Synthesis of an amorphous Geobacter-manganese oxide biohybrid as an efficient water oxidation catalyst

Abstract

The development of a low cost and efficient oxygen evolution reaction (OER) catalyst has paramount importance to meet the future sustainable energy demand. Nature's photosynthetic machinery deploy manganese-based complex in the photosystem II to oxidize water. Inspired by nature, herein, we synthesized a high performing manganese-based OER catalyst using an electrochemically active and iron-rich bacterium, Geobacter sulfurreducens. The as-synthesized biohybrid catalyst (amorphous Geobacter-Mn₂O₃) produced a current density of 10 mA cm⁻² at an overpotential of 290 ± 9 mV versus a reversible hydrogen electrode with a low Tafel slope of 59 mV dec⁻¹. The catalyst exhibited remarkable stability, evidenced through a long-term chronopotentiometry experiment. Multiple evidence showed that G. sulfurreducens contributed OER active elements (iron and phosphorus) to the biohybrid catalyst, and the as-synthesized Geobacter-Mn₂O₃ is amorphous. The amorphous structure of the biohybrid catalyst provided a large electrochemically active surface area and excess catalytic sites for the OER catalysis. In addition, Mn³⁺ present in the biohybrid catalyst is believed to be the precursor for oxygen evolution. The OER activity of the biohybrid catalyst outperformed commercial-Mn₂O₃, commercial-IrO₂ and most of the benchmark precious OER catalysts, thus supporting its suitability for large-scale applications. The proposed green approach to synthesize a biohybrid catalyst paves a new avenue to develop robust and cost-effective electrocatalysts for energy-related applications.

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Introduction

There is an increasing global demand for energy, which is mainly supported by burning fossil fuels.^1,2 The current rate of consumption of fossil fuels, which are limited resources, may lead to their ultimate depletion and further increase in atmospheric carbon dioxide (CO₂) concentration, resulting in serious environmental pollution and climate change issues.³ Hence, exploring alternative energy sources, in particular renewable energy sources, is highly warranted to protect our environment. Water is a unique source of energy since it serves as a cheap and abundant source of electrons.⁴ Water oxidation or oxygen evolution reaction (OER) is a key process in solar fuel production, rechargeable metal–air batteries, and microbial electrosynthesis to produce either hydrogen or high-value chemicals/fuels from CO₂ reduction.^2–6 However, OER is kinetically sluggish and needs high energy input, which remain a bottleneck in water splitting.⁵ Catalysts play a significant role in OER by lowering the overpotential required to oxidize water. To facilitate OER, great efforts have been made to develop efficient and affordable catalysts. Metal oxides of iridium and ruthenium are the most common electrocatalysts that are employed for OER.⁷ However, their high cost, poor stability, and scarcity hinder the large-scale applications of these materials as OER catalysts.⁷ Thus, there is an urgent need to develop earth-abundant, cost-effective, and efficient electrocatalysts for the facilitation of OER.

Manganese oxide (MnO_x)-based OER catalysts provide an alternative to the high-cost catalysts because of their earth abundance, non-toxicity, low cost and versatile redox properties.⁴ Manganese (Mn) is a natural choice for OER as a Mn-based complex is the OER unit in the process of photosynthesis (PS II).^4,8 Thus, inspired by nature, numerous efforts have been made to develop Mn-based OER catalysts.⁸ Mn occupies a unique position in catalysis because of its ability to exhibit different oxidation states and each state demonstrates distinct catalytic properties.^4,9 The Production of MnO_x heavily depends on chemical and physical methods used.^10,11 Chemical methods require rigorous reaction conditions such as high temperature and copious amount of toxic chemicals, while physical methods consume high energy to produce MnO_x.

