Rare-metal-free Zn–air batteries with ultrahigh voltage and high power density achieved by iron azaphthalocyanine unimolecular layer (AZUL) electrocatalysts and acid/alkaline tandem aqueous electrolyte cells

Metal–air batteries have energy densities more than three times higher than the existing lithium-ion batteries and, thus, are expected to become the next generation of energy devices.¹ Zn–air batteries are the most common type of metal–air battery because they are stable in aqueous systems with little gas generation during the dissolution of the anode. However, Zn–air batteries have only been used in limited applications, such as hearing aid batteries, due to their low power density and standard voltage of around 1.4 V. Therefore, to use Zn–air batteries as a drive power source in cutting-edge devices such as drones,² it is essential to improve the drive voltage and output power density.

The cell voltage of a battery is determined by the potential difference between the cathode and anode. Changing the anode potential is difficult because the potential of Zn at the anode is determined solely by the pH of the electrolyte. Therefore, the overvoltage of the oxygen reduction reaction (ORR) at the cathode must be reduced to improve the open-circuit voltage (OCV) value. Consequently, various kinds of high-performance ORR catalysts have been developed and their application in Zn–air batteries has been investigated. The ORR is the same process as that at the cathode in polymer electrolyte fuel cells, and catalysts using rare metals such as Pt/C are highly active electrocatalysts for ORR. However, low-cost, highly efficient alternative electrocatalysts are required because of resource constraints and the high cost of rare metal electrocatalysts.

Various alternative catalysts have been investigated, including inorganic oxides and alloys other than manganese oxides,^3–5 and carbon alloy electrocatalysts^6,7 consisting of heteroatom-doped carbon, such as FeN₄ structures. In particular, iron phthalocyanines (FePcs) with intramolecular FeN₄ structures have been considered as low-cost ORR catalysts.^8–10 However, although they have an activity comparable to that of manganese oxides, their ORR catalytic activity is too low. Electrocatalysts with FePcs immobilized on carbon at the molecular level that show an ORR activity comparable to that of Pt/C have been reported,^11,12 and some of them have been employed in metal–air batteries.^13,14 Furthermore, we have reported that the immobilization of iron azaphthalocyanine (FeAzPc) on multi-walled carbon nanotubes or Ketjen black (KB) can produce ORR electrocatalysts with a higher ORR activity than FePc electrocatalysts.^15,16 This catalyst was prepared by dissolving FeAzPc molecules in a solvent, and the catalyst molecule was molecularly adsorbed and immobilized on the carbon surface (Fig. 1). Compared to other FePc derivatives, both electrochemical measurements and density functional theory (DFT) calculations revealed that nitrogen substitution optimized the electronic structure of the catalyst for ORR, resulting in a high catalytic activity. Because this metal azaphthalocyanine unimolecular layer (AZUL) electrocatalyst is also highly durable and it can be prepared from low-cost pigments and KB without the high-temperature pyrolysis required by carbon alloy systems as reported in previous studies,^15,16 this is a promising cathode electrocatalyst for Zn–air batteries.

Wen et al. reported Zn–air and Al–air batteries that generated an OCV of over 2.0 V and comprised an acidic electrolyte at the cathode and an alkaline electrolyte at the anode separated with an anion exchange membrane.^17,18 Some related studies have also achieved high-voltage metal–air battery cells.^19,20 This dual electrolyte system generates higher cell voltages and higher power densities than the conventional single-electrolyte system. However, these systems used high-cost, resource-constrained rare-metal Pt/C as a cathode electrocatalyst, which does not meet the demand for ultrahigh voltage and ultrahigh power density metal–air batteries without rare-metal electrocatalysts. Furthermore, the discharge profiles of the dual electrolyte cells at high current densities were not determined.

Against this background, in this work, we fabricated a Zn–air battery using AZUL electrocatalysts as the cathode catalyst. First, we examined normal alkaline Zn–air battery cells and found that the AZUL electrocatalyst had a high molar mass activity and high cell performance, especially in the high-current-density region. Furthermore, tandem acid/alkaline electrolyte Zn–air battery cells were fabricated and their ultrahigh voltage and ultrahigh power density were demonstrated. The OCV of the cell was 2.25 V, and an extremely high-power density (318 mW/cm²) was achieved. Furthermore, we observed the instability of the Pt/C electrode under highly acidic conditions, whereas the AZUL electrocatalyst showed a much higher stability and better electrochemical performance than Pt/C in tandem cells. The discharge potential of the AZUL Zn–air battery was 1.9 V at 10 mA/cm² and over 1.0 V at 200 mA/cm², and the output power density was 1139 mWh/g_(Zn) at 100 mA/cm² discharge.

