1. Introduction
Easy storage and handling, high energy density and wide availability are features that make alcohols attractive fuel for the most promising alternative power sources for transportation, portable electronics and stationary applications[1-4]. Direct methanol fuel cells (DMFCs) are the most studied systems and the closest to commercialization, since methanol is the simplest alcohol among small organic molecules (SOM) and the electrooxidation of methanol is relatively simple compared to other alcohols. However, DMFCs still have a number of drawbacks, including the usage of high amounts of platinum as anode and cathode catalysts, resulting in an extremely high price for the final devices, and methanol crossover from anode to cathode through cation exchange membranes, significantly decreasing the fuel cell performance and durability.
Direct oxidation alkaline fuel cells (DOAFCs), which oxidise fuels directly on the anode in alkaline media, have attracted attention again in recent years because of the potential solutions to overcome the problems associated with the use of platinum group metal (PGM) catalysts and fuel efficiency[1-3]. The increased number of studies in the past few years indicates a growing interest in the research community, driven by the several advantages of the anion exchange membrane fuel cells (AEMFCs) technology over the currently commercialized proton exchange membrane fuel cells (PEMFCs)[5]. Technically, AEMFCs are similar to PEMFCs, with the main difference being that the solid membrane is an alkaline AEM instead of an acidic PEM. With an AEM in an AEMFC, the OH- anion is transported from the cathode to the anode, opposite to the H+ conduction direction in a PEMFC as shown in Figure 1. In an AEMFC, water as well as ions are not transported in the same directions as in PEMFCs.
Hydrazine, N2H4 is a strong reducing agent that can be used as a rocket fuel. As it can exist in the form of stable aqueous solutions in concentration of up to 100% hydrazine hydrate (N2H4⋅H2O or N2H5OH), it can be transferred easily like alcohols. Therefore, hydrazine as an inorganic fuel for alkaline fuel cells has been studied since the 1970s[6,7]. The main advantages of hydrazine fuel cells are the high hydrogen contents (12.5 wt%) as a hydrogen carrier, high reactivity which leads to higher cell voltages (about 1.61 V) and no emission of any green house gases[3,8-10]. The characteristics of alkaline hydrazine fuel cells are also described and compared with DMFCs in terms of customer benefits in Table 1. Direct hydrazine fuel cells (DHFCs) can be the most promising candidate if the technologies for DMFCs are effectively implemented to avoid potential safety hazards due to high toxicity of hydrazine.
This paper reviews the recent developments of the catalysts and effective electrode structure affecting the performance and stability of alkaline hydrazine fuel cells. Cathode catalysts and their physico-chemical property, however, are ruled out to better focus on the characteristics of DHFCs rather than entire AEMFCs.
2. Anode Catalysts for Hydrazine Oxidation
Platinum (Pt) is considered to be the best electrocatalyst for hydrazine oxidation with the lowest onset potential, but the socioeconomic problem associated with Pt catalysts (cost and scarcity) pose serious challenges for its widespread use in commercial applications. Thus, the need for a cost-effective and efficient electrocatalysts for hydrazine oxidation is essential for the further development of alkaline hydrazine fuel cells.
Since many researches regarding noble metal free catalysts revealed that relatively low cost transition metals and their alloy exhibit higher activities for hydrazine oxidation than Pt, most of the studies on DHFCs have focused on the development and characterization of no-ble-metal-free catalysts over the last few decades[10-20].
Among the promising candidates, Ni-based catalysts for hydrazine oxidation have been extensively explored by various research groups, despite that these catalysts suffer from poor stability due to the formation of inactive oxide layer on the surface during or before the oxidation step of hydrazine[20]. As a representative example, Figure 2(a) presents a typical current-voltage polarization curves when using Ni-Zn catalysts for anode of direct hydrazine fuel cells[12]. It should be noted that peak power density for 60 wt% Ni-Zn/KB (Ketjenblack) of 450 mW/cm2 is the highest ever reported under humidified air supply as an oxidant. The extraordinary power performance is associated with the increased retention time of hydrazine hydrate in contact with the active sites of the catalysts due to the increase of surface area in supported catalysts.
