Direct Alcohol Fuel Cells: A Comparative Review of Acidic and Alkaline Systems

Abstract

In the last 20 years, direct alcohol fuel cells (DAFCs) have been the subject of tremendous research efforts for the potential application as on-demand power sources. Two leading technologies respectively based on proton exchange membranes (PEMs) and anion exchange membranes (AEMs) have emerged: the first one operating in an acidic environment and conducting protons; the second one operating in alkaline electrolytes and conducting hydroxyl ions. In this review, we present an analysis of the state-of-the-art acidic and alkaline DAFCs fed with methanol and ethanol with the purpose to support a comparative analysis of acidic and alkaline systems, which is missing in the current literature. A special focus is placed on the effect of the reaction stoichiometry in acidic and alkaline systems. Particularly, we point out that, in alkaline systems, OH⁻ participates stoichiometrically to reactions, and that alcohol oxidation products are anions. This aspect must be considered when designing the fuel and when making an energy evaluation from a whole system perspective.

Graphical Abstract

1 Introduction

Fuel cells (FCs) are an electrochemical energy technology that directly convert the chemical energy of a fuel into electrical energy [1]. Such energy conversion is based on the concurrent electrochemical oxidation of a fuel at the anode and the reduction of an oxidant at the cathode. The most widely investigated FCs use hydrogen as fuel and oxygen (pure or within environmental air) as oxidants. Hydrogen FCs are already available in the market; in the future, they will play a relevant role in the decarbonization of transportation, residential usage, and industry [2]. However, hydrogen FCs require a complex balance of plant and are not suitable for powering, e.g., small portable devices [3]. Oppositely, fuels that are liquid at ambient temperature and pressure show the potential for portable power applications, as they allow operation at ambient pressure, with the advantage of a much easier storage [4].

Among the potential liquid fuels for FCs, alcohols have attracted most of the attention. This is due to the fact that they are thermodynamically unstable and, in principle, easy to oxidize with reduction potentials close to that of hydrogen [5, 6]. This basic fact guarantees that an FC fed with alcohol, known as the direct alcohol fuel cell (DAFC), can show open circuit potentials exceeding 1 V. The most investigated DAFCs use methanol (direct methanol fuel cells (DMFCs)) and ethanol (direct ethanol fuel cells (DEFCs)). The use of methanol and ethanol in FCs has several key advantages: (1) methanol and ethanol are easy to store and distribute; (2) they possess high volumetric energy density [7, 8]; (3) both can be obtained by fermentation or transformation of biomass and waste biomass with environmentally friendly processes that contribute to circular economy [9].

However, there are critical issues that prevent the commercial exploitation of DAFCs. Alcohols have much lower mass-weighted energy density than hydrogen [10, 11]. Being alcohols relatively more complex compared to hydrogen, their electrooxidation has knottier kinetics requiring higher activation energy than hydrogen oxidation. The kinetics also affects the oxidation products. Indeed, while methanol is usually fully oxidized to CO₂ or carbonates depending on pH values, the cleavage of the C–C bond in ethanol is difficult and the most relevant oxidation pathways lead to the transfer of only 2 or 4 electrons instead of the theoretical maximum number of 12 electrons. This aspect limits the Faradaic efficiency of the device, significantly reducing the ability of DEFCs to exploit the energy content of the fuel. Possibly, the complete oxidation of ethanol to CO₂ is the most relevant kinetic issue in DAFCs [12]. Many researchers have indeed worked for develo** electrocatalysts able to cleave the C–C bond in ethanol, but despite of some interesting advances and successful examples [13, 14], the complete oxidation is still the less probable pathway.

A further important drawback of DAFCs is the permeability of membranes to the alcohols; this phenomenon is known as fuel crossover. Crossover affects much the performance of DAFCs for two reasons: (i) the loss of fuel and (ii) the decrease of the cell potential because of the mixed potentials at the cathode [15]. In the latter case, the alcohol is oxidized at the cathode, with its electrocatalyst not selective for the sole reduction reaction but active toward the oxidation of alcohols. So far, important research has been carried out on the anodic electrocatalysis, and in parallel, important effort has also been spent on the cathodic reaction to avoid mixed potentials. In addition, membranes play a crucial role as both an ion conductor and a physical separator to create a barrier for diminishing the negative effects derived from the alcohol crossover.

In DAFCs, two leading technologies have emerged based on: (i) proton exchange membranes (PEMs) [16] and (ii) anion exchange membranes (AEMs) [17]. DAFCs based on PEMs have been developed mainly for the availability of commercial membranes (e.g., Nafion) previously developed for PEMFCs and inherited entirely from that technology. Later, the increasing availability of efficient commercial AEMs (e.g., Tokuyama) has opened new venues in develo** alkaline-based DAFC systems. While the two systems share the same aim of providing reliable and easy handling power sources, they have different features and related issues. Among them, it has to be mentioned that, during the oxidation of alcohols, alkaline systems provide product mixtures in the form of dissolved cations (e.g., carbonates for methanol oxidation and acetate for ethanol oxidation). This implies that alkaline DAFCs require stoichiometric amounts of alkali in the fuel. Indeed, according to the complete reaction scheme of the cell, hydroxides (OH⁻) are consumed to balance the charge that forms from the oxidation of the neutral molecule of an alcohol to a negatively charge ion [18, 19].

