1

First insights on the electrochemical valorisation of black liquor R. Oliveira,* M. Mateus, D.M.F. Santos**

Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal

*E-mail: raisa.oliveira@tecnico.ulisboa.pt

**E-mail: diogosantos@tecnico.ulisboa.pt

Abstract Weak black liquor is a pulp mill effluent from wood cooking and has a solid content of 15-18 wt.% that

is mostly lignin, an organic compound that finds wide applications in the market. Black liquor is currently

used for steam generation purposes, by burning it, which is not an efficient process due to the

undifferentiated combustion of the liquor, losing most of the lignin potential. Thus, developing a process

for the lignin recovery from black liquor would be an enormous breakthrough. Black liquor electrolysis is

an option with environmental and economic advantages, as it simultaneously generates a clean fuel

(hydrogen) and a precipitated material with economic value (lignin). The present work focuses a study

on the electrolysis of black liquor from Portucel, S.A. This effluent was characterised by determining its

dry solids content, organic to inorganic ratio, conductivity, pH, total lignin content (by thermogravimetric

analysis). Furthermore, platinum (Pt), nickel (Ni) and AISI 304 stainless steel bulk electrodes are tested

for black liquor electrolysis. Cyclic voltammetry, chronopotentiometry and chronoamperometry are used

to study the lignin anodic oxidation at these electrodes, at 25 ºC. Kinetic parameters are calculated. The

hydrogen evolution reaction in the black liquor is also evaluated, by linear sweep voltammetry, at 25 ºC.

A small-scale laboratory black liquor electrolyser using Ni plates, both for anode and for cathode, was

assembled and its operation parameters are evaluated.

1. Introduction Black liquor is a byproduct of the papermaking process.

It consists of the remaining substances after the

digestive process where the cellulose fibres have been

cooked out from the wood [1]. The Kraft pulping process

is dominant in pulp and paper industry. This process

involves cooking the cellulosic raw material in a solution

of sodium hydroxide and sodium sulphide to obtain the

fibre suspension. In conventional processes the spent

black liquor is washed from the pulp and part of it is

treated to recover the cooking chemicals and

regenerate the cooking liquor [2], while the other part is

burned in a recovery boiler in order to generate power.

In the last case, the so-called weak black liquor, with a

solid content of 15 wt.%, is concentrated through a

series of evaporators. When the resulting strong black

liquor reaches the recovery unit (boiler or gasifier) it has

a solid content of around 75 wt.%, followed by

combustion in a recovery boiler [3].

Black liquor is comprised of a complex mixture of both

inorganic and organic constituents. The inorganic

constituents are derived from cooking liquor, being

mainly inorganic salts in a percentage between 15-25

wt.% of the solids. Organic compounds come from the

wood; they can be natural wood extractives that are

released as a result of the pulping process or materials

formed during the reaction of the pulping liquors with

lignin or cellulose components of wood. Typical

composition of Kraft liquor includes lignin derivate (39-

54 wt.%), cellulose derivate (25-35 wt.%) and extractive

derived (3-5 wt.%) [4].

With today’s increasing energy and chemical costs,

coupled with stringent environmental regulations, the

black liquor evaporation process is a critical economic

factor in Kraft pulp mill operation. About 90 % of the

lignin contained in the black liquor from Kraft process is

simply burned [5]. Still, lignin has a broad applicability in

the market, especially as a dispersant in inks, pesticides

and insecticides, as binding agent and as an additive for

soil breeding and conditioning. Therefore, Kraft lignin

may be used more in a more profitable way [6].

The emission of greenhouse gases have caused

serious climate change, leading to worldwide warning.

This has changed the awareness level of different

sectors, like energy, economy, industry and,

nevertheless, at the social and individual level [7]. Thus,

research has been done to obtain a clean fuel that can

replace existing fossil fuels, responsible for the vast

majority of atmospheric and environmental pollution.

This has led to a great interest on hydrogen economy.

2

A major advantage on the use of hydrogen as a fuel is

that it has a very high gross calorific value, about three

times higher than that of gasoline [8], and it causes

virtually no pollution when burned, as the product of

combustion is only water (Eq. 1) [7].