Figure 2 shows the Pourbaix diagram of Zn,²¹ which is the phase diagram of the redox potentials of Zn dissolution in aqueous solutions of various values of pH. Because alkaline aqueous solutions are used in commercially available Zn–air batteries, the theoretical cell voltage of the Zn–air battery is determined by the redox potential of reaction (v) [Fig. 2(a)] and the ORR under high pH conditions and this voltage is normally around 1.9 V. Compared with alkaline conditions, acidic conditions give much higher theoretical cell voltages, but the rapid chemical dissolution of Zn in an acidic aqueous solution results in a low cell capacity and violently generates H₂ gas. Therefore, alkaline conditions are usually used in Zn–air batteries. The Pourbaix diagram shows that the theoretical cell voltage was maximized to around 2.7 V with alkaline electrolytes for the Zn anode (anolyte) and acidic electrolytes for the cathode (catholyte).

A schematic of the simple single-electrolyte alkaline cell is shown in Fig. 3(a), and a schematic of the tandem-electrolyte cell we designed to maximize the cell voltage is shown in Fig. 3(b). Although dual-electrolyte cells have been reported,^19,22 these contain a mixture of different electrolytes in a solvent;^23,24 therefore, we refer to our cell as a tandem-electrolyte cell to avoid confusion. The electrolyte chamber of the tandem cell was separated in the center with an alkaline ion exchange membrane (AEM) and the catholyte and anolyte were 6M KOH aq. and 3.5M HCl aq., respectively. This configuration generates a larger theoretical potential difference between the cathode and anode than an alkaline cell.

To compare the performance of the AZUL catalyst with Pt/C, the ORR activities of the electrocatalysts were evaluated by using rotating ring-disk electrode (RRDE) measurements under alkaline conditions (pH 13, KOH aq.). Figure 4(a) shows the linear sweep voltammetry (LSV) curves obtained with the glassy carbon (GC) (black), Pt/C (orange), and AZUL-coated (red) disk electrodes at a rotation speed of 1600 rpm. The GC electrode showed the lowest performance, whereas the Pt/C and AZUL (20 wt. %) electrodes showed good ORR activities. From the LSV curves, the onset potentials (E_onset), half-wave potentials (E_1/2), and current densities at E = 0.6 V (J_E=0.6V) of the two electrocatalysts were obtained (Table I). The electrocatalysts had almost identical E_onset values, and the AZUL electrocatalyst had higher E_1/2 and J_E=0.6V values than Pt/C. From the current densities of the RRDE, the H₂O₂ ratios and reaction electron numbers (n) were calculated with Eq. (2). Figure 4(b) shows the H₂O₂ ratios and n values from 0.4 to 0.6 V vs reversible hydrogen electrode (RHE). The H₂O₂ ratio of the GC electrode was over 30%, whereas those of AZUL and Pt/C were less than 1%. The n values of AZUL and Pt/C were 3.99 and 3.96, respectively, and these values were constant in this potential regime. These results demonstrated the high electrocatalytic performances of AZUL and Pt/C. Figure 4(c) shows the rotation speed dependence of LSV curves of the AZUL catalyst (Pt/C as the reference; see the supplementary material, S1). The current density increased linearly with increasing rotation speed, and these data gave the K–L plot (the inset plot). Based on the K–L plot and Eq. (3), the n value was estimated as ∼4.0, which was the same as that calculated from Eq. (2). These results showed that the AZUL catalyst had a higher catalytic activity than Pt/C under alkaline conditions. Additionally, as reported in a previous work,¹⁵ the AZUL catalyst has a higher electrochemical stability in a chronoamperometric test and stability to methanol crossover. The Tafel slope of the AZUL catalyst using KB was 59 mA/decade, which was close to that of Pt/C (62 mA/decade).¹⁶ Compared to previously reported ORR electrocatalysts, the AZUL catalyst has a prominent performance (see the supplementary material, S7).

The present data obtained from RRDE are equivalent to the normal three-electrode RDE experiments^29,30 because it can be used as an RDE when the signal from the ring electrode is ignored. In the RRDE experiment, electrochemical measurements were performed using the three-electrode configuration with a Pt counter electrode and an Ag/AgCl reference electrode. This measurement is a proven method and is normally used in the evaluation of many electrocatalysts both for fuel cells and for metal–air batteries.