Here, we propose a microbial method using the bacterium Geobacter sulfurreducens to synthesize an amorphous Geobacter-Mn₂O₃ biohybrid as an efficient OER catalyst. G. sulfurreducens is a naturally abundant dissimilatory metal reducing bacterium and has the highest electricity producing capacity in microbial electrochemical systems.^12–15 It shows a great respiratory versatility using a variety of soluble (e.g., fumarate) and insoluble (e.g., metal oxides) terminal electron acceptors.¹⁵G. sulfurreducens communicates with solid electron acceptors such as electrodes, by discharging the metabolically generated electrons through a unique respiratory pathway called extracellular electron transfer (EET).^12,13 EET in G. sulfurreducens involves outer-membrane c-type cytochromes (OM c-Cyts) and nanowires/pili (extension of c-Cyts) as electron conduits to transport the electrons from the cell interior to the external environment.^14,15 c-Cyts in G. sulfurreducens are rich in iron (Fe) due to the presence of heme groups, which are the key components in the EET. The hallmark of G. sulfurreducens is their ability to uptake Fe to maintain intracellular Fe concentration using a homodimeric protein, ferric uptake regulator (Fur).¹⁶ Isotopic experiments showed a three-fold higher Fe concentration in G. sulfurreducens as compared to Escherichia coli, suggesting the Fe richness in the cells.¹⁷ It is known that Fe-based electrocatalysts usually show the highest OER activity¹⁸ and a recent study found that Fe is the actual OER site in Fe/Ni layered double hydroxides.¹⁹ Moreover, G. sulfurreducens can act as a microbial factory to produce diverse nanomaterials and nanostructures. For example, G. sulfurreducens can produce noble metal nanoparticles (palladium, silver and gold), magnetite nanoparticles and reduced graphene oxide by employing the EET pathway.^5,20–23 Moreover, G. sulfurreducens plays a crucial role in the geo-biochemical cycling of Mn by reducing or oxidizing Mn compounds depending on their availability in nature and redox behavior.²⁴ The above-mentioned physiological advantages of G. sulfurreducens inspired us to choose G. sulfurreducens as an inoculum source for synthesizing an efficient Geobacater-Mn₂O₃ biohybrid electrocatalyst for the OER.

Experimental

Bacterial culture

G. sulfurreducens PCA (DSM 12127) was purchased from DSMZ-German Collection of Microorganisms and Cell Cultures. G. sulfurreducens was cultured in anaerobic serum bottles with 10 mM acetate as the electron donor and 50 mM fumarate as the electron acceptor in a defined medium (pH 7.0).²⁵ A gas purged (for 1 h with N₂ : CO₂—80 : 20%) anaerobic medium was transferred into the serum vials, followed by adjusting the solution pH under an N₂ environment in an anaerobic glove box (Labconco, USA). After media sterilization (121 °C, 20 min at 15 psi), the serum vials were inoculated with a G. sulfurreducens (3% v/v; OD_{600 nm}) culture and incubated in a shaking incubator (Innova 40, New Brunswick Scientific, USA) for three days (200 rpm, 30 °C). Growth was monitored by measuring optical density (OD_{600 nm}) using a UV-vis spectrometer (Varian Cary 50, Agilent Technologies).

Synthesis of amorphous Geobacter-Mn₂O₃

The as-grown 100 mL G. sulfurreducens solution (OD_{600 nm} = 0.6) was centrifuged (7000 rpm, 4 minutes) and the resulting red pellets (due to the presence of Fe containing c-Cyts on the cell membrane)⁵ were washed with the defined medium (lacking fumarate), followed by re-suspending the pellet into the medium and centrifugation. The washed pellets were added to an anaerobic serum vial containing 100 mL of the anaerobic growth medium with acetate (20 mM, as the electron donor) and potassium permanganate (KMnO₄, 5 mM) as the sole electron acceptor. The serum bottles were incubated in a shaking incubator (200 rpm, 30 °C) for one day until the solution color changed from violet to dark brown, suggesting the reduction of Mn. The as-synthesized Geobacater-Mn₂O₃ was collected by centrifugation (10 000 rpm, 5 minutes). The collected material was washed six times with MilliQ water by gentle vortexing to remove any contribution of the media to the Geobacater-Mn₂O₃ biohybrid electrocatalyst. After washing, Geobacater-Mn₂O₃ was dried overnight (50 °C) and this dried sample was used for all the experiments. Several batches of experiments were conducted to reproduce the results. In addition, control tests using dead bacterial cells (heat-killed by autoclaving) and without the bacterial cells were conducted to evaluate the biological activity of G. sulfurreducens on KMnO₄ reduction.

Suppression of c-type cytochromes in G. sulfurreducens

The role of c-type cytochromes (c-Cyts) in the Mn reduction was deciphered by suppressing the formation of c-Cyts in G. sulfurreducens. The suppression of c-Cyts in G. sulfurreducens was performed as previously reported.¹⁷G. sulfurreducens was initially grown in an Fe-lacking defined medium. After three days of growth, the bacterium was re-inoculated in the defined medium (without Fe) with the addition of 30 μM bipyridine (an Fe chelator to suppress the production of OM c-Cyts) and grown for three days. The bipyridine treated cells were used as inoculum for the reduction of KMnO₄ by following the same procedure as mentioned above.

