Direct N₂H₄/H₂O₂ Fuel Cells Powered by Nanoporous Gold Leaves

Abstract

Dealloyed nanoporous gold leaves (NPGLs) are found to exhibit high electrocatalytic properties toward both hydrazine (N₂H₄) oxidation and hydrogen peroxide (H₂O₂) reduction. This observation allows the implementation of a direct hydrazine-hydrogen peroxide fuel cell (DHHPFC) based on these novel porous membrane catalysts. The effects of fuel and oxidizer flow rate, concentration and cell temperature on the performance of DHHPFC are systematically investigated. With a loading of ~0.1 mg cm⁻² Au on each side, an open circuit voltage (OCV) of 1.2 V is obtained at 80°C with a maximum power density 195 mW cm⁻², which is 22 times higher than that of commercial Pt/C electrocatalyst at the same noble metal loading. NPGLs thus hold great potential as effective and stable electrocatalysts for DHHPFCs.

Introduction

The direct liquid N₂H₄/H₂O₂ fuel cell (DHHPFC) is known to be a unique power source for air-independent applications under extreme conditions such as outer space and underwater environments. Comparing to fuel cells using oxygen as the oxidizer, the replacement of oxygen by H₂O₂ allows much improved reaction kinetics on the cathode as well as higher power density and theoretical open circuit voltage^1,2. The use of liquid fuel is also beneficial to the construction of more compact and portable power sources. A variety of materials have been designed and investigated as the hydrazine fuel cell catalysts^{3,4,5,6,7,8,9}. For example, both anion and cation exchange membrane electrode assemblies (MEAs) have been fabricated and studied by Yin and co-workers, using Zr/Ni alloy and carbon supported Pt as anode and cathode catalysts respectively and a high power density of 84 mW cm⁻² was observed⁵. Lao and co-workers reported, with a Pt-based anode and an Au/C cathode, a DHHPFC could generate a high power density of 1.02 W cm⁻² and an open circuit voltage (OCV) of 1.75 V at 80°C with a total catalyst loading of ~10 mg cm^{−2 9}. Although notable power densities and high OCVs have been achieved, the critical obstacle to the development of fuel cell technology is still the high cost of the catalysts due to excess use of the resource-limited platinum. Furthermore, the current nanoparticle-based electrodes suffer from additional disadvantages, such as structural discontinuity, non-uniform dispersion and serious agglomeration during operation. Therefore, there is still great demand to pursue alternative materials and structures to replace the Pt-based catalysts, while maintaining high enough fuel cell performance and efficiency. In such context, building novel structures facilitating electrons/ions transferring and enhancing the catalytic efficiency of non-platinum catalysts are highly desired in fuel cell community.

Recently, there are rapidly growing interests in Au-based nanotechnology and catalysis¹⁰. It is quite well known that nanostructural Au can be good electrocatalysts for N₂H₄ oxidation^11,12 and H₂O₂ reduction^13,14. This inspired us to develop an Au-based DHHPFC so that Pt can be replaced by this more easily available coinage metal. While a majority of electrocatalysts are based on supported nanoparticles, recently dealloyed porous metals such as nanoporous gold (NPG) were proven to be interesting self-supported nanostructured catalysts and electrocatalysts with some intriguing properties^{15,16,17,18,19,20,21}. Unlike many other porous materials, dealloyed nanoporous metals possess excellent structural continuity, mechanical rigidity and conductivity, with structural dimension readily tunable from several nanometers to many microns. They have demonstrated great potential for use in heterogeneous catalysis, SERS, actuator, supercapacitors, electrochemical sensors, etc^{15,16,17,18,19,20,21}.

In this paper, we report on the electrocatalytic performance of nanoporous gold leaves (NPGLs)²¹ toward N₂H₄ oxidation and H₂O₂ reduction, with an emphasis on the construction and evaluation of an NPGL-based DHHPFC. Working as both anode and cathode catalysts, NPGLs were found to be able to power a fuel cell with a specific power at least one order of magnitude higher than that of Pt/C under the same testing conditions.

