diff --git a/README.md b/README.md
index 3d12e3f..494e7ea 100644
--- a/README.md
+++ b/README.md
@@ -58,6 +58,12 @@ from pyfcm import FCMNotification
 
 fcm = FCMNotification(service_account_file="<service-account-json-path>", project_id="<project-id>")
 
+# Google oauth2 credentials(such as ADC, impersonate credentials) can be used instead of service account file.
+
+fcm = FCMNotification(
+    service_account_file=None, credentials=your_credentials, project_id="<project-id>"
+)
+
 # OR initialize with proxies
 
 proxy_dict = {
diff --git a/pyfcm/__meta__.py b/pyfcm/__meta__.py
index 932e00d..8fdb2d3 100644
--- a/pyfcm/__meta__.py
+++ b/pyfcm/__meta__.py
@@ -2,7 +2,7 @@
 __summary__ = "Python client for FCM - Firebase Cloud Messaging (Android, iOS and Web)"
 __url__ = "https://github.com/olucurious/pyfcm"
 
-__version__ = "2.0.3"
+__version__ = "2.0.4"
 
 __author__ = "Emmanuel Adegbite"
 __email__ = "olucurious@gmail.com"
diff --git a/pyfcm/baseapi.py b/pyfcm/baseapi.py
index 7fb334f..ad4ef98 100644
--- a/pyfcm/baseapi.py
+++ b/pyfcm/baseapi.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import json
 import os
 import time
@@ -26,8 +28,9 @@ class BaseAPI(object):
 
     def __init__(
         self,
-        service_account_file: str,
+        service_account_file: str | None,
         project_id: str,
+        credentials=None,
         proxy_dict=None,
         env=None,
         json_encoder=None,
@@ -38,6 +41,7 @@ def __init__(
         Attributes:
             service_account_file (str): path to service account JSON file
             project_id (str): project ID of Google account
+            credentials (Credentials): Google oauth2 credentials instance, such as ADC
             proxy_dict (dict): proxy settings dictionary, use proxy (keys: `http`, `https`)
             env (dict): environment settings dictionary, for example "app_engine"
             json_encoder (BaseJSONEncoder): JSON encoder
@@ -49,10 +53,11 @@ def __init__(
         self.FCM_REQ_PROXIES = None
         self.custom_adapter = adapter
         self.thread_local = threading.local()
+        self.credentials = credentials
 
-        if not service_account_file:
+        if not service_account_file and not credentials:
             raise AuthenticationError(
-                "Please provide a service account file path in the constructor"
+                "Please provide a service account file path or credentials in the constructor"
             )
 
         if (
@@ -85,7 +90,12 @@ def requests_session(self):
             self.thread_local.requests_session = requests.Session()
             self.thread_local.requests_session.mount("http://", adapter)
             self.thread_local.requests_session.mount("https://", adapter)
+            self.thread_local.token_expiry = 0
+
+        current_timestamp = time.time()
+        if self.thread_local.token_expiry < current_timestamp:
             self.thread_local.requests_session.headers.update(self.request_headers())
+            self.thread_local.token_expiry = current_timestamp + 1800
         return self.thread_local.requests_session
 
     def send_request(self, payload=None, timeout=None):
@@ -120,17 +130,20 @@ def send_async_request(self, params_list, timeout):
 
     def _get_access_token(self):
         """
-        Generates access from refresh token that contains in the service_account_file.
+        Generates access token from credentials.
         If token expires then new access token is generated.
         Returns:
              str: Access token
         """
         # get OAuth 2.0 access token
         try:
-            credentials = service_account.Credentials.from_service_account_file(
-                self.service_account_file,
-                scopes=["https://www.googleapis.com/auth/firebase.messaging"],
-            )
+            if self.service_account_file:
+                credentials = service_account.Credentials.from_service_account_file(
+                    self.service_account_file,
+                    scopes=["https://www.googleapis.com/auth/firebase.messaging"],
+                )
+            else:
+                credentials = self.credentials
             request = google.auth.transport.requests.Request()
             credentials.refresh(request)
             return credentials.token
diff --git a/requirements.txt b/requirements.txt
index c1e330e..ef4d0df 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,6 @@
 aiohttp>=3.6.2
 cachetools==5.3.3
-google-auth==2.29.0
+google-auth==2.22.0
 pyasn1==0.6.0
 pyasn1-modules==0.4.0
 rsa==4.9
diff --git a/tests/test_fcm.py b/tests/test_fcm.py
index f7f3b7c..ecabc93 100644
--- a/tests/test_fcm.py
+++ b/tests/test_fcm.py
@@ -4,7 +4,7 @@
 
 def test_push_service_without_credentials():
     try:
-        FCMNotification(service_account_file="", project_id="")
+        FCMNotification(service_account_file="", project_id="", credentials=None)
         assert False, "Should raise AuthenticationError without credentials"
     except errors.AuthenticationError:
         pass