Characterization

Scanning transmission electron microscopy (STEM) analysis was performed on a FEI Titan 80–300 equipped with an energy dispersive X-ray (EDX) detector. For TEM analysis, samples were sonicated in ethanol solution for 30 minutes and a few drops of the sample were drop casted on a TEM Cu-grid. An electron energy loss spectrum (EELS) was obtained via scanning transmission electron microscopy-high angle annular dark-field (STEM-HAADF) mode with a probe size of ∼1 nm. Scanning electron microscopy (SEM) analysis was performed on a TESCAN MIRA3 FEG-SEM equipped with the EDX detector, operating at 5 kV. An X-ray diffraction (XRD) analysis was conducted on a Bruker D8 Advance X-ray diffractometer using Cu Kα irradiation. The survey and high-resolution X-ray photoelectron spectroscopy (XPS) spectra were obtained at fixed analyzer pass energies of 160 and 20 eV, respectively (model: ESCA 3400).

Inductively coupled plasma atomic emission microscopy (ICP-OES; Thermo Scientific) analysis was performed to estimate the elemental composition of samples. Bacterial cells were centrifuged, and the pellets were washed with MilliQ water six times to avoid any contribution from the media. The washed cells were soaked in nitric acid solution (70%) overnight and diluted to 5% prior to ICP-OES. The standard Fe solutions of 1 ppm and 10 ppm were used for the calibration. The wavelengths used were 238.20 nm, 239.56 nm and 259.94 nm, and the sample uptake used was 3 ml per minute. Triplicate measurements were conducted at each wavelength. UV-Vis spectra of KMnO₄ and Geobacter-Mn₂O₃ were recorded from 800–200 nm using a UV-Vis spectrometer (Varian Cary 50, Agilent Technologies).

Electrochemical experiments

The OER activity of amorphous Geobacter-Mn₂O₃ was measured by employing a rotating disc electrode (RDE). The catalyst for the OER was prepared as follows: the amorphous Geobacter-Mn₂O₃ (∼2 mg) was dispersed in 500 μl of ethanol (96% v/v), 500 μl of deionized water and 15 μl of Nafion (as a binder). The solution was sonicated for 30 min. The obtained monodispersed solution (2 μl) was drop-casted onto a 3 mm glassy carbon disc electrode (loading concentration ∼0.05 mg cm⁻²) and was dried under a lamp for 1 h. The same protocol was followed to make a working electrode with commercial-Mn₂O₃ and commercial-IrO₂ (Sigma Aldrich). All the electrochemical analyses were performed in 1 M KOH (Sigma Aldrich, semiconductor grade, pellets, 99.99% trace metals basis) using a VMP3 potentiostat (BioLogic, France) at room temperature in a three-electrode system, where Pt coil and mercury/mercury oxide (Hg/HgO) were used as the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) experiments were conducted at a scan rate of 5 mV s⁻¹ at a constant rotational speed of 1600 rpm. The durability of the biohybrid catalyst was investigated by running a long term chronopotentiometry measurement at a fixed current density of 10 mA cm⁻² while recording the potential under a constant rotating speed of 1600 rpm. The electrochemically active surface area of the catalysts was determined by measuring double layer capacitance by performing cyclic voltammetry (CV) at various scan rates. All the measured potentials vs. the Hg/HgO were converted to RHE by the Nernst equation (E_RHE = E_Hg/HgO + 0.0591 pH + 0.140). All the experiments were done at least in triplicates to reproduce the data.

