A Precious‐Metal‐Free Hybrid Electrolyzer for Alcohol Oxidation Coupled to CO2‐to‐Syngas Conversion

Abstract Electrolyzers combining CO2 reduction (CO2R) with organic substrate oxidation can produce fuel and chemical feedstocks with a relatively low energy requirement when compared to systems that source electrons from water oxidation. Here, we report an anodic hybrid assembly based on a (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) electrocatalyst modified with a silatrane‐anchor (STEMPO), which is covalently immobilized on a mesoporous indium tin oxide (mesoITO) scaffold for efficient alcohol oxidation (AlcOx). This molecular anode was subsequently combined with a cathode consisting of a polymeric cobalt phthalocyanine on carbon nanotubes to construct a hybrid, precious‐metal‐free coupled AlcOx–CO2R electrolyzer. After three‐hour electrolysis, glycerol is selectively oxidized to glyceraldehyde with a turnover number (TON) of ≈1000 and Faradaic efficiency (FE) of 83 %. The cathode generated a stoichiometric amount of syngas with a CO:H2 ratio of 1.25±0.25 and an overall cobalt‐based TON of 894 with a FE of 82 %. This prototype device inspires the design and implementation of nonconventional strategies for coupling CO2R to less energy demanding, and value‐added, oxidative chemistry.


Introduction
Theelectrosynthesis of fuels is being pursued as apotential solution to intermittent electricity production via renew-able wind and solar technologies. [1] Conventional fuel-generating electrolyzers couple the oxygen evolution reaction (OER) at the anode to the hydrogen evolution reaction (HER) or CO 2 Ra tt he cathode. [2,3] However, the kinetic hurdles of the anodic four-electron process and consequently large overpotential for the OER, tied to the limited commercial value of O 2 ,a re spurring interest in employing more synthetically useful and facile organic oxidative reactions. [4][5][6][7][8][9][10][11] Recent technoeconomic studies have shown that % 90 % of the overall energy requirements for current commercial approaches in CO 2 electrolysis stem from the OER, and that lower cell potentials for fuel-generating reductive chemistry can be achieved by substituting the OER for AlcOx. [12] In particular,bycombining theory and experiment, it was shown that glycerol, abiomass-derived platform chemical and abyproduct from the production of biodiesel and soap, [13,14] is an attractive candidate that can greatly improve the economics of the overall redox process.
Reports on dual AlcOx-CO 2 Re lectrolyzers suffer from two main drawbacks to date. [7,9] Firstly,p recious metalcontaining components are employed in the electrolysis cells. Secondly,homogeneous catalysts and mediators are required in excess in the electrolyte solution, which complicates postreaction processing of the liquid products.T he use of dissolved catalysts presents an additional challenge during electrosynthesis,b ecause only at iny fraction of the catalyst (positioned in the Helmholtz layer) can turnover, while the rest remains inactive in the bulk solution.
Here,w ec onsider glycerol as ac ommercially viable resource to develop ar obust and precious-metal-free anodic assembly for coupling with the cathodic CO 2 Rr eaction. We modified TEMPO,a ne fficient, non-toxic,m olecular (electro)catalyst that can oxidize awide range of alcohol substrates under mild conditions, [15,16] with as ilatrane anchor (giving STEMPO), for robust immobilization on a mesoITO scaffold ( Figure 1). Thep orous electrode enables high catalyst loading, while the immobilization procedure permits direct electronic communication between the electrode and the molecular species,l eading to constant catalytic turnover, easier product isolation and catalyst recyclability.The performance of the mesoITO j STEMPO assembly was assessed under different conditions with arange of alcohols.