The current density difference between AZUL and Pt/C originated from the difference of carbons. Since the relative surface areas of carbons used in Pt/C and AZUL were different, the maximum current densities of those values changed even though the densities of electrocatalysts were constant. The commercially available Pt/C uses Vulcan XC72 (Cabot, USA) as a carbon support, and its Brunauer--Emmett--Teller (BET) surface area is ∼240 m²/g. On the other hand, AZUL uses KB (ECP600JD, Lion, Japan) and has a higher BET surface area of around 1300 m²/g. These surface area differences caused the difference in the maximum current densities.

Additionally, the turnover frequency (TOF) values of electrocatalysts were calculated (supplementary material, S14).^31,32 From the calculation, the TOF_(AZUL) and TOF_(Pt/C) values were 0.701 and 0.197 at 0.6 V vs RHE, respectively, and the TOF ratio between AZUL and Pt/C reached the maximum value when the TOF_(AZUL) was 6.1 times higher than the TOF_(Pt/C) at 0.9 V vs RHE. This difference was caused by the difference in catalyst formulation between AZUL and Pt/C electrocatalysts. In the case of AZUL, FeAzPc-4N molecules are molecularly adsorbed on the surface of carbons and those molecules act as a catalytic center for ORR. On the other hand, Pt was formulated as nanoparticles; therefore, only surface atoms of Pt nanoparticles can act as catalytic sites. Those actual catalytic site density differences offer a significant difference in TOF values and the high electrocatalytic performance of AZUL for ORR.

Additionally, the LSV curves in 6M KOH aq. were also measured (see the supplementary material, S12).

The ORR electrocatalyst cathodes were fabricated by coating carbon sheets with different amounts of electrocatalysts. Figure 5(a) shows the configuration of Zn–air batteries containing an alkaline electrolyte (6M KOH aq.). A cathode carbon sheet and metal mesh, which functioned as a current corrector, were sandwiched with a resin plate containing a 0.95 cm² hole and an electrolyte chamber, and a Zn foil was sandwiched with a resin plate and the electrolyte chamber at the opposite side. The assembly was fixed together with screws at the corners. The battery performance was measured by using a potentiostat with a connecting metal mesh cathode and a Zn foil anode.

Figures 5(b)–5(d) show the I–V and I–P characteristics of Zn–air cells with different loadings of Pt/C and AZUL electrocatalysts immobilized on the cathodes. From the I–V polarization curves, the OCV of the Pt/C cell was 1.45–1.70 V and that of the AZUL cell was 1.55–1.80 V and then the voltage dropped to around 1.4 V, which is the nominal voltage of the Zn–air battery. A potential drop was found in the high-current-density region over 200 mA/cm² for the Zn–air battery cell with a Pt nanoparticle density of 59 μg/cm². This potential drop originated from the ORR electrochemical pathways changing in the high-current-density region. The ORR performance of Pt/C is strongly affected by the Pt nanoparticle density on the electrode surface. Fabbri et al.²⁵ reported that oxygen reduction occurred via the two-electron pathway, which generated hydroxyl peroxides rather than hydroxyl ions, when the Pt nanoparticle density on a flat amorphous carbon electrode was lower than 10 μg/cm². When the Pt nanoparticle density increased above 50 μg/cm², the synergetic effect of the Pt nanoparticles being in an intimate contact allowed the four-electron reaction that converts hydrogen peroxide to hydroxide ions to proceed. The minimum amount of Pt nanoparticles examined in the present experiments was similar to the lower threshold for the four-electron pathway reported in the literature. However, the surface density of the Pt nanoparticles used in this experiment should be lower because the Pt nanoparticles were coated on carbon black, which has a higher surface area than the amorphous carbon (Vulcan XC-72, relative surface area of 254 m²/g). Therefore, in the high-current-density region, a higher Pt nanoparticle density is required to ensure an efficient four-electron reaction, which requires a rapid diffusion of O₂ molecules. This potential drop gradually disappeared as the catalyst loading increased, and finally, the I–V curve became a straight line for a loading of 488 μg/cm² and the maximum current density reached 640 mA/cm². The current density values at 1.0 V changed from 115 to 150 mA/cm² as the Pt nanoparticle density increased. These results indicated that the Pt nanoparticle loading should be over ∼500 μg/cm² to ensure a high current density.

