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• Lithium-ion batteries, priced for their high energy density and cycle stability, have emerged as the leading battery technology for electric vehicles [1-4]. However, given its limited capacity, safety concerns, and the high cost of Li resources, alternative battery technologies must be developed [5-8]. Among these, metal-air batteries (MABs) have been garnered for their high energy density and environmental friendliness [9-14]. However, certain MABs, such as Li-air and zinc (Zn)-air batteries, are prone to dendrite formation during cycling, reducing battery performance, and leading to internal short circuits, which limits their wide applicability [15-17]. In comparison with MABs, research on semiconductor-air batteries such as silicon (Si)-air and germanium (Ge)-air batteries remain relatively underexplored. Particularly, Ge-air batteries offer high theoretical energy density and excellent safety features; moreover, the corrosion reaction of Ge in an alkaline environment is significantly slower compared with Si, resulting in higher actual anode utilization [18,19]. They are anticipated for use in portable electronic products such as hearing aids and electronic watches.

  The discharge mechanism of Ge-air batteries is depicted in Fig. 1a. At the initial stage of discharge, Ge reacts with hydroxyl ions (OH⁻) to form Ge hydroxide in the electrolyte, releasing four electrons through an external circuit to the air electrode (Eq. 1). Some of the Ge hydroxide dissolves gradually and dehydrates, forming a Ge dioxide passivation layer on the Ge surface (Eq. 3). The KOH electrolyte in Ge-air batteries can dissolve a portion of the GeO₂ passivation layer formed during discharge, thus achieving a dynamic equilibrium (Eq. 4). Simultaneously, oxygen from the air migrates due to concentration gradients toward the cathode catalyst surface through the gas diffusion layer, where it is adsorbed and converted into OH⁻ through a reduction reaction (Eq. 2) [20-23]. The discharge reaction formulas are as follows:

  $ \text { Ge anode: } \mathrm{Ge}+4 \mathrm{OH}^{-} \rightarrow \mathrm{Ge}(\mathrm{OH})_4+4 \mathrm{e}^{-} $

  (1)

  $ \text { Air cathode: } \mathrm{O}_2+4 \mathrm{e}^{-}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{OH}^{-} $

  (2)

  $ \text { Passivation reaction: } \mathrm{Ge}(\mathrm{OH})_4 \rightarrow \mathrm{GeO}_2+2 \mathrm{H}_2 \mathrm{O} $

  (3)

  $ \text { Passivation dissolution: } \mathrm{GeO}_2+2 \mathrm{OH}^{-} \rightarrow \mathrm{GeO}_3{ }^{2-}+\mathrm{H}_2 \mathrm{O} $

  (4)

  Figure 1

  

  Figure 1.  Diagram of discharge mechanisms of (a) flat Ge and (b) etched Ge as anodes in Ge-air batteries.

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  Furthermore, similar to the Si-air cells, although the KOH electrolyte may gradually dissolve the oxide on the anode surface over time if oxide formation outpaces the dissolution in the KOH solution, a passivation layer will inevitably accumulate on the anode surface [24]. This accumulation not only reduces anode utilization rate, limiting battery efficiency, but also reduces operational voltage and battery lifespan [20]. To address these challenges and extend the discharge life of Ge-air batteries, several studies on Ge anode have been conducted [22,23,25]. For example, Lee et al. successfully prepared porous Ge with an ordered hierarchical structure using electrochemical etching (ECE) technology, enhancing the Ge surface area and reducing passivation layer formation. Consequently, the discharge time of prepared Ge-air batteries reached 150 h [25].