Results and Discussion
Metal oxide (MO x )electrodes present asuitable platform for catalyst immobilization as they offer affinity for avariety of anchoring units,and the possibility to morphologically tune the surface to enhance the loading of molecular components. [18,19] Metal oxides can exhibit different electronic properties,a sd emonstrated by the metallic behavior of ITO and the semiconductive properties of TiO 2 ,t hus offering av ersatile electroactive platform to combine with surfaceanchored molecular catalysts. [20,21] Several mechanistic studies have highlighted the effect of pH on the TEMPO catalytic cycle,w ith enhanced oxidation rates observed under more basic conditions. [22][23][24] This stringent criterion implies that some of the more commonly used anchoring groups compatible with MO x scaffolds,s uch as carboxylic acids and phosphonic acids (pH stability < 4a nd 7, respectively), [25] may not be suitable for TEMPO immobilization on an ITO electrode.W et herefore designed STEMPO,w hich contains ac aged silatrane unit to improve binding to the MO x .T he silatrane moiety can hydrolyze on the MO x surface to form strong siloxane bonds,w hich provide an increased anchor stability under more alkaline conditions (Figure 1). [26,27] STEMPO was synthesized in good yield by reacting the acyl chloride of 4-carboxy-TEMPO with 3-aminopropylsilatrane.F ull synthetic and characterization details (highresolution mass spectrometry,i nfrared spectroscopy (Figure S1) and elemental analysis) are provided in the Supporting Information.
The mesoITO j STEMPO anode was assembled by incubating the mesoITOe lectrode (film thickness % 4.5 mm, Figure S2) in as olvent bath mixture containing STEMPO, and heating the solution to 70 8 8Cu nder aN 2 atmosphere for 6h.Multi-scan cyclic voltammetry (CV) measurements were used to deduce the optimal mixture,i nw hich the surface loading of STEMPO (G STEMPO )onthe mesoITOscaffold was both maximal and stable,w ith G STEMPO being determined by integrating the charge passed in the oxidation wave of the consecutive cyclic voltammograms (see Supporting Information, Equation (S1)). Thebest mixture consisted of a STEM-PO solution (2 mm)w ith 2%(v/v) acetic acid (AcOH) and 1%(v/v) H 2 Oi na cetonitrile (MeCN). With regards to the stability of the immobilized STEMPO compound, MeCN was the most suitable solvent from those attempted ( Figure S3). Thecombination of AcOH and H 2 Ofacilitated the hydrolysis of the silatrane cage on the mesoITOs urface, [28] and was deemed necessary for instigating the anchoring process ( Figure S4 and S5). Optimal G STEMPO was typically found to be 20-25 nmol cm À2 ,w hich is in the expected range for nanostructured ITOs urfaces. [29,30] X-ray photoelectron spectroscopy (XPS) showed binding signals in the Si 2p and N 1s regions ( Figure 2a and Figure S6, respectively), where the Si 2p signal agrees with XPS reference spectra for the siloxane-bearing group. [31]  Multiple CV scans of the mesoITO j STEMPO electrode reveal ar eversible redox wave at E 1/2 = 0.83 Vv s. NHE (Figure 2b), which corresponds to the nitroxide/oxoammonium species,a nd is only slightly more positive than that of dissolved TEMPO (E 1/2 = 0.74 Vv s. NHE, Figure S7a). At low scan rate (10 mV s À1 ), the peak-to-peak separation is below 20 mV and is thus in good agreement with the ideal value of 0mVfor areversible response of asurface-adsorbed species ( Figure S7b). Thef ull width at half-maximum is 116 mV ( Figure S7b), only slightly broader than the theoretically predicted value of around 91 mV for a1e À process (at 25 8 8C). [32] This slight deviation from ideal behavior can be attributed to multilayer formation, [33,34] stemming from the cross-polymerization of Si-O-Si bonds between adjacent anchoring units in the mesoporous scaffold and film resistance of the mesoITOe lectrode.
Adeeper analysis of the electron-transfer dynamics of the mesoITO j STEMPO system was inferred using the Laviron method, [35] which relies on the change in the peak potential (DE p )w ith scan rate (n). Ther esulting trumpet plot for the mesoITO j STEMPO assembly is portrayed in Figure 2c.T he intercepts of the linear regions of the plot can be used to deduce the critical scan rate (n c )a nd the apparent electron transfer rate constant (k app )f or the system (see Supporting Information for further details). Va lues for n c and k app were determined to be equal to 72 AE 2mVs À1 and 0.68 AE 0.02 s À1 , respectively.T he rate of electron transfer appears to be low (hence the low value for n c ), but is comparable with other covalently linked redox species in the literature. [36] Figure S8 depicts the CV scans measured over ar ange of scan rates to highlight the change in the peak-to-peak separation for the STEMPO redox wave as the applied scan rate exceeds n c .The linear relationship between the peak current (i p )a nd n,f or n < n c (Figure 2d), is characteristic for asurface-immobilized redox entity. [32] Thep Hs tability of the mesoITO j STEMPO assembly was investigated using am ulti-scan CV approach, whereby the electrode was subjected to several redox cycles in solutions of differing pH (Figure 2e and Figure S9). Ag ood stability was obtained after 200 scans at pH 7and 8(decrease in signal intensity of 34 %a nd 39 %, respectively,r elative to scan 1), and the decay curve only began to be more severe at pH 10. These results support that the assembly is suited to operate under the basic conditions required for enhanced TEMPO catalysis.
