Quasi-perpetual discharge behaviour in p-type Ge-air battery

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Introduction
2][3][4] This arises from the unique feature of metal-air batteries, when compared with other batteries, such that one of the electroactive materials (i.e.oxygen) does not need to be stored within the system.With abundant supply of oxygen in the air, metal-air batteries have extremely higher theoretical energy densities than its traditional aqueous and lithium-ion counterparts. 5Furthermore, metal-air batteries, particularly those with aqueous chemistries, are generally classified as safe and inexpensive, fire-prone alternatives to hazardous lithium batteries.Different metals (e.g.zinc (Zn), 6 aluminum (Al), 7 and lithium (Li) 8 ) and metal-based compounds (e.g.borides, 9 silicides, 10 and phosphides 11 ) have been studied as anode materials for metal-air batteries.Although the chemistries of these metal-air batteries offer high-energy capacities, the anode utilization efficiency in most of these cells (i.e.3] Immediate passivation of the metal anodes (e.g.Al, Mg) during discharge severely cuts the available energy and makes the rest of the anode unusable.It is therefore of great technological importance to improve the discharge behaviour of these anodes, or alternatively, to find new chemistries with better discharge kinetics than the previously reported anodes.
Of late, semiconductor-air batteries, a term we first define here as metal-air batteries that use a semiconductor anode such as silicon (Si, 3822 mAh g Si -1 ) [14][15][16][17] and germanium (Ge, 1477 mAh g Ge -1 ) 18 , have been introduced as high capacity anode materials.Of the semiconductor-air batteries based on pure Si as anode, two types of electrolytes have been used: an aqueous alkaline electrolyte and a room temperature ionic liquid (RTIL) electrolyte.5][16] Although various p-type and n-type doped Si wafers, with different doping levels and crystal orientations, have been initially screened, only polarization voltammograms were used to evaluate the anodes, leading to the conclusion that n-type Si is the best candidate.On the other hand, the Si-air battery with an alkaline electrolyte utilized only porous n-type Si wafers of various doping levels. 17They reported a discharge capacity, however, of only ~30 % of the theoretical anode capacity, and no full discharge curves were provided.Hence, the extent of the effect of the doping type and level on the discharge kinetics in semiconductor anodes has neither been discovered nor understood yet.
In our previous study, we first reported the successful operation of a novel semiconductor-air battery, running on a ptype Ge (0.019-0.024Ω cm) anode in alkaline electrolyte.The formation of ordered hierarchical porous structures, via controlled etching, improved the energy capacity of the Ge-air cell, showing a stable long-term discharge profile. 18Our discussion, however, was limited only to the formation of the nanoporous structure and the surface oxide coverage after etching and its effect on the discharge kinetics of Ge-air cell.Nevertheless, earlier studies and our results suggest that doped semiconductors (Ge and Si) are indeed viable high energy density anodes for metal-air energy conversion cells, in addition to being suitable candidates for next generation lithium ion batteries. 19n this paper, we explore the effect of the doping type and level on the discharge kinetics of Ge anodes in a semiconductor-air cell.By studying the discharge of the semiconductor-air cells with various anodes, we demonstrate the unexpected excellent performance of p-type Ge in comparison with other Ge anodes.In addition, our results indicate that the semiconductor-air battery, based on flat Ge anode, offers as high as 1,302.5 mAh g Ge -1 of practical energy density, which is two and three times higher than commercial Zn-air and Al-air batteries, respectively.

Results and discussion
Following the design of a semiconductor-air cell we have previously used, we modified the cell to decrease the gap between the anode and cathode.Semiconductor anodes were tested using the simple device architecture, consisting of Ge wafer as the anode, a gelled KOH as the electrolyte, and 46.7 wt.% platinum on carbon (Pt/C) as the cathode (Fig. S1).Pt/C was used as the oxygen reduction reaction (ORR) catalyst in order to make sure that the performance difference will be coming solely from the different anodes only.Meanwhile, the gelled KOH electrolyte was prepared by adding carboxymethyl cellulose (CMC) into the KOH solution.Gelling agents have been widely used to enhanced the electrochemical performance of zinc-air cells and prevent leakage of the KOH electrolyte. 20ighly concentrated KOH solutions were used in order to limit the ohmic polarization and to make sure that the discharge products, mainly Ge(OH) 2 , are easily dissolved in the electrolyte.
Fig. 1a shows the potentiodynamic curves comparing n-type Ge and p-type Ge.As seen in the curves, the expected cell voltage of the two anodes at low current drains does not differ much, contrary to what was observed in Si-air battery with RTIL electrolyte, where n-type Si exhibited relatively higher operating potential than p-type Si at the low current regime. 15n fact, the two curves are close with one another throughout the entire range of current densities, implying that the discharge kinetics could not be too different.The anodic dissolution current densities rapidly increase (-1.0 V to -0.7 V) at first and then flattened at the more positive potential.Both types of doped Ge, however, have relatively large oxidation overpotentials at the high current density region, which can lead to lower operating potential. .Both n-type Ge and p-type Ge can be considered as heavily doped Ge wafers, with the following average resistivities: n-type Ge (0.007 Ω cm), p-type Ge (0.0145 Ω cm).