Results and discussion

Fig. 1a shows a schematic of the synthesis of the Geobacter-Mn₂O₃ biohybrid by incubating G. sulfurreducens with MnO₄⁻ as the sole electron acceptor. G. sulfurreducens dissipates the respiratory electrons from the acetate oxidation to reduce MnO₄⁻ through the EET respiratory chain. The appearance of dark brown color (Fig. 1b) suggested the reduction of Mn took place. The EET pathway in G. sulfurreducens deploys a series of intra and extracellular conductive protein networks, which are aligned together from the cell-interior to cell-exterior.^13–15 Among the conductive protein networks, Fe (heme) rich OM c-Cyts play a key role in the final step of EET process.^13–15 The midpoint potential of the majority of OM c-Cyts involved in EET is in the range of −0.15 to −0.2 V vs. SHE,^26,27 while the reduction potential of MnO₄⁻ is 1.51 V vs. SHE. This huge difference in the potential gradient enables OM c-Cyts to reduce MnO₄⁻ relatively easily. To investigate the role of c-Cyts in the production of Geobacter-Mn₂O₃ biohybrid, we suppressed the production of c-Cyts in G. sulfurreducens by the addition of bipyridine. Bipyridine suppresses the production of c-Cyts in G. sulfurreducens by chelating with Fe at the growth stage.¹⁷ The ICP-OES analysis showed a significant reduction in the Fe content of the suppressed (Δcyts) cells compared to the wild type (WT) cells (Fig. 1c). The reddish color substantially decreased in Δcyts cells (inset Fig. 1c) due to low Fe content. The Δcyts cells did not yield any Mn₂O₃ formation (Fig. S1†), suggesting that c-Cyts are essential to produce the Geobacter-Mn₂O₃ biohybrid. Further, there was no Mn reduction in the absence of live G. sulfurreducens cells or with heat-killed G. sulfurreducens cells. Taken together, these observations support that bacterial respiration by G. sulfurreducens was responsible for the production of Geobacter-Mn₂O₃ biohybrid.

Fig. 1 (a) Schematic for the synthesis of amorphous Geobacter-Mn₂O₃ by G. sulfurreducens; (b) a digital photograph displaying the color change from violet (KMnO₄ precursor) to brown, suggesting the manganese reduction; and (c) Fe concentration data measured by ICP-OES for wild type (WT) and Cyts-suppressed cells (Δcyts), inset shows the fading of color for Δcyts cells compared to the WT because of low Fe content.

An SEM analysis was performed to visualize the formation and morphology of the as-synthesized Geobacter-Mn₂O₃ biohybrid. The SEM image showed a uniform deposition of Mn₂O₃ nanocrystals on the surface of G. sulfurreducens cells (Fig. 2). The SEM-EDX spectrum (Fig. S2†) showed the elemental composition of Geobacter-Mn₂O₃ biohybrid as Mn (68 wt%), O (29.1 wt%), P (2.5 wt%) and Fe (0.4 wt%). P and Fe found in the hybrid originated from G. sulfurreducens cells.⁵ STEM was used to investigate the chemical and structural nature of the as-synthesized Geobacter-Mn₂O₃ biohybrid. Fig. 3a shows the STEM-HAADF image of an individual G. sulfurreducen cell before incubation with MnO₄⁻ ions. The image depicts a typical morphology of the G. sulfurreducens cell, which is rod-shaped with a size of ∼2 μM in length and ∼0.5 μM in diameter.⁵ After incubation with MnO₄⁻ ions, Geobacter-Mn₂O₃ biohybrid formed (Fig. 3b). Here, the micron-size bacterial cells served as a support for the decoration of Mn₂O₃ nanocrystals with uniform size and morphology on the surface of the cells. Elemental mapping demonstrated that Geobacter-Mn₂O₃ biohybrids were mainly made of Mn and oxygen (Fig. 3c and d). The high resolution TEM (HRTEM) image of Geobacter-Mn₂O₃ displayed no lattice fringes, suggesting the amorphous phase of the material (Fig. S3†), while the STEM image of commercial-Mn₂O₃ showed crystalline nature (Fig. S4†). The selected area electron diffraction (SAED) pattern of Geobacter-Mn₂O₃ exhibited halo diffraction rings, a characteristic observed for amorphous materials (Fig. 4a), while commercial-Mn₂O₃ showed well-defined sharp Braggs diffraction rings confirming its high crystallinity (Fig. 4b). An XRD analysis was performed to examine the crystal structure of Geobacter-Mn₂O₃ biohybrids and commercial-Mn₂O₃. The XRD profile of commercial-Mn₂O₃ showed sharp Bragg peaks, indicating high crystallinity, while Geobacter-Mn₂O₃ biohybrids showed broader peaks with poor resolution, a signature of the amorphous behavior (Fig. 4c). This observation is in well-agreement with the SAED results (Fig. 4a and b). The TEM-EDX spectroscopy analysis showed that in addition to Mn and oxygen, P and Fe were also present in the Geobacter-Mn₂O₃ biohybrid (Fig. S5†), which were derived from G. sulfurreducens, while Cu was derived from the TEM Cu-grid. The ICP-OES analysis further confirmed the presence of Fe (463 ± 21 ppm) in the amorphous Geobacter-Mn₂O₃ biohybrid.

