Microbial electrolysis cell (MEC)

The MEC system equipped with three different polymeric membranes and pre-mature biofilms were operated for 18 days (12 days with acetate, 6 days with a mixture of volatile fatty acid (VFA)) as displayed by the progress curves in Figure 28. The average peak current densities were similar throughout the measurements regardless the used membrane separator, such as 7.2 A m^-2 (PSEBS SU22¹), 6.8 A m^-2 (CF22 R14, Mega, Czech Republic) and 7.4 A m^-2 (Fumatech, Germany (FBM)), using the acetate substrate for eleven successive feeding cycles.

Figure 28 Chronoamperometric progress curves obtained in MEC with the three different membranes and two different substrates

Whilst the H2 yields (YH2) were comparable, for mean volumetric H2 productivities (Q) were observed more variations from in regard to one system to another, as follows:

1010 (PSEBS SU22), 1158 (CF22 R14) and 1527 mL H2 L^-1 d^-1 (FBM), (Figure 29).

1 For the purpose of this publication the membrane PSEBS SU was coded as PSEBS SU22

Figure 29 Hydrogen productivities and yields as a function of the membrane and used feedstock According to the results in Figure 30, the r^H2 and ηE depending on the actual membrane, were in the range of 63–70 % and 58–108 %, respectively. When it was switched to the VFA mixture substrate for several feeding cycles (Figure 28), the following average peak of current densities were recorded: 9 A m^-2 (PSEBS SU22), 7.6 A m^-2 (CF22 R14) and 5.8 A m^-2 (FBM). With respect to the used membrane, as indicated in Figure 29, the corresponding Q and Y^H2 values were: 1521 mL H2 L^-1 d^-1 and 1164 mL H2 g^-1 COD (PSEBS SU22), 1310 mL H2 L^-1 d^-1 and 678 mL H2 g^-1 COD (CF22 R14), 1152 mL H2 L^-1 d^-1 and 1076 mL H2 g^-1 COD (FBM). In addition to the r^H2 and η^E, for particular membrane, reached values of 82% and 110% (PSEBS SU22), 48 % and 102 % (CF22 R14), 76 % and 64 % (FBM) (Figure 30).

Figure 30 Overall H² recovery and net energy recovery of MEC with various substrates in case of various membrane separators

Overall, based on the various technology-oriented measures of efficiency, MEC’s behavior was quite different, and a further assessment is required to say which of these three membranes should be chosen for continuous operation with a particular substrate (acetate vs. a mixture of VFA). For that purpose, is presented a complex ranking approach in 4.2.3.1, which can clarify whether one membrane is more suitable than the another.

4.2.3.1 Ranking of membrane materials used in MEC

A ranking was imposed using the EXPROM-2 method, where the criteria were selected as cumulative-type MEC parameters to choose the most promising alternative membrane materials for MEC. Firstly, rH2 represents the efficiency of COD-to-H2 conversion process. Secondly, as the indicator of the energy balance (or energy efficiency), ηE was considered. Finally, Q was added to the list of criteria to assess a reaction rate-associated variability. These parameters were analyzed in terms of substrate and membrane. Furthermore, another (anion exchange membrane)

materials and their relevant data (obtained with the identical MEC reactors, electrodes, experimental conditions, and operating strategy) were taken over from previous work (57) for diversification purposes and to ensure a more complex comparison. Accordance with above mentioned the decision matrix (and its normalized form) in Table 13 was created.

Table 13: Initial and normalized decision matrix for material ranking Initial

Membrane type

Acetate VFA mixture

rH2 (%) ηE (%) Q (mL HL^-1cat d^-1) ² rH2 ηE Q (mL H2

L^-1cat d^-1)

PSEBS SU22 69 108 1010 82 110 1521

CF22 R14 63 103 1158 48 102 1310

FMB 70 58 1527 76 64 1152

AMI-7001 60 76.4 1422 67 73 1375

AF49 R27 55 107 1135 65 112 1461

PSEBS-CM-DBC 79 105 1157 64 99 956

Normalized membrane type

Acetate VFA mixture

rH2 (%) ηE (%) Q (mL HL^-1cat d^-1) ² rH2 ηE Q (mL H2

L^-1cat d^-1)

PSEBS SU22 0.5833 1 0 1 0.9583 1

CF22 R14 0.333 0.9 0.2863 0 0.7917 0.6265

FMB 0.6250 0 1 0.8235 0 0.3469

AMI-7001 0.2083 0.368 0.7969 0.5588 0.1875 0.7416

AF49 R27 0 0.98 0.2418 0.5 1 0.8938

PSEBS-CM- DBC 1 0.94 0.2843 0.4706 0.7292 0

The relative weights of the criteria (determined by entropy method) were: w(rH2)Ac = 0.2139;

w(ηE)Ac = 0.5455; w(Q)Ac = 0.2406; w(rH2)VFA = 0.2484; w(ηE)VFA = 0.4193; w(Q)VFA = 0.3322.