Immobilization and direct wiring of STEMPO to the mesoITOe lectrode was confirmed by film electrochemical electron paramagnetic resonance (FE-EPR) spectroscopy (see Supporting Information). [37] Thec ombined FE-EPR spectroelectrochemical technique allows for the appearance and disappearance of paramagnetic species to be monitored as af unction of the applied potential in the absence of any mediators.The high electrical conductivity combined with the . c) Trumpet plot deduced from the variable scan rate CV measurements;conditions:pH8aq. HCO 3 n plot, for n < n c . e) Stability curves as afunction of pH (data fitted to amono-exponential decay), formulated by tracing the change in G STEMPO (obtained through integration of the oxidation wave in the CV) over scan number in the multi-scan CV experiment. f) FE-EPR potentiometric titration of C-mesoITO j STEMPO.Peak area of the STEMPO EPR signal as afunction of potential (colored dots), fitted to 1e À Nernst equation (solid line). Inset:X-band (9.5 GHz) EPR spectra of STEMPO at different applied potentials. Measurements performed at 100 K, 2mWmicrowave power,100 kHz modulation frequency and 2G modulation amplitude.
surface-modification properties of ITOm ake it as uitable platform for carrying out FE-EPR spectroscopy.C arbonbased electrodes tend to give rise to large radical signals and are thus unsuitable for such studies. [37] ForF E-EPR spectroscopy,c ylindrical mesoITO (C-mesoITO) electrodes were employed for use in the EPR spectroelectrochemical cell. Theu npaired electron in the TEMPO moiety is delocalized around the Na nd Oa toms with nuclear spins (I)o f1and 0, respectively,a nd thus only couples with Nn uclei. This interaction gives rise to at riplet pattern in which the peaks,for the case of adiffusional species tumbling rapidly in solution at room temperature,are all the same intensity (EPR spectrum for diffusional TEMPO presented in Figure S10a, black trace). At riplet pattern is also discernible for the C-mesoITO j STEMPO assembly,but the peak intensities are distorted in this case ( Figure S10a, red trace). This change in line-shape of the EPR spectrum relative to the diffusional case arises from as lower tumbling rate which can be ac onsequence of the impaired mobility of the TEMPO moiety upon STEMPO immobilization. [38] Figure 2f highlights the results from the FE-EPR investigation. C-mesoITO j STEMPO samples were poised at aparticular potential, using athree-electrode setup,and then flash-frozen to allow for low-temperature EPR characterization. Examples of EPR spectra, at three different potentials,a re presented in Figure 2f (inset) (full range in Figure S10b), where an increase in the applied bias is accompanied by ad rop in signal intensity,t hat eventually vanishes due to the oxidation of the radical to EPR-silent STEMPO + . Thes hape of the EPR spectra for E < 1.0 Vv s. NHE are typical of nitroxide radicals measured at low temperatures (100 K). [39] Thenormalized signal area of each individual EPR spectrum was plotted as af unction of the potential, and is ac lose fit to the anticipated 1e À Nernst equation (solid line, Figure 2f).
Next, we investigated the catalytic performance of the mesoITO j STEMPO assembly. Figure 3adepicts the catalytic behavior of the system as afunction of the solution pH, where 4-methylbenzyl alcohol (MBA) was chosen as am odel substrate.T he current density increases with increasing pH, accompanied by al ower onset potential for catalysis (from 0.75 Va tp H7.3, to 0.68 Va tp H10, vs.N HE), which is comparable to previous reports for immobilized TEMPO on carbon-based electrodes. [40,41] This observation is in-line with the established TEMPO-mediated oxidation mechanism, whereby alcohol deprotonation leads to formation of ap reoxidation complex via nucleophilic attack of the alkoxide on the electrophilic nitrogen of the oxidized TEMPO moiety (the oxoammonium cation), prior to aldehyde formation. [24,[41][42][43] However,t he enhancement starts to plateau between pH 9a nd pH 10, contrary to what is observed for TEMPO, and related nitroxyl derivatives,i ns olution. [43] Thep lateau shown in Figure 3a for the mesoITO j STEMPO system could be due to ac ombination of factors,a nd we rationalize this behavior to stem from the relatively slow electron transfer between the ITO electrode and immobilized STEMPO,a s well as from mass transport limitations of the substrate in the mesoporous film.