H2+ ½O2 → H2O (1)

Electrolysis of black liquor holds promise as an option

for hydrogen production. Initial experiments suggest

energy efficiencies that are competitive and in some

cases even better than those achieved in water

electrolysis [9,10]. Given these factors, we are currently

studying the electrolysis of black liquor in order to obtain

precipitation of lignin at the anode and simultaneous

generation of hydrogen at the cathode [9,10], as

described by (Eq. 2).

H2O + e- → ½H2 + OH- (2)

It is known that the presence of lignin in wastewater

streams significantly increases the chemical oxygen

demand (COD) and biological oxygen demand (BOD),

which are responsible for serious damages to the

environment and human health [5]. Thereby, after the

proposed black liquor electrolysis concept, and

consequent removal of the lignin content, the pulp mill

effluent only requires a much simpler and inexpensive

treatment, as there is previous removal of most of the

organic pollutants.

Herein, the electrolysis of weak black liquor from a

Portuguese pulp and paper company (Portucel, S.A.)

has been investigated. The black liquor contain lignin

exclusively from Eucalyptus Globulus, used in the Kraft

process. Both the organics oxidation and the hydrogen

evolution reaction have been studied in platinum (Pt),

nickel (Ni) and AISI 304 stainless steel electrodes, by

using cyclic voltammetry and chronopotentiometry

measurements. A laboratory black liquor electrolysis

cell has also been assembled and tested. The

preliminary results allowed determination of several

kinetic and diffusional parameters that enabled a first

insight on the underlining mechanisms that control black

liquor electrolysis.

2. Experimental 2.1 Characterisation of the samples

Samples of Kraft weak black liquor were obtained from

Eucalyptus Globulus pulp mill and its physical

properties were determined at room temperature. The

pH was determined using a Hanna pH20 pH meter.

Conductivity of the black liquor samples (σsample) was

determined using a Hanna Instruments HI8733

conductivity meter, after dilution of the samples with

Millipore water (1:10), measuring their conductivity

(σmixture) and then by applying Eq. 3, where σwater = 0.2

mS cm-1 [11].

σsample = (11σ

mixture- 10σ

water)

11 (3)

The dried solids content was determined by drying the

samples in a Nabertherm 2/11/R6 muffle at 120 °C, until

constant weight is achieved, after previous evaporation

of most of the water in a Heidolph VV200 rotary

evaporator. The solid ash content was obtained

according to typical determination of biomass ash

procedure [12].

The thermogravimetric analysis of the black liquor dried

solids was made in inert atmosphere. A 10.9 mg of the

sample was inserted in a Netzsch STA 409 PC

thermogravimetric balance and the heating was

performed at a rate of 10 ºC/min (from 25 to 1200 ºC).

The cooling process was performed by a JULABO 5

water bath. A NETZSCH Proteus Thermal Analysis

software was used for total control of the experiment.

2.2 Electrochemical measurements

Electrochemical measurements were performed in a

glass cell, using a sample volume of 125 mL. The cell

temperature was maintained at 25 °C. A PAR 273A

computer-controlled potentiostat/galvanostat (Princeton

Applied Research, Inc.) and the associated

PowerSUITE package were employed for total control

of the experiments and data acquisition. Cyclic

voltammetry (CV) and chronopotentiometry (CP)

experiments were carried out using a typical three

electrodes arrangement, comprising a platinum (Pt,

Metrohm 60305100, A = 1 cm2), a nickel (Ni, A = 0.79

cm2) or a AISI 304 stainless steel (AISI 304, A = 8 cm2)

working electrode (according to the experiment), a

saturated calomel electrode (SCE) as a reference

electrode (Metrohm 60701100), and a Pt mesh auxiliary

electrode (A = 50 cm2).

Between each electrochemical measurement, the black

liquor was magnetically stirred in order to homogenise

the solution and remove possible deposits on the

electrode surface.

CV was performed to obtain the full voltammogram,

between -1.5 V and 1 V, at a potential scan rate of 50

mV s-1. Then, to study the obtained oxidation peaks, the

potential was scanned in the anodic direction, from the

open circuit potential (OCP) to 0.5 V (except in stainless

steel electrode, wherein a higher potential range were

used), at different applied scan rates, ranging from 5 to

1000 mV s-1.

Currents ranging from 2.5 to 24 mA cm-2 were applied

in the CP measurements, with the electrode potential

moving from the OCP to a value where lignin oxidation

occurred. After completion of the organics oxidation, the

potential further increases until the following oxidation

process is possible, that is, the OH- oxidation to O2.