Physical Chemistry Chemical Physics Accepted Manuscript
As a preliminary test of Ge-air cell performance, Ge-air cells were constructed using different gelled KOH concentrations (1.0 M, 3.0 M, and 6.0 M) and discharged at different current densities (1 µA cm -2 -500 µA cm -2 ).The typical discharge plots of the n-type Ge-air cell at different KOH concentrations, measured under constant current mode of operation, are shown in Fig. 1b.As the KOH concentration increases, n-type Ge operates at higher potentials.Increasing the KOH concentration, however, has a drawback since it can lead to elevated corrosion rates in the anode, decreasing the energy available for the cell's operation.Fluctuation in the operating potential, which increases as the electrolyte concentration is lowered, has been observed during discharge, indicating a possible instability in the anode.This behaviour has been likewise observed in alkaline Si-air batteries that are running on n-type Si anodes. 17n contrast, p-type Ge-air cell operated at the highest potential when using 3.0 M gelled KOH electrolyte (Fig. 1c).It is good to note also that the voltage fluctuation observed in ntype Ge is absent in p-type Ge, as shown in the very flat discharge profiles.In order to confirm the flat discharge profiles of p-type Ge, we tested the discharge behaviour of ptype Ge at various current drains.Inset of Fig. 1c demonstrates the flat discharge profiles of p-type Ge-air cells, operating at higher voltage when decreasing the amount of current drawn.In fuel cell operation, which is somehow similar to how semiconductor-air cells work, the drop in potential with increasing current can be attributed to the increase in the internal resistance of the cell, especially in between the electrode/electrolyte interfaces. 21o fully explain the discharge behaviour of the different Ge anodes, it is important to revisit the three main reactions during the Ge-air cell discharge in alkaline electrolyte, as written below.During discharge, Ge is oxidized to produce four electrons, which are accepted by the cathode to reduce oxygen molecule into hydroxide ions.Slow dissolution of the Ge(OH) 4 into the electrolyte, in combination with fast anode oxidation kinetics, results in the passivation of the anode. 18ode discharge: ‫݁ܩ‬ ‫ܪܱ4‬ ି → ‫݁ܩ‬ሺܱ‫ܪ‬ሻ ସ 4݁ ି (1) Oxygen reduction reaction: The observed fluctuations in the discharge of n-type Ge can be assumed to arise from the interplay between the dissolution of Ge(OH) 4 into the electrolyte and the dehydration reaction that passivates the Ge surface.The increase in the amplitude of the fluctuations with decreasing KOH concentration provides ample evidence to this hypothesis.At low KOH concentrations, formation rate of Ge(OH) 2 exceeds its dissolution rate, decreasing the cell voltage due to surface passivation.Surface passivation, however, has not yet reached a critical level and the oxide is intermittently removed with the continuous discharge, causing an increase in the operating voltage, and the cycle goes on.In contrast, p-type Ge has a very flat discharge profile, which denotes that the surface passivation proceeds at a slower rate, possibly resulting from p-type Ge's higher passivation overpotential than n-type Ge.These results indicate that the electrochemical activity of p-type Ge is significantly more reliable than that of n-type Ge, even if they have comparable dopant concentrations.In order to account for the self-discharge of the Ge-air cells at open circuit potential (OCP), we measured the corrosion rates of both heavily doped n-type Ge and p-type Ge in the 6.0 M gelled KOH electrolyte by linear polarization experiments in the range of +/-20 mV relative to the OCP.As seen in Fig. 2a, p-type Ge corrodes faster than n-type Ge, contrary to the trend found in Si anodes in RTIL electrolyte, where n-type Si generally shows higher corrosion currents than p-type Si.The low overpotential for passivation in n-type Ge might explain this extremely low corrosion rates.Once passivated, electrolyte ions can no longer attack the metallic Ge in the surface.However, although p-type Ge has relatively faster corrosion kinetics than n-type in the cell environment, the measured corrosion rates are not that high and are comparable to that of This journal is © The Royal Society of Chemistry 2012 Si in RTIL electrolyte, 15 and even in highly resistive metals in extreme environments. 22ig. 2b shows the Nyquist plots of n-type Ge and p-type Ge in the semiconductor-air cell with the gelled KOH electrolyte at OCP before any discharge experiment.As shown in the impedance spectra, p-type Ge and n-type Ge behaves similarly, but n-type Ge has lower value of the real axis intercept.At the high frequency region, the intercept with the real axis corresponds to the equivalent series resistance of the cell.As shown in the inset of Fig. 2b, however, p-type Ge (6.3 Ω) has a relatively lower electrical resistance than n-type Ge (49.8 Ω), when the impedance was measured after the cell was discharged at 50 µA cm -2 for 8 hr.This might have arisen from the higher rate of passivation in the n-type Ge anode's surface, increasing the cell's resistance.
The operating voltage of metal-air cells can be seen as a measure of the cell's efficiency, since maintaining high battery voltage -even at higher current loads -is critical to the successful implementation of the battery technology.From the results above, the difference in discharge kinetics in Fig. 1 is not much evident.Hence, we measured the potential response of various Ge anodes with an incremental increase in current density drawn from the cell, in order to generate the currentpotential (j-V) and the power curves of the Ge-air cell with various anodes.The current of the Ge-air cell was held fixed in time and the pseudo steady-state value of the cell voltage was recorded after a long equilibration time (~10 min).
As seen in Fig. 3, various Ge wafers revealed different behaviour as semiconductor-air anodes.For instance, n-type Ge has a slightly better discharge kinetics over any p-type Ge at the low current regime (<20 µA cm -2 ).There is, however, a large drop in operating potential beyond this region, indicating a strong passivation behaviour in n-type Ge, whereas, p-type Ge displays a gradual decrease in potential with increasing current.This is contrary to the small variation in electrochemical behaviour of n-type Ge and p-type Ge, as observed in the potentiodynamic curves in Fig. 1a.Unexpectedly, n-type Ge behaves similarly as the undoped Ge, raising an important question as to why, despite having good electrical conductivity (almost five orders of magnitude) and electronegativity (high dopant concentrations); n-type Ge's discharge kinetics is much poorer than expected.
On the contrary, as seen in the representative j-V plots, ptype Ge anodes have increasingly improving discharge kinetics as the doping degree increases.This is manifested by the higher operating voltage and the more gradual decrease of potential at high current densities.Unlike in fuel cell operation, where the voltage quickly drops due to mass diffusion limitation and ohmic resistance at higher currents, we assume that the decrease in operating voltage in semiconductor anodes can be largely explained by the faster passivation of the anode's surface.As shown in the power curves in Fig. 3b, the heavily doped p-type Ge (p+++) exhibits the best discharge kinetics, especially at higher currents.At the low current regime (inset of Fig. 3b), however, medium doped p-type Ge (p++) demonstrates the highest power density.
Fig. 3 (a) Current-potential (j-V) curves and (b) power curves for Ge-air cells operated using different anode types: heavily doped n-type Ge, undoped Ge, and p-type Ge with various doping levels: heavily doped (p+++), medium doped (p++), and lightly doped (p+).P-doped Ge anodes were doped with gallium (Ga) while the heavily doped n-type Ge was doped with antimony (Sb).The inset figure in (b) displays the power behaviour at the low current region.All Ge-air cells were operated using a 6.0 M gelled KOH electrolyte at ambient conditions.Furthermore, the cells were aged for 3 hr to ensure that the electrodes are already in full contact with the gelled electrolyte.