Controlled potential electrolysis (CPE) was then conducted at an applied potential (E app )of1.0 Vvs. NHE at room temperature,t of urther probe the effect of pH on the mesoITO j STEMPO system. Figure 2e shows that the stability of the anodic,molecular assembly is high at pH 7and 8, but less so at pH 10. However, the TEMPO-mediated catalysis, and hence reaction kinetics,are favored under more alkaline conditions (Figure 3a). To compare the overall mesoITO j STEMPO performance as af unction of pH, the TONa nd FE (Supporting Information, Equations (S4) and (S5), respectively) were calculated after a3hC PE experiment with MBA( 30 mm)a st he substrate,a tf our different pH values (Figure 3b,F igure S11). Them oles of product, 4methylbenzaldehyde (n MBAd ), originating from selective MBA oxidation, were quantified by high performance liquid chromatography (HPLC) (Supporting Information).
TheTON for STEMPO experiences amaximum at pH 8, reaching av alue close to 3000, highlighting the fine balance between immobilization stability and catalytic activity in long-term electrolysis experiments.O ne ither side of the maximum, there is ac orresponding decrease in the TON. At lower pH, this can be attributed to al ower rate of substrate oxidation thereby resulting in less n MBAd ,w hereas higher pH adversely affects the stability of the mesoITO j STEMPO assembly,likely leading to aloss of the catalytic sites from the electrode over reaction time.P ost-CPE (at pH 8) XPS conducted on the mesoITO j STEMPO electrode reveals peaks in the Si 2p and N 1s regions ( Figure S12), similar to those observed on af reshly assembled electrode (Figure 2a and Figure S6), and thus indicates that the gradual drop in activity could be primarily due to hydrolysis of the amide bond and subsequent loss of the TEMPO moiety from the assembly.Onthe other hand, the FE is invariant with the pH (average of 86 AE 3% as calculated across the pH range, Figure 3b), implying that the charge passed at the electrode j catalyst interface is utilized in the same,s elective manner (being directed towards substrate oxidation) throughout the pH range.
Thev ersatility of the hybrid electrode was demonstrated by extending the substrate scope to glycerol, cellulose-derived hydroxymethylfurfural (HMF), and the lignin model compound 2-phenoxy-1-phenylethanol (PP-ol;T able S2). [44] A turnover frequency (TOF) analysis based on the sigmoidal catalytic response of the CV trace was performed for the STEMPO system in the presence of the different substrates (Supporting Information). [45] Figures 3c and 3d depict concentration profiles obtained for glycerol and HMF,r espectively,a nd the concentration profile for MBAi ss hown in Figure S13a (corresponding "maximum current density vs. concentration" plots for these three substrates are presented in Figure S13b-d). PP-ol was poorly soluble in pure aqueous electrolyte,a nd thus aC Vt race for this compound was recorded in aM eCN-water mixture ( Figure S14). The estimated TOFs for the four compounds,a nd the relevant experimental conditions,a re summarized in Table S2. The results show that the mesoITO j STEMPO system can be used to oxidize av ariety of alcohol-based substrates,w ith the primary benzylic alcohols MBAa nd HMF showing the highest activity (TOF = 0.677 and 0.680 s À1 ,r espectively),

Angewandte Chemie
Research Articles followed by the aliphatic triol, glycerol (0.557 s À1 ). Theresults from this analysis therefore encourage the use of low-cost and abundant alcohols such as glycerol for electrocatalytic applications with the mesoITO j STEMPO electrode.P P-ol gave the lowest TOF( 0.268 s À1 ), which agrees with the expected trend that primary alcohols are oxidized more rapidly than secondary alcohols by TEMPO in basic solution. [42] Having characterized the anodic assembly and demonstrated the electrocatalytic compatibility of mesoITO j STEMPO with av ariety of biomass representative alcohols, we turned towards applying this system within ac oupled electrolyzer. Conversion of CO 2 -to-syngas as the cathodic half-reaction presents an attractive strategy to utilize the electrons from alcohol oxidation by mesoITO j STEMPO.T o facilitate ac ost-efficient redox cycle,u se of robust, earthabundant catalysts for selective CO 2 Ri se ssential. While many 3d transition metal-based molecular