Employment of Fiber-Shaped Co Modified with Au Nanoparticles as Anode in Direct NaBH4-H2O2 and N2H4-H2O2 Fuel Cells

In this study, fiber-shaped cobalt (Cofiber) was electroplated on the copper surface and modified with various amounts of gold nanoparticles (denoted as AuCofiber/Cu) with the aim to use it as the anode material in direct NaBH4-H2O2 (DBHPFC) and N2H4H2O2 (DHHPFC) fuel cells. The galvanic displacement technique has been used for the deposition of Au nanoparticles onto the Cofiber surface. The AuCofiber/Cu catalysts were prepared with the Au loadings in the range of 10.9–84.4 μgAu cm−2. Single fuel cell tests were performed by employing the prepared Cofiber/Cu and different AuCofiber/Cu catalysts as the anode and a Pt sheet as the cathode. It was found that the peak power densities up to 162 mW cm−2 for DHHPFC and 188 mW cm−2 for DBHPFC were obtained at a temperature of 25◦C using the AuCofiber/Cu catalyst with the Au loading of 84.4 μgAucm as the anode. The highest specific peak power density values of 12018 mW mgAu for DHHPFC and 14954 mW mgAu for DBHPFC were obtained, when employing the AuCofiber/Cu catalyst with the lowest Au loading of 10.9 μgAu cm–2 as the anode material. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.1261814jes]

The interest of low-temperature fuel cells (FC), including direct borohydride fuel cells (DBFCs), as well as direct hydrazine fuel cells (DHFCs), have been growing over the past decades. 1,2 The use of sodium borohydride (NaBH 4 ) and hydrazine (N 2 H 4 ), those have hydrogen content of 10.6 and 12.5 wt%, respectively, as a fuel in FC is a good alternative to methanol or hydrogen fuel cells. [2][3][4] Operation of DBFC is typically based on borohydride (BH 4 -) oxidation involving eight electrons at the anode (Eq. 1) and oxygen reduction at the cathode (Eq. 2). BH 4 − + 8OH − → BO 2 − + 6H 2 O + 8e − E 0 = − 1.24 V vs. SHE [1] 2O 2 + 4H 2 O + 8e − → 8OH − E 0 = 0.40 V vs. SHE [2] The overall cell reaction is as follows: The DHFC is based on the oxidation of hydrazine in an alkaline media, occurring at the anode (Eq. 4) and oxygen reduction at the cathode (Eq. 2). N 2 H 4 +4OH − → N 2 +4H 2 O+4e − E 0 = −1.16 V vs. SHE [4] The total cell reaction can be written as Eq. 5: 5,6 N 2 H 4 + O 2 → N 2 + 2H 2 O E 0 = 1.56 V [5] Usually oxygen (O 2 ) is used as an oxidant in fuel cells, but recently, fuel cells with hydrogen peroxide (H 2 O 2 ) as the oxidant have also been proposed and are finding various potential applications. 3,[7][8][9] The reduction of hydrogen peroxide at the cathode in an acidic media is given in Eq. 6: 4H 2 O 2 + 8e − + 8H + → 8H 2 O E 0 = 1.77 V vs. SHE [6] Here we can discern one more type of FCs -direct borohydridehydrogen peroxide fuel cell (DBHPFC) and direct hydrazinehydrogen peroxide fuel cell (DHHPFC The fact that hydrogen peroxide is a liquid oxidant at normal temperature and pressure, simplifies its transportation, storage, and handling, when compared to that of oxygen. It has the additional advantage of being good to use in a small volume single stack fuel cell. Although using H 2 O 2 as oxidant enables to take advantage of the pH difference between anode/cathode and thus gain some "free" Nernst potential difference (leading to higher OCV compared to O 2 as oxidant), the pH gradient raises a lot of practical issue (junction potential, degradation of the membrane, etc.). Since borohydride and hydrazine are used as fuel, the development of electrocatalysts with reasonable costs and a high electroactivity toward its oxidation is industrially vital. The electro-oxidation of borohydride [10][11][12][13][14][15] and hydrazine 5,16-20 on the surface of different catalysts has been properly studied recently. Herein, we use a fiber-shaped cobalt coating modified with small amounts of Au nanoparticles as the anode in direct borohydride-hydrogen peroxide and hydrazinehydrogen peroxide fuel cells.
