Cost-effective ion-tuning of Birnessite structures for efﬁcient ORR electrocatalysts

(cid:1) K-, Cu-Birnessite nanostructures by cheap oone pot method for ORR. (cid:1) Birnessite showed high activity in ORR with the 4-electron pathway. (cid:1) Cu effecting the Mn 3 þ /Mn 4 þ ratio resulting in good ORR activity. Birnessite structured MnO x with tuned potassium, copper and water content in the interlayer spacing is produced by a simple and cost-effective method. The new structures are investigated by XRD, Raman spectroscopy, SEM, EDX, HRTEM, DLS, TG, and DSC. Our study demonstrates a successful intercalation process to produce birnessites with mixed interlayer cations. Both Birnessite and Cu 2 þ /Birnessite structure have a nanosheet-like morphology where the sizes of the copper-treated birnessite nanoparticles are drasti-cally decreased compared to the copper ion free structures. The speciﬁc surface area is increased from 21.6 m 2 /g to 77.8 m 2 /g in the presence of copper as a result of a longer ageing process. Our study reveals that the electron transfer numbers of Birnessite and Cu 2 þ /Birnessite are about 3.40 and 3.65, respectively in the oxygen reduction reaction. Both as-synthesized pristine Birnessite and copper tuned Birnessite are a promising candidate for a cheap, noble metal-free electrocatalyst for fuel cell applications.


Introduction
The world's most pressing concerns today are the future exhaust of conventional energy reserves [1] and the increasing environmental pollution from the byproduct of these sources, such as greenhouse gas emission from fossil fuel combustion [2]. Researchers are focusing to find sustainable energy resources and to provide ecologically friendly and cost-effective technologies and processes for further energy production. The energy conversion from renewable sources sharply increased in the last several decades using various advanced processes such as the electrochemical fuel cell technology including two key steps: hydrogen oxidation reaction (HOR) [3], and oxygen reduction reaction (ORR) [4]. The electrochemical energy conversion in a fuel cell is a promising candidate for sustainable energy production [5,6] due to a wide range of application, and high energy efficiency [7,8]. Most of the energy conversion processes including ORR have slow reaction kinetics [9], which can be boosted by catalysts. Noble metalbased catalysts are one of the most common heterogeneous catalysts for ORR such as PteM 2 O 3 /C (M ¼ Y or Gd) [10], Pd-RuSe/C [11], and Pt/CeO 2 /C [12], but their relative rarity and high cost is making them unpractical. Therefore, electrochemical ORR catalyst research is focusing more on earthabundant and cost-effective noble-metal free materials such as ZIF-67-derived CoO [13], iron nitride/nitrogen-doped graphene [14], nitrogen-doped carbon nanofiber [15], copper/ graphite oxide [16], and NiCo 2 O 4 [17].
Since the manganese is the second most abundant transition metal in the earth's crust, manganese oxide-based materials are increasingly studied as the ORR catalyst, for instance, MnO x [18,19], Ag/MnO x [20], NiOx-MnOx-graphene [21], Mn 3 O 4 decorated N-doped carbon [22], and porous Mn 2 O 3 [23]. However, manganese oxides have numerous different crystal structures even if they have the same number of oxidation state, for example, manganese (IV) oxide has several polymorphs: a-MnO 2 , g-MnO 2, d-MnO 2 , and l-MnO 2 . From these polymorphs, Delta(d)-manganese oxide, also known as birnessite, is one of the most common manganese minerals in nature, which was first described in 1956 [24], and it has the following constitution: (Na 0$3 Ca 0$1 K 0.1 )(Mn 4þ , Mn 3þ ) 2 O 4 $ 1.5 H 2 O). As a consequence of layered structure and earth-abundance, an increasing number of studies have investigated birnessite as a water oxidation catalyst [25] and as a supercapacitor [26]; but, only limited research was done on birnessite as an efficient cathode catalyst for the oxygen reduction reaction [27,28] Recently, one study revealed that birnessite type oxide has higher ORR catalytic activity than other polymorphs [29]. Interlayer molecules such as water, and cations (K þ , Na þ , Ca 2þ , Mg 2þ ) are playing an essential role in electrochemical property of birnessite. Numerous studies have reported the effect of the interlayer cation exchanges, and the number of water molecules on the catalytic activity such as for water oxidation [30] and oxygen evaluation reaction [31]. According to literature, copper doping is enhancing the catalytic activity of other types of manganese oxide-based catalysts in ORR such as Cu x Mn 3-x O 4 spinel particles/polypyrrol composite [32], graphene-Cu-a-MnO 2 nanowire blends [33,34], and PtNiCu [35]. Despite these results, no research was done up until now on copper doped delta-manganese oxide catalyzed ORR processes.
Our goal was not only to investigate copper doping influence on the catalytic activity of birnessite but also to find a reliable method to synthesize birnessite. Birnessite contains Mn 3þ and Mn 4þ ions, where Mn 3þ can be reduced to Mn 2þ , or oxidized into Mn 4þ , and M 4þ can be reduced to Mn 3þ and Mn 2þ . This alternation of oxidation state particularly Mn 3þ species increases catalysis of the ORR process [36], but it is challenging to control phase changes, and oxidation state ratios of the birnessite, which has been an essential part of the production and application. On account of this problem, a typical synthesis of birnessite requires hydrothermal condition, constant cooling, organic template, and the conventional reflux synthetic method [37].
In this report, we produced birnessite with a cost-effective and straightforward method described by Boumaiza et al. [38]. Modified by tuning the ageing time, which affects the amount of exchange from potassium (K þ ) cation to copper (Cu 2þ )-ions as well as the concentration of the water in the interlayer space of the structure. The characterization of the assynthesized material was done and the activity for oxygen reduction reaction was investigated with voltammetry and chronoamperometric methods. The study revealed that copper ion tuning is changing the Mn 3þ /Mn 4þ ratio and longer ageing process is increasing surface area and decreasing particle size, those are providing more active sites for the contact between catalyst and electrolyte.