However, alcohol oxidation in acidic conditions has a more demanding kinetics than in alkali [20]. Moreover, the use of alkaline electrolytes increases the durability of the components of the fuel cells. In fact, it is easier to tailor alkaline fuel concentrations to minimize bipolar plate degradation [21], carbon support dissolution [22] or anodic catalyst leaching [23]. Alkaline FCs also have a crucial advantage related to the cathodic reaction. Indeed, in alkaline media, the gap in the oxygen reduction reaction (ORR) electrocatalytic activity between platinum group metal (PGM) and platinum group metal-free (PGM-free) electrocatalysts is significantly reduced compared to in acidic systems, enabling the use of PGM-free electrocatalysts at the cathode. The utilization of PGM-free cathodes led to significant advantages, as PGM-free electrocatalysts are not active toward the oxidation of the alcohols and avoid potential drops (mixed potentials) due to the alcohol crossover from the anode to the cathode [24,25,26].

A comparative analysis of alkaline and acidic systems for DAFCs is currently missing in the existing literature. We believe that such comparison is of paramount importance to highlight the peculiarity of the systems. The purpose of this review is to present for the first time such a comparative analysis, also covering the recent advances on materials, operation, and technology of DAFCs. Moreover, so far, no analysis has been proposed on the effect of the nature of the oxidation products in acid and alkaline environment. Remarkably, in acids, the oxidation products are generally neutral, and the system operates with aqueous solutions of the alcohols in the anode compartment. Oppositely, alkaline systems require high concentrations of OH⁻ that is consumed stoichiometrically. Accordingly, the stoichiometric OH⁻ consumption happens to guarantee the reaction charge balance. This aspect must be considered in the design of the fuel of alkaline systems and may severely affect the energetics of the reactions. For example, it has been demonstrated that the kinetics of ethanol electrooxidation in alkali is affected more by the OH⁻ concentration than by the ethanol concentration [27]. The need for using OH⁻ also affects the energy evaluation of the system as the production of alkali that happens most with the chloro-soda industry is highly energy intensive and may contribute importantly to the energy assessment of the technology. This aspect has already been demonstrated by the analysis of the case of electrochemical reforming of ethanol [28, 29] but has not yet been considered for DAFCs.

This review provides an initial brief description of the components of the DAFC membrane electrode assembly (MEA), the core of the fuel cell assembly. A description of the reactions occurring on the anode and the cathode of acid and alkaline DAFCs is elucidated. The thermodynamics and kinetics of alcohol oxidation reactions in alkaline and acid conditions are described, focusing the attention on the main differences between the two operating conditions. Then, the materials currently used for the membrane electrode assemblies (MEAs), named electrolytic membranes and electrocatalysts (anodes and cathodes), are introduced and described with an emphasis regarding their actual limitations. Membrane technologies used in acid and alkaline DAFCs are examined, introducing the most relevant and promising polymeric materials. Notably, the progress achieved by utilizing PGM-free electrocatalysts substituting PGM cathode electrocatalysts is deeply discussed, and its extraordinary tolerance towards alcohol oxidation is presented. Advancements in terms of operating condition optimization and transport phenomenon mitigation/control are deeply reviewed. The state-of-the-art electrochemical performance in terms of power density output is reported and discussed. Durability studies are also presented and considered. Ultimately, strategies adopted for overcoming problems are displayed, and future perspectives and outlooks are provided and suggested drawing a possible roadmap towards wide commercialization.

1.1 The Core of a DAFC: The Membrane Electrode Assembly

The MEA is the core of the fuel cell. This first section will briefly summarize its main constituents (a precise description of the fuel-cell components and assembly can be easily found in the already published literature and is out of the purpose of this review). Minor MEA constituent changes can be addressed between different FCs, depending on the fuels adopted for the different half-reactions [30]. In general, the MEA is constituted by three distinct elements: (1) an electrolytic membrane; (2) the electrocatalytic layers; (3) the diffusion layers (Fig. 1). The electrolytic polymeric membrane is positioned at the center of the MEA; it is made by a polymeric material which acts as: (a) a separator between the anodic and cathodic compartments of the fuel cell and (b) an ionic conductor for certain species (H⁺ for proton exchange membranes, OH⁻ for anion exchange membranes) which can migrate between the two compartments, closing the circuit and permitting the external current flowing in the cell. The membrane is also the prime responsible for fuel crossover, a detrimental phenomenon occurring in DAFCs, in which the fuel crosses the membrane from the anodic compartment, occupying catalyst active sites otherwise used for the cathodic reactions. The anodic and cathodic electrocatalytic layers are placed on the two sides of the membrane and present themselves in form of thin compact layer composed by a mixture of a finely dispersed electrocatalyst powders and ionomers. Particularly, DAFC electrocatalysts can be divided into two main and general groups: (1) metallic nanoparticles (for both anodic and cathodic reactions) supported over a carbon backbone and this is the typical case of platinum based electrocatalysts (Pt/C); and (2) electroactive functional groups of the type M–N–C with M being a transition metal such as Mn, Fe, Co and Ni grafted onto the backbone of the support material (mainly for the cathodic reactions [31]) and this is the typical example of PGM-free electrocatalysts. The MEA is completed by the two diffusion layers (DLs), placed externally, and in contact with the two electrocatalytic layers. DLs have four main features that are the desired requisites: (i) porosity, (ii) electrical conductivity, (iii) chemical stability, and (iv) mechanical stability. These specific characteristics are required to: (a) permit the flow of fuel/byproducts to/from the electrocatalysts, (b) act as a current collector for the external leads of the FC, and (c) grant structural stability to the MEA, by sandwiching the electrocatalysts at the two sides of the membrane and avoiding electrocatalyst loss into the fuel. Lab-grade FCs usually operate with carbon cloth or carbon paper as DLs, while industrial cell stacks exploit more rigid and durable metallic sponges or meshes for this particular purpose [32,33,34,35].