catalysts have been developed over the years, [46] CoPc has emerged as one of the most promising catalysts for CO 2 -to-CO reduction because of its enhanced performance upon immobilization onto polymers and carbon-based electrodes. [47][48][49] In the coupled electrolyzer, we employed aC NT-CoPPc hybrid, fabricated by in situ polymerization, that was subsequently deposited on CP. [17] TheC P j CNT-CoPPc cathode catalyzes the electrochemical reduction of CO 2 to syngas,w ith aC O:H 2 ratio dependent on the applied potential. [17,50] TheC Vt race recorded for CP j CNT-CoPPc under N 2 displays ab road quasi-reversible redox process (Figure 4a, E 1/2 %À0.71 Vv s. NHE), which corresponds to the metalcentered Co II /Co I reduction of CoPPc.T he surface concentration of electroactive cobalt centers was estimated to be 18.3 AE 1.6 nmol cm À2 from integration of the Co I /Co II oxidation wave ( Figure S15). This corresponds to 5.6 AE 0.5 %cobalt sites being electrochemically accessible,w hereby the total amount of Co was determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements (Supporting Information, Eq. (S6)).
Acatalytic onset from the CP j CNT-CoPPc electrode was observed in aC O 2 -saturated solution at ap otential close to À0.84 Vv s. NHE (Figure 4a). Electrocatalytic performance of the cathode was probed by stepped constant potential chronoamperometry in the range of À0.70 to À1.00 Vv s. NHE, with 50 mV increments and 30 min steps ( Figure S16). Product formation was monitored via ac ontinuous flow gas chromatography (GC) method (Supporting Information). H 2 was the only product until À0.80 Vv s. NHE and CO evolution started at more negative potentials (%À0.85 Vv s. NHE). Theselectivity of the electrode towards CO increases sharply at more negative potentials,reaching 76 %atÀ1.00 V vs.N HE (overpotential, h = 0.46 V, where E(CO 2 /CO) = À0.54 Vv s. NHE at pH 7.3). [51] Within the same potential range,t he blank CNT electrode did not generate any H 2 or CO ( Figure S16a, purple trace).
To elucidate the working principle of the coupled mesoITO j STEMPO-CP j CNT-CoPPc electrolyzer,i nitial experiments were conducted using MBA. Ac atalytic wave for the mesoITO j STEMPO assembly in the presence of MBA( 30 mm)w as observed, which appeared to plateau at around 3mAcm À2 ,a ta na pplied potential just above 1Vvs. NHE (Figure 4b). The mesoITO j STEMPO electrode displayed slightly lower current densities than CP j CNT-CoPPc and was therefore selected as the working electrode (WE) in the coupled electrolyzer,while the cathode assumed the role of the counter electrode (CE). At wo-compartment electrochemical cell was employed with aS elemion-AMV anionexchange membrane to separate the compartments.AAg/ AgCl reference electrode (RE) was placed in the working compartment and the three-electrode configuration was adopted prior to studying at wo-electrode system, to be able to precisely control the E app at the WE versus ak nown reference (Supporting Information). This also allowed us to record the exact potential at the CE (E CE )during electrolysis against the same reference,t hus providing am ore detailed description of the cell parameters over reaction time.
ACO 2 -saturated carbonate buffer (0.5 m)was used in both compartments,w hich yielded as olution pH close to 7.3 that remained relatively constant throughout the experiment. Figure 4c depicts the results from the coupled electrolysis (three-electrode configuration), with E app = 1.0 Vvs. NHE at room temperature.A lcohol conversion to the corresponding aldehyde,MBAd, was quantified by HPLC,whereas CO and H 2 were quantified by ac ontinuous flow GC method (Supporting Information). Catalytic metrics obtained for the respective anode and cathode highlight the effectiveness of the combined system. MBAo xidation resulted in aT ON STEMPO of 1515 and FE close to 90 %a fter the 3h CPE experiment. TheT ON STEMPO was lower than expected from the TOFanalysis from CV scans (Table S2) due to the modest stability of the anodic assembly,a sd emonstrated by the multiple CV scan measurements and prolonged CPE (cf. Figure 2e and Figure 3b,r espectively). Ac obalt-based TON for syngas generation of 1360 (TON CO = 599 and TON H 2 = 761) and overall FEs for CO and H 2 of 35 %a nd 45 %, respectively,were achieved for the CP j CNT-CoPPc cathode.