In contrast, for the Zn–air battery cells with the AZUL electrocatalyst cathode, the current density increased with increasing catalyst density with no potential drop in the high-current-density region. The current density values at 1.0 V changed from 140 to 160 mA/cm² as the catalyst density increased from 17 to 201 μg/cm². This result implied that the AZUL catalyst did not require the inter-catalyst synergetic effect found in the Pt nanoparticles and the AZUL cells showed a higher performance than the Pt/C cells. The maximum current density of the AZUL cells was up to 554 mA/cm². The difference between the Pt/C and AZUL electrocatalysts was that the Pt nanoparticles have been fixed on the carbons in Pt/C (see the supplementary material, S8), but Fe azaphthalocyanine molecules are adsorbed on the carbons. In the case of Pt/C, the active site of ORR has been limited to the surface of Pt nanoparticles; therefore, the Pt atoms located inside the nanoparticles have not contributed to the catalyst reactions. To ensure the four-electron process of ORR, a synergetic effect of neighboring Pt nanoparticles is required to compensate for the low active site density on Pt/C. However, whole catalyst molecules of the AZUL electrocatalyst are basically active on the surface of the carbon. Therefore, the active site density is high enough and this led to a high performance on the high current density regime compared with the Pt/C case.

Figures 5(d) and 5(e) show the I–P curves calculated from the I–V curves of the Pt/C Zn cells and AZUL Zn cells, respectively. Due to the potential drop in the high-current-density region, the I–P curve of the Pt/C Zn cells with a cathode catalyst loading lower than 125 μg/cm² decreased above 200 mA/cm² and the maximum power density was 215 mW/cm² at 328 mA/cm². For the AZUL Zn cell, the power density curve was uniform and parabolic and its maximum was 225 mW/cm² at 325 mA/cm², which was almost identical to that of the Pt/C Zn cell. These results show that the AZUL Zn cells have an equivalent performance to the Pt/C Zn cells (Table II).

An interesting feature of AZUL Zn cells is that the cathode catalyst densities are much lower than those of Pt/C Zn cells. Figures 5(f) and 5(g) show the maximum power density values and maximum current density values plotted against the molar cathode catalyst density, respectively. The power density values of Pt/C Zn cells increased linearly with the cathode catalyst density, and the maximum power density was obtained at a density of 488 μg/cm². In contrast, AZUL Zn cells showed the highest power density, which was similar to that of the Pt/C Zn cell, at a density of only 85 μg/cm². From the data fitting curves, the AZUL Zn cell required at least only 1/9.6 quantity to achieve the same performance as the Pt/C Zn cells.

The RRDE measurements of the Pt/C and AZUL catalysts showed that their original ORR performances were similar. Therefore, the difference in the required catalyst densities between Pt/C and AZUL to achieve the highest cell performance did not arise from the catalyst performance itself and the high mass activity of the AZUL catalyst originated from its structural features. The Pt/C catalyst comprises Pt nanoparticles (∼2 nm) immobilized on the surface of carbon black carriers. Therefore, Pt nanoparticles are the active sites for the ORR and the number of Pt nanoparticles on the carbon black surfaces affects the catalyst performance. The 2 nm Pt nanoparticles consisted of ∼400 Pt atoms and the calculated active site density was 3.8 × 10¹⁵ cm⁻² at a catalyst density of 488 μg/cm². In contrast, the AZUL catalyst consisted of FeAzPc-4N molecularly adsorbed onto the carbon black carrier. Avogadro’s number of catalyst molecules was available as the active sites for the ORR on the carbon black surface. The active site number was calculated as 8.5 × 10¹⁶ cm⁻² for a catalyst density of 85 μg/cm². This simple calculation implied that the active site density of the AZUL catalyst was much higher than that of Pt/C, which increased the mass activity of the AZUL catalyst.

The high molar activity of the AZUL catalyst was also demonstrated by the effect of the loading quantity on P_max and J_max. Figure 5(f) shows the relationship between the molar loading of the electrocatalysts on the carbon sheets and P_max. The P_max values increased with increasing Pt/C and AZUL loading, and the AZUL electrocatalyst achieved the same P_max value as Pt with a smaller molar loading. The fitted regression line showed that the AZUL electrocatalyst required 5.4 times fewer catalyst molecules than Pt/C, and thus, the AZUL electrocatalyst had a much higher molar mass activity than Pt/C.

This trend was also observed in the relationship between the molar loading of the electrocatalysts and J_max [Fig. 5(g)]. A high J_max value indicates a high electrochemical durability in the high-current-density region. The high activity in the high current density region, where oxygen supply tends to be rate-limiting, suggested that the AZUL catalyst had a high ORR activity and that the performance degradation due to limited oxygen supply, a reason for the low output of conventional metal–air batteries, can be overcome with a highly active ORR catalyst.