Table 1 summarizes the doping information and electrochemical discharge kinetics of undoped and doped semiconductor anodes.The resistivity data were gathered from the manufacturer of semiconductor wafers, while the carrier concentration can be derived from the resistivity versus impurity concentration plot. 23For instance, for the same dopant concentration, n-type Ge has a relatively lower resistivity than p-type Ge.In order to facilitate an easier comparison of the discharge kinetics of semiconductor anodes, heavily doped ntype Si and p-type Si wafers were also tested to emphasize the inherently favourable kinetics in flat Ge anodes, especially at higher current densities.For example, both n-type Si and p-type Si anodes are already passivated at a discharge current of 50 µA cm -2 .As shown in the j-V curves in Fig. S2, Si-air cells initially ran at higher operating voltages than Ge-air cells but the voltage quickly reversed to a negative potential, signifying the passivation of the anode.Despite the high doping levels and low resistivities for both n-type Si and p-type Si anodes, the maximum power density is limited by the faster passivation in Si anodes.Although this has a serious implication on the future use of Si anodes in alkaline semiconductor-air batteries, this can be overcome -albeit at a limited degree -by increasing the exposed area of the Si anodes through etching. 17In contrast, as the carrier concentration in p-type Ge increases, more power is available from the cell: p+, 38.9 µA cm -2 ; p++, 208.1 µA cm -2 ; and p+++, 251.7 µA cm -2 .The results above prove the excellent discharge kinetics of heavily doped p-type Ge anodes over ntype Ge and doped Si anodes, especially at high current densities.4] We believe that the high initial concentration of holes in p-type Ge might be the reason why it exhibits excellent discharge kinetics as an anode over n-type Ge, regardless if ntype Ge has higher Fermi level.Evidently, the discharge performance improved considerably when the carrier concentration in p-type Ge increased.In order to demonstrate the response of the p-type Ge-air cell (p+++) with abrupt change in current density, a step test was performed from 1 µA cm -2 -1.0 mA cm -2 .As shown in Fig. 4a, the cell exhibited a very fast response after current cutoff and after a new current density was drawn.This is in agreement with our previous results. 18In addition, there was a steady voltage drop as the current was increased up to 0.1 mA cm -2 , consistent with the results in the j-V curve.The OCP notably increases with the discharge time, arising from the increased exposed area and enhanced anode-electrolyte interface as the discharge process proceeds.This just shows how important the surface area of the anode is to the overall discharge process, allowing the cell to operate at higher voltage and preventing early passivation of the cell.
Fig. 4b demonstrates the big difference in the long-term discharge profiles of n-type Ge and p-type Ge (p+++) anodes.The heavily doped p-type Ge-air cell exhibited a flat voltage profile and operated for almost 650 hr before the potential gradually dropped.In contrast, n-type Ge operated only for a short time and showed voltage fluctuations, leading to extremely low anode utilization.We surmise that upon n-type Ge oxidation, the concentration of Ge(OH) 2 exceeds its solubility quickly, resulting in dehydration and deposition as GeO 2 and passivating Ge surface.The poor performance of ntype Ge, however, has been strongly established already from the j-V curves above.In terms of the practical gravimetric capacity, p-type Ge-air cell exhibited a full discharge energy capacity of 1,302.5 mAh g Ge -1 for the 700 hr operation, which is 17 times higher than ntype Ge of 77.4 mAh g Ge -1 (Fig. 4b).This extremely high anode utilization is close to 88 % of the theoretical energy density of 1,477.45mAh g Ge -1 based on the anode alone.In fact, the Geair cell's capacity is two-folds and three-folds higher than commercial Zn-air cells (650 mAh g Zn -1 , Energizer), 26 and Alair cells (320 mAh g Al -1 , Altek Fuel Group Inc.), 27 respectively.Considering that we are only using a flat p-type Ge anode, this practical energy capacity is even relatively higher than in a Znair cell using a porous, monolithic, three-dimensional (3D) zinc sponge anodes at 728 mAh g Zn -1 , 28 Mg-air cell using a nanomesoscale structured anode (188 mAh g Mg -1 ), 29 and an advanced Zn-air cell with high performance hybrid electrocatalyts (570 mAh g Zn -1 ). 30 Furthermore, p-type Ge-air cell's energy capacity and anode utilization efficiency is notably higher than porous Si-air cells with alkaline electrolyte (1206 mAh g Si -1 ), which has a capacity corresponding to only This journal is © The Royal Society of Chemistry 2012 31.6 % of Si-air cell's theoretical energy capacity. 17As summarized in Fig. S3, the long discharge behaviour in p-type Ge-air battery displays key advantages in terms of high practical specific capacity and anode utilization efficiency, which can offset its disadvantages (e.g.cost of Ge, lower potential relative to other metal-air chemistries).In order to account for the cell's self-discharge in the long-term operation, we estimated the Ge mass loss due to corrosion from the average corrosion current in p-type Ge (5.02 µA cm -2 ) and compared it with the actual total mass loss during discharge (see Fig. S4).Based on our computations, corrosion mass loss accounts to only 6.4 % of the total mass loss during discharge, or less than 1 % of the initial mass of Ge anode.Even with an increase in the actual corrosion rate as the surface of Ge is roughened during discharge, the self-discharge mass loss is still well within the 12 % unaccounted Ge, based on the anode utilization efficiency of 88 %.
Post discharge characterization on the p-type Ge cell electrodes in Fig. 4b were performed to ascertain if structural and chemical changes occurred after the discharge.As expected, the Ge anode underwent morphological changes, from a flat morphology to a grainy texture (Fig. S5a-d), similar to the observed morphology in our previous study. 18eanwhile, the Pt/C electrode remained almost the same, especially when examined at higher magnifications in SEM, as seen in Fig. S5e-f.Further proof of the unchanged nature of the air cathode is shown by the similar X-ray diffractograms in Fig. S5g, prior to and after discharge.In addition, XPS studies confirmed that some Ge atoms, although it is not yet clear if it's in hydroxide or oxide form, are deposited on the Pt/C electrode (Fig. 5a).In Si-air batteries, formation of reaction products in the cathode during discharge and the conversion of MnO 2 ORR catalyst to inactive MnF 2 are considered as major factors in the cell discharge termination. 31- 32However, there seemed to be no adverse effect on the cathode, as there was no shift in the binding energy of Pt4f in Fig. 5b.If Ge somehow affects the activity of the cathode, especially if non-Pt ORR catalysts are used, this problem could be alleviated by using an anion exchange membrane (AEM) in between the two electrodes.On the other hand, the metallic Ge signal is still visible and the ratio of the metallic Ge and GeO 2 in the Ge2p spectra in Fig. 5c, before and after discharge, even increased slightly.This can mean that the p-type Ge anode might not have been totally passivated and can still continue to discharge.This is consistent with our hypothesis that since the cell is not hermetically sealed, since drying out of the electrolyte might have caused the end of discharge.Despite the failure of the cell after 700 hr, however, a full discharge capacity of 1,302.5 mAh g Ge -1 for an anode utilization of 88 % represents the highest among semiconductor-air cells already, and even higher than conventional metal-air batteries such as Zn-air and Al-air.