Fig. 1

Schematic diagram of a direct alcohol fuel cell

In this review, the state-of-the-art research on both membrane electrolytes (Sect. 4) and electrocatalytic materials (Sect. 5) is reported.

2 Alcohol Oxidation Reactions in Alkali and Acid: Thermodynamics and Kinetics

2.1 Theoretical Reactions and Potentials

Figure 2a and b illustrates the schematic diagrams of DMFCs operating in acid and alkaline conditions, while Fig. 2c and d reports the schematic diagrams of DEFCs operating in acid and alkaline conditions. In acidic operations, the protons (more precisely the hydronium H₃O⁺) move from the anode to the cathode while in alkaline operations, the hydroxides move from the cathode to the anode balancing the redox reactions. In both cases, the electrons move through the external circuit producing electricity. Figure 2 lists the theoretical half reactions and the overall reactions occurring in DMFCs and DEFCs operating in acid and alkaline environment, together with the relative potentials.

Fig. 2

Schematic diagrams of a acid DMFC, b alkaline DMFC, c acidic DEFC and d alkaline DEFC, together with the main reactions taking place in the cell, and their potentials (expressed as V vs. the SHE for the half reactions). The reactions are for acidic DMFCs [36], alkaline DMFCs [37, 38], acidic DEFCs [36, 37, 39], alkaline DEFCs [38, 40, 41]. The molecules are not in scale

2.2 Thermodynamic Considerations: Theoretical Cell Potentials in Acid and Alkaline Conditions

According to Fig. 2, the theoretical cell potential (E₀) for methanol that undergoes complete oxidation both in acid and alkali is practically the same at pH 0 and pH 14. The only difference is that, in acid, the oxidation leads directly to CO₂, while in alkali at pH 14, the formation of carbonate leads to a reduction in valuable potential. The situation varies importantly when ethanol is the fuel. Indeed, partial oxidation may lead to significant changes in the cell potential. In DEFCs, if we assume a complete conversion to CO₂, the theoretical cell potential in acid is 1.145 V versus the RHE. However, such a complete conversion is a minor pathway. The most likely products coming from the oxidation of ethanol are acetaldehyde and acetic acid. A cell that would produce solely acetaldehyde would generate a maximum potential of 1.032 V, slightly lower than that of the complete conversion to CO₂. However, acetic acid is an important product in these conditions and a cell that would oxidize ethanol selectively to acetic acid would lead to a potential of 0.432 V with a more than two-fold reduction compared to the oxidation to CO₂. In real systems, as the oxidation results in a distribution among 2, 4 and 12 electrons, the thermodynamic potential will be a combination of the previously mentioned values, resulting in a potential which is significantly lower compared to the theoretical value. In alkaline DEFCs, the complete oxidation of ethanol leads to the formation of carbonate. A cell capable of converting ethanol to carbonate has a theoretical potential of 1.171 V, slightly higher than the potential of acidic cells. However, the dominant pathway in alkali is the one that leads to the formation of acetate that still possesses an important theoretical potential of 1.121 V.

2.3 A Breakdown of Direct Alcohol Fuel Cell Potentials

The overall potential (E) that an FC could practically deliver is lower than its theoretical potential (E⁰). This is due to a series of efficiency loss contributions summarized in the term of overpotential ($\eta$). These contributions can be consequent to fundamental and technological factors. Electrolyte and fuel concentration, fuel crossover, reaction overpotentials and reaction pathways could be named among the fundamental factors that affect the cell potential. Fuel crossover on PGM cathodic electrocatalysts is one of them; this could impair ORR efficiency by the occupation of the cathodic catalyst active sites by the alcohol molecules. On the other hand, the main technological limitations could be directly ascribed to the architecture of the MEA. The sandwich structure that composes the cell, and the nature of the electrolytic membrane are in fact the main cause for the ohmic resistance (named as iR drop), and for the flux of chemicals to/from the electrode surfaces. All these factors are considered in the overall overpotential $\eta$ as a linear combination, as stated in Eq. (1) [42]:

$$\eta = \eta_{{\text{m}}} + \eta_{{\text{k}}} + \eta_{{\text{r}}} + \eta_{{\text{t}}}$$

(1)

where $\eta$_m takes into consideration the mixed potentials due to fuel crossover, $\eta$_k is the activation overpotential dependent on the chemical nature of the electrocatalyst and on the distribution of its active sites, $\eta$_r is the overpotential due to iR drop taking into consideration all the electrical losses (mainly due to technological factors), and $\eta$_t is the overpotential due to fuel transport phenomenon (the depletion of the fuel concentration in the solution, hindering the reactive species in reaching the electrocatalyst). The different $\eta$ terms have different weights, which are strictly dependent on the extracted current. At low currents, an increase in $\eta$ is caused by the electrocatalyst activation losses, while at high currents the depletion of electroactive species from the double layer plays a major role and could induce a strong increase in overpotential. A constant contribution to the overpotential could be attributed to the mixed potential effect, while a linear increase in $\eta$ with the current (i) can be finally expected by the iR drop component. The influence of all the parameters on the overall i–V curve of alkaline  fuel cells as an example is summarized in Fig. 3, together with the single contributions.