  Chemical etching (CE), ECE, and metal-assisted chemical etching (MACE) are common methods employed to increase the specific surface area of Ge [25-27]. Figs. 2a-c depict the etching of the Ge surface using CE, ECE, and MACE, respectively. CE, also commonly known as wet etching, is an etching method that uses a chemical solution to dissolve the surface of a material and requires a certain amount of time to complete the etching process [28]. In the CE process, the metal is immersed in hydrofluoric acid (HF), nitric acid (HNO₃), organic solvent or other strong acid solution etching agent, which chemically reacts with the surface of the metal to produce products that are soluble in the solution, and which are subsequently rinsed away, thus achieving removal of the material and modulating the metal surface configuration. For example, the CE of Ge uses a solution of HF, which reacts with Ge to form water-soluble germanium fluoride (GeF₄) and water. The CE method is easy to operate and does not require external bias; however, the rate of nano-Ge pyramid structures (GePS) formation in this method is slow. In comparison with CE, the ECE process is more complex, necessitating an external bias voltage, promoting Ge surface oxidation [28]. Specifically, in ECE, Ge, and an inert electrode are connected to the positive and negative externals of a voltage, respectively; Ge is injected by the hole, undergoing oxidation, while electrons consumed at the inert electrode. Besides its accuracy limitations, ECE consumes substantial energy, potentially leading to energy waste. Furthermore, building upon CE, the MACE method introduces metal particles or ions as a local cathode to catalyze solution reaction; this facilitates anode reaction at metal particle-Ge surface contact points, facilitating Ge substrate oxidation [29]. Additionally, the introduced metal particles or metal ions not only catalyze the oxidant for reduction, thus producing abundant holes to oxidize the Ge but also induce semiconductor band bending and a built-in electric field upon contact with Ge. This effect can facilitate hole collection and injection, thereby facilitating the etching reaction to form uniform Ge nanostructures more rapidly [28]. Lee et al. employed MACE to create Ge nanostructures with an octagonal groove pattern, enhancing light absorption and improving the efficiency of Ge-based solar cells [30]. Subsequently, Zhang et al. obtained porous Ge layers (PGe) with different morphologies on single-crystal Ge through MACE, noting that the aperture diameter of the PGe film decreased while thickness increased with etching time [31]. In 2021, Dutta et al. applied MACE to a single-crystal p-Ge(100) substrate with H₂O₂ solution, observing the emergence of randomly distributed pyramidal textures on the Ge surface as the etching time increased, indicating the tunability of these textures under different etching times [32]. However, the application of MACE to enhance the specific surface area of Ge anodes for improving the passivation resistance of Ge-air batteries remains underexplored.

  Figure 2

  

  Figure 2.  Schematic diagram of (a) chemical etching, (b) electrochemical etching and (c) metal-assisted chemical etching.

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  In this study, both the CE and MACE methods were employed to prepare GePS on p 〈100〉 Ge wafers. The morphology of GePS was controlled by adjusting the etching time and solution composition (5 mol/L HF and 0.02 mol/L AgNO₃). This study systematically investigated the discharge behavior and passivation mechanism of the Ge-air battery anode in a 6 mol/L KOH electrolyte. Additionally, prolonging the etching time and increasing the volume of HF in the mixed solution during MACE not only accelerated GePS formation but also yielded GePS with larger dimensions and sharper edges. These optimized GePS structures increased the specific surface area of the Ge anode, enhancing contact with the electrolyte and enabling more effective dissolution of GeO₂ to mitigate passivation (Fig. 1b). Specifically, in the MACE method, when the volume of HF in the etching solution exceeded 30 mL and the etching time reached 10 h, thicker GePS structures resulted in a significantly expanded specific surface of the Ge anode, accelerating the dissolution rate of the GeO₂ passivation layer. Furthermore, the stable ratio of Ge and Ge oxide in the Ge anode prepared using MACE enhanced chemical stability and self-corrosion reactivity, resulting in significantly prolonged discharge time (9240 h) and higher peak power density (3.03 mW/cm²) compared with the flat Ge anodes.

  To prepare excellent GePS, thorough research on optimal etching conditions is essential. Initially, the variation in Ge surface morphology during the CE process (without AgNO₃ solution) was investigated through SEM (Figs. S2–S4 in Supporting information). Notably, numerous Ge rods appeared in Figs. S2a, S3a and S4a. As the etching time progressed, these structures evolved into bean-like structures, eventually forming GePS. In an etching solution containing 40 mL HF, when the etching time reached 20 h, the GePS structure became more pronounced (Fig. S4f). However, it was observed that the GePS structure was destroyed and holes appeared (Fig. S3f), indicating that the surface morphology of Ge correlated with the etching time and etching solution. The size of GePS in Fig. S4f was larger compared with those in Figs. S2f and S3f. The differences in Ge morphology induced by varying HF volumes were attributable to the distinct redox rate of the Ge surface. Generally, with the extension of etching time and increased HF volume, GePS became more apparent, although their size remained relatively small, with less defined pyramid edges and slower etching rates of the Ge surface.