Results

Similar to the direct borohydride fuel cell (DBFC), DHHPFC is a powerful all-liquid fuel cell that takes advantages of highly active liquid fuel (hydrazine hydrate) and strong liquid oxidizer (hydrogen peroxide). In a DHHPFC, the following electrode reactions occur at the anode and cathode:

Anode:

Cathode:

And overall reaction can be written as:

During cell operation, electrons transfer from anode to cathode accompanied with Na⁺ migration in an opposite direction through a Nafion membrane (Figure 1). Considering the working mechanism, the overall reaction can also be understood as . This configuration is similar to the direct borohydride/hydrogen peroxide fuel cell that uses concentrated NaOH as the anolyte and H₃PO₄ as the catholyte^22,23. An ideal electrocatalyst for this type of fuel cells should have a high surface area open framework (nanoporosity) to allow fast ion/molecule diffusion and highly active surfaces to initiate the electrocatalytic reactions and is intrinsically highly conductive for electron transportation^17,24. NPGL happens to be such a material that fulfills all these key structural properties.

Figure 1

Electrode reactions and working mechanism of the DHHPFC.

To evaluate the catalytic activities of NPGL, cyclic voltammetric (CV) tests of various samples toward N₂H₄ and H₂O₂ were conducted. Three NPGL samples were prepared and compared by dealloying 12-carat white gold leaves²¹ in concentrated nitric acid for 15, 30, 60 minutes, which were denoted as NPGL15, NPGL30 and NPGL60, respectively. Figure 2A shows CV plots of these NPGL samples in 0.01 M NaOH containing 10 mM N₂H₄. In the range of −0.6 ~ −0.1 V vs. SCE, the anode peaks for N₂H₄ oxidation on these three electrodes locate around −0.45, −0.41 and −0.34 V, respectively and a slight decrease in current is seen upon increase in dealloying time. These results are generally consistent with the previous report that shorter etching time often leads to smaller pore size and higher activity²⁵. For NPGL30, the oxidation peak current density was up to 10.5 mA cm⁻², with a peak potential at about −0.41 V, which was 11 times higher and 480 mV more negative than that on a bulk Au electrode (inset of Figure 2A, 0.9 mA cm⁻² at 0.07 V). In comparison, no noticeable current peak can be observed for bare GCE (curve d), indicating that NPGL is the major contributor to N₂H₄ oxidation.

Figure 2

Cyclic voltammetry of NPGL15 (a), NPGL30 (b), NPGL60 (c) and GCE (d) in (A) 0.01 M NaOH containing 10 mM N₂H₄ and in (B) 0.5 M H₂SO₄ containing 10 mM H₂O₂.

Scan rate: 50 mV s⁻¹. The insets show the respective voltammograms of bulk Au electrode for these two reactions. Digital photographs of (C) a NPGL-based MEA and (D) a DHHPFC single cell.

Figure 2B shows the CVs of NPGLs in 10 mM H₂O₂. A quite similar trend can be found for H₂O₂ reduction for NPGL samples of different type. Accompanying the decrease in pore size, catalytic activity increases. For NPGL30, the onset reduction potential was about 95 mV, which was nearly 100 mV more positive than that on a bulk Au electrode (~0 V, inset of Figure 2B), suggesting that NPGL30 can efficiently catalyze the reduction reaction. Again, there is negligible activity for GCE (curve d) toward this reaction.

For electrochemical redox reactions of N₂H₄ and H₂O₂, NPGL15 shows the highest catalytic activity in both cases. However, considering that there is appreciable amount of residual Ag (4.88%, Table 1) in the NPGL15 sample, which might get lost during electrochemical and fuel cell testing and thus influence the evaluation of sample performance and stability²⁶, NPGL30 was selected instead to assemble MEAs for fuel cell tests as discussed below.