Fig. 3

Cell potential distribution (anode activation, cathode activation, iR drop, mass transport) for alkaline fuel cells

2.4 Alcohol Oxidation Kinetics in Acidic and Alkaline Conditions

Figure 2 reports the oxidation potentials for the complete and the partial oxidation of the alcohols considered and, in this case, methanol and ethanol. While these reactions well schematize the oxidation of a C1 alcohol as methanol, they did not represent a real screenshot of the reactions that are actually occurring in the fuel cells operating with alcohols with 2 or more carbon atoms (e.g., ethanol, glycerol, etc.). Unfortunately, as stated in Sect. 2.2, the oxidation is more often partial than complete, in other words, not all the electrons are harvested from the initial molecules and this, in turn, also reflects negatively on the actual thermodynamic potential of the systems. In the following sections, the expected different mechanisms for methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) are reported.

2.4.1 Methanol

Methanol oxidation reaction (MOR) is still a challenging reaction despite the relative simplicity of the theoretical reaction pathway, which does not involve C–C bond cleavage. Methanol reaction which leads to complete oxidation to CO₂, as reported in Fig. 2, proceeds according to Eq. (2).

$${\text{CH}}_{3} {\text{OH}} + {\text{H}}_{2} {\text{O}} \to {\text{CO}}_{2} + 6{\text{H}}^{ + } + 6{\text{e}}^{ - }$$

(2)

Unfortunately, the oxidation mechanism is rather complex and, according to Leger [43], it involves eleven possible steps. Still today, the most effective electrocatalysts for the MOR are based on noble metals, especially Pt and its alloys (e.g., Pt–Ru). A short survey of the mechanism for the MOR in acidic conditions on Pt as the state of the art for DMFCs is presented. The first step of the mechanism is the adsorption of a methanol molecule on the Pt, according to Fig. 4 Reaction A. After the adsorption step, two main pathways (Reaction B1 or B2) can be taken by the molecule: the first step after adsorption is the formation of radical species with the release of 1 e⁻ according to Fig. 4. Further monoelectronic oxidation (Reaction C1 or C2) occurs. Both the products of Reactions C1 and C2 can be oxidized to Pt-(CHO)_ads through Reaction D1 or D2. The Pt-(CHO)_ads is then oxidized to Pt-(CO)_ads according to Reaction E (Fig. 4). This reaction is responsible for the poisoning of Pt-based catalysts in DMFCs as CO is so strongly adsorbed to Pt blocking the active sites, thus hampering further oxidation. In parallel with the formation of the formyl like species and CO adsorption, OH adsorption could occur as well at the Pt surface (Reaction F, Fig. 4). Adsorbed OH species may react with the adsorbed formyl to (i) directly produce CO₂ (Reaction G) or (ii) form adsorbed COOH groups (Reaction H) which, according to Reaction I, are successively oxidized to CO₂. To oxidize CO, larger overpotentials are required. Under such conditions, production of CO₂ occurs directly via Reaction J or indirectly via Reaction K followed by Reaction I (Fig. 4).

Fig. 4

Methanol electrooxidation reaction mechanism [40]

This discussion on the mechanism is limited to the acidic environment, as in these operating conditions, platinum exhibits the lowest overpotentials for the MOR, thus preventing the electrocatalyst poisoning, and prolonging the life of the electrocatalyst. Platinum is often alloyed to ruthenium for the MOR reaction, due to a synergistic effect between the two metals. Indeed, platinum is very active for the dissociative chemisorption of methanol, while the oxidation of the carbonaceous adsorbate to CO₂ is favoured by the presence of the oxidized form of ruthenium [44]. At present, platinum-ruthenium alloys are the state-of-the-art electrocatalysts for the MOR.

2.4.2 Ethanol

Ethanol is receiving much attention for its exploitation as fuel in DEFCs, largely for its renewable nature, its well-established distribution infrastructure and lower toxicity as compared to methanol. Figure 5 reports the variety of oxidation products attainable in principle through electrocatalysis. The pathway which leads to complete oxidation to CO₂ would give 12 e⁻ but it is challenging to obtain because it implies the breakage of the C–C bond. It is then much commoner to obtain acetic acid and acetaldehyde (Reactions A, B and C in Fig. 6), delivering 4 e⁻ and 2 e⁻, respectively [45].

Fig. 5

Possible ethanol oxidation pathways. Readapted with permission from Ref. [1]. Copyright © 2013, Springer

Fig. 6

Ethanol electrooxidation reaction mechanism [42, 43]

As pointed out, complete oxidation is difficult to obtain. Next, a description of the complex mechanism leading to the formation of the above-mentioned products is reported. Aldehydes may react through the following pathway to render acetic acid (Reactions C and E in Fig. 6): CO₂ is difficult to obtain; this pathway leads to the formation of methane from the adsorbed aldehyde (Reactions F and G). The adsorbed methyl may recombine with adsorbed hydrogen (produced by water adsorption, Reaction D1) to produce methane, freeing both catalytic metal sites (Reaction I). On the other side, CO may react with the hydroxyl adsorbed at the platinum surface to produce CO₂ (Reaction H).

It is important to highlight the role that the adsorbed hydroxyl species play in the oxidation of ethanol. As for the methanol oxidation, the presence of adsorbed CO species at the platinum surface may hamper the electrocatalytic activity. Nevertheless, its occurrence is essential to producing a full oxidation to CO₂. Indeed, it is the presence of adsorbed hydroxyl which allows CO to be oxidized to CO₂. Coupling materials capable of increasing the rate of formation of the adsorbed hydroxyl at the electrocatalyst surface are indeed a key for increasing the effectiveness of the ethanol electrooxidation.