Fig. 2 Scanning electron microscopy (SEM) images of G. sulfurreducens cells decorated with Mn₂O₃ nanocrystals. (a) Low resolution, (b) high resolution SEM images.

Fig. 3 Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images of a G. sulfurreducens cell before and after incubation with KMnO₄ (precursor). (a) G. sulfurreducens cell, (b) Geobacter-Mn₂O₃ biohybrid with G. sulfurreducens cell decorated with Mn₂O₃ nanocrystals. Elemental mapping of Geobacter-Mn₂O₃ showing (c) Mn and (d) oxygen.

Fig. 4 Selected area diffraction (SAED) patterns of the (a) Geobacter-Mn₂O₃ biohybrid, (b) commercial-Mn₂O₃. (c) X-ray diffraction (XRD) patterns of Geobacter-Mn₂O₃ and commercial-Mn₂O₃.

EELS is a powerful tool to determine the oxidation state and bonding environment of metals in their oxide forms.^28–31 The EELS analysis shows the highest spatial and energy resolutions, which makes it suitable to characterize heterogeneous samples such as Mn and Fe oxides. Fig. 5a and b show oxygen K-edge and Mn-L_2,3 edge spectra of the amorphous Geobacter-Mn₂O₃ biohybrid and commercial-Mn₂O₃ captured on the nanointerface (see Fig. S4 and S6† for details). The spectra were recorded from 500 to 700 eV region using an energy dispersion of 0.1 eV. The EELS spectra of the Geobacter-Mn₂O₃ biohybrid and commercial-Mn₂O₃ showed similar patterns, suggesting that both have similar structures (Fig. 5a and b). The first peak ∼529.2 eV in the O-K edge (O-K_a) represents the electron transition from 1s core states to oxygen 2p states hybridized with Mn 3d orbitals.²⁸ The second peak ∼540 eV (O-K_b) corresponds to probable unoccupied oxygen 2p states mixed with Mn 4sp states.³⁰ The energy separation between the first and second peaks is ∼10.8 eV, which confirms the Mn₂O₃ phase in the biohybrid catalyst and the observation is consistent with earlier reports.^30,31 Mn-L_2,3 spectrum displays two white lines L₂ and L₃ because of the transition from 2p_3/2 and 2p_1/2 to 3d unoccupied states localized on the excited Mn ions.²⁸ The Mn valence state can be easily predicted by measuring the energy difference between M-L₃ and O-K_a (ΔE = M-L₃ − O-K_a).^28–31 The ΔE value measured for the amorphous Geobacter-Mn₂O₃ biohybrid is 111 eV, which was very close to a previously reported value for Mn₂O₃ (111.2 eV).³⁰ We further probed the oxidation state of Mn via XPS. The XPS spectra of commercial-Mn₂O₃ and Geobacter-Mn₂O₃ display similar profiles, indicating that they have similar structures (Fig. 5c and Fig. S7, S8†). The Mn 3s region in the XPS spectra can provide the oxidation state of Mn in its oxide form. Fig. 5c shows the Mn 3s peaks of commercial-Mn₂O₃ and Geobacter-Mn₂O₃ containing two, multiplet split components. The energy difference (ΔE) of the peak splitting for both the catalysts was determined to be 5.5 eV, which matches well with the Mn³⁺ oxidation state.³² UV-Vis spectra can also provide some clues to understand the oxidation state of metal oxides. The Geobacter-Mn₂O₃ showed strong absorption in the visible region (300–600 nm) with a prominent peak at 430 nm (Fig. S9†). This peak arises from the d–d transitions of Mn³⁺ in the Geobacter-Mn₂O₃ as Mn³⁺ (d⁴) forms a high spin complex.³³ Combined together, EELS, XPS and UV-Vis analyses support that the oxidation state of Mn in Geobacter-Mn₂O₃ is +3, and that the most probable chemical structure of the oxide in as-synthesized sample is Mn₂O₃.