Table 1 EDS data of NPGL samples before and after fuel cell tests

Discussion

In view of the high catalytic activities of NPGL, MEAs for DHHPFCs were constructed by sandwiching a Nafion 115 membrane with two pieces of NPGL30 on each side (see Methods for details). As observed in Figure 2C, 100 nm thick NPGLs can be readily attached onto both sides of the membrane without breakage. Its excellent mechanical rigidity and structural continuity are also beneficial to the decrease of mass and electron transferring resistance in discharging processes. Figure 2D depicts a single cell within which such an NPGL-based MEA was assembled and the device performance was investigated under various conditions such as fuel flow rate, fuel concentration and cell temperatures. The durability of this interesting electrocatalyst was also evaluated.

As illustrated in a scheme shown in Figure 1, Na⁺ is the main charge carrier during DHHPFC operation. To prove this critical point, flame atomic absorption spectroscopy (FAAS) measurements were first carried out and the results were collected in Table 2. It shows that the concentration of Na⁺ in the catholyte increases dramatically with the increase of discharging time, which implies that Na⁺ has effectively transported to the cathode side and is indeed the main charge carrier during DHHPFC operation.

Table 2 pH value, temperature and concentration of Na⁺ in the catholyte at different discharging stages

Considering the distinct electrolytes on each side of Nafion membrane, possible acid-base neutralization might occur which would lead to heat production and fast deterioration of the entire system. However, pH and temperature measurements showed that our system was rather stable. From Table 2, there is only slight variation of pH values during 300 min operation, which could be ascribed to the consumption of the respective ions on the electrodes. And the temperature variation of below 0.5°C proves that the mixing of OH⁻ or H⁺ did not occur in this system, a further evidence for the stability of the DHHPFC.

Considering the varying activity with pH and to optimize the fuel cell performance, experiments were conducted at various concentrations of NaOH and H₂SO₄. As observed in Table 3, in the absence of fuel and oxidizer, the OCV values increase as the concentration of H₂SO₄ and/or NaOH gets higher. For example, the cell could demonstrate a voltage of 0.489 V when 4 M NaOH and 0.5 M H₂SO₄ were separated by the membrane. However, the cell could not discharge due to the lack of an effective and continuous redox reaction, even at a very small current density of 1 mA cm⁻². In comparison, adding liquid fuel and oxidizer to each side would lead to dramatic increase of both OCV and the current output. It is noted that NaOH addition in anolyte and H₂SO₄ addition in catholyte are beneficial to improvement of the DHHPFC performance, which is in agreement with literature report of Lao et al⁹. According to the optimization of cell performance, 4 M NaOH and 0.5 M H₂SO₄ were selected as the appropriate electrolytes.

Table 3 Performance of DHHPFC with various concentrations of electrolyte

To optimize the fuel concentrations, the experimental temperature was controlled at 60°C and the anode and the cathode flow rates were set at 1.4 and 4.2 mL min⁻¹, respectively. As shown in Figure 3A, with a fixed H₂O₂ concentration of 20 wt.%, the OCVs locate at around 1.06 V regardless of the varying of the N₂H₄ concentration. However, their MPD sees evident improvement from 50 to 98.2 mW cm⁻² as the concentration of N₂H₄ increased from 5 to 10 wt.%. Interestingly, when the concentration of N₂H₄ further increased to 20 wt.%, the MPD would decrease to 82.1 mW cm⁻².

Figure 3

Performance of the DHHPFC under various electrolyte conditions with NPGL30 as the catalyst at 60°C.

(A) Dependence of the N₂H₄ concentration, the H₂O₂ concentration was fixed at 20 wt.%; (B): Dependence of the H₂O₂ concentration, N₂H₄ concentration was fixed at 10 wt.%. The flow rates of the fuel and oxidizer were 1.4 and 4.2 mL min⁻¹, respectively. (C) Dependence of the N₂H₄ flow rate as the cathode flow rate was fixed at 4.2 mL min⁻¹; (D) Dependence of the H₂O₂ flow rate as the anode flow rate was fixed at 1.4 mL min⁻¹. The concentrations of N₂H₄ and H₂O₂ were 10 wt.%.