The influence of the KOH concentration on fuel cell performance is also shown in Figure 2(b)[11]. The hydrazine hydrate concentration was set to 4 M. It was found that the addition of KOH improved the cell performance remarkably. The cell performance was very poor in the absence of KOH and for 0.1 M KOH. It is, therefore, considered that the addition of KOH improves the ion conductivity of anion exchange membranes as well as ionomer in the electrodes[21]. However, the cell performance decreases when the KOH concentration is 3 M. This decrease can be attributed to mass transfer limitation due to the specific viscosity of the high concentration of KOH that influences the cathode polarization. The cell performance is also influenced by hydrazine hydrate concentration. The increase in the hydrazine concentration to some extent results in an increase in the reactivity. However, an excess concentration of hydrazine adversely affects the cell performance because of a decrease in ion conductivity.
Carbon-based nanomaterials (graphene, carbon nanotubes and mesoporous carbon, etc.) can be good candidate for anode catalyst partially because of their stability in strongly alkaline and acidic conditions. Moreover, these materials possess high intrinsic electrical conductivity that facilitates the electron transfer processes for hydrazine oxidation along with high surface area[22].
The surface property of the N-doped carbon nanofibers (NDCs) is selectively controlled by poly(methyl methacrylate) (PMMA) and water vapor treatments to tune the pore size, structure and surface morphology as shown in Figure 3[18]. The PMMA and water vapor treatments contribute to the development of both microporous (< 2 nm) and meso/macroporous (>10 nm) structures on the surface of the NDCs coincidentally, resulting in much higher accessibility of KOH electrolyte toward active sites for hydrazine oxidation. However, only the active sites in the mesoporous structures, not those in the microporous structures, mainly contribute to the cell performance as shown in Figure 3(e). This is in good agreement with that the meso/macroporous structures were predominantly utilized as the active catalyst sites in membrane- electrode-assembly (MEA) level performance[23].
3. Effective Electrode Structure for Stable Operation
The anodic reaction of hydrazine in an alkaline environment takes place through the following reactions
(a) Electrochemical reaction
(b) Chemical reaction
In the electro-oxidation reaction of hydrazine, the most desirable reaction is the four electron reaction given by Equation 1. However, especially on noble metals, such as Pt, an electroless (nonfaradaic) purely chemical catalytic decomposition pathway takes place. This is due to the fact the thermodynamically hydrazine is unstable. Therefore, like the oxygen electrode, the open circuit potential of a hydrazine electrode deviates from the thermodynamic value and in certain case comes close to that the hydrogen electrode. At temperature above 60 ℃, traces of ammonia are also detected in the gases evolved. This indicates that a slow catalytic decomposition of hydrazine might occur according to Equation 3.
One of the important issue regarding gas evolution is the catalytic decomposition of hydrazine hydrate by electrochemical promotion of chemical catalysis (EPOC effect) according to Equation 2[13]. Moreover, the applied electrochemical potential and the associated faradaic current appear to promote the purely chemical catalytic decomposition of hydrazine hydrate as shown Figure 4(a). However, the fact that the highly active Ni-Co alloy catalysts does not electrooxidize molecular hydrogen as shown in Figure 4(b) indicates parasitic fuel loss at open circuit voltage (OCV) conditions as well as reduced fuel efficiency under operating conditions, despite the expected significant improvement in fuel cell output power.
A novel approach was proposed to alleviate the undesirable fuel loss of chemical decomposition of hydrazine hydrate and mass transfer limitation by gas bubbles even at OCV conditions[24]. Anode for multi- step reaction (MSR) consists of both hydrazine decomposition catalysts and hydrazine & hydrogen oxidation catalysts as shown in Figure 5(b). In the MSR-DHFCs system, anode catalyst layer is readily filled with gases such as N2 and H2 formed by hydrazine chemical decomposition. Next, generated H2 can be electrooxidized in hydrogen oxidation reaction (HOR) with power generation while unreacted liquid hydrazine is also electrooxidized on the bi-functional electrocatalysts contributing into overall power generation. The durability of DHFC and MSR-DHFC was compared in the AEMFC configuration as shown in Figure 5(c,d). In case of operation with 1 M KOH electrolyte (after 350 h), cell voltage at 5 mA/cm2, OCV are significantly improved as well as very stabilized enough, demonstrating the potential advantages of this approach for prevention of fuel loss and crossover.