The EXPROM-2 method uses preferential indices, exit-, entry- and net revaulation flows (designated as φ⁺, φ^- and φ). The outranking flows, as well as the ranking of materials, are given in Table 14.

Table 14: Outranking flows and ranking of membrane materials for various substrates Membrane type Acetate

φ⁺ (i) φ^- (i) φ (i) Rank

PSEBS SU22 0.481 0.290 0.190 2.

CF22 R14 0.384 0.269 0.114 3.

FMB 0.44 0.877 -0.433 6.

AMI-7001* 0.327 0.669 -0.343 5.

AF49 R27* 0.374 0.374 0.000 4.

PSEBS-CM-DBC* 0.611 0.140 0.471 1.

Membrane type VFA

φ⁺ (i) φ^- (i) φ (i) Rank

PSEBS SU22 0.841 0.007 0.834 1.

CF22 R14 0.365 0.497 -0.131 3.

FMB 0.221 0.776 -0.555 6.

AMI-7001* 0.268 0.575 -0.307 4.

AF49 R27* 0.651 0.102 0.549 2.

PSEBS-CM-DBC* 0.260 0.649 -0.389 5.

* Membranes and related data were taken from (57).

It should be noted that in the case of acetate, the PSEBS SU22 achieved the a high rating and only an AEM from previous study (57), the PSEBS-CM-DBC – synthetized with the same polymer backbone – was rated higher. The CF22 R14 membrane ranked 3rd, while the FBM proving to be the least attractive choice. With the VFA substrate, the membrane material PSEBS SU22 was the best, followed by a heterogeneous AEM (AF49 R27). The CF22 R14 qualified for 3rd place, while the FBM finished at the end of the line again. As far as FBM is concerned, its comparatively less favorable contribution to the operation of MEC could be attributed to a lower energy yield (the relative statistical weight of energy yield as a criterion was the highest due to a stronger dispersion of data). Thus, even if the FBM MEC showed sufficient hydrogen production (e.g. in terms of hydrogen yield and hydrogen productivity), the high energy input to overcome cathodic losses makes this material the least preferred among the six membranes tested (2 CEM, 1 FBM, 3 AEM). Nevertheless, it is worth to mention that other studies under specific conditions, such as a strongly acidic cathode environment, found the bipolar membrane as a suitable membrane candidate for MEC (58). The evaluation also showed that for both acetate and VFA mixture, the use of PSEBS SU22 in MEC can be proposed as an acceptable alternative. In addition, it would be interesting to address why PSEBS-CM-DBC showed such a different evaluation for two substrates in a follow-up study. Since PSEBS-CM-DBC is an AEM, the

passage of VFA (especially their deprotonation, and thus negatively charged form) may be much more significant than that of CEMs (59). It can be assumed that either the higher mass transfer of propionate and butyrate – and the associated consequences – is the source of the lower score in the case of VFA mixture, or the mass transfer of the VFA, in general, has increased more and more during the period of the operation due to change in membrane stability. Both assumptions are worth to study in joint mass transfer and stability measurements.

4.2.3.2 Ion transport and potential losses

The output of MEC is a function of, among others, the composition of the electrolytes and the transfer of ions between the anolyte and catholyte that occurs through the applied membrane (60). Therefore, the evaluation of the membrane performance in terms of ion transfer can be of great importance. In this study, both CEMs exhibited similar behaviors, i.e. decrease in pH was observed in the anolyte due to the insufficient proton transport rate through the CEMs (the acidic pH of anolyte together with the decreasing conductivity (Table 15) shows the transport of mainly other cations, such as Na⁺, K⁺, Ca²⁺, Mg²⁺, NH⁴⁺ over H⁺), citation (61), (62).

Table 15: pH and conductivity obtained in the MEC with different membranes Membrane

Anolyte Catholyte

pH initial pH final σinitial (mS

cm^-1) σfinal

(mS cm^-1) pH initi al

pH final σinitial

(mS cm^-1) σfinal

(mS cm^-1) PSEBS SU22

Acetate 8.0 5.9 ± 0.3 7.8 ± 0.1 5.7 ± 0.2 6.0 12.8 ± 0.1 12.2 ± 0.3 19.5 ± 0.9 PSEBS SU

Ace/Prop/But 8.0 6.3 ± 0.1 7.8 ± 0.3 5.4 ± 0.1 6.0 12.8 ± 0.2 12.3 ± 0.1 18.3 ± 0.3 CF22 R14