In CA, potentials between -0.5 V and 0.6 V were

applied, with the transient currents following a stepwise

change of the electrode potential from the OCP (no

oxidation) to a value where the lignin oxidation was

sufficiently fast so that its concentration at the electrode

surface was essentially zero, i.e. diffusion-controlled.

The hydrogen evolution reaction was also studied in the

black liquor by scanning the working electrode (either

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Pt, Ni or AISI 304) potential from the OCP to -1.4 V, at

a scan rate of 0.5 mV s-1.

A small-scale laboratory black liquor electrolyser (V =

250 mL) using Ni plates with an area of 19.2 cm2, both

for anode and cathode, was tested at 25 ºC by applying

cell potentials ranging from 0.5 to 2.5 V.

Furthermore, a more lengthy (190 min) experiment was

made in this electrolyser, at a ∆E = 2.2 V, with the aim

of obtain lignin.

3. Results and discussion 3.1 Chemical characterisation of the samples

The value obtained for the dried solids content was 16.7

%. The total lignin content in dried solids was

determined by thermogravimetric analysis (Figure 1).

Considering the results and that the degradation

temperature of lignin is 400 ºC [13], a value of 29% was

obtained.

The conductivity of the black liquor was 460 mS cm-1,

its density and pH was 1100 g cm-3 and 12.4

(respectively) and it had a organic/inorganic ratio of 1.0.

The values found for the dried solids content and the

organic/inorganic ratio of the weak black liquor are in

agreement with the results previously reported [14–16]. The density of black liquor sample (ρblack liquor) was

calculated using Eq. 4 [15],

ρblack liquor

= [Css

ρss

+ 1-Css

ρliq

]

-1

(4)

where Css is the dry solid content, ρss (1936 kg m-3) is

the density of the black liquor’s solids [15] and ρliq (1012

kg m-3) is the density of the black liquor’s liquid [15]. A value of 1100 kg m-3 was obtained for ρblack liquor, which

is in agreement previously reported values [14,15].

Knowing the density of the black liquor and the lignin

content in the black liquor dried solids, it is possible to

estimate the lignin concentration (C) in the effluent by

using Eq. 5,

C = Css×ρ

black liquor × 0.29

1000 × MMlignin

(5)

where MMlignin (1111 g mol-1) is the Kraft lignin molecular

weight [15]. A value of C = 4.8 x 10-2 mol cm-3 was

calculated by Eq. 5.

3.2 Electrochemical characterisation of lignin

oxidation

3.2.1. Cyclic Voltammetry

A typical shape of the full cyclic voltammograms (CVs)

obtained for the black liquor electrooxidation on the

platinum (Pt), nickel (Ni) and AISI 304 stainless steel

electrodes in natural diffusion conditions is shown in

Figure 2. It can be observed for Pt (Figure 2A) two well-

defined oxidation peaks, at potentials of -0.24 V and 0.1

V, with current densities of 6.0 mA cm-2 and 8.6 mA cm-

2, respectively. Regarding the Ni electrode (Figure 2B)

it is possible to observe a single peak, with lower

intensities (4.7 mA cm-2) when compared to Pt and at

slightly different potential than the second peak of

platinum (0.03 V). The AISI 304 electrode presents an

anodic activity at 0.5 V, with a current density of 4.3 mA

cm-2 (Figure 2C).

Reduction peaks were not observed in the reverse

scan, which indicates that the oxidation processes are

irreversible, in agreement with previous reports [10].

Figure 1. Thermogravimetric analysis of black liquor dried solids.

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Figure 2. (A) Pt, (B) Ni and (C) AISI 304 full CVs in black liquor,

obtained at 50 mV s-1 and 25 ºC.

Considering that the oxidation peaks are located at

potentials ranging from the open circuit potential (OCP)

and 0.5 V, the CV studies were limited to this anodic

zone. Only in the case of the stainless steel electrode,

the potential range between OCP and 1.5 V. Figure 3

shows the effect of the potential scan rate on the anodic

peaks obtained at Pt (Figure 3A), Ni (Figure 3B) and

AISI 304 (Figure 3C) working electrodes.