Chemicals
Manganese(II)-chloride-tetrahydrate (MnCl 2 *4H 2 O), potassium-permanganate (KMnO 4 ), copper(II)-chloride (CuCl 2 ) and potassium hydroxide (KOH) were purchased from Sigma Aldrich. All chemicals and reagents used in this study were at least in analytical grade and used without further purification. Ultrapure water was used for all synthesis.

Synthesis of Birnessite
Three aqueous solutions were used for the synthesis of potassium-birnessite: 1.75 ml of KOH (8.8 mol L À1 ), 2.125 ml KMnO 4 (0.1 mol L À1 ) and 3.75 ml MnCl 2 (0.6 mol L À1 ). The synthesis method [38] used in the present work consisted of mixing (MnCl 2 ) and KMnO 4 solutions under vigorous stirring for 2 h then KOH-solution was dropwise added into this mixture. The final reaction mixture was then stirred for another 30 min and aged at 60 C for 16 h. The final product was centrifuged and washed until the pH of the solution settled between 8 and 9 and dried at 60 C overnight. Synthesis scheme showed in Supplementary material (Fig. S1). For the Cu 2þ /Birnessite structure, MnCl 2 (0.5 mol L -1) and KMnO 4 (0.1 mol L -1) mixed with CuCl 2 (0.1 mol L -1) for 2 h then KOH (8.8 mol L À1 ) solution was added dropwise into this mixture. The final reaction mixture was then stirred for another 30 min and aged at 60 C for 48 h. The intercalated birnessite was finally centrifuged and washed until the pH of the solution settled between 8 and 9 and dried at 60 C overnight.

Basic characterization of the Birnessite structures
The intercalation of Birnessite samples was investigated by Xray diffractometer (XRD) and Raman spectroscopy. The morphology was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the chemical composition was calculated from energy-dispersive X-ray spectroscopy (EDS). The specific surface area and pore diameter were measured by nitrogen adsorption, while the thermal properties were investigated using thermogravimetry i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 6 2 6 6 e1 6 2 7 6 (TG) and differential scanning calorimetry (DSC). The pHdependent surface charge and aggregation state were studied by zeta potential measurement, and by dynamic light scattering (DLS). The detailed description of these techniques and methods are given in the supplementary information.