  Given the above preliminary experimental observation, to further improve the etching rate of the Ge surface and achieve excellent GePS, the MACE method was conducted by introducing AgNO₃ solution into HF the solution. Top-view SEM images (Figs. 3a–e) revealed that GePS morphologies of different sizes appeared with increasing etching time. Notably, after 10 h of etching, GePS exhibited significantly increased size and thickness, featuring clear shapes and pronounced vertices (Figs. 3f–o). The images also indicated that the surface coverage of GePS increased with etching time, owing to their increased size. Additionally, a large number of GePS was observed in Fig. 3k, which was not observed in Fig. 3a. Meanwhile, at constant etching times and AgNO₃ vol, GePS sizes increased with increasing HF volume (Figs. 3e, j and o). Furthermore, cross-sectional SEM images of the GePS anodes (Figs. 4a–c) indicated that GePS height increased from 3 µm to 8.19 µm with increasing HF volume under the same etching time (10 h) and AgNO₃ (30 mL) volume, exhibiting sharper edges. Upon comparing Figs. S2–S4 and Fig. 3, it was found that without AgNO₃ solution in the etching solution, pronounced GePS was observed only after etching for 20 h, whereas introducing AgNO₃ yielded excellent GePS in just 10 h. These results indicated that the morphology of GePS was adjusted by changing the amount of etching solution and the reaction time. Specifically, excellent GePS was obtained by extending the etching time, adding AgNO₃, and increasing the HF volume of the etching solution. The addition of AgNO₃, expedited Ge surface etching by carrying holes generated during cathode reactions via Ag particles, facilitating Ge surface oxidation. The combination of various HF volumes and 30 mL AgNO₃ mixed etching solution as well as the critical etching time of 10 h produced pyramid structures with different sizes (from 6.5 µm to 11 µm) (Figs. 4d–f). The formation of these GePS increased the total surface area of the GE surface. In the following experiments, only three anode samples, Etch Ge@20:30, Etch Ge@30:30, and Etch Ge@40:30 —etched for 10 h, were studied and analyzed.

  Figure 3

  

  Figure 3.  Top-view SEM images of Ge etched in a mixed solution of varying HF volumes and 30 mL AgNO₃ for different times. Top-view SEM images of Ge etched in a mixture of 20 mL HF and 30 mL AgNO₃ for (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h, and (e) 10 h. Top-view SEM images of Ge etched in a mixture of 30 mL HF and 30 mL AgNO₃ for (f) 2 h, (g) 4 h, (h) 6 h, (i) 8 h, and (j) 10 h. Top-view SEM images of Ge etched in a mixture of 40 mL HF and 30 mL AgNO₃ for (k) 2 h, (l) 4 h, (m) 6 h, (n) 8 h, and (o) 10 h.

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  Figure 4

  

  Figure 4.  Cross-sectional SEM images of Ge etched for 10 h in a mixture of (a, d) 20 mL HF and 30 mL AgNO₃, (b, e) 30 mL HF and 30 mL AgNO₃, (c) as well as (f) 40 mL HF and 30 mL AgNO₃.

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  To investigate potential changes in the overall crystallinity of etched single-crystal Ge wafers, XRD measurements were performed. It was found that all XRD signals exhibited peak splitting corresponding (400). Fig. 5a depicts the diffraction pattern, indicating that the crystal structure of GePS remained unchanged despite immersion of the Ge wafer in three different etching solutions for 10 h, correlating with the previously reported results [26]. As shown in Fig. 5b, the Raman of both flat Ge and prepared GePS via MACE exhibited a conspicuous peak at 296.6 cm⁻¹, which is assigned to the Ge-Ge bond [23,33].

  Figure 5

  

  Figure 5.  (a) XRD, (b) Raman and (c) XPS diagrams of flat Ge, Etch Ge@20:30, Etch Ge@30:30 and Etch Ge@40:30. XPS Ge3d spectrum of (d) flat Ge, (e) Etch Ge@20:30, (f) Etch Ge@30:30, and (g) Etch Ge@40:30.