Lately, ethanol electrooxidation has been widely explored in alkaline media mainly because Pt can be effectively substituted by Pd indeed leading to even higher performance. Nevertheless, the C–C bond cleavage in alkaline environments has proved to not occur for ethanol at pH > 13 [46], acetate being the only oxidation product according to Eq. (3).

$${\text{C}}_{2} {\text{H}}_{5} {\text{OH}} + 5{\text{OH}}^{ - } \to {\text{ CH}}_{3} {\text{COO}}^{ - } + 4{\text{H}}_{2} {\text{O}} + 4{\text{e}}^{ - }$$

(3)

To obtain the full oxidation to acetate, the adsorption of hydroxyl is essential. Indeed, it has been demonstrated that palladium hydroxyl adsorption is the rate determining step [27], at least at low overpotentials. The addition of materials which may increase the hydroxyl adsorption rate on palladium has proved to be effective in enhancing ethanol electrooxidation. This is the case of Ceria, which is capable of improving the kinetics via spill-over of the primary oxide [47].

2.5 Fuel Composition: Acidic Versus Alkaline

Figure 2 summarizes the complete and half reactions occurring in acidic and alkaline systems. The reactions show that in acidic conditions, the products are always neutral (without charge), while in alkali, they are in form of anions. For methanol, the oxidation in alkali leads to the formation of carbonate with two negative charges; accordingly, two moles of OH⁻ are required to oxidize one mole of methanol. Concerning ethanol, the partial oxidation to acetate requires an equimolar consumption of the alcohol and OH⁻. In passive systems, where the fuel is held in a tank directly in contact with the anodic electrocatalyst, the consumption of the fuel leads to the drop of potential with the time. In active systems, where the fuel is continuously renewed and flowing at the electrocatalyst, local variation in the thermodynamic conditions can occur in the flow field, with a more significant effect in alkaline systems compared to acidic ones. Remarkably, in the case of oxidation to carbonate, OH⁻ depletes more rapidly than the fuel. Indeed, the oxidation of one mole of ethanol provides two moles of carbonates with the consumption of 4 OH⁻ for each mole of ethanol. Accordingly, this affects the thermodynamics of the cell more than the molarity of alcohol itself. This fundamental aspect has been poorly highlighted in the literature; however, it is of crucial importance as it has important significance on the system engineering. The fuel must contain stoichiometric amounts of OH⁻, possibly exceeding the concentration of the alcohol. This consideration is due to the faster oxidation kinetics in the presence of the significant amount of OH⁻ [48], especially in the low overpotential regions where, at least for ethanol, the oxidation kinetics is entirely dominated by the OH⁻ concentration on the electrode surface [49].

2.6 Device Energy Density

As liquid fuels, alcohols show a relatively high energy density. Table 1 summarizes the energy density of ethanol and methanol compared to common fuels. The comparison shows that alcohols have a weight energy density roughly 30% lower than gasoline or diesel. There are important differences between methanol and other alcohols with two or more carbon atoms. Indeed, the energy density of methanol is lower. Oppositely, alcohols show an undoubtful advantage when compared to hydrogen having much larger volumetric energy density, especially at low pressure.

Table 1 Mass and volumetric energy densities of ethanol and methanol compared to various fuels

However, this is a point in favor of alcohols only in principle. Indeed, there are factors hampering the capability to convert the energy of the fuel into electric energy. In 2012, a study aiming to investigate the system efficiency was conducted on alkaline devices for ethanol oxidation in passive systems [50] showing a maximum efficiency of 12%. For DMFCs, efficiency up to 40% has been reported [148]. At T > 100 °C, these membranes possess high proton conductivity, low electroosmotic drag (ca. 0 compared to 0.6 for Nafion^®), low methanol crossover (an 80-μm-thick PBI membrane has a methanol crossover of 1/10 with respect to that of a 210-μm-thick Nafion^® membrane) [148]. The major disadvantage is the leaching of the acid used for the do** once exposed to methanol solutions at high temperatures. Better performance can be achieved by using a silica impregnated phosphoric acid doped polybenzimidazole (PA/PBI/SiO₂) composite membrane [149]. The composite membrane shows a proton conductivity of 29–41 mS cm⁻¹ at temperatures between 200 and 250 °C allowing DMFCs delivering a peak power density of 136 and 237 mW cm⁻² at 260 °C using the Pt/C and PtRu/C as the anode electrocatalysts, respectively.

Sulfonated polyimides (sPIs) are another type of ionomers extremely suitable as polymer electrolyte membranes (PEMs) for fuel cell applications, except for their poor water stability. Crosslinking is a method that is commonly used to improve the weak hydrolytic stability of sPI membranes allowing to get performance in DMFCs comparable to that of Nafion^® 212 [150].

Polyvinyl alcohol (PVA) based membranes are one such class which has receiving increasing attention in DMFC application [151]. Because PVA is not a proton-conducting polymer, it should be sulfonated by using different sulfonic-carboxylic acids forming crosslinked polymers. The research efforts on this kind of membranes as well as their performance in terms of methanol crossover and proton conductivity are summarized in [152, 153].