Electrochemical (ORR) measurements
The ORR electrode is prepared by the modification of a glassy carbon electrode with a mixture of the birnessite sample and carbon black slurry. The ORR measurements were performed in a three-electrode cell using the electrochemical workstation. The ORR preparation and measurements are detailed in the supplementary material.

Result and discussion
Structural determination and chemical characterization XRD patterns of the potassium birnessite (Birnessite) and copper intercalated one (Cu 2þ /Birnessite) are shown in Fig. 1a. The two main reflection peaks observed at 12.63 (dspacing ¼ 0.69 nm calculated from Bragg's equation [39]) and 24.8 (dspacing ¼ 0.36 nm) correspond to the (001) and (002) crystal planes of Birnessite, respectively [38,40]. The addition of the copper-ions resulted in a slight shift of (001) reflection from 12.81 (dspacing ¼ 0.69 nm) to 12.63 (dspacing ¼ 0.70 nm) (Fig. 1b.). The interlayer distance expansion can be attributed to the bigger size of the copperions as well as the higher amount of total interlayer cations [41] in Fig. 1(c-d). In the case of the pristine Birnessite, the reflection at 27.0 (dspacing ¼ 0.32 nm) corresponds to the impurity raised from manganite (MnO(OH)) which is an intermediate phase during the birnessite formation. However, this phase is not observable for the copper intercalated birnessite structure. A weak reflection at~19 and~35 appears, which refers to the presence of a small amount of the tunnel structured(a-MnO 2 ) cryptomelane [42] as a result of a longer ageing process. Only complete phase changes observed in XRD (Fig. S2) after thermal gravimetric analysis where samples heated up until 750 C.
Raman spectra of the samples are shown in Fig. 2a. The major peak at 582 cm À1 corresponds to the n 3 (MneO) stretching vibration in the basal plane of the MnO 6 sheets [43], which is slightly shifted in case of Cu 2þ /Birnessite to 580 cm À1 . It correlates with interlayer distance changes as observed in XRD results (Fig. 1b.). Further six peaks at 285, 411, 478, 513 and 640 cm À1 can be obtained for both samples which are correlating well with typical Raman shifts of birnessite in the literature [44]. Notably, a copper cation being present in the interlayer structure of Cu 2þ /Birnessite, the intensity of the peak positioned at 565 cm À1 is significantly decreased [30,45]. Besides, in case of the Cu 2þ -ion intercalated birnessite, a peak observed at 697 cm À1 (out-of-plane symmetric stretching of MneO of MnO 6 ) groups are attributing to changes of Mn 4þ / Mn 3þ ratio in the birnessite structure and disordering of the octahedral sheets when Cu 2þ -ion are integrated into the interlayer space and illustration of the cryptomelane structure forming [46].
Energy-dispersive X-ray spectra of birnessite showed in Fig. 2b. The atomic percentage of oxygen is about 73e75 at%, manganese content around 18 at% and the interlayer cationic content (including potassium for Birnessite and copper/potassium for Cu 2þ /Birnessite) is about 3e7 at% for both samples, which is similar to typical birnessites [47]. The calculated formula is K 0$18 MnO    respectively [48] based on molar ratio detected from the quantitative analysis showed in Table S1. Thermal analysis of the samples at RT-750 C in both air and nitrogen atmosphere with a heating rate of 5 C/min are depicted in Fig. 3a. Thermal stability of birnessite increased significantly due to the longer ageing time and the decreasing of the interlayer water molecule and the insertion of copper cations into the interlayer. The first massive decomposition appears at~120 C with a mass loss of 10% and 6% for Birnessite and Cu 2þ /Birnessite, respectively. These phenomena can be attributed to the desorption of physisorbed water and the removal of interlayer water [49]. The second decomposition occurs in 2e3 steps at 120e550 C with a weight loss of 8% for Birnessite and 2% for Cu 2þ /Birnessite, which can be attributed to the loss of lattice oxygen species. Slight weight increment of~1% occurred at about 500 C for the Cu 2þ /Birnessite corresponding to the oxidation of manganese. In the literature, weakly crystallized synthetic birnessite shows similarly slight weight gain at 500e600 C in the thermogravimetric analysis [50]. Thermal stability of both Birnessite sample is quite high, and only 8e18% of the total weight is lost before reaching 750 C. For the understanding slight weight increment in the case of Cu 2þ /Birnessite at elevated temperature, TG analysis was performed in a nitrogen atmosphere, too (Fig. S3). During the inert condition, no weight gain has appeared. In the case of the air-based measurements, the Cu 2þ /Birnessite shows the presence of manganese oxidation of Mn 3þ to Mn 4þ , which is attributed to the increased amount of the Mn 3þ and decreases of the amount of crystallization in the samples. Differential Scanning Calorimetric results are shown in Fig. 3b. Endothermic peaks around 100 C and 150e350 C refer to the dehydration of the physical and the chemically adsorbed water molecules respectively, located between layers. In   the case of Cu 2þ /Birnessite, an exothermic peak around 500 C was observed which corresponds to the transformation of the layered structure to the tunnel structured cryptomelane due to the oxidation of Mn (III) to Mn (IV) [51,52].
N2 adsorption isotherms and pore diameter of the samples showed in Fig. 3c and d. Type IV N 2 adsorption isotherm was observable in the case of both samples. Birnessite displays a smaller specific surface area of 21.6 m 2 /g as typical birnessites [49], while the specific surface area of Cu 2þ /Birnessite was calculated to be 77.8 m 2 /g, respectively in Fig. 3c. As can be seen in Fig. 3d, the pore diameter is 5 nm and 10e15 nm for Birnessite and Cu 2þ /Birnessite, respectively showing that the copper ion modification resulted in a more mesoporous-like structure beside the increased surface area. The development of a more mesoporous-like structure for Birnessite and Cu 2þ /Birnessite can be attributed to the doping of copper ion, which refines layered MnO 2 nanoparticles into tiny grains, while longer ageing process results in higher specific surface areas.