Fabrication of catalysts.-The catalysts were prepared by a twostep process, which involves electrodeposition of cobalt on the copper surface followed by a spontaneous gold displacement from the 1 mM HAuCl 4 + 0.1 M HCl solution (denoted as gold(III)-containing solution). Prior to the electroplating of Co coating, the surface of the Cu foil (1 × 1 cm) was pretreated with SiC emery paper (grade 2500) and MgO powder, etched in 10% H 2 SO 4 and rinsed with deionized water. A rectangular glass cell that has a Cu foil cathode parallel to two stainless steel sheets anodes was used. A Co coating that has a fiber structure was electroplated on a Cu foil from an electrolyte containing 40 g l −1 CoSO 4 , 100 g l −1 NaOH and 60 g l −1 N-(2-Hydroxyethyl) ethylenediamine using a galvanostatic control at a cathode current density of 40 mA cm −2 for 20 min at a temperature of 20 ± 2 • C.  The prepared Co fiber /Cu and AuCo fiber /Cu catalysts with a geometric area of 2 cm 2 were employed as the anode and a Pt sheet as the cathode. Each compartment of the cell contained 100 ml of the corresponding aqueous electrolyte. The anolyte was composed of an alkaline mixture of 1 M NaBH 4 or 1 M N 2 H 4 in 4 M NaOH. The catholyte was contained of 5 M H 2 O 2 in 1.5 M HCl. The anolyte and catholyte solutions were prepared immediately before the measurements. A Nafion N117 membrane was used to separate the anodic and cathodic compartments of the single direct NaBH 4 -H 2 O 2 or N 2 H 4 -H 2 O 2 fuel cells. The load was applied in steps of 50 mV. Each step lasted 30 second (one point per second) and the cell voltage was continuously applied from one value to next without disconnecting the cell. Power density values were calculated from the applied cell voltage and steady-state current. The presented current and peak power densities were normalized with respect to the geometric area of catalysts.

Results and Discussion
In the present study, the performance of direct alkaline NaBH 4 -H 2 O 2 and N 2 H 4 -H 2 O 2 single fuel cells has been investigated by using the fiber-shaped Co modified with Au nanoparticles as the anode material. The simple and cost-effective electrochemical and chemical methods were used for fabrication of the catalysts. At first, the Co coating with a fiber structure and the thickness of approximately 3 μm was electrodeposited on the Cu surface. As seen from the data of SEM analysis in Fig. 2a, the Co coating, that has the fibers in the order of tens of nanometers in thickness and hundreds of nanometers in length, was electrodeposited on the Cu surface. Then, the prepared Co fiber /Cu electrodes were immersed into the gold(III)-containing solution for various time periods. This simple procedure is known as galvanic displacement, during which the deposition of a noble metal occurs by the oxidation of a precursor metal adlayer deposited on the substrate at the open-circuit potential. 5,10 Immersion of Co fiber /Cu into the gold(III)-containing solution for 0.5, 1 and 5 min results in the formation of Au nanoparticles on the Co fiber /Cu surface in size of 10 up to 50 nm (Figs. 2b-2d). The Au and Co loadings in the prepared catalysts were determined using ICP-OES. The summarized data are given in Table I. The Au loadings in the prepared AuCo fiber /Cu catalysts were in the range of 10.9 up to 84.4 μg Au cm −2 ( Table I).