This performance encouraged the substitution of MBA for glycerol, on account of its advantages as ap otential substrate for coupling with CO 2 Rinreal-life applications.A similar setup to that used for coupled MBAo xidation was employed, except in this case,t he anode compartment consisted of ac arbonate buffer (0.5 m)a tp H8.3 (under N 2 ), whereas the catholyte was comprised of aC O 2 saturated carbonate buffer (0.5 m)atpH7.3. This was deemed necessary for glycerol, as the STEMPO-mediated catalysis involving this substrate was observed to be too sluggish at the quasineutral pH of CO 2 -saturated carbonate buffer (i.e.p H7.3), but increased in activity under more alkaline conditions (as evidenced by the CVs recorded at pH 7.3 and 8.3, Figure S17). Figure 5a illustrates the reaction time plot obtained with glycerol as the substrate,w ith E app = 1.0 Vv s. NHE. HPLC analysis revealed that glyceraldehyde (GlyAd) was the primary anodic product from the coupled electrolysis experiment.
Thet wo compartments maintained their individual pH values for the duration of the electrolysis,a nd aT ON STEMPO and FE of 997 and 83 %, respectively,were measured for the anodic half-reaction. Although precautions were taken to minimize overoxidation or further reaction of GlyAd, trace amounts of some side-product can potentially form (not detected by HPLC), leading to the observed % 7%drop in the FE relative to the MBAe lectrolyzer. With regards to the cathode metrics,the cobalt-based TONwas determined to be equal to 894 (TON CO = 444 and TON H 2 = 450), while similar FEs for the gaseous products,r elative to the MBA-based electrolyzer, were measured (FE = 41 %for CO,41%for H 2 ). Aside-by-side comparison of the calculated FEs for the liquid and gaseous products over reaction time,f or the MBA-and glycerol-based electrolyzers,i sp rovided in the Supporting Information ( Figure S18).
Thethree-electrode configuration allowed for E CE (i.e.the potential at the CP j CNT-CoPPc electrode) to be monitored throughout the course of the electrolysis experiment. From the traces shown in Figure 5b,t here is an alteration in the CO:H 2 ratio at the cathode over time,which seems to reflect the change in E CE .T his decrease in the reducing potential at the cathode is itself aresult of the gradual decline in activity at the anode over time.T he change in the CO:H 2 ratio as afunction of the cathodic potential is in-line with the stepped chronoamperometric experiments carried out for the CP j CNT-CoPPc electrode (with Pt mesh as CE), as discussed above ( Figure S16). Thet ime-lag between the minima of the E CE trace and the maximum value of CO:H 2 ratio on Figure 5b is likely caused by the slow diffusion of CO from the porous cathode.
We furthered our investigation into coupled glycerol oxidation and CO 2 R, and performed as eries of experiments in amore practical two-electrode configuration, while varying the applied cell potential (V cell ). Va lues for V cell in the range of 1.8 to 2.1 Vw ere chosen, based on the rationale that: j E cathode ÀE anode j%j E À CE ÀE app j= 1.85 V, where E À CE is the average potential at the CE, over reaction time,asmeasured in the three-electrode configuration (i.e.F igure 5b). Figure 5c depicts the combined FE at the cathode (for CO and H 2 )and the CO:H 2 ratio,o ver reaction time,f or V cell = 2.0 V. Thet rends agree with those obtained for the three-electrode setup.T he increase in the maximum of the CO:H 2 ratio for the twoversus three-electrode configuration (shown in Figure 5b) could be ar esult of the increased driving force provided by the 2.0 Vp otential. This bias most likely leads to more reductive potentials at the cathode,a nd, in accordance with the stepped chronoamperometry data for CP j CNT-CoPPc ( Figure S16), would translate to ahigher CO:H 2 ratio.