Conclusions
Our results indicate that the doping type and the degree of doping in semiconductor anodes are crucial in the discharge kinetics in a metal-air battery.We first show that contrary to the conventional wisdom that n-type Ge should show favorable discharge kinetics, p-type Ge behaves extremely better.Heavily doped p-type Ge operates at higher voltage at the high current regime and discharges for a significantly longer time, leading to better utilization and higher energy capacity.It should be emphasized that even if flat semiconductor wafers were used in this study, a very high energy density of 1,302.5 mAh g Ge -1 for an anode utilization of 88 % can be derived from the p-type Geair cells.We believe that it is possible to significantly decrease the anodic polarization by downsizing the flat Ge anode to nanoscale Ge.4][35] .In addition, increasing the doping degree of semiconductors and enhancing the surface area (e.g.3D structured semiconductor anodes) might potentially lead to better utilization efficiency of the cells and high operating voltage.While a deeper understanding, especially on the molecular level, of the effect of doping on the discharge kinetics is still needed, the observations here might be extended to Si-air batteries and other semiconductor-air cells.Subsequent studies using careful discharge measurements in the dark (to avoid photocurrents), in addition to density functional theory (DFT) calculations, can help shed light on the nature of the j-V behaviour of semiconductor anodes in metalair cells.In the future, semiconductor-air batteries using SiGe alloys could combine the two extremes of advantages and disadvantages, in terms of cost, discharge kinetics, and operating voltage.SiGe-air cells could provide inherent advantages that are not found individually in Si and Ge.Overall, our results here provide a better understanding on the discharge kinetics of semiconductor anodes in chemical energy conversion devices and might lead to more studies on the effect of doping on semiconductors as they are utilized in other applications (e.g.semiconductors in thermoelectrics).

