side of the Naï¬on-115 membrane at 135 °C for 60 s. The active area of the anode and cathode was 5 cm2. The Pd loading of Pd/CFC cathode is 0.3061 mg cm−2 (the data from ICP measurement). The prepared MEA was sandwiched between two graphite plates with serpentine ï¬‚ow ï¬eld. The anolyte (a solution of KOH and H2O2) and the catholyte (a solution of H2SO4 and H2O2) were pumped into the bottom of the anode and the cathode compartments, respectively, by two individual peristaltic pumps and exited at the top of the compartments. The anolyte and catholyte were circulated during the cell test. The discharge performance of the DPPFC was measured using a computer-controlled E-load system (Arbin, USA) .
3. Results and discussion
3.1 Characterization of the Ni/Ni-foam electrode
The 3D porous nano-Ni film is successfully prepared by a facile cathodic electrodeposition accompanying hydrogen evolution. The as-prepared Ni film exhibited a 3D porous structure with highly porous nanoramified walls and relatively uniformly distributed on the surface of Ni foam substrate (Figure 1A). The details in each Ni deposit walls consist of numerous interconnected nanoparticles with diameters of 300-400 nm (Figure 1B and 1C). The unique 3D porous structure was synthesized with hydrogen bubbles as a dynamic template which makes the Ni/Ni-foam electrode have larger surface area than Ni foam substrate. All the Ni particles surfaces are easy accessible to H2O2 and electrolytes, so they have a full utilization. Besides, oxygen gas generated by H2O2 electrooxidation or hydrolysis is able to diffuse away from the electrode surface quickly, prohibiting the blockage of Ni surface active sites by trapped gas bubbles. In order to determine the composition and structure of Ni/Ni-foam electrode, XRD analysis was shown in Figure 2. The XRD pattern of the Ni/Ni-foam electrode (Figure 1D) exhibited three diffraction peaks at 44.5°, 51.8° and 76.3°, which can be indexed to the diffraction from the (111), (200) and (220) plane of Ni metal according to the standard crystallographic spectrum of Ni (JCPDS card No. 40-0850). The result demonstrated that the Ni micro-particles are pure Ni rather than other Ni species.
Catalytic effects are influenced strongly by the structure and composition of the surface of nanoparticles. It can be seen from the insert of Figure 2, a pair of typical redox peaks was observed for both Ni substrate and Ni/Ni-foam electrode, which can be ascribed to the interconversion of Ni(OH)2 and NiO(OH) species as following .
The larger peak area of Ni/Ni-foam than Ni substrate indicated that the Ni/Ni-foam electrode has larger surface area than Ni substrate. The cyclic voltammograms (CV) of Ni/Ni-foam electrode at 50 mV s-1 in the potential range -1.2~0.75 V in 2.0 mol L-1 KOH with and without H2O2 was shown in Figure 2. As can been seen that, in the presence of H2O2, the onset potential of the Ni2+ oxidation shifted to positive value and enhanced upon increasing the concentration of H2O2. In addition, the redox peaks of the interconversion of Ni(OH)2 and NiO(OH) species were not obvious at high concentration of H2O2. At Ni/Ni-foam electrode, H2O2 oxidation easily occurs rather than H2O2 reduction, which is agreement with that in literature . In fact, it can be inferred that a strong interaction of H2O2 with the surface already covered by low valance nickel species, that both H2O2 electrooxidation and the conversion reaction of Ni2+/Ni3+ take place on the surface of electrode.
Polarization curves for H2O2 electrooxidation in 3.0 mol L-1 KOH containing 0.1 mol L-1 H2O2 were recorded in the temperature range of 298-338 K (Figure 3A). With the increase of temperature, a shift of the onset oxidation potential towards low cathodic value and an increase in limiting currents were observed, demonstrating the dependence of reaction rate on the reaction temperature. The activation energy for the electrooxidation of H2O2 on Ni/Ni-foam electrode was obtained from the current density of the same potential at different temperatures using the Arrhenius relationship (Eq. 6):
The current density at 0V increased from 110 mA cm-2 to 300 mA cm-2 with the temperature increased from 298 to 338 K. The logarithm of peak current densities (lnj) were plotted against the reciprocal of absolute temperatures (1/T) (Figure 3B) and linear relationships were observed at the oxidation potential of 0V. The activation energies (Ea) were calculated to be 21.2 kJ mol-1, which is comparable with the electrooxidation of methanol and hydrazine, and certainly higher than that of H2 . Furthermore, the high volume of H2O2 production results in very low cost. It is reported that DPPFC could be more competitive than the PEMFC, DMFC and DBFC systems in terms of the total cost of the energy production (fuel cost: 6.9 $/kg for H2, 10.2 $/kg for methanol, 55 $/kg for NaBH4 and 1.8 $/kg for H2O2) .