The stability of the Au nanoparticles modified fiber-shaped Co catalysts deposited on the Cu surface was examined in DBHPFC and DHHPFC single fuel cells. An alkaline direct NaBH 4 -H 2 O 2 and N 2 H 4 -H 2 O 2 fuel cells were constructed with the Co fiber /Cu and different AuCo fiber /Cu catalysts as the anode and Pt sheet as the cathode (Fig. 1). The solutions of 1 M NaBH 4 or 1 M N 2 H 4 in 4 M NaOH and 5 M H 2 O 2 in 1.5 M HCl were used as the anolyte and catholyte, respectively. A Nafion N117 membrane was used as a separator. Fuel cell measurements were performed at a temperature of 25 • C. It should be noted that during the discharge process, small bubbles of hydrogen and oxygen were observed on the Co fiber /Cu and AuCo fiber /Cu catalysts due to the homogeneous decomposition of the anode and cathode. The obtained fuel cell polarization curves and the corresponding power density curves against the current density by employing the prepared Co fiber /Cu and AuCo fiber /Cu catalysts as the anodes for DBHPFC and DHHPFC are presented in Fig. 3. The main obtained parameters are summarized in Table II Table II).
To compare the power density of the prepared AuCo fiber /Cu catalysts, the obtained power density values were normalized by the Au

Type of fuel Cell
Catalysts Au loading (μg cm −2 ) E peak (V) j peak (mA cm −2 ) P (mW cm −2 ) Specific P (mW mg −1 Au ) Specific P (mW mg −1 cat ) loadings for each catalyst. The summarized date are given in Fig. 4 and Table II. As seen, the highest specific power density (mV mg -1 Au ) was obtained using the catalysts with the lowest used Au loading, e.g. 10.9 μg Au cm -2 in both types of FC (Table II). Highest specific peak power density values of 12018 mW mg -1 Au for N 2 H 4 -H 2 O 2 and 14954 mW mg -1 Au for NaBH 4 -H 2 O 2 were obtained, when employing the AuCo fiber /Cu catalyst with the Au loading of 10.9 μg Au cm -2 as the anode material. The obtained specific peak power density values for DHHPFC using the AuCo fiber /Cu catalyst with the Au loading of 10.9 μg Au cm -2 are ca. 1.9 and 6.3 times higher as compared to that at the AuCo fiber /Cu catalysts with the Au loadings of 22.4 and 84.4 μg Au cm -2 , respectively. In the case of DBHPFC, the specific peak power density is ca. 2.0 and 6.7 times higher at the AuCo fiber /Cu catalyst, which has the lowest Au loading, as compared to that at the AuCo fiber /Cu catalysts, prepared with higher Au loadings.

DHHPFC
To evaluate the performance of DHHPFC and DBHPFC by employing the Co fiber /Cu and AuCo fiber /Cu as the anodes, the peak power density values were normalized by the Co and total catalyst (Au+Co)  mass, respectively. The summarized data are given in Table II. It should be noted that the Co fiber /Cu catalyst seems to be an attractive material on its own, whereas the decoration of it by Au nanoparticles significantly increases performance of the both DHHPFC and DBHPFC fuel cells (Table II). The AuCo fiber /Cu anode catalysts with the Au loadings in the range of 10.9-84.4 μg Au cm -2 exhibited ca. 1.7-3.2 times higher specific power density (mW mg −1 cat ) as compared to those for the employed Co fiber /Cu anode in the DHHPFC tests. In the case of DBHPFC, ca. 1.7-2.9 times higher specific power density values were obtained using the investigated AuCo fiber /Cu anode catalysts as compared to that for pure Co fiber /Cu anode.