Figure 5(h) shows the discharge curves of Pt/C (dashed lines) and AZUL (solid lines) cells with discharge current densities of 10–300 mA/cm². With a 68.5 mg Zn plate as an anode, the total discharge times were 6.4, 3.2, and 1.7 h at discharge currents of 10, 20, and 40 mA/cm², respectively. Based on these results, the weight capacity of the AZUL Zn cell per Zn plate was 700–750 mAh/g and the weight energy density was 818–847 mWh/g. Figure 5(i) shows the average cell voltage at different discharge current densities. The Pt/C and AZUL cells had similar cell voltages; however, the AZUL cell had higher cell voltages than the Pt/C cell in the high-discharge-current-density region. This result also suggests that AZUL cells had a superior performance in the high-current-density region.

Normally, the battery voltage at the open circuit of Zn–air cells is determined by the difference between the potential of anode Zn and that of the cathode electrode. The potential of Zn is constant; therefore, the cathode potential strongly affects the battery voltage and it was determined by the overpotential of the electrocatalyst immobilized on the cathode.

When increasing the current density of the cell, it required an efficient conversion of oxygen reduction. It depends on how many active catalytic sites exist on the surface of the cathode. Therefore, catalyst loading is crucially important to keep the voltage at the high current density region.

We have performed electrochemical impedance spectroscopy (EIS) of Zn–air battery cells using AZUL and Pt/C electrocatalysts (see the supplementary material, S11 and Fig. S8). In both cases, they showed the resistance components in a typical battery cell. Among these, the resistance originating from mass diffusion is called the Warburg impedance (Z_w) and corresponds to the linear component extending from the semi-circle in the Nyquist plot. It is noteworthy that both cells showed almost identical curves in the plots. This result shows the resistivities of the components and the mass transfers in those cells were almost the same even though the coated electrocatalysts on gas diffusion layers (GDL) were different, and those parameters did not affect the performance of the cells. Therefore, the difference in the performance of zinc–air battery cells depends almost entirely on the catalyst performance.

Owing to the tandem-electrolyte cell configuration, the cathode catalyst performance should be evaluated in HCl aq. There are many reports of high-performance ORR catalysts under alkaline conditions, but there are few of the ORR activities of electrocatalysts in highly acidic HCl aq. Therefore, we measured the ORR performance of electrocatalysts in 3.5M HCl aq. Figure 6(a) shows the LSV curves of Pt/C, AZUL (20%), and GC in 3.5M HCl aq. using an RRDE. No electrochemical reaction was observed for GC, but Pt/C and 20% AZUL showed clear current generation due to the ORR. However, the E_onset of Pt/C was low (0.589 V) compared with that measured in an alkaline electrolyte (Fig. 4) and in the low pH region of pH 0, as reported in the literature. This result is unusual because Pt/C is normally used as a reference for measuring the ORR because it has a high ORR activity and stability. From the ORR kinetics of Pt/C, the E_onset and current densities gradually decreased with increasing rotation speed (see the supplementary material, S2). This trend is the opposite of normal ORR electrocatalysts and means that the ORR performance of Pt/C gradually decreased during measurements owing to the dissolution of Pt in highly acidic HCl aq. Normally, Pt does not react with HCl in the pH region above pH 0, but chlorination of Pt occurred, and Pt was converted to PtCl₆²⁻ or PtCl₄²⁻ above an E of 0.5 V.²⁶ The observed E_onset was almost identical to the potential of Pt chlorination in HCl aq. of E = 0.5 V, and this indicates that the dissolution of Pt decreased the ORR activity in 3.5M HCl aq.