Gas evolution reactions (GERs), which are processes of converting liquid reactants to gas products accompanied with electron transfer in aqueous media, are of great importance in energy storage and conversion systems[25]. In particular, GER is important in anodic reaction in direct hydrazine fuel cells, which exhibit relatively much higher current density region in comparison with any other direct liquid fuel cells. It is because at especially high reaction rates, the gas slugs accumulated on the electrode surface may not only hinder the liquid fuels transport to the catalyst sites, but also impose a higher pressure in the flow field, thus restraining the liquid fuel supply[26-28].
It was reported that adhesion behavior of gas bubbles could be flexibly tuned by designing the surface architecture of electrodes [19,20,29-31]. Direct construction of electrochemically active materials into micro-/nanostructured film can offer a superaerophobic effect to improve GERs. The discontinuous three phase (solid-liquid-gas) contact line (TPCL) built by creating the high surface porosity would significantly reduce the adhesion force resulting in accelerated gas evolution and eventually much improve electrocatalytic performance even at high reaction rates.
3D nanostructured Cu metal catalyst was fabricated by electrodeposition to obtain a “superaerophobic”surface for diminishing the negative effects induced by the gas bubble adhesion[19]. The highly porous nanostructured Cu showed a negligible adhesion force to the gas bubbles, resulting in much smaller bubble size and fast removal of as-formed nitrogen gas bubbles at high reaction rates, in contrast to the planar counterpart with continuous TPCL as shown in Figure 6(a,b). The practical application was demonstrated by testing a DHFC assembled by the nanostructured Cu film on Cu foam and commercial Pt/C catalyst as the anode and cathode, respectively in Figure 6(c). The open-circuit voltage of the cell was close to 1 V, and the maximum power density of 160 mW/cm2 was achieved at 353 K, showing that the value is more than 2 times higher than that of the DHFCs assembled with the commercial Pt/C catalysts (58 mW/cm2). Therefore, the inherent superaerophobic property by ultra-rough, vertical and nano-architectured surface will further facilitate and enhance the electro- oxidation of hydrazine on the electrode surface by swiftly removing the nitrogen bubbles produced on its surface at even higher current density region.
Apart from the well-designed catalyst layer, efforts for the ideal configuration within an electrode are also important because electrodes are highly complex systems with many interfaces. The major criteria that need to be considered in electrode structural design include the transport resistance in the networks of various phases[32].
For the stable operation of alkaline hydrazine fuel cells in particular, water balance is of great importance along with gas management[33]. In PEMFCs [Figure 7(b)], water is only generated (at the cathode as a product of the oxygen reduction reaction, ORR) and not electrochemically consumed. Water is also moved to the cathode from the anode by electro-osmotic drag as H+ produced by the hydrogen oxidation reaction (HOR) moves through the PEM. Thus, removing cathode water is the prime concern in the PEMFC to avoid catastrophic electrode flooding. In the AEMFC [Figure 7(a)], water is both electrochemically generated (at the anode from the HOR) and consumed (at the cathode by the ORR) during cell operation. Water is also moved from the cathode to the anode by electro-osmotic drag (in the reverse direction of a PEMFC). This distinctive water transport scenario, together with the high alkaline medium in AEMFCs, represent a unique feature of AEMFCs. Consequently, different approaches are indispensable for the stable operation of alkaline hydrazine fuel cell system to provide adequate water to maintain AEM and electrode hydration, without flooding or drying out the catalyst or gas diffusion layers as shown in Figure 7(c). Furthermore, special care must be taken to select the anode diffusion media because the diffusion of hydrazine hydrate can be affected by the physical properties of anode components. Hydrophobicity, thickness and density of the diffusion media and catalyst layer should be tuned, because of the use of highly concentrated hydrazine and KOH electrolyte, which is considered to have something to do with ion conductivity as well as cathode polarization[11,34].
4. Concluding Remarks
Since the nature of energy conversion system is trade-offs between many parameters, various approaches on reliability, availability, cost, compactness and user operability should be considered with respect to targeted applications and markets. In this regard, a clear understanding of the underlying processes of the entire system should be established before developing individual components. Otherwise, it remains impossible to determine whether or not alkaline hydrazine fuel cells will be a good solution for portable power source in the future.
Therefore, research efforts into the synthesis and design of new electrocatalysts, diffusion media and membranes should be motivated by the search for more general concepts, where the extensive study of the underlying processes in related components would be a key step on the way to faster implementation of alkaline hydrazine fuel cells.