Clear differences regarding the potential and intensity of

the anodic peaks for the different potential scan rates

are shown in the Figure 3. In general, with the increase

of the scan rate, the peak potential is shifted to the right,

i.e., to more positive potentials, and peak current

density increases. It is also shown that Ni electrode

presents less intense peaks than Pt. In the case of the

AISI 304 electrode, it presents less intense peaks than

Ni. For Pt, the first and second peaks are located in the

potential range of -0.27 to -0.11 V and 0.02 to 0.27 V,

respectively. For the Ni and AISI 304 electrodes, its

peaks ranges from 0.02 to 0.35 V and from 0.32 to 1.50

V, respectively.

Figure 3. Effect of the potential scan rate on the anodic peaks

obtained with (A) Pt, (B) Ni and (C) AISI 304 working electrodes at 25

ºC.

The shift of the peak potentials with the increase of the

scan rate is typical of irreversible electrochemical

processes. Considering the observed peak potential

value for each potential scan rate, it is possible to

calculate the anodic charge transfer coefficient, α, by

applying Eq. 6,

Ep = E0+ [

RT

(1-α)naF] × {0,78+ ln

D

ks+ ln [

(1-α)naFν

RT]}

1/2

(6)

where Ep is the peak potential, R is the universal gas

constant (8.314 J K-1 mol-1), T is the temperature (K), na

is the number of electrons involved in the rate

determining step (being 1 the most likely value), F is the

Faraday constant (96485 C mol-1), D is the diffusion

coefficient of the species that will be oxidised (cm2 s-1),

ks is the standard heterogeneous rate constant (cm s-1)

and ν is the scan rate (V s-1). By rearranging Eq. 6, we

get a simplified expression (Eq. 7) that allows obtaining

α via the slopes of the Ep vs. ln ν plots (Figure 4),

Ep= [RT

2(1-α)naF] ln ν+b (7)

where b is a constant. Figure 4 shows the Ep vs. ln ν

plots for the peaks obtained with the different working

electrodes. From the slopes of the regressions present

in the Figure 4, and by application of Eq. 7, α values of

0.53 and 0.72 were obtained for the first and second

peaks of Pt electrode, respectively. The value for the

first peak of Pt is in agreement with the value of 0.5

previously reported by Ghatak et al [10]. Regarding the

Ni and AISI 304 electrodes, the values found for α were

0.73 and 0.94, respectively.

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Figure 4. Ep vs. ln ν plots for Pt, Ni and AISI 304 electrodes at 25 ºC.

It is also possible to estimate the number of exchanged

electrons, n, in the anodic processes, by applying Eq. 8

[17,18] to the jp vs. ν1/2 plots (Figure 5),

jp = 2.99 × 10

5

[(1-α)na]1/2× nC (Dν)

1/2 (8)

where jp is the peak current density in A cm-2, C is the

lignin concentration in mol cm-3, and D is the lignin

diffusion coefficient in the black liquor (cm2 s-1).

The results were n = 2.2 and n = 5.0 for the first and

second peaks of Pt electrode, respectively. Regarding

the Ni and AISI 304 electrodes, n values of 3.8 and 5.7,

respectively, were obtained.

As expected from the shape of the Pt CVs (Figure 3),

the n values for the 2nd anodic peak were always higher.

This fact suggests that lignin oxidation in Pt electrode

occurs on two consecutive steps, with the second one

being less hindered.

As expected, the n values for Ni and AISI 304 electrodes

were lower than the sum those obtained for Pt.

Figure 5. jp vs. ν1/2 for Pt, Ni and AISI 304 electrodes at 25 ºC.

3.2.2. Chronopotentiometry

In chronopotentiometry (CP), the current flowing in the

cell is instantaneously stepped from zero to some finite

value [19] and it is recorded the potential vs. time

response [14]. Current densities, j, ranging from 2.5 to

24 mA cm-2 were applied in the CP studies. Figure 6

shows typical CP curves obtained for Pt, Ni and AISI

304 electrodes.

As illustrated in the figure, in CP measurements,

following the application of a current pulse, the potential

increases from the OCP until a value where the

oxidation of the species near the electrode is possible.

Specifically, lignin at the electrode surface is oxidised at

this potential and, during that process, the diffusion

layer grows, which explains the small slope in the

oxidation step. Eventually, diffusion can no longer

supply enough species to provide the required current

and the potential goes up until the next electrochemical

process can occur, in this case, the OH- oxidation to O2

[20]. The time required for the complete lignin depletion

from the electrode surface (before reaching the next

potential swing) is called the transition time, . As

expected, higher currents lead to shorter transition

times.