Non-fluorinated acid membranes can be also prepared in blend with polyvinylidene fluoride (PVDF), whose mechanical strength, thermal stability and good chemical resistance make it suitable to improve the properties of sulfonated polymers. Blend of PVDF with sulfonated polyimide [154], sulfonated polyether sulfone [155], sulfonated polyether ether ketone [156], and sulfonated poly [bis(benzimidazobenzisoquinolinone)] [157] has been used as polymer electrolyte for DMFCs. More recently, copolymers of poly(methyl methacrylate)-co-poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PMMA-co-PAMPS) have been blended with polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF) to prepare proton conducting membranes [158], showing low methanol permeability (22 × 10⁻⁷ cm² s⁻¹).

3.5 Anion Exchange Membranes for Alkaline DAFCs

Alkaline DAFCs are very promising due to the several advantages with respect to the acidic counterparts. Indeed, the kinetics of both anodic oxidation of alcohols and cathodic oxygen reduction in alkaline media is more favourable than in acidic solution. Moreover, in alkaline media the cathodic catalysts are more methanol tolerant and the methanol crossover due to electro-osmotic drag would be reduced in OH⁻ conducting membranes [22, 159]. Anion exchange membranes (AEMs), which act as both the ionic conductor and electronic insulator, need to have high hydroxide conductivity, high electrochemical and mechanical stability [160,161,162].

Some commercially available AEMs have been used in DAFCs. MORGANE^®-ADP membranes from Solvay, ordinarily used for salt electrodialysis, were used as the solid polymer electrolytes for DMFCs [163,164,165,166]. They are cross-linked fluorinated polymers, whose exchange group is quaternary ammonium. They have thickness ranging between 150 and 160 μm when fully humidified with a resistance of 0.5 Ω cm² in 1 M (1 M = 1 mol L⁻¹) NaOH [164]. Encouraging performance were obtained, with the highest power density of 20 mW cm⁻². They show to be less permeable to methanol compared to Nafion^®, but they are not very stable in strong alkaline media.

Tokuyama Co. Japan is studying few ammonium-type AEM membranes and particularly, A201 and A901 were used for alkaline DEFCs [167,168,169], while AHA has been tested in DAFCs by using other alcohols [170].

Ongoing research on AEMs is mainly focused on polyvinylidene fluoride (PVDF, –[CH₂CF₂]_n–) and poly(tetrafluoroethene-co-hexafluoro propylene) (FEP, –[CF₂CF₂]_n[CF(CF₃)CF₂]_m–) films grafted with vinylbenzyl chloride using radiation-grafting [171, 172]. The FEP membranes showed good properties suitable for the fuel cell application, but their cost is relatively high. Cationic polymers with high ion exchange capacity (IEC), such as quaternized cationic polymers, are the preferred materials for the preparation of AEMs with high hydroxide conductivity [173, 174].

Many kinds of AEMs based on quaternized polymers containing a quaternary ammonium group have been developed and tested in DAFCs, such as polysiloxane containing quaternary ammonium groups [175], aminated poly(oxyethylene) methacrylates [176], quaternized polyethersulfone cardo [177], quaternized poly(phthalazinon ether sulfone ketone) [178], and quaternized polyvinyl alcohol [179]. The best performance obtained with the latter in DMFCs and DEFCs with utilizing QPVA-based membranes allowed to get peak power densities of 272 and 144 mW cm⁻², respectively.

An alternative approach to the production of AEMs is do** or reacting polymer films with KOH. Polybenzimidazole membrane was doped in alkaline solution and showed at room temperature higher conductivity than acid doped PBI [409] at temperatures up to 90 °C. The use of multiwall carbon nanotubes subjected to specific heat-treatment allowed to reach PPD up to 108 mW cm⁻² [409]. In this specific case, Pt nanoparticles have been deposited on medium-unzipped multiwall carbon nanotubes (Pt/MU-MWCNT) with a tailored 3D nanoarchitecture which allowed to better exploit the performance of Pt towards ORR.

Considering the alcohol crossover phenomenon, bimetallic electrocatalysts are preferred. The use of Pt- and Pd-based alloys or bimetallic electrocatalysts (Pt–Pd, Pt–Ru, Pt–Ni, Pt–Co, Pt–Fe) was investigated with good success [410,411,412,413]. As an example, Pd–Pt bimetallic electrocatalysts highly rich in Pd demonstrated to be highly active and methanol tolerant in DMFCs, reaching a performance of 112 mW cm⁻² when the amount of Pt is the lowest [410]. In fact, electrocatalysts with a higher Pt/Pd ratio showed a lower performance due to the high activity of Pt towards MOR. Evidence of DMFCs working at temperatures above 90 °C is also present in literature. Such devices showed a dramatic increase of power densities when cathodic Pt is alloyed with Ru (400 mW cm⁻² using pure oxygen [216], 280 mW cm⁻² using air [414]). Experimental evidence also suggested a strong influence of membrane materials on power density. The use of composite membranes could enhance power density output up to 400 mW cm⁻² [415].

PGM-free electrocatalysts are very promising because of their high alcohol tolerance [381, 392, 416,417,418]. However, their ORR activity is relatively low due to the poor ORR kinetics in acidic medium, reaching PPD between 40 and 70 mW cm⁻² at 60 °C. However, with PGM-free electrocatalysts, the MEA fabrication process plays a crucial role. Wang et al. [418] demonstrated that realizing a good triple-phase interface in the catalyst layer is necessary to boost the ORR activity. A good mixing of micropores, mesopores and macropores is necessary to assuring an optimal oxygen transport to reach the active sites, enhancing the electrode performance [419]. They demonstrated that with an engineered MEA with sufficient hydrophobicity and mass transport properties, the performance of a DMFC can increase by more than 40% on the maximum power peak density. They reached this result by introducing dimethyl silicon oil (DMS) in the electrode fabrication, increasing the performance of their Fe–N–C catalyst from 72 to 102 mW cm⁻². Osmieri et al. [393] showed that a proper optimization of the electrocatalyst loading and the ionomer content in the electrocatalytic layer is also crucial to enhancing the DMFC performance by using a Fe–N–C electrocatalyst at the cathode. An engineered MEA can also prevent flooding of the cathode, enhancing the performance of the DMFC [381, 418, 420].