Morphological characterization
Scanning electron microscopic images showed that the birnessite consists of larger well-crystallized plates (sheet-like) with length from 100 nm to a few micrometres (Fig. 4a.). In the case of Cu 2þ /Birnessite due to copper intercalation and longer ageing process, the sheets are smaller, and aggregated miniature sheets were formed with rod-like wires (Fig. 4b.). Transmission electron microscopic images of the samples ( Fig. 4c and d) show similar morphological structures as images made by the SEM. HRTEM images (Fig. 4e and f.) reveal that the leading lattice distance of the Birnessite was 0.69 nm while it was measured to be 0.70 nm in case of Cu 2þ /Birnessite pointing to the presence of a slight interlayer distance expansion due to the presence of Cu 2þ -ions. These results correlate with results obtained by XRD (Fig. 1b.). Morphology of these samples is similar to the typical birnessite structures as published in the literature [53].

The pH-dependent aggregation state and zeta potential
In water-based systems, the metal oxide particles have variable surface charges. The most critical parameters of this phenomena are the pH [54] and ionic strengths [54,55]. The pH-dependent zeta potential (z) and hydrodynamic diameter (Z AVE ) of the original Birnessite particles, as well as the effect of Cu 2þ -ion additions on the surface charges and the aggregation state of the birnessites, are shown in Fig. 4. The surfaces of the Birnessite particles become charged in water due to the reactions of their surface hydroxyl groups (≡MneOH), controlled by both the pH and the ionic strengths of the medium [56]. The protonation/deprotonation reactions of (≡MneOH) sites lead to the formation of positive (≡MneOH 2 þ ) or negative (≡MneO -) surface charges. The pH is at pH~2.5 for Birnessite particles at the isoelectric point (I.E.P.), where z ¼ 0 mV, which is in good agreement with previously published measurement [56]. Around this pH value, the particles are aggregating (Z AVE > 1000 nm) because of the lack of the electrostatic stabilization. Due to presence of (≡MneO -) surface groups, colloidally stable dispersion can be observed at pH values between 4 and 10, accompanied by a low Z AVE and high absolute value of z (see Fig. 5a.). The large amount of Na þion (~100 mmol L À1 ) added into the system as NaOH solution to set the pH around 13, which results in the so-called screen effect. The success of the dispersion process was confirmed by the~200 nm measured Z AVE value in the temporarily stable regime, which is very close to the lower value of the particle size determined by SEM.
The I.E.P. of Cu 2þ /Birnessite is shifted from pH~2.5 to pH~3.5 (see Fig. 5b). The addition of Cu ions can lead to the number of chemical alterations in the mixture. First, the intercalated K ions can be exchanged by polyvalent Cu cations (justified by the lattice distance changes determined by XRD). The charge of cations can modulate the oxidative state of the manganese in the layer, namely, they are partially converted from the stable Mn 4þ to Mn 3þ (proved by Raman spectroscopy, TG, EDX). This phenomenon affects the surface charge properties due to the shift of the (≡MneOH) groups' pK. Secondly, a small part of the added Cu ions could form an oxide/hydroxide nanoparticle. These nanoparticles have own surface functional groups (≡SeOH) with representative protonation/ deprotonation equilibrium. Based on the literature, the I.E.P. of copper-oxide nanoparticles is about pH~6.8 [57], so the contribution of the protonated (≡CueOH 2 þ ) surface groups (pH > I.E.P.) can explain the shift of the z values presented in Fig. 5b. Furthermore, these changes result in the increase of the aggregation regime for Cu 2þ /Birnessite (see Fig. 4c) probably due to heterogeneous coagulation.
According to the z measurements, we could improve and prepare samples with the best parameters for the ORR investigations. As seen in z results, the Birnessite particles are aggregated in alkaline conditions, so the usage of carbon black during the preparation of the electrodes for ORR measurements is necessary to get an appropriate layer under the condition of ORR experiments.