Assembly of semiconductor-air cells
Semiconductor wafer anodes were cut from commercial wafers of different doping types and levels (MTI Corp.).Ge and Si wafers were cleaved to produce 1 cm x 1 cm anodes with 0.785 cm 2 of the flat polished side exposed to the electrolyte via an Oring.All samples were cleaned first using 48 wt.% hydrofluoric (HF) solutions (Sigma-Aldrich) to remove the native oxide layer, then afterwards rinsed with deionized (D.I.) water and dried.It should be noted that only flat semiconductor wafers were used in this work.Meanwhile, the air-breathing cathode was constructed by spraying a mixture of 46.7 % Pt/C (Tanaka, 1.0 mg cm -2 ) and Nafion solution (Sigma Aldrich, 10 wt.%) on a gas diffusion layer (SGL, 25AA).The gelled KOH electrolyte was prepared by adding sodium carboxymethyl cellulose (CMC, Sigma Aldrich) into the KOH solution (5 wt.%) with vigorous stirring and letting the mixture age for three days to ensure complete mixing.Initially, different gelled KOH electrolytes were prepared from 1 wt.% to 15 wt.% CMC (see Fig. S5), but the 5 wt.% CMC in KOH showed the best consistency for use as a gelled electrolyte.In order evaluate the electrochemical discharge kinetics of different semiconductor anodes, a single cell -consisting of the Ge anode, gelled KOH electrolyte, Pt/C on GDL cathode, and thin copper (Cu) meshes as current collectors -was assembled.The gelled alkaline electrolyte was injected into the cell through a small hole at the top of the cell.All the cells were assembled and operated in ambient conditions.