Bimetallic Fe–Co electrocatalysts can enhance DMFC performance as well. Shi et al. [421] reached a performance of 130 mW cm⁻² (in air) with a dual-site Fe/Co–N–C electrocatalyst. The electrocatalyst was synthesized as a Co–N–C from a Co-doped ZIF-8 precursor with a controlled Zn/Co ratio, subsequently doped with Fe ions. A Zn/Co ratio equal to 11/2 was identified an optimal value. In fact, with an excessive Co-do**, the inactive Co-based metal species led to a promising ORR activity. The authors demonstrated that do** the basic Co–N–C catalyst with Fe allowed increasing the specific surface area and mesoporosity, which both favor oxygen diffusion to the active sites in the MEA.

An interesting approach to overcome the actual low performance of PGM-free electrocatalysts is the mixed approach adopted by Kosmala et al. [422]. A hybrid Pt/FeNC electrocatalyst, rich of FeN_x sites and Pt@FeO_x particles, showed excellent durability in acid media, together with a high tolerance to methanol. The physical–chemical characterization of the Pt/FeNC electrocatalyst containing only 2 wt% Pt showed the coexistence of Pt nanoparticles encapsulated by a thin Fe-oxide shell and FeN_x sites. The core/shell structure of the electrocatalyst, where Pt is covered by a layer of Fe-oxides, makes this material inert to methanol oxidation and highly stable thanks to the stability of FeO_x in acid environment. The high activity demonstrated by the hybrid materials can be attributed to the tunneling effect of Pt electrons through the FeO_x ultrathin layer at the core/shell interface. In fact, electrons from the surface of Pt can tunnel through the Fe oxide thin layer and catalyze the oxygen reduction. Such an effect is present only when the thickness of FeO_x surrounding the Pt nanoparticles is ultralow, otherwise the tunneling effect decreases progressively with the increase of thickness of the FeO_x. Thus, the core/shell Pt/FeNC electrocatalysts can be envisaged as promising materials for stable PGM-free surfaces in acid media, and tolerant to methanol. Interestingly, these Pt@FeO_x electrocatalysts could also be interesting HOR electrocatalysts for PEMFC anodes, thanks to their tolerance to species known to poison exposed Pt nanoparticles.

5.3.2 Performance of DEFCs in Acid Electrolytes

The use of ethanol in acid DAFCs is somehow limited because of the sluggish kinetics of the anodic EOR (involving C–C bond breaking) reaction compared to MOR, and of course the crossover of ethanol from the anode to cathode across the membrane. Similar to methanol, ethanol crossover results in parasitic EOR at the cathode, with the formation of a mixed potential and consequent lower fuel cell efficiency. With a Pt/C ORR electrocatalyst, the PPD does not overcome 9 mW cm⁻² [25, 167] (Table 7). An ethanol-tolerant ORR electrocatalyst can mitigate the oxidation of ethanol at the cathode side. Few examples are available in the literature, where the most interesting electrocatalyst is the one described by Meenakshi et al. [423]. They developed a mixed Pt-TiO₂/C electrocatalyst, using the so-called strong “d–d type” metal support interaction (SMSI) effect between Pt and TiO₂. With this electrocatalyst, they obtained a PPD three times higher, reaching 31 mW cm⁻² at 60 °C. Thanks to the SMSI effect, the Pt–H and Pt–CO bonds which limit the ORR are substantially weakening till suppression of intermediate formation. Transition metal oxides like TiO₂ and WO₃ are well-known for this effect. In fact, water molecules trapped inside the oxide network provide a hydrophilic behavior, which favors the OH⁻ transfer within the system, avoiding the formation of intermediates, with consequent spillover of primary oxide over metallic electrocatalyst particles [424] and improvement of the ORR performance.

Table 7 Activities in DAFCs: acid environment (the fuel: MeOH or EtOH)

An example of acid DEFCs with PGM-free cathode electrocatalysts is reported by Sebastian et al. [25]. In this work, they compared MEAs with a Pt/C and a Fe–N–C electrocatalyst at the cathode. With the Fe–N–C catalyst, they reached a PPD of 12 mW cm⁻² at 60 °C by feeding a 2 M ethanol solution (14 mW cm⁻² at 90 °C with 5 M ethanol solution). They also performed a chronoamperometric test of 75 h to assess the stability of the PGM-free electrocatalyst. The MEA had a rapid decrease of performance in the initial 10 h of the experiment, with a current density loss rate of 3.5% per hour. Then, the current density loss was lower, stabilizing at a rate of about 0.2% per hour. Investigations on the reasons of the irreversible performance decay with time highlighted that it could be rather caused by the degradation of other MEA components, such as the anodic electrocatalyst for the alcohol oxidation catalyst (anode), or the ionomer or the membrane, rather than the cathodic electrocatalyst. It is well known that Ru present in the anodic electrocatalyst is susceptible to leaching out [425].

Figure 9 shows the morphology of some of the electrocatalysts listed in Table 7.