As observed in Figure 3B, both the OCV and MPD underwent dramatic changes in value with increase in concentration of H₂O₂ from 5 to 20 wt.% when the anode concentration of N₂H₄ was fixed at 10 wt.%. Within this concentration range, the cell OCV and MPD increased from 0.84 V and 36.1 mW cm⁻² to 1.08 V and 98.2 mW cm⁻².

Two factors simultaneously influence the cell performance when the N₂H₄ concentration changes. On one hand, fuel transfer rate will increase due to the higher concentration gradient of N₂H₄, leading to higher power density. On the other hand, the amount of fuel crossover and the degree of N₂H₄ hydrolysis may become serious, which will reduce cell performance. Therefore, an optimized fuel concentration for the MEAs used in this work should be around 10 wt.%. On the cathode side, the rate-determining step should be the transfer of H₂O₂ since no decay of the DHHPFC was found even when the concentration of H₂O₂ increased to 20 wt.%. In terms of cell performance optimization, 10 wt.% N₂H₄ and 20 wt.% H₂O₂ were selected as the optimal concentration in the following tests.

In order to study the influence of fuel flow rate on the cell performance, the experimental temperature was kept at 60°C and the concentrations of both anode and cathode electrolyte solutions were maintained at 10 wt.%. The current-voltage and current-power polarization curves of a DHHPFC under different flow rates were then investigated. As shown in Figure 3C, when the H₂O₂ flow rate was set at 4.2 mL min⁻¹, by varying the N₂H₄ flow rate from 0.7 to 1.4 mL min⁻¹ there was very little change for the OCV and maximum power density (MPD). When the anode flow rate increased to 2.8 mL min⁻¹, the OCV decreased ~30 mV, resulting in a concomitant 4 mW cm⁻² decrease in MPD. In Figure 3D, when the N₂H₄ flow rate was maintained at 1.4 mL min⁻¹, by varying the H₂O₂ flow rate from 2.1 to 6.3 mL min⁻¹, the OCV remained nearly unchanged while the MPD showed a gradual increase from 44.5, 61.8 to 64 mW cm⁻².

As shown in Figure 3C, the MPD slightly decreased upon the increase of the anode flow rate. This could be attributed to an increase of N₂H₄ permeation at the high flow rate, which led to the decline of the overall performance. Therefore, the anode flow rate of 1.4 mL min⁻¹ was chosen as an appropriate value in the following tests.

It was noted that the decomposition of H₂O₂ on the cathode side was quite severe as compared with that of N₂H₄ on the anode. And the released oxygen bubbles may block the transfer of H₂O₂ molecules to the catalytic sites of NPGLs. The larger flow rate of H₂O₂ is therefore beneficial to flush away these obstructing oxygen bubbles on the catalyst surface, which in turn results in a positive effect on the cell performance. As the H₂O₂ flow rate increased to over 4.2 mL min⁻¹, the voltage gain became negligible. Therefore, 4.2 mL min⁻¹ was selected as an appropriate cathode flow rate in the following experiments.

While temperature is an important factor influencing the device performance, tests were carried out at 40, 60 and 80°C. Figure 4A presents typical polarization curves of DHHPFCs at various cell temperatures. It can be seen that the cell performance can be significantly improved at higher temperature, generating the respective MPD values at around 42.5, 94.5 and 195.4 mW cm⁻². Accordingly, their OCVs were 0.89, 1.05 and 1.21 V, respectively. The increase in cell performance at higher temperature may be attributed to the enhanced kinetics of both the anodic and cathodic reactions and higher conductivity of the electrolyte^27,28.

Figure 4

Performance of DHHPFC at various temperatures with NPGL30 (A) and Pt/C (B) as the catalysts for both anode and cathode, 4 M NaOH + 10 wt.% N₂H₄ as anolyte and 0.5 M H₂SO₄ + 20 wt.% H₂O₂ as catholyte.