Figure 6. Chronopotentiograms of black liquor oxidation at (A) Pt, (B)

Ni and (C) AISI 304 electrodes at 25 ºC.

So, the transition times obtained in the CP studies were

recorded for each applied current. Figure 7 shows the

τ1/2 vs. j-1 plots for the three electrodes. The good

linearity of the plots allows the application of Sand

6

equation (Eq. 9) for the determination of kinetic

parameters for the black liquor oxidation [17,20],

τ1/2 = nFC(πD)1/2

2j (9)

From the slopes of the j-1 vs. τ1/2 plots, Eq. 9 leads to n

values of 7.6, 3.8 and 5.6 for Pt, Ni and AISI 304

electrodes, respectively. It is interesting to notice that,

for Pt electrode, the n value obtained by CP matches

approximately the sum of the n values obtained for the

first and second peaks of the CV study. This may be

justified by the fact that two consecutive electrochemical

steps occur and, since CP forces the flow of a constant

current, it will lead to consecutive oxidation of the

species involved in both steps, before the potential

swings to more positive potentials. In the case of Ni and

AISI 304 electrodes, the n values from CP are in

agreement with CV results.

Figure 7. Plots of τ1/2 vs. j-1 for black liquor oxidation at 25 ºC in Pt, Ni

and AISI 304 electrodes.

3.2.3 Chronoamperometry

Chronoamperometry (CA) measurements involve

following the current response with time after applying a

specific electrode potential [21]. Herein, electrode

potentials imposed ranged between -0.5 and 0.6 V.

Figure 8 shows a typically current density-time (j vs. t

plot) transient for oxidation of black liquor in Pt, Ni and

AISI 304 electrodes, at 25 ºC and 0.1 V.

It is possible to observe that highest values of j were

obtained for Pt and Ni at the beginning of experience.

The stabilised current is very similar for all electrodes.

For example, in the Figure 9 is represents a typical CA

experiment and the corresponding j-t−1/2 plot for the AISI

304 electrode, at a potential of 0.6 V.

Figure 8. Current density transient for Pt, Ni and AISI 304 electrodes

at 25 ºC and 0.1 V.

The linearity of the j-t−1/2 plots, for all electrodes, proves

that for the selected experimental conditions, the

potential is in the diffusion-controlled region; the current

transients exhibit Cottrellian behavior, allowing use of

Cottrell’s equation [17,21] (Eq. 10),

j = nFD

1/2C

π1/2

t1/2 (10)

where j is the transient current density and t is the time.

Figure 9. Current density transient and corresponding j-t1/2 plot for the

stainless steel AISI 304 electrode, at 25 ºC and 0.6 V.

The application of Cottrell equation for each electrode

resulted in a potential distribution of the number of

exchanged electrons, n (Figure 10).

It is possible to observe that platinum presents two

maximums values, at -0.1 V (n = 2.5) and 0.1 V (n =

3.5). The sum of these n values is n = 6.0. This result is

close to the cyclic voltammetry, with an error of 16.7%.

Regarding the nickel, a maximum are present at 0.1 V

(n = 2.9) and there is a discrepancy between these

7

results and those of cyclic voltammetry (n = 3.8), with

an error of 23.7%.

In the AISI 304 stainless steel electrode a peak as

noticed at a potencial of 0.2 V (n = 2.7), a very different

value when compared with the results of CV (5.7), with

an error of 52.6%.

Figure 10. Number of exchanged electrons, n, as a function of the

applied potential, for each electrode material.

It should be noted that electrochemical reactions usually

occurs at a restricted potential. Thus, for each applied

potential exists a different response of the system,

depending on the oxidation processes which are

involved. Since lignin is a large molecule and the

complete oxidation of this compound involves a higher

electron exchange, it is possible that this reaction takes

place in several stages. Thus, the existence of a

maximum value of n, at a specific potential is justified.

In the potential values above and below this, it is

observed lower values of n (Figure 10).