Fig. 5 (a) and (b) Electron energy loss spectra (EELS) recorded at the nanointerface of amorphous Geobacter-Mn₂O₃ and commercial-Mn₂O₃. The corresponding EELS images of the Geobacter-Mn₂O₃ and the location for the EELS spectra recorded are shown in Fig. S6.† (c) Mn 3s spectra of commercial-Mn₂O₃ and Geobacter-Mn₂O₃.

The crystal structure of MnO_x and the oxidation states of Mn in the oxide form are crucial for the catalytic performance. Amorphous structures perform with higher OER activity over crystalline counterparts,³⁴ and the Mn³⁺ state triggers the OER more efficiently than the other oxidation states of Mn.³⁵ Most of the existing synthetic methods usually yield crystalline MnO_x where the oxidation state of Mn mainly stays in the +2 state.³⁴ Using our approach (i.e., bacterial synthetic route), we were able to synthesize an amorphous structure of Mn oxide with Mn³⁺ state. In addition to acting as a reducing agent, G. sulfurreducens provided OER active elements (such as Fe and P) to the biohybrid catalyst (Fig. S2 and S5†) and acted as a micron size carbon support to the as-produced Mn₂O₃ nanocrystals (Fig. 3b). The carbon template provided by bacteria can render additional surface area, porosity and conductivity to the as synthesized biohybrid.

We employed the as-synthesized amorphous Geobacter-Mn₂O₃ biohybrid as an OER electrocatalyst and compared its performance with a commercial-Mn₂O₃ and commercial-IrO₂ catalyst. The OER activities of the catalysts were determined by conducting the LSV analysis at a scan rate of 5 mV s⁻¹ under a constant rotating speed of 1600 rpm. A scan rate of 5 mV s⁻¹ was chosen in the current study to compare the performance of our Geobacter-Mn₂O₃ biohybrid with previously reported OER electrocatalysts (Table 1 and Table S1†), which were also analyzed at a scan rate ≥5 mV s⁻¹. The overpotential for the OER is calculated by measuring the potential needed to generate a current density of 10 mA cm⁻², which is considered as a common practice in the literature.³⁶ The amorphous Geobacter-Mn₂O₃ biohybrid catalyst produced a geometrical current density of 10 mA cm⁻² with an overpotential of 290 ± 9 mV (vs. RHE), while the commercial-Mn₂O₃ and commercial-IrO₂ displayed an overpotential of 370 ± 15 mV and 390 ± 18 mV (vs. RHE), respectively (Fig. 6a). Glassy carbon did not show any OER activity, as expected. A Tafel plot is useful to probe the kinetics and catalytic active sites of electrocatalysts.^5,36 The Tafel plot of amorphous Geobacter-Mn₂O₃ showed a lower slope value (59 mV dec⁻¹) than that of commercial-Mn₂O₃ (81 mV dec⁻¹) and commercial-IrO₂ (112 mV dec⁻¹), suggesting its superior OER activity (Fig. 6b). The chronopotentiometry experiment confirmed that the amorphous Geobacter-Mn₂O₃ biohybrid is highly stable for the OER even after 24 h of continuous measurements at a fixed current density of 10 mA cm⁻² (Fig. 6c). TEM measurement after the stability test showed no significant changes in the structure of the catalyst, suggesting the structural robustness of the biohybrid catalyst (Fig. S10†). The absence of sharp diffraction peaks in the XRD pattern after chronopotentiometry further confirmed that the biohybrid kept its amorphous phase even after the long-term stability test (Fig. S11†).

Fig. 6 (a) Linear sweep voltammetry (LSV) plots of amorphous Geobacter-Mn₂O₃, commercial-Mn₂O₃, commercial-IrO₂ and glassy carbon support at a scan rate of 5 mV s⁻¹; (b) Tafel plots of the used catalysts; and (c) stability test for the oxygen evolution reaction (OER) performance of amorphous Geobacter-Mn₂O₃ at a constant current density of 10 mA cm⁻². All experiments were conducted at a constant rotating speed of 1600 rpm.