The ORR performance of 20% AZUL was not affected by the highly acidic conditions. To examine the stability of FeAzPc-4N in HCl aq., solid FeAzPc-4N was added to various concentrations of HCl aq. Figure 7(a) shows the UV–Vis spectra obtained from the supernatant of FeAzPc-4N in different concentrations of HCl aq., and Fig. 7(b) shows the concentration dependence of the peak intensity at λ = 640 nm, which is attributed to the Q-band of FeAzPc-4N.²⁸ The inset in Fig. 7(b) is a photograph of FeAzPc-4N in various concentrations of HCl aq. The solubility of FeAzPc-4.0M in HCl aq. increased gradually with the HCl concentration. The peak intensity increased above 4.0M HCl aq. These results show the unique low solubility and chemical stability of the AZUL electrocatalyst under highly acidic conditions. Figure 7(a) shows that FeAzPc-4N started to dissolve at HCl concentrations of 3.5–4.0M, the pH values of which were −0.5 to −0.6. In other words, FeAzPc-4N was stably immobilized on KB around this concentration. Furthermore, FeAzPc-4N molecules were stable under the highly acidic conditions because the Q-band, which is attributed to the absorption of the C–T complex comprising the iron ion and organic ligands, was clearly observed, even though FeAzPc-4N was dissolved in HCl at concentrations over 4.0M. These results demonstrated that the AZUL electrocatalyst, in which FeAzPc-4N was adsorbed on KB, functioned as a stable electrocatalyst in 3.5M HCl aq. From the Pourbaix diagram, the higher acidic condition gives a higher cell voltage for Zn–air batteries. Therefore, we have to choose the highest acidic conditions without damaging the AZUL electrocatalysts. As shown in Fig. 7, FeAzPc-4N dissolved in over 4.0M HCl aq. Therefore, we chose 3.5M HCl aq. as the best acidic condition to achieve the highest cell voltage without damaging the AZUL electrocatalysts. Additionally, the ORR activities of the AZUL and Pt/C electrocatalysts in 0.1M H₂SO₄ aq. were also measured by using the RRDE (supplementary material, S13 and Fig. S10).

Figure S6 shows the cyclic voltammogram (CV) curves of Pt/C, 20 wt. % AZUL, and 50 wt. % AZUL measured in 6M KOH aq. (a), 3.5M HCl aq. (b), and 0.1M H₂SO₄ aq. (c), respectively, at the scanning rate of 100 mV/s after N₂ bubbling. In the 6M KOH aq. and 0.1M H₂SO₄ aq. cases, Pt/C shows the usual high redox activities; however, in only 3.5M HCl aq., the redox activities of Pt/C were clearly lower than those of AZUL cases. From CV, the redox peaks of Pt/C were prominent in 0.1M H₂SO₄ aq. but decreased in 3.5M HCl aq., which indicated the decomposition of Pt nanoparticles due to Pt dissolution in the HCl aq. On the other hand, AZUL electrocatalysts show clear redox peaks in each case. These results are also essential to understand why AZUL electrocatalysts showed a high ORR performance in both acidic and alkaline electrolytes. Those results indicate that Pt nanoparticles are not stable in a high concentration of HCl aq.

Figure 8(a) shows a schematic and the configuration of the tandem-electrolyte cells. Compared with the single-electrolyte cell, an additional electrolyte chamber was used and an AEM was sandwiched with O-rings as a gasket between the two electrolyte chambers to prevent the acid and alkaline electrolytes from mixing. The other parts of the tandem-electrolyte cell were the same as the single-electrolyte cell. The chambers were filled with the 3.5M HCl aq. catholyte and 6.0M KOH aq. anolyte. Figure 8(b) shows the J–V and J–P polarization curves of the tandem-electrolyte Zn–air battery cells with AZUL or Pt/C cathodes.

The tandem-electrolyte Zn–air battery cells showed a much higher performance than the single-electrolyte cells. The OCV, J_E=1.0, and P_max of the tandem-electrolyte Pt/C Zn–air cell were 2.11 V, 225 mA/cm², and 238 mW/cm², respectively (Table II). The tandem configuration increased the OCV value, and high current and power densities were achieved. These results provide a proof of concept that the potential control of the anode and cathode by the pH of the anolyte and catholyte provide a high cell voltage, even though the ORR performance of the Pt/C catalyst is insufficient in 3.5M HCl aq.

Furthermore, the tandem-electrolyte AZUL Zn–air cell showed a much higher performance than the tandem-electrolyte Pt/C Zn–air cell, with an OCV, J_E=1.0, and P_max of 2.25 V, 316 mA/cm², and 318 mW/cm², respectively. Because the AZUL electrocatalyst showed a higher ORR performance than Pt/C in 3.5M HCl aq., the OCV value was higher than that of Pt/C. Importantly, the OCV value was close to that of tandem-electrolyte Al–air battery cells, which may have higher voltages than Zn–air batteries, and the current and power densities are much higher than those of the Al one. Compared with previously reported state-of-the-art Zn–air and Al–air battery cells, this cell is the most powerful, high-performance Zn–air battery that we know of that does not use Pt/C (see the supplementary material, S4).