Finally,w ec alculated the cell energy efficiency (e), as afunction of V cell using Equation (1): [12] e where E H þ =H 2 , E CO 2 =CO ,a nd E GlyAd/glycerol denote the reduction potentials for H + ,C O 2 ,a nd glyceraldehyde,r espectively, under non-standard conditions (Table S3). Am ore detailed breakdown regarding the thermodynamic analysis required to compute e is provided in the Supporting Information. Figure 5d illustrates the FEs for the anodic and cathodic processes,a long with the corresponding e calculations,f or different V cell values.There is aslight improvement in the CO selectivity upon increasing from 1.8 to 1.9 V( FE CO = 36 and 46 %, respectively), presumably aresult of the higher driving force at these applied voltages.This enhancement is met with an improvement in e (from 16 to 18 %), since the 100 mV additional bias is offset by the increase in FE CO ,a sg overned by Equation (1). However,f or V cell ! 2.0 V, the combined effects of al argely unchanged CO:H 2 ratio and anodic FE, causes ac orresponding drop in the cell efficiencyt o% 16 %, similar to that obtained for V cell = 1.8 V. Thec ell energy efficiency values measured for our hybrid electrolyzer are in accordance with those reported in the literature,w here for example an efficiency of 17 %, at 1.8 Vc ell potential, was measured for ad ual electrolyzer featuring benzyl alcohol oxidation coupled with the reduction of aqueous CO 2 to CO and H 2 . [9] However,t he previously reported system was comprised of Ru-based molecular catalysts for the reductive and oxidative half-reactions,and additionally,only one of the catalysts was immobilized. In contrast, we have incorporated immobilized cathodic and anodic catalysts in our electrolyzer, both free of any precious metals,and have also demonstrated the applicability of the tandem AlcOx-CO 2 Rdevice to couple the oxidation of more commercially viable substrates,such as glycerol, with CO 2 -to-syngas conversion.

Conclusion
We have designed, fabricated and characterized an anode featuring as ilatrane-modified TEMPO molecule on a meso-ITOscaffold, and demonstrated the electrocatalytic ability of the molecularly engineered MO x system to efficiently oxidize av ariety of biomass representative substrates.T he siloxane anchor, formed upon hydrolysis of the silatrane cage on the MO x surface,displays robust binding.T he catalytically active site (i.e.the oxoammonium cation) is both stable and readily regenerated under electrocatalytic conditions, [52] and we believe that the long-term stability of the hybrid electrode assembly is currently limited by the amide bond in STEMPO. Improvements to the molecular design of the linker employed for STEMPO will provide ap ossibility to enhance the stability and overall activity of the anodic assembly.
We further showed the advantage and versatility of our novel STEMPO anode by coupling alcohol oxidation with an efficient CO 2 Rc athode (CP j CNT-CoPPc), to construct an AlcOx-CO 2 Re lectrolyzer based on immobilized, preciousmetal-free, molecular catalysts.T he functionality and performance of the device was investigated using at hreeelectrode configuration, first employing MBAa samodel substrate,a nd later, using the commercially applicable substrate,glycerol. It was found that in both cases,stoichiometric amounts of as elective oxidation product (the corresponding aldehyde) and syngas were generated at the anode and cathode,r espectively.F Es were typically excellent for the hybrid system, exceeding 80 %f or both anode and cathode. TONs were also high, approaching 1000 for mesoITO j STEMPO and 900 for CP j CNT-CoPPc (with glycerol as substrate). TheT ON of the cathode in the electrolyzer is currently limited by the prolonged stability issue of the anodic assembly during continuous CPE experiments and the CoPPc-cathode on its own is known to maintain activity over alonger time-period. [17] Further studies were then made using ademonstrator-type,two-electrode setup for coupled glycerol oxidation at the anode and syngas generation at the cathode, showing similar performance metrics as the three-electrode system. Cell energy efficiencyc alculations also revealed the advantages of operating at al ower V cell ,w ith am aximum efficiency of 18 %being measured at acell potential of 1.9 V. This molecular hybrid system is therefore asuitable model for the development of future AlcOx-CO 2 Re lectrolyzers based on earth-abundant materials,w hich can provide chemical feedstocks (aldehydes and syngas) from sustainable and abundant resources,s uch as biomass-derived alcohols,C O 2 , and renewable electricity.
Qian Wang and Dr. Sam Cobb for helpful discussions,and the Centre for Advanced ESR (University of Oxford) for EPR measurement time.