The flow rates of the anolyte and catholyte were 1.4 and 4.2 mL min⁻¹, respectively.

The experimentally observed 1.21 V is still much lower than the theoretical OCV of DHHPFC (2.13 V)⁹. This large deviation was mainly due to the high over-potential, which can in part be reduced by the elevation of cell temperature. This also explains the higher OCV and better performance obtained at higher temperature. On the other hand, with the 125 microns thick Nafion membrane, crossover of N₂H₄ and H₂O₂ is possible, which would reduce the cell performance. Indeed, by effectively separating the anode and cathode in a cell configuration using two Nafion membranes, an even higher OCV of 1.45 V was observed. However, discharge performance was significantly lower due to the high cell resistance.

Because the Au loading for NPGL is around 0.1 mg cm⁻² on each side, the highest performance observed at 80°C can thus be calculated to give a specific power of 975 mW mg⁻¹. The power density of 195.4 mW cm⁻² is twice higher than that of binary alloy catalysts⁵ and if the catalyst efficiency is counted, the specific power is nearly one order of magnitude higher than the mixed Pt-based electrode⁹. For comparison purpose, we also tested a DHHPFC with the same loading of platinum (0.1 mg cm⁻²) on each side by using the commercial 60 wt.% Pt/C catalyst (Johnson Matthey, UK). From Figure 4B, the MPD was only 8.8 mW cm⁻² at 80°C, which is nearly 22 times lower than that of NPGL. Considering the comparatively larger world resources of Au compared to Pt, our results show the potential of using Au nanostructures for certain applications, such as DHHPFC.

Stability of the DHHPFC was evaluated by discharging at 150 mA cm⁻² for 6 hours a day within three consecutive days at 60°C. The result is shown in Figure 5, where the fluctuation of cell voltage was due to the oxygen generated from the cathode by H₂O₂ decomposition or the hydrogen generated from the anode by N₂H₄ hydrolysis. The generated gas bubbles may accumulate on the surface of electrodes and block the transferring of N₂H₄ or H₂O₂ solution, which results in instant performance loss. However, the cell voltage can be quickly restored to its normal discharge voltage as shown in Figure 5. During the first day of testing, the initial cell potential was 0.89 V and it decreased to 0.87 V after 6 hours continuous discharging. The reason for performance deterioration may be related to the accumulation of some reaction intermediates onto the surfaces of NPGL catalyst rather than the irreversible structural change of the catalyst itself. This is supported by a totally recovered cell performance in the following two days (Figure 5). Although the long term durability of our catalysts and DHHPFCs needs further exploration, our preliminary results showed that the catalytic activity of NPGL30 toward N₂H₄ oxidation and H₂O₂ reduction was relatively stable in DHHPFCs.

Figure 5

Durability test of the DHHPFC within three consecutive days with NPGL30 as the catalyst at 60°C.

The flow rates of the anolyte and catholyte were 1.4 and 4.2 mL min⁻¹, respectively.

To confirm the catalyst stability, the SEM images of NPGL before and after fuel cell tests are presented in Figure 6. From the surface and cross-sectional SEM images shown in Figure 6A&B, it can be seen that NPGL30 has a 30 nm pore size structure across the whole thickness of about 100 nm. After the fuel cell tests, the sample was carefully detached from the MEA and characterized by SEM. Figure 6C&D illustrate the morphology of NPGL30 on the anode and cathode, respectively. Compared to the fresh samples, there is only a slight increase in pore size to around 35 nm. We analyzed the compositional information with Energy Dispersive X-ray Spectroscopy (EDS) (Table 1) and found the changes of Ag content for the anode and cathode were quite different upon fuel cell test. The silver content of NPGL30 on the cathode decreased more obviously than that on the anode. This may be attributed to the formation of Ag₂O during a positive potential sweep in H₂O₂²⁶. However, Ag content did not greatly change at the anode, indicating that Ag is rather stable at negative potentials in the presence of N₂H₄.

Figure 6

Scanning electron microscopy (SEM) images of NPGL30.