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  Furthermore, the oxide composition on the surface of Ge anodes was critical to their electrochemical reaction rate and chemical stability with electrolytes [24]. Therefore, the oxidation state of the Ge surface post-MACE was detected through XPS. The four significant peaks at 30.0, 284.6, 531.2, and 1219.9 eV of the full XPS spectrum in Fig. 5c belong to the orbitals of Ge 3d, C 1s, O 1s, and Ge 2p, respectively, manifesting the presence of these elements. Among these, the presence of elemental O was attributable to sample exposure to air. All binding energies were calibrated concerning the peak position of the C–C bond (284.8 eV). In the XPS spectrum of the Ge 3d in Figs. 5d–g, two significant peaks at 32–33 eV and 30 eV corresponded to the O-Ge-O bond of Ge oxide and Ge-Ge bond of the Ge element, respectively [34]. The relative intensities of the two stable oxidation states changed with the varied etching conditions. From Fig. 5d, it was observed that there was a significant Ge oxide peak in this spectrum, indicating that the flat Ge wafer was almost covered by natural oxides, correlating with the previous literature reports [35]. Notably, when the HF volume increased to 40 mL during MACE, more Ge oxide was removed. Lee et al. demonstrated that while a large amount of Ge oxide on the Ge surface enhanced chemical stability, it reduced electrochemical reactivity during Ge-air battery operation [25]. In Fig. S5 (Supporting information), the open-circuit voltage value of the Etch Ge@40:30 sample (0.97 V) was the highest, whereas that of the flat Ge sample (0.63 V) was the lowest, indicating that a large amount of Ge oxide on the surface of the flat Ge resulted in a decreased electrochemical reactivity of the surface. As the reaction intermediate between Ge elemental and Ge⁴⁺ oxidation state, Ge²⁺ not only provided chemical stability to restrain self-discharge but also oxidized into electrochemically active Ge⁴⁺. Therefore, the content of GeO_x on the Ge surface was controlled to ensure stability. It was neither too high nor too low, indicating that the Ge anode exhibited excellent chemical stability and reaction rates.

  Electrochemical tests on Ge PS anodes were conducted to demonstrate the effect of etched Ge on the property of Ge-air batteries. The actual current-potential (J–V) and power density curves of the flat Ge, Etch Ge@20:30, Etch Ge@30:30, and Etch Ge@40:30 were obtained, respectively (Fig. 6a). The maximum power density of the flat Ge, Etch Ge@20:30, Etch Ge@30:30, and Etch Ge@40:30 were 1.67, 2.02, 2.57, and 3.03 mW/cm², respectively. The maximum peak power density of GePS (3.03 mW/cm²) prepared using the MACE method exceeded over four times higher compared with that obtained by Lee et al. (0.73 mW/cm²), and over 2.7 times higher than that by Zhao et al. (1.1 mW/cm²) [19,22]. This enhanced performance was attributable to the formation of a micro-nano pyramid structure on the surface of Ge using the MACE method. This increased the effective Ge surface area, enhancing contact with electrolyte, and reducing surface passivation rate. Additionally, interfacial charge transfer impedances of Ge-air batteries assembled with different anodes were assessed by electrochemical impedance spectroscopy (EIS) (Fig. 6b). The experimental Nyquist diagram fitted with the equivalent circuit in Fig. S6 (Supporting information), where R_s and R_ct represent the solution and charge transfer impedance, respectively. In Fig. 6b, it was observed that the R_ct of the three GePS anodes prepared using MACE were all lower compared with those of the flat Ge. Meanwhile, with increased HF volume in the reaction solution using the MACE method, R_ct gradually decreased. Thus, the R_ct of Etch Ge@40:30 decreased most significantly. The results indicated that the GePS prepared by MACE increased the specific surface area of the Ge anode, inhibiting the accumulation of the passivation layer, and facilitating charge transfer.

  Figure 6

  

  Figure 6.  (a) J-V and power density curves of Ge-air batteries. (b) EIS curves of Ge-air batteries assembled with four different Ge anodes after 1000 h of discharge. Constant current discharge curves of Ge-air batteries containing (c) Flat Ge, (d) Etch Ge@20:30, (e) Etch Ge@30:30, and (f) Etch Ge@40:30 at 100 µA current.