The discharge polarization curves of the tandem-electrolyte Pt/C and AZUL cells are shown in Fig. 8(c). For a discharge current density of 10 mA/cm², the initial cell voltage was 1.99 V and it remained over 1.70 V. Notably, the tandem-electrolyte cell worked even at discharge current densities of 100 and 200 mA/cm². Normally, a main drawback of metal–air batteries is their low performance under high-current-density regimes. The cell voltage at a discharge current of 100 mA/cm² was initially 1.64 V, and the voltage remained above 1.40 V. Even at 200 mA/cm², the initial cell voltage was 1.19 V and it remained above 1.00 V. At 300 mA/cm², the cell voltage of the AZUL cell was above 0.60 V, whereas that of the Pt/C cell was initially 0.50 V and gradually decreased to less than 0.50 V. The maximum capacity and total power density of the AZUL cell were 728 mAh/g_(Zn) and 1139 mW/g_(Zn), respectively. The capacity was three times higher than that of the normal Li ion batteries, and the total power density was close to the theoretical upper limit for metal–air batteries. It is noteworthy that the OCV of the fabricated cells is basically very stable (supplementary material, S15). The result shows that the OCV value (over 2.1 V) was kept for a long time after the injection of electrolytes.

Because the ORR is the rate-determining process of metal–air batteries, the slow diffusion of oxygen and sluggish ORR limit the power generation, and thus, high-performance ORR catalysts are required. However, normal alkaline Zn–air cells usually generate voltages of less than 1.5 V and it is difficult to achieve both high voltages and high current densities. The high-performance AZUL ORR catalyst and the tandem-electrolyte cell provide a promising solution for these two requirements.

To demonstrate the high performances of the AZUL single and tandem electrolyte cells, a video image of battery driven experiments is shown in the supplementary material, S5.

We fabricated Zn–air batteries with a high potential and high-power density by using the AZUL electrocatalyst and a tandem Zn–air battery cell. The AZUL electrocatalyst in this new type of cell had a high electrochemical stability and high ORR performance in the ultralow pH region, in which Pt and other metallic and inorganic electrocatalysts cannot be used. Furthermore, the tandem-electrolyte cells had a cell voltage over 1.0 V at high discharge current densities of up to 300 mA/cm². A sufficient voltage output under these high current density regimes has not been achieved previously by conventional and dual electrolyte Zn–air batteries.²⁷ We believe this work is a considerable advance toward realizing high-performance Zn–air and other metal–air batteries for next-generation energy devices that use abundant resources. In the present report, we focus on the development of ultrahigh voltage and high-power density primary Zn–air battery cells using AZUL electrocatalysts and acid/alkaline tandem electrolytes, but this knowledge can be extended to realize more lightweight and high-power density secondary battery cells. However, several issues need to be resolved, such as chlorine generation from the cathode during charging, development of oxygen evolution reaction (OER) catalysts to reduce overpotential during charging, and cell design to realize thinner flexible cells. Each of these issues is considered worth investigating, and those issues are also future perspectives.

FeAzPc-4N (29H,31H-tetrapyrido[2,3-b:2′,3′-g:2″,3″l:2‴,3‴-q]porphyrazine iron complex, Fig. 1(a)) was synthesized according to the literature.¹⁶ The same batch of FeAzPc-4N was used throughout the study. KB (ECP600JD) was purchased from Lion Specialty Chemicals, Co., Ltd. (Tokyo, Japan). An alkaline ion exchange membrane (AEM; FAA-3-50, Fumasep) was purchased from Fuel Cell Store (College Station, TX, USA). The Zn foil (5 × 10 × 0.2 mm) was purchased from The Nilaco Corporation (Tokyo, Japan). The carbon sheet (MCNW 30H) was purchased from MFC Technology, Inc., Tokorozawa, Japan. The 20 wt. % Pt/C catalyst (738549-1G) was purchased from Sigma-Aldrich (St. Louis, MO, USA). The 20% Nafion dispersion solution (DE2020 CS), isopropyl alcohol (IPA), and other solvents were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan).

The AZUL electrocatalyst was prepared according to the literature (Fig. 1).¹⁶ FeAzPc-4N (1.0 g, 20 wt. %, or 4.0 g, 50 wt. %) and KB (4.0 g) were mixed in a good solvent, such as dimethyl sulfoxide (DMSO) (250 ml), with stirring for 1 h, and then, the catalyst molecule was adsorbed with drying at 80 °C for 3 h. Stirring was stopped, and the mixture was dried at 90 °C for 1 h. After the evaporation of the remaining solvent at 120 °C for 3 h, the AZUL catalyst was obtained.