Surface (A) and cross-sectional (B) micro-morphology of a fresh sample; Anode (C) and Cathode (D) surface micro-morphology after fuel cell tests.

In summary, we demonstrated that dealloyed nanoporous gold in a free-standing leaf form can function as novel electrocatalysts on both anode and cathode in a hydrazine/hydrogen peroxide fuel cell. This is due to NPGL's intrinsically high electrocatalytic activities toward N₂H₄ oxidation and H₂O₂ reduction, a behavior not readily seen on planar Au electrode. Under optimized conditions, a high power density of 195.4 mW cm⁻² can be realized on an MEA using 0.1 mg Au on each electrode, generating an impressive specific power of 975 mW mg⁻¹. Considering there are a big selection of nanoporous metals (NPMs) including alloys²⁹ and less precious metals³⁰ and these new materials all possess tunable porosity and tailored surface chemistry³¹, NPMs might become a new generation nanoparticle-free catalysts or electrodes for a wide variety of energy and environmental technologies.

Methods

Preparation and characterization of NPGL

NPGLs with a thickness of 100 nm were prepared by etching 12-carat white gold leaves (Au₅₀Ag₅₀ wt.%, Sepp Leaf Products, USA) at 303 K in concentrated nitric acid (~65%) for 15, 30, 60 minutes, which were denoted as NPGL15, NPGL30 and NPGL60, respectively. The morphology and composition were characterized by scanning electron microscopy (SEM) on an FEI NOVA NanoSEM 230 equipped with Energy Dispersive X-ray Spectroscopy.

Electrochemical tests

To fix NPGL onto the surface of glassy carbon electrodes (GCE), NPGL was first transferred into a beaker containing 500 mL water with a glass slide. A polished GCE was then dipped into the water underneath to allow the entire conducting GC circle covered by NPGL on top. The NPGL-coated GCE was then carefully taken out of the water, air-dried, trimmed and finally fixed by Nafion solution. The electrochemical measurements of NPGLs were carried out on a potentiostatic (CHI760C, China) in a three-electrode configuration with a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Supporting electrolyte was deoxygenated by bubbling high purity nitrogen for at least 20 minutes before each experiment.

Fuel cell assemble and tests

Nafion 115 membrane was used as the solid electrolyte. In order to remove organic contaminants and heavy metal ions, the membrane was treated with 3 wt.% H₂O₂ solution, ultra pure water, 0.5 M H₂SO₄ and ultra pure water in turn at 80°C for 1 h.

The NPGL-based MEA was made by first lifting NPGL onto a carbon paper (TGP-H-060, Toray, Japan) and after drying they were hot-pressed onto both sides of a Nafion 115 membrane at 110°C and 0.5 MPa for 3 minutes.

Pt/C (60 wt.%, Johnson Matthey, UK) catalysts were dispersed in ethanol in ultrasonic bath for 10 min before mixing with 5 wt.% Nafion solution. The ink was precisely sprayed on gas-diffusion layer by ultrasonic spraying coating system (Exacta. Coat, Sono-Tek, USA) with Pt loading of 0.1 mg cm⁻². The MEA was obtained by hot-pressing onto both sides of a pre-treated Nafion 115 membrane at 140°C and 0.5 MPa for 3 min.

To maintain a stable operating condition, on the anode side the fuel was mixed with 4 M NaOH, while on the cathode side 0.5 M H₂SO₄ was used in the electrolyte. N₂H₄ and H₂O₂ were pumped to the anode and cathode at desired concentration and flow rate by individual peristaltic pumps and silicone tubes. The cell temperature was controlled via a temperature controller and monitored by thermocouples buried in the graphite blocks. The steady state polarization curves were recorded by an automatic Electric Load (PLZ 70UA, Japan).

Flame atomic absorption spectroscopy (FAAS 3510, China) measurements were performed in order to prove that Na⁺ is the main charge carrier during DHHPFC operation. The pH values of anolyte and catholyte were collected with a Sartorius PB-10 pH meter.