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  The discharge behaviors of various Ge-air batteries with 100 µA current are shown in Figs. 6c–f. The discharge curves of the four batteries exhibited a large potential drop and voltage fluctuation in the initial stage, primarily attributable to the intermittent removal of Ge(OH)₄ accumulated on the surface during discharge. Notably, the voltage fluctuation of the planar Ge-air battery was the largest during the whole discharge process. The Ge-air battery assembled with the Etch Ge@20:30 stopped discharging after 5800 h, surpassing that of the Flat Ge (4300 h) by more than 1.3 times. Furthermore, the discharge time of the Ge-air batteries assembled with Etch Ge@30:30 and Etch Ge@40:30 is 9189 h (~383 days) and 9240 h (385 days), respectively, which is more than twice of the discharge time of the flat Ge-air batteries. Upon comparing the discharge time of three etched Ge anodes, it was observed that the increased specific surface area of the Ge anode effectively prolonged the discharge time of Ge-air batteries. Generally, GePS increased the surface area of Ge, facilitating Ge contact with the KOH electrolyte, and reducing the passivation rate of the Ge anode surface. Thus, it enabled prolonged discharge in Ge-air cells. The discharge time of germanium air battery is summarized as shown in Table 1. It can be found that the Ge-air battery prepared in this work has the longest discharge time.

  Table 1

  

  Table 1.  Summary of discharge time of Ge-air batteries.

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  Anode	Electrolyte	Current (µA)	Discharge time (h)	Ref.
  EE-ELE-PGe	6 mol/L KOH	880	150	[25]
  Ge (P100)	Gelled KOH	39	> 700	[21]
  P+++Ge	Gelled KOH	39	> 72	[19]
  Ge/Gr	Gelled KOH	150	250	[22]
  Ge@MOS	Gelled KOH	100	360	[23]
  Etch Ge	6 mol/L KOH	100	9240	This work
  Note: The meanings of some abbreviated words in the table are as follows: EE-ELE-PGe: electrochemical and electroless etched porous Ge; P+++Ge: heavily doped Ge.

  To study the variation of Ge anodes during discharge, SEM was employed to observe the morphologies of different anodes prepared by MACE after discharging at 100 µA for 40 h (Fig. S7 in Supporting information). Upon comparing the morphology of the passivation layers on the surface of the flat Ge, Etch Ge@20:30, Etch Ge@30:30, and Etch Ge@40:30, it was observed that numerous disordered and large Ge oxide particles appeared on the flat Ge (Fig. S7a), while the passivation layer of the prepared GePS using MACE was uniform (Figs. S7b–d). Moreover, the particle size on Etch Ge@40:30 anode was the smallest, indicating that the larger HF volume utilized in MACE increased the specific surface area of Ge, significantly enhancing the anti-passivation ability of the Ge anode. Fig. S7e and Table S2 (Supporting information) show the EDS spectra and element contents of flat Ge, Etch Ge@20:30, Etch Ge@30:30, and Etch Ge@40:30 anodes after discharging for 40 h. The oxygen content on the surface of the prepared GePS using MACE is much lower compared with the flat Ge (26.12 wt%). The oxygen content on Etch Ge@40:30 (8.64 wt%) was the lowest, indicating that increasing the exposed area of the Ge anode using MACE, effectively reduces passivation rates by overcoming surface accumulation of the passivation layer.

  Given the above characterization, it was observed that MACE on the Ge anode enhanced the specific surface area and utilization rate of the anode. Furthermore, when the flat Ge served as the anode of the battery, the limited specific surface area led to the formation rate of Ge(OH)₄, exceeding the dissolution rate over the discharge time. This resulted in a large number of Ge oxide passivation layers, forming on the Ge surface through dehydration, ultimately ceasing discharge. Conversely, the MACE method not only adjusted the oxide content on the Ge surface but also increased the specific surface area of the Ge anode. This enhanced the exposure area of the Ge-air battery during discharge and improved the contact between the anode and electrolyte interface, facilitating the dissolution of more passivation layers.