Acetate 8.0 6.1 ± 0.2 7.8 ± 0.1 5.7 ± 0.4 6.0 13.0 ± 0.1 12.3 ± 0.3 17.3 ± 1.0 CF R14

Ace/Prop/But 8.0 6.0 ± 0.1 7.8 ± 0.3 5.3 ± 0.5 6.0 13.0 ± 0.1 12.2 ± 0.4 18.9 ± 0.7 FMB Acetate 8.0 7.1 ± 0.1 7.8 ± 0.1 7.5 ± 0.2 6.0 12.3 ± 0.1 12.2 ± 0.1 13.3 ± 0.3 FMB Ace/Prop/But 8.0 7.1 ± 0.1 7.8 ± 0.3 7.5 ± 0.1 6.0 12.4 ± 0.1 12.1 ± 0.2 14.0 ± 0.4

The final pH of the catholyte for both CEMs reached 12.8–13.0 due to the enrichment of OH^- ions in the cathode chamber. The catholyte conductivities in CEM-equipped MEC showed increasing trends over the batches (Table 15), especially in the case of PSEBS SU22-MEC (e.g., catholyte conductivities of 19.5 ± 0.9 mS cm^-1 and 18.3 ± 0.3 mS cm^-1 were observed with acetate and VFA mixture, respectively). Table 16 shows that the pH imbalance was similar for

PSEBS SU22 (ΔpH = 6.7) and CF22 R14 (ΔpH = 6.9). However, the use of the FBM membrane in MEC resulted in lower pH-imbalance (ΔpH) than in case of CEMs. The pH of anolyte decreased by 0.9 unit, while the final pH of catholyte at the end of batches was ~ 12.35 for both substrates tested (ΔpHAc = 5.2 and ΔpHVFA = 5.3). The anolyte and catholyte’s conductivities ranged between 7.8 and 7.5 and 12.25–13.5 mS cm^-1, respectively (Table 15). As a result of pH division and electrolyte conductivity shifts, various potential losses occur (63). In the case of acetate, the potential loss due to pH-splitting (EΔpH) was the same for PSEBS SU22-MEC and CF22 R14-MEC (414 mV), whilst it was significantly lower (315 mV) in the case of FBM-MEC (Table 16). In terms of VFA feeding period, the FBM-MEC showed a comparable EΔpH (318 mV), while it decreased in PSEBS SU22-MEC (392 mV) and increased in CF22 R14- MEC (420 mV), with respect to the operation only acetate was applied. The electrolyte resistances determine the ionic loss (Eionic) was lower in the MEC assembled with PSEBS SU22 and CF22 R14 in case of the acetate substrate (93 mV and 88 mV compared to 101 mV for FBM-MEC), the change of the substrate was accompanied by the lowest Eionic, obtained for FBM-MEC (77 mV). Cathode surplus potentials for FBM-MEC were significantly higher in the (-2.0 V irrespectively of the substrate), almost twice as high as those obtained for CEMs (Table 16).

Table 16: List of pH imbalance- and ionic-related losses, cathodic overpotentials and open circuit potentials

PSEBS SU 22 CF22 R14 FMB

EΔpH/mV, acetate 414 ± 21 414 ± 19 315 ± 6

EΔpH/mV, Ace/Prop/But 392 ± 14 420 ± 6 318 ± 10

Eionic/mV, acetate 93 ± 16 88 ± 21 101 ± 9

Eionic/mV, Ace/Prop/But 128 ± 22 109 ± 17 77 ± 11

Ecat/mV, acetate -946 ± 58 -1184 ± 29 -2095 ± 29

Ecat/mV, Ace/Prop/But -904 ± 133 -1243 ± 44 -1958 ± 41

EOCP/mV, acetate -237 ± 18 -232 ± 12 -457 ± 14

E^OCP/mV, Ace/Prop/But -228 ± 26 -235 ± 18 -428 ± 18

In addition, PSEBS SU22-MEC induced the lowest Ecat in VFA mixture (- 904 mV), which is highly beneficial in terms of application. It is clear from the results of this study that the potential losses attributed to pH-splitting contributed to the total losses more significantly than ionic losses. However, among these factors, further transport and Ohmic losses may occur. The high cathode overpotential in the case of FBM-MEC is an applicable example: the reduced pH- imbalance as result of usage of bipolar membrane (and the associated migration of protons and hydroxide ions of water splitting at the transition region of FBM) does not compensate for the additional losses resulting from its functional and ion transport character (64), (58). As suggested in the literature describing bipolar membrane MEC, unconventional cathodic conditions (e.g., initial acidic pH) should be maintained to utilize the advantages of bipolar membranes and minimize pH-imbalance (58).

According to the reported results have been proven that membrane PSEBS-SU is reasonable choice for operating in MEC with regard to estimation of ion transport and potential losses together with high yield production of hydrogen.