Electrochemical characterization of ORR activity
ORR activity of the samples was examined using cyclic voltammetry and linear sweep voltammetry and their stability were tested by the chronoamperometric method. All electrochemical measurement was performed in alkaline media (0.1 M KOH) in a three-electrode glass cell, using glassy carbon electrode modified with the samples as the working electrode, Ag/AgCl electrode as reference electrode and platinum wire as a counter electrode.
As Fig. 6 shows, a reduction peak appears on the voltammograms measured in the oxygen-saturated electrolyte compared to the background measured in nitrogen saturated electrolyte. The peak position was 0.73 V vs RHE in case of pristine Birnessite (Fig. 6a) while it was 0.77 V vs RHE in case of the Cu 2þ /Birnessite (Fig. 6b) modified GCE. The onset potential of the pristine Birnessite was measured to be 0.8 V vs RHE while this value was shifted in case of the Cu 2þ /Birnessite modified GCE to 0.87 V vs RHE, similarly to the shift of the reduction peak position. These values are comparable to the results observed for common ORR catalysts [58]. Notably, the voltammograms in Fig. 6b show two oxidation peaks at 0.57 and 0.82 V vs RHE, which correspond to the oxidation peaks of Cu to Cu(I) and Cu(I) to Cu(II), respectively [16]. Elemental Cu can be originated from the reduction process of the Cu(II) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 6 2 6 6 e1 6 2 7 6  during the electrochemical measurement [59]. The reduction peaks are not visible on the measured voltammograms, likely because of the small amount of copper presented in the sample, which gives relatively small current values during the reduction process compared to that of the parallel oxygen reduction reaction [60]. Fig. 7a and b shows the LSV curves of the glassy carbon electrodes modified with the as-prepared samples. As it shows, the current density was increased with increasing rotation rates. Fig. 7c shows the comparison of the LSV curves at 1500 rpm rotation rate. The reduction peak shifted from 0.8 V to 0.87 V, although the reduction current densities (taken at 0 V vs RHE) did not change after the intercalation, which was measured to be 4.6 mA cm À2 for both pristine Birnessite and Cu 2þ /Birnessite. Electron transfer numbers were calculated from the linear sweep voltammograms at different rotation rates by using the Koutecky Levich equation (for details see supplementary information) [61]. The Koutecky-Levich plot of the Birnessite-modified GCE electrodes at various potentials is depicted in Fig. S4. The parallel straight   trend lines can be attributed to the first-order reaction kinetics of the ORR [62]. The extrapolated K-L plots have non-zero intercepts, thus mixed kinetic-diffusion control exists in the investigated potential range [63]. The calculated electron transfer numbers were summarized in Fig. 7d. It is seen, that the calculated electron transfer numbers are close to 4 (~3.4 and 3.6) in both Birnessite and Cu 2þ /Birnessite case, and the oxygen reduction reaction mainly proceeds through the fourelectron pathway with their use, proving that both samples show great promise as a cathode catalyst material for fuel cells.