  This study investigated the effects of two methods, CE and MACE, on the morphology and etching rate of Ge anodes. It analyzed in detail the morphology and discharge behavior of various GePS prepared using MACE as the anode in alkaline electrolyte for Ge-air batteries. The Ge anode surface morphology and surface metal oxide content were regulated by adjusting the etching time and solution composition. In comparison with CE, the introduction of Ag ions in MACE significantly accelerated the etching rate of the Ge surface, facilitating the rapid formation of GePS. In MACE, optimal results were achieved with an HF volume exceeding 20 mL and an etching time of 10 h, indicating enhanced discharge performance of Etch Ge@30:30 and Etch Ge@40:30 as anodes.

  Furthermore, compared with flat Ge anodes, GePS-assembled Ge-air batteries demonstrated more than 2-fold increase in discharge life to 9240 h (385 days) at a 100 µA current and a 4.5-fold increase in peak power density to 3.03 mW/cm². The prolonged discharge time was attributable to the clear edge profile of GePS prepared using MACE, which increased the exposed area of the Ge anode, facilitating excellent contact with the electrolyte during the discharge process. Thus, it resulted in more GeO_x passivation layer formation in the electrolyte. This study elucidated the promising application potential of the MACE method for modifying Ge anodes in Ge-air batteries, offering experimental insights for enhancing Ge anode utilization in future applications.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Ya Han: Writing – original draft, Methodology, Investigation. Yingjian Yu: Writing – review & editing, Supervision, Project administration, Funding acquisition.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. 61904073), Spring City Plan-Special Program for Young Talents (No. K202005007), Yunnan Talents Support Plan for Yong Talents (No. XDYC-QNRC-2022-0482), Yunnan Local Colleges Applied Basic Research Projects (No. 202101BA070001-138), and Frontier Research Team of Kunming University 2023.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110144.

  
  
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• 

  Figure 1  Diagram of discharge mechanisms of (a) flat Ge and (b) etched Ge as anodes in Ge-air batteries.

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  Figure 2  Schematic diagram of (a) chemical etching, (b) electrochemical etching and (c) metal-assisted chemical etching.

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  Figure 3  Top-view SEM images of Ge etched in a mixed solution of varying HF volumes and 30 mL AgNO₃ for different times. Top-view SEM images of Ge etched in a mixture of 20 mL HF and 30 mL AgNO₃ for (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h, and (e) 10 h. Top-view SEM images of Ge etched in a mixture of 30 mL HF and 30 mL AgNO₃ for (f) 2 h, (g) 4 h, (h) 6 h, (i) 8 h, and (j) 10 h. Top-view SEM images of Ge etched in a mixture of 40 mL HF and 30 mL AgNO₃ for (k) 2 h, (l) 4 h, (m) 6 h, (n) 8 h, and (o) 10 h.

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  Figure 4  Cross-sectional SEM images of Ge etched for 10 h in a mixture of (a, d) 20 mL HF and 30 mL AgNO₃, (b, e) 30 mL HF and 30 mL AgNO₃, (c) as well as (f) 40 mL HF and 30 mL AgNO₃.

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  Figure 5  (a) XRD, (b) Raman and (c) XPS diagrams of flat Ge, Etch Ge@20:30, Etch Ge@30:30 and Etch Ge@40:30. XPS Ge3d spectrum of (d) flat Ge, (e) Etch Ge@20:30, (f) Etch Ge@30:30, and (g) Etch Ge@40:30.

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  Figure 6  (a) J-V and power density curves of Ge-air batteries. (b) EIS curves of Ge-air batteries assembled with four different Ge anodes after 1000 h of discharge. Constant current discharge curves of Ge-air batteries containing (c) Flat Ge, (d) Etch Ge@20:30, (e) Etch Ge@30:30, and (f) Etch Ge@40:30 at 100 µA current.

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  Table 1.  Summary of discharge time of Ge-air batteries.

  Anode	Electrolyte	Current (µA)	Discharge time (h)	Ref.
  EE-ELE-PGe	6 mol/L KOH	880	150	[25]
  Ge (P100)	Gelled KOH	39	> 700	[21]
  P+++Ge	Gelled KOH	39	> 72	[19]
  Ge/Gr	Gelled KOH	150	250	[22]
  Ge@MOS	Gelled KOH	100	360	[23]
  Etch Ge	6 mol/L KOH	100	9240	This work
  Note: The meanings of some abbreviated words in the table are as follows: EE-ELE-PGe: electrochemical and electroless etched porous Ge; P+++Ge: heavily doped Ge.

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