Table 1 OER activities of various Mn-based electrocatalysts

Catalyst	Overpotential (mV) @ 10 mA cm^−2	Tafel (mV dec^−1)	Ref.
a NCNT: nitrogen-functionalized carbon nanotube. b Mn-NG: mononuclear manganese embedded in nitrogen-doped graphene. n.a: not available in the literature.
Amorphous Geobacter-Mn2O3	290	59	This study
Commercial-Mn2O3	370	80	This study
Amorphous manganese oxide	590	179	36
Mn oxide	540	n.a	43
Mn2O3	460	94	45
Calcined MnOx on NCNT^a	570	147	46
MnOx/NCNT	410	91	47
Dandelion-like α-MnO2	550	155	48
Fe doped-MnO2	692	94	49
MnO2	770	85	50
Mn2O3	410	99	51
Mn3O4	570	n.a	52
Mn-NG^b	337	55	53
γ-MnO2	427	n.a	54
Co- and Mn mixed oxides	450	35.8	55

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The OER activity of the amorphous Geobacter-Mn₂O₃ biohybrid outperformed most of the Mn-based electrocatalysts (Table 1) and other benchmark OER catalysts including IrO₂ and RuO₂ (Table S1†). The OER activities of MnO_x largely depend on its chemical composition and crystallographic structure.³⁷ The amorphous structure of the Geobacter-Mn₂O₃ biohybrid offers a large surface area and high density of surface defects, which provide excess active sites for OER.³⁸ Previous studies have demonstrated that the amorphous phases of metal oxides outperform crystalline counterparts for the OER catalysis.³³ The X-ray absorption spectroscopy analysis of amorphous MnO_x has shown longer Mn–O distances due to Jahn–Teller-elongated Mn^III–O bonds.^38,39 This allows structural flexibility on the surface of MnO_x and enhances the OER activity. Electrochemically active surface area (ECSA) of the electrode is the actual surface area that is involved in a catalytic reaction.⁴⁰ ECSA is usually determined by measuring the double layer capacitance (C_dl) of the catalysts via performing CV at various scan rates.⁴⁰ The amorphous Geobacter-Mn₂O₃ biohybrid catalysts showed a C_dl of 26.8 mF cm⁻², which is nearly two times higher than that of the commercial-Mn₂O₃ (14 mF cm⁻²) (Fig. S12†). The increased ECSA indicated highly porous nature of the Geobacter-Mn₂O₃ biohybrid, which provided excess catalytic sites for the OER. An in situ UV-vis spectroelectrochemical measurement revealed that Mn³⁺ serves as the precursor for the O₂ evolution.⁴¹ Mn in the Geobacter-Mn₂O₃ biohybrid was present as Mn³⁺, which can trigger O₂ evolution in the OER. In addition to its amorphous structure and the fact that Mn was present as Mn³⁺, the superior OER activity of the Geobacter-Mn₂O₃ biohybrid could be due to the presence of Fe and P (Fig. S2 and S5†), which were provided by G. sulffurreducens cells during the MnO₄⁻ reduction process. Fe and P are known OER candidates that facilitate the catalytic activity.^42,43 A trace amount of Fe (ppm level) in the catalyst can drastically enhance the OER catalytic activity.⁴³ When pure G. sulfurreducens was used alone, no significant OER activity was observed (Fig. S13†). This was expected because one of the inherent problems of bacteria is that they possess low electrical conductivity,⁴⁴ which may suppress their catalytic activity.

Conclusions

In summary, we demonstrated a direct synthetic route to produce amorphous Geobacter-Mn₂O₃ biohybrid catalyst by G. sulfurreducens without the use of any toxic chemicals or high energy input. Further, we demonstrated that the as-produced amorphous Geobacter-Mn₂O₃ biohybrid is a promising OER electrocatalyst with an overpotential of 290 ± 9 mV vs. RHE to produce a current density of 10 mA cm⁻² and a low Tafel slope of 59 mV dec⁻¹. The catalyst showed an excellent stability even after a long term chronoamperometry experiment. The OER activity of the biohybrid outperformed commercial-Mn₂O₃ and precious benchmark metal oxide electrocatalysts such as IrO₂ and RuO₂. The remarkable OER activity of the Geobacter-Mn₂O₃ biohybrid is attributed to the combination of its amorphous structure and the presence of additional OER active elements (Fe and P). The proposed cost-effective and green approach to synthesize Mn-based electrocatalysts at ambient experimental conditions using EET-capable bacteria opens the door for the synthesis of other high-performing and low-cost biohybrid electrocatalysts for various energy-related applications.

Conflicts of interest

There are no conflict of interests to declare.

Acknowledgements

This work was supported by Competitive Research Grant (URF/1/2985-01-01) from King Abdullah University of Science and Technology (KAUST).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9gc04353e

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