Figure 4 also systematically researched the influnce of H2O2 concentration towards H2O2 electrooxidation. KOH concentration fixed at 3.0 mol L-1 with different H2O2 concentration from 0.25 mol L-1 to 2.0 mol L-1. The insert is the comparative polarization curves of Ni foam and Ni/Ni-foam electrode in 3.0 mol L-1 KOH+1.0 mol L-1 H2O2. Clearly, the Ni/Ni-foam electrode exhibited significantly improved electrocatalytic activity for H2O2 electrooxidation over the Ni foam substrate. Ni substrate and Ni/Ni foam electrode also have electrocatalytic performance to H2O2 electrooxidation. The onset potential for H2O2 electrooxidation at the Ni/Ni foam started at -0.2V, while that shifted to negative value by around 100 mV at Ni substrate. Moreover, the novel Ni/Ni foam anode also exhibited higher performance for H2O2 electrooxidation than that at the Pd/CFC reported in our previous work . For example, the oxidation current density at 0.2 V in the solution of 3.0 mol L-1 KOH+1.0 mol L-1 H2O2 is 777 mA cm-2 for the Ni/Ni-foam electrode, and it is only 580 mA cm-2 for the Pd/CFC electrode. In addition, the Ni/Ni-foam electrode is much cheaper than Pd/CFC electrode. So it can be concluded that, as the anode of a DPPFC, the Ni/Ni-foam electrode outperformed the state-of-the-art Pd/CFC electrode. The excellent catalytic performance of the Ni/Ni foam electrode may result from the unique electrode structure such as the porous and interconnected electrode surface and the 3D support skeleton (Ni foam substrate), which make the surface of catalyst fully contact the electrolyte and allow the gaseous product quickly to diffuse away from the electrode. The catalytic activity of the Ni/Ni-foam electrode enhanced with the increase of H2O2 concentration from 0.25 mol L-1 to 1.0 mol L-1, however, decreased with the further increase. It may be due to the higher decomposition rate of H2O2 at high concentration of H2O2. The H2O2 decomposition produces gas bubbles that attach to the electrode surface and thus reduce its effective surface area. It is noticed that when H2O2 concentration reached to 1.0 mol L-1, the decomposition of H2O2 became apparent. More importantly, the rate of chemical decomposition of H2O2 is much faster in alkaline than in acid medium. So in order to maintain high utilization of H2O2 fuel, its concentration in KOH solution should be kept as low as possible. Interestingly, lowering the H2O2 concentration did not obviously reduce its oxidation performance on the Ni/Ni foam electrode. So this is beneficial for the operation of a DPPFC.
The effects of KOH concentration towards H2O2 electrooxidation at the Ni/Ni-foam electrode were discussed in Figure 5. The electrooxidation of KOH varied from 1.0 mol L-1 to 5.0 mol L-1 when fixing the H2O2 concentration at 0.5 mol L-1. As seen, with the increase of KOH concentration, the onset oxidation potential slightly shifted to more negative values and the oxidation current density increased, demonstrating that H2O2 electrooxidation performance was improved by increasing the KOH concentration. Notably, the performance enhancement became insignificant at high KOH concentration.
In order to examine the stability of Ni/Ni foam electrode, the chronoamperometric curves for H2O2 electrooxidation at different potentials in 3.0 mol L-1 KOH+1.0 mol L-1 H2O2 was revealed in Figure 6. Clearly, larger oxidation current density was obtained at higher oxidation potential and the oxidation currents remained nearly constant within the 30 min test period at both low and high potentials. At higher potentials (e.g. 0.2 V), the current-time curves for H2O2 electrooxidation in KOH became quite rough with obvious current fluctuations. This phenomenon is caused by the formation and desorption of O2 gas bubbles, which have been observed during the test. The current density af ter 30 min test at -0.1, 0.1 and 0.2 V was 100, 400 and 630 mA cm-2 respectively.
A DPPFC was built using the Ni/Ni-foam as anode and the Pd/CFC as cathode, and its performance was shown in Figure 7. The DPPFC demonstrated an OCV of 0.9 V and a peak power density of 19.4 mW cm-2, corresponding to a cell voltage of 0.5 V and a cell current density of 39 mA cm-2. This cell performance is obviously higher than that of our previously reported DPPFC using Pd/CFC both as the anode and cathode (14.3 mW cm-2) , and it is much higher than that of DPPFC using nickel supported on carbon paper as the anode that reported by Sanli et al.(3.8 mW cm-2) . The high performance can be attributed to the unique 3D porous structure of the Ni/Ni-foam anode, which has large catalytic surface area and superior mass transport property, allowing O2 product to quickly diffuse away from the electrode, preventing surface active sites of Ni from blocking by adsorbed gas bubbles.
The porous 3D Ni film consisting of interconnected microparticles was successfully deposited on the commercial Ni foam via a simple electrochemical method. The obtained electrode possesses a unique structure enabling the full utilization of Ni surfaces and allows the easy transportation of reactants to the catalyst as well as the quick removal of gaseous products from the electrode. The electrode demonstrated higher catalytic activity towards H2O2 electrooxidation than Pd/CFC electrode prepared in our previous work. The DPPFC with the Ni/Ni-foam anode displayed a peak power density of 19.4 mW cm-2 at 20°C. The high catalytic performance and the low cost make the Ni/Ni-foam electrode a very promising new type anode for DPPFC.
We gratefully acknowledge the financial support of this research by Fundamental Research Funds for the Central Universities (HEUCFT1205), Harbin Science and Technology Innovation Fund for Excellent Academic Leaders (2012RFXXG103).