The ORR activity was improved with Cu 2þ ion-doped Birnessite catalyst compared to the activity with pristine Birnessite catalyst, based on the calculated~10% electron transfer number increment, and the measured positive shift in the oxygen reduction potential peak and onset potential. These phenomena can be explained by several causes based on the literature. As others already stated, Mn 3þ /Mn 4þ oxidation state ratio plays an important role in ORR catalytic activity and ion-tuning (Ag þ and Zr 4þ ) increases Mn 3þ ratio in birnessite type manganese oxides [28,64,65]. As shown in Raman spectra (Fig. 2a) the peak observed at 697 cm À1 corresponds to changes of Mn 4þ /Mn 3þ ratio in the birnessite structure and disordering of the octahedral sheets when Cu 2þion are integrated into the interlayer space [46]. Wang et al. found that the mutual action between the intercalated ion Zr 4þ (in our case Cu 2þ ) and the Mn 3þ ions results in increased O 2 adsorption for ORR [64]. A similar connection can be hypothesized between the birnessite and the intercalated copper ion, as Yaday et al. have already investigated the adsorption of oxygen species to the surface of similar material and found that the intercalated copper ion has beneficial effects on the birnessite type manganese-oxide [59]. Likewise, the smaller particle size is boosting the activity owing to the larger surface-to-bulk ratio and numerous surface defects. Higher surface areas provide more active sites for the contact between catalyst and electrolyte [66]. Fig. 8a shows the LSV derived Tafel slopes, where a slight increase can be experienced and the value was found to be À99,8 mV/dec for pristine Birnessite mv/dec and À94,4 for the Cu 2þ /Birnessite. These values are also similar to those reported in the literature (ref: [20,28] from Table .1). The stability of the catalysts was evaluated by measuring the current retention-operating time curves with the chronoamperometric method at 0.4 V (vs RHE) potential in oxygen-saturated 0.1 M KOH solution at 1500 rpm rotating rate as seen in Fig. 8b. The stability study reveals that the as-synthesized birnessites have high stability with over 10,000 s operating time. It is exciting to note that Cu 2þ /Birnessite shows higher stability (94.8% current retention) than pristine Birnessite sample (87.4% current retention), which can be the results of the interlayer copper ion stabilizing the Mn 3þ /Mn 4þ species in the catalyst during ORR [67]. As Yadav et al. explain, the reversibly intercalated copper stabilizes and enhances its charge transfer characteristics. As can be seen in Fig. 6b, a portion of copper ions can be reduced by oxygen and form an oxidized copper state, which is typical phenomena for the copper included ORR catalysts [16].  i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 1 6 2 6 6 e1 6 2 7 6 The comparison of the ORR catalytic activity test results of the two synthesized samples with those of the literature can be seen in Table 1 The pristine birnessite and Cu 2þ /birnessites samples both have the possibility to be utilized as a cathode catalyst in fuel cells as the results are comparable to other already published manganese oxide-based catalysts.

Conclusions
This work presented an alternate synthesis method for birnessite with different interlayered potassium and copper cations. The addition of copper and a longer ageing process resulted in a significant specific surface area increment (21.6 m 2 /g to 77.8 m 2 /g) as well as small nanoparticle-type morphology. It was also experimentally validated, that copper ion enhances the main active site Mn 3þ in the birnessite structure, which results in an improved electron transfer number and higher stability during the ORR test. Significant four-electron transferability was observed for Cu 2þ /Birnessite and for Birnessite at a smaller degree. Both catalysts proved to be highly stable with above 87e95% retention after 10,000-s measurement. These results prove that the synthesized Birnessite samples described in this paper, show great promise as an efficient, cheap, noble-metal-free electrochemical catalyst.