Fig. 9

Morphology of some of the cathodic electrocatalysts for acid DMFCs listed in Table 7. a Scheme and TEM image of the unzipped Pt/MU-MWCNT. Adapted with permission from Ref. [409]. Copyright © 2019, American Chemical Society. b TEM image and EDS spectrum of the carbon-supported Pd₁₉Pt₁. Adapted with permission from Ref. [410]. Copyright © 2015, Elsevier Ltd. c TEM image of the Fe–N_x–C. Adapted with permission from Ref. [416]. Copyright © 2016, John Wiley & Sons, Inc. d STEM-EDXS images of the Pt_2.0Fe_1.0–N–C, with the highlight on a single Pt-rich nanoparticle. Adapted with permission from Ref. [422]. Copyright © 2020, American Chemical Society. e HAADF-STEM image and relative EELS analysis of the Fe/Co–N–C electrocatalyst, with Zn/Co = 11/2. Adapted with permission from Ref. [421]. Copyright © 2020, the Royal Society of Chemistry

5.3.3 Performance of DMFCs in Alkaline Electrolytes

Concerning DMFCs in alkaline environment with PGM-free cathode electrocatalysts the performance is relatively low, mostly because methanol oxidation leads to CO₂ generation, which causes a rapid carbonation of the OH⁻ exchange membrane. For example, Ratso et al. [426] did not overcome 7 mW cm⁻² as PPD at 50 °C with Fe–N–C or Co–N–C electrocatalysts. A typical reference material for ORR in alkaline environment is MnO₂, thanks to the effectiveness of the redox couple Mn^3+/4+ [388, 427]. MEAs with a cathode composed of only MnO₂ reached a PPD of 27 mW cm⁻² at 40 °C [428]. The addition of M–N–C materials to MnO₂ allowed reaching 41 mW cm⁻² as PPD at 40 °C [428]. In this specific case, Fe–N–C electrocatalysts were prepared from urea–formaldehyde resins mixed with MnO₂. Alkaline DMFCs displayed a superior ORR electrocatalytic activity thanks to the formation of Fe–N_x active sites and high amounts of pyridine-N. A chronoamperometric test showed an excellent durability with a minimal cell voltage decay after 39 h of testing.

Other examples in the literature show that a mix of oxides such as Fe₂O₃ and Mn₂O₃ (in the ratio 3:1) can reach a PPD of 46 mW cm⁻² at 60 °C [429]. The good performance is ascribed to the formation of numerous heterojunctions between the two oxides, which form an intensive internal electric field favoring the electron transfer and thus the ORR. They demonstrated that the heterojunctions increased with the increase of the Fe/Mn ratio. In addition, the presence of Fe₂O₃, which is an n-type semiconductor, enhanced the oxygen storage capability of the electrocatalyst, favoring the Fe^3+/2+ redox couple.

The highest performance reported so far in the literature has been reached with a hybrid electrocatalyst composed of three-dimensional Mn₂O₃ oxide deposited on ultrathin and porous nanosheets of Co_1.2Ni_1.8O₄. With this electrocatalyst, Liu et al. [430] reached the remarkable PPD value of 120 mW cm⁻² at 55 °C (70 mW cm⁻² at 28 °C). The tolerance to methanol was very high, also at a concentration of 2 M. The durability of the performance was monitored by kee** the MEA at a constant current density of 30 mA cm⁻² for 120 h. The voltage remained practically constant, showing almost no degradation effects. XRD analysis after the durability tests conducted on the cathodic electrocatalyst showed unchanged diffraction peaks, demonstrating the high stability of this hybrid electrocatalyst in alkaline environment. The excellent electrochemical activity of the Mn₂O₃@Co_1.2Ni_1.8O₄ could be attributed to the narrow band gap of Co_1.2Ni_1.8O₄, which accelerates the electrons jum** from the valence band to the conduction band. Consequently, Mn³⁺ is fast oxidized into Mn⁴⁺ providing an electron to the absorbed oxygen. Indeed, the excellent stability can be attributed to the high stability of Mn₂O₃ and to the partial substitution of Ni with Co in the Co_1.2Ni_1.8O₄ mixed oxide.

Interestingly, Liu et al. [431] developed a highly performing and stable cathodic electrocatalyst based on NiC₂O₄. They obtained 151 mW cm⁻² as PPD at 65 °C, compared to 18 mW cm⁻² reached with a commercial Pt/C at the cathode side. Stability tests performed monitoring the voltage during galvanostatic test at 10 and 50 mA cm⁻² at room temperature for 90 h demonstrated a very stable electrocatalyst, with limited cell voltage loss. The high activity towards ORR is provided by Co³⁺ surface ions, which act as functional active sites [428]. Copyright © 2019, Elsevier B.V. b SEM of Mn₂O₃, SEM, TEM, HRTEM, SAED patterns and EDX map** of Mn₂O₃@Co_1.2Ni_1.8O₄. Adapted with permission from Ref. [430]. Copyright © 2018, American Chemical Society. c FESEM and EDS elemental map** of Fe₂O₃/Mn₂O₃ in the ratio 3:1. Adapted with permission from Ref. [429]. Copyright © 2018, the Royal Society of Chemistry. d SEM, TEM and HRTEM of NiCo₂O₄. Adapted with permission from Ref. [431]. Copyright © 2017, Elsevier B.V. e SEM of NiCo₂O₄. Adapted with permission from Ref. [433]. Copyright © 2019, American Chemical Society. f STEM and TEM of Fe–Co–N–C. Adapted with permission from Ref. [438]. Copyright © 2018, John Wiley & Sons, Inc