Data given in Table III presents the comparison of the power performance data of DHHPFC and DBHPFC in recent years. 4,[21][22][23][24][25][26][27][28][29][30][31][32][33] As we can see, the obtained peak power density values for DHH-PFC that operated using the Co fiber /Cu and AuCo fiber /Cu (84.4 μg Au cm −2 ) catalysts as the anodes and at a temperature of 25 • C, are significantly greater compared to those using the Co@Au/C and nanoporous Au leaves as the anodes. 4,21 The obtained peak power density values are ca. 1.9 and 2.6 times higher using the Co fiber /Cu anode in DHHPFC, compared with those obtained on Co@Au/C 4 and nanoporous Au leaves, 21 respectively. Notably, a DHHPFC with the employed AuCo fiber /Cu (84.4 μg Au cm −2 ) anode catalyst generates ca. 2.7-3.8 times greater peak power density values compared with those using the Co@Au/C 4 and nanoporous Au leaves, 21 respectively, catalysts as the anodes. The specific peak power density value for AuCo fiber /Cu (10.9 μg Au cm −2 ) is 12018 mW mg −1 Au , which is ca. 28 times higher than that for nanoporous Au leaves 21 (425 mW mg −1 Au ) (Table III). It should be noted that the AuCo fiber /Cu anode catalysts prepared with the Au loadings in the range of 10.9-84.4 μg Au cm −2 exhibited ca. 1.4-2.5 times greater specific power density values based on total mass of catalyst as compared to those for Co@Au/C 4 (Table III).
Significantly higher power densities were also obtained in the DBHPFC with the Co fiber /Cu or AuCo fiber /Cu anodes as compared to those on the previously reported CoO, 22 CoB powder on Ni foam, 27 Au/C, 23,29,31 electrodeposited nano Au on Ni grid, 30 Au/Ti, 32 nanoporous Au leaves 33 or Au-M 24,25 catalysts (Table III). Notably, the Co fiber /Cu anode catalyst exhibited higher specific power density values based on total mass of catalyst as compared to those of Co-based catalysts. 22,27,28 The DBHPFC with the AuCo fiber /Cu (10.9 μg Au cm −2 ) anode catalyst shows the highest specific power density (mW mg −1 Au ) as compared to that for Au-based catalysts. [29][30][31][32][33] Therefore, the highest specific power density based on total mass of catalyst shows the AuCo fiber /Cu anode catalyst with the Au loading of 84.4 μg Au cm −2 as compared to that on the previously reported Au-based catalyst. [23][24][25] Therefore, a fiber-shaped Co coating and that modified with low amounts of Au nanoparticles would be promising anode catalysts for the application of direct N 2 H 4 -H 2 O 2 and NaBH 4 -H 2 O 2 fuel cells.

Conclusions
The simple and cost-effective electrochemical method was used for the deposition of the fiber-shaped Co coating onto the Cu surface. Au nanoparticles were then deposited onto the fiber-shaped Co coating by the galvanic displacement technique. It was found that the Au loadings in the prepared AuCo fiber /Cu catalysts were in the range of 10.9-84.4 μg Au cm −2 . Pure Co fiber /Cu and AuCo fiber /Cu catalysts were tested as the anode material in the direct N 2 H 4 -H 2 O 2 and NaBH 4 -H 2 O 2 fuel cells. The direct N 2 H 4 -H 2 O 2 and NaBH 4 -H 2 O 2 fuel cells exhibited an open circuit voltages of ca. 1.7 and 1.9 V, respectively. The deposition of low amounts of Au nanoparticles on the fiber-shaped Co coating resulted in an enhanced peak power densities for the direct N 2 H 4 -H 2 O 2 and NaBH 4 -H 2 O 2 fuel cells as compared to those of pure Co fiber /Cu. Peak power densities up to 162 mW cm −2 for N 2 H 4 -H 2 O 2 and 188 mW cm −2 for NaBH 4 -H 2 O 2 were obtained at a temperature of 25 • C using the AuCo fiber /Cu anode catalyst with the Au loading of 84.4 μg Au cm -2 . The highest specific peak power density values of 12018 mW mg -1 Au for N 2 H 4 -H 2 O 2 and 14954 mW mg -1 Au for NaBH 4 -H 2 O 2 were obtained, when employing the AuCo fiber /Cu anode catalyst prepared with the lowest Au loading of 10.9 μg Au cm -2 .