3.2.4 Hydrogen evolution reaction studies

Voltammetry scans in the cathodic direction were

performed in order to analyse the hydrogen evolution

reaction (HER) during black liquor electrolysis. Figure

11 presents Pt, Ni and AISI 304 polarisation curves for

HER in black liquor at 25 ºC and at a scan rate of 0.5

mV s-1. From these results, it is possible to determine

several kinetic parameters that characterise the HER on

these electrodes. The analysis is based on the

application of the Tafel expression (Eq. 11) [17,22],

η = a + b log j (11)

where η is the overpotential, j is the current density,

parameter a is the intercept associated to the exchange

current density, j0, which reflects the electron transfer

intrinsic rate, and b is the Tafel slope, corresponding to

the rate of change of j with η.

Figure 11. Cathodic polarisation curves of black liquor for Pt, Ni and

AISI 304 electrodes at 25 ºC.

Figure 12 shows the η vs. log j plots – known as Tafel

plots – necessary for application of Tafel analysis.

Figure 12. Tafel plots of black liquor for Pt, Ni and AISI 304 at 25ºC.

From the Tafel slope, b, and applying Eq. 12, it is

possible to calculate the charge transfer coefficient, α.

Then, from the intercept, a, of the Tafel plots, and by

application of Eq. 13 [17,23], j0 may also be obtained,

with the calculated parameters being shown in Table 1.

α =2.3RT

bF (12)

j0 = 10

−𝑎/𝑏 (13)

8

Table 1. Values of b, α, and j0 for platinum (Pt), nickel (Ni) and

stainless steel (AISI 304) electrodes.

b / mV dec-

1 α j0 / mA cm-2

Pt 88 0.67 6.27 x 10-2

Ni 224 0.26 1.87 x 10-2

AISI 304 205 0.29 1.28 x 10-2

It is observed that the Tafel slope for Pt is lower than

that for Ni and AISI 304, indicating that this material has

higher activity for HER. The b value obtained for the Pt

electrode is about 35% lower than the previously

reported by Ghatak et al. for HER in black liquor [10].

3.2.5 Laboratory electrolyser

Black liquor electrolysis was attempted in a small-scale

electrolyser using Ni electrodes, both for the anode and

for the cathode. CA was used to follow the cell current

after application of cell voltages, E, ranging from 0.5 to

2.5 V. The obtained cell polarisation curve is shown in

Figure 13. As expected, the current flow increases for

increasing voltages. The corresponding power density

curve, calculated by multiplying E by j is also shown in

Figure 13. The power density curve shows a relatively

linear behavior with j until a potential of 2.3 V. From that

point, an increase in the applied cell voltage is not

followed by an equivalent increase in current. Further

cell studies are necessary to optimise the black liquor

electrolyser performance, including the evaluation of the

flow of produced hydrogen gas.

Figure 13. Cell voltage (left axis) and power density (right axis) as a

function of current density for a small-scale black liquor electrolyser at

25 ºC.

A longer experiment (190 min) was made at a potential

of 2.2 V, and 1.9768 g of lignin was obtained in this

procedure. Moreover, it is noted that the current density

stabilized in a value of 5.7 mA cm-2 after 62 min.

4. Conclusions In this work a preliminary study on the electrochemical

valorisation of black liquor was performed. The anodic

oxidation of the lignin content was analysed by cyclic

voltammetry, chronopotentiometry and

chronoamperometry, at room temperature, at platinum,

nickel and AISI 304 stainless steel electrodes. Kinetic

and diffusional parameters were calculated (e.g.,

charge transfer coefficient and number of exchanged

electrons), with the values calculated by different

methods showing good agreement between them. The

hydrogen evolution reaction in the black liquor was also

evaluated using Tafel analysis, for the calculation of

Tafel slopes, exchange current densities, and charge

transfer coefficients. The performance of the Pt

electrode was always superior to that of Ni and AISI

304, although Ni showed reasonable activity for both the

liquor oxidation and reduction processes. A small-scale

laboratory black liquor electrolyser using Ni plates, both

for anode and for cathode, was assembled and tested.

The process seems to operate with reasonable

efficiency but further studies are necessary to optimise

the black liquor electrolyser performance, namely by

measuring the amount of produced hydrogen and

testing the effect of temperature. Still, present work has

shown that electrolysis has a huge potential to recover

lignin from black liquor, mainly due to its features that

include high ionic conductivity, alkaline pH, and

presence of many organic anions.

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