Electrochemical Analysis
Potentiodynamic studies were conducted in an electrochemical cell consisting of Pt reference and counter electrodes, n-type Ge or p-type Ge as working electrode and 6.0 M gelled KOH electrolyte.All the discharge curves were measured in a multichannel high performance battery tester (Maccor 4300 K, Korea Thermo Tech.Co., Ltd.), using various current densities.Electrochemical impedance spectroscopy was performed with a two-electrode configuration (Zahner Zennium, Zahner Elektrik GmbH & Co.).A 5 mV RMS excitation signal was used with a frequency range of 10 mHz to 1MHz.

Post-discharge electrode characterization
Following full discharge of the Ge-air cell, various characterization techniques were used to study the electrodes.For instance, the surface morphology of the pristine and postdischarge electrodes was investigated using a scanning electron microscope (SEM, JEOL JSM 5200), in plain and cross-section views.In addition, X-ray diffraction (XRD, Rigaku D/MAXIIA) study was performed to confirm possible structural changes after the long-term battery discharge.The surface chemical properties of the electrodes were examined by X-ray photoelectron spectroscopy (XPS) with an Mg Kα X-ray source (1253.6 eV) at a base pressure of 2 x 10-9 torr.After the full discharge, the used electrodes were carefully washed with D. I. water and vacuum-dried fully before the XPS analysis.

Fig. 1 1 .
Fig. 1 (a) Potentiodynamic curves of n-type Ge and p-type Ge in 6.0 M KOH electrolyte at a scan rate of 5 mV s -1 .Galvanostatic discharge curves of Ge-air battery using n-type Ge and p-type Ge in a gelled electrolyte at various KOH concentrations and discharge current densities: (b) n-type Ge-air cell operating at a discharge current density of 10 µA cm -2 at various KOH concentrations and (c) p-type Ge-air cell operating at a discharge current density of 10 µA cm -2 at various KOH concentrations.The inset shows the p-type Ge-air cell in 6.0 M gelled KOH electrolyte at various discharge current densities, with the decreasing operating voltages corresponding to the following current densities: 1, 50, 100, 200, 300, and 500 µA cm -2

Fig. 2
Fig. 2 (a) Corrosion rates of the n-type Ge and p-type Ge (both heavily doped) in the semiconductor-air cell as a function of the exposure time in the 6.0 M gelled KOH electrolyte at open circuit potential.(b) Nyquist plots for n-type Ge and ptype Ge in the semiconductor-air cell with a 6.0 M gelled KOH electrolyte and Pt/C cathode.The inset shows the behaviour of the Ge anodes under the high frequency region after discharging for 8 hr at a current density of 50 µA cm -2 .

Fig. 4
Fig. 4 (a) Dynamic step discharge response of the p-type Ge-air cell (p+++), when operated at increasing current drains.(b) Long-term discharge profiles of n-type Ge and p-type Ge anodes (both heavily doped), showing the huge difference in the electrochemical performance in a Ge-air cell using a 6.0 M gelled KOH electrolyte at a discharge current of 50 µA cm -2 .

Fig. 5 X
Fig. 5 X-ray photoelectron spectra of the cathode and anode after full discharge for 700 hr.(a) XPS survey scan of the Pt/C cathode after discharge, while the inset shows the X-ray photoelectron spectrum of Ge2p3/2, fitted with the Ge 4+ spectrum.(b) Pt4f XP spectra of Pt/C cathode (i) before and (ii) after full

Table 1
Comparison of the electrochemical discharge performances of semiconductor-air anodes with various doping types and levels