AZUL catalyst inks were prepared by mixing a AZUL catalyst (0.5 g) in IPA (18 ml), deionized water (4.5 ml), and 20% Nafion dispersion (0.9 ml) by ball milling (P-6, Fritsch, Idar-Oberstein, Germany) using a zirconia pot and zirconia balls (5 mm) at a rotation speed of 300 rpm for 20 min. The sample was collected and diluted with IPA (18 ml) and deionized water (4.5 ml) to obtain 1.29 wt. % catalyst ink. The Pt/C ink was prepared in the same way by mixing 20 wt. % Pt/C (1.0 g) with IPA (4.2 ml), deionized water (4.2 ml), and 20% Nafion dispersion (1.8 ml) by ball milling. IPA (12.6 ml) was added to obtain 5.0 wt. % catalyst ink.

The catalyst ink was dropped on a glassy carbon electrode (BAS, Inc., Tokyo, Japan) to form a catalyst layer with a catalyst density of 300 μg/cm². A Pt wire and an Ag/AgCl electrode were inserted into the electrolyte as reference and counter electrodes, respectively. The rotating ring-disk electrode (RRDE) measurements for each catalyst were performed with an RRDE system (RRDE-3A, ALS Co., Ltd.) equipped with a bipotentiostat (2325, ALS Co., Ltd.).

The same catalyst ink as used in the RRDE measurements was coated on hydrophobic carbon sheets (70 × 70 mm²), which were used as GDL, with different coating times with a spray coater (SimCoat, Sono-Tek Corporation, Milton, NY, USA). The Pt or FeAzPc-4N loading was 0.059–0.658 or 0.017–0.635 mg/cm², respectively. The surface morphologies of the cathodes were observed by scanning electron microscopy (S-5200, Hitachi High-Tech, Co., Ltd., Tokyo, Japan). The coated carbon sheets were cut to 10 × 10 mm² for the battery performance measurements.

The configurations of single- and tandem-electrolyte Zn–air battery cells are shown in Figs. 5(a) and 8(a), respectively. The cell components were self-made using a photocuring resin 3D printer (formlab 3, formlabs, Somerville, MA, USA).

The single-electrolyte cells were prepared by sandwiching the Zn foil anode, the catalyst-coated carbon sheet cathode, and a stainless-steel (SUS) mesh current collector with 3D-printed parts. Silicone O-rings and rubber sheets were used as the gaskets, which were sandwiched between resin parts to prevent liquid leakage and were screwed in place. The anode was held in place by a plate of light-cured resin, and the cathode was held in place by a component with a circular hole 0.99 cm in diameter. The center electrolyte chamber was filled with 6M KOH aq., and the I–V polarization curves and discharge polarization curves were measured with potentiostats (VersaSTAT-3 and VersaSTAT-4, AMETEK, Berwyn, PA, USA).

The tandem-electrolyte cells were built in the same way as the single-electrolyte cells, and two electrolyte chambers were used. The AEM was sandwiched between the two electrolyte chambers. The positive electrode consisted of a catalyst-coated carbon sheet and current collector SUS mesh, and the negative electrode was a Zn plate. The anode and cathode electrolyte (anolyte and catholyte) chambers were filled with 6 N KOH aq. and 3.5M HCl aq., respectively. The I–V polarization curves and discharge polarization curves were measured in the same way as for the single-electrolyte cells.

Alkaline type Zn–air battery cells were fabricated using AZUL (105 μg/cm²) and Pt/C (116 μg/cm²) electrocatalysts. EIS measurements were performed by using a potentiostat (VersaSTAT-4, AMETEK, Berwyn, PA, USA) equipped with impedance measurement software. The details are shown in the supplementary material, S11.

Detailed information regarding kinetic LSV data, a comparison of battery performances, chemical and microscopic analysis of AZUL catalysts, EIS of batteries, TOF values, etc., are shown in the supplementary material.

H.Y. thanks Mr. Koki Nakamura at AZUL Energy for synthesizing FeAzPc-4N and acquiring preliminary data and Professor Yasutaka Matsuo, Research Institute for Electronic Science (RIES), Hokkaido University, for EELS observation. This work was partly supported by NEDO (Grant No. 2020-0115004) and KAKENHI, JSPS, Japan (Grant Nos. 18H05482 and 19KK0357).

The authors have no conflicts to disclose.

Kosuke Ishibashi: Data curation (supporting); Formal analysis (supporting); Methodology (supporting); Validation (supporting). Koju Ito: Project administration (equal); Validation (equal). Hiroshi Yabu: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead).

The data that support the findings of this study are